Wikijunior:Solar System/Complete

Wikijunior

The Solar System

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The Volunteers and Editors at
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Introduction

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

Introduction

These planets are little bigger than dots in the night sky, but if you call them dots, then Earth (in the marble-sized group, topleft) is smaller than a grain of sand.

Wikijunior books welcomes you to the children's book Solar System. Outer space is perhaps the final frontier for humanity. Even though the rest of the solar system objects may seem like tiny dots from Earth, our celestial neighbors are still important to learn about. If, when you grow up, you are going to be an astronaut and travel in space, you will need to know quite a bit about the solar system. And even if you don't travel to space, the things other people do there will affect you, so you need to know about it. Also, if you meet an astronomer or an astronaut, you do not want to sound ignorant! The importance of learning about the solar system has led many experts here at Wikijunior to donate their time and talents to bring this volume together.

Wikibooks is a project of the Wikimedia Foundation, aimed at providing free, easily available, quality reading for adults and children to promote the global spread of knowledge. Traditional publishing houses make the bulk of their income from re-issues of classic books, new books by authors with long track records, or celebrities who are famous in their own right. The chances of a truly good new work being published solely on the basis of merit skyrocket when the traditional business model is overturned and the wellspring of new talent out there is tapped using the Internet.

With this project we have reached a crossroads between the books of yesterday and the encyclopedia of everything for tomorrow. Simply by reading this book and telling others about it, you have advanced the cause of free access to information and of democratizing the field of publishing. Thank you, and once again, welcome.

Studying the Solar System

A stamp issued by the Soviet Union, showing Sputnik's planned orbit path around the Earth.

Scientists are still exploring the universe. Whether things are very tiny, like the cells of plants and animals, or very big, like a solar system or a galaxy, there is still a lot that scientists don’t know.

Scientists who study space are called Astronomers or Astrophysicists. They explore the solar system in two different ways. Astronomers do it by observing celestial bodies through telescopes, while astrophysicists (a specialized class of astronomers) try to explain the observed phenomena using physics, as suggested by the name, and theorize about what is still unseen or unknown.

Telescopes were invented in the early 1600s in Europe and allowed curious scientists like Galileo Galilei to look at very distant things in close-up and see details of the solar system and the universe that nobody had ever seen before. Using his telescope, Galileo was the first person to see the rings around Saturn and draw a very detailed picture of the moon. He also saw the four largest moons of Jupiter, sometimes called the Galilean moons, and saw spots on the Sun. Telescopes on Earth and in space are still used to explore the Solar System. There are several types of telescopes. The most common ones are optical telescopes, such as Galileo's (optical means relating to light, which is what these telescopes see), and radio telescopes, that pick up radio waves from outer space (radio waves occur naturally; they don't have to be made by humans).

Until the 1950s, humans were limited to exploring the Solar System from the ground. However, in 1957, the Soviet Union (now Russia and several other countries) launched the very first satellite, Sputnik 1 (pronounced like spoo-tneek). Since then, humans have been launching vehicles into space to explore the Solar System—some manned (with people) and some unmanned (without people).

Now, the Solar System is full of human-made probes exploring the planets and moons of the solar system. The probes send back information to Earth that scientists study to figure out what it means. Every year, scientists learn more about the Solar System. Sometimes they learn things about other worlds that remind us of Earth. Other times, the things they learn are very strange. Everything they learn helps us understand more about Earth, Earth's history, and Earth's neighborhood.

How is the Solar System measured?

It’s important that scientists use measurement to tell how big, how hot or cold, or how far away something is. In science, people use the metric system, which is named after its basic unit the metre. Below is a description of all the types of measurement used in this book.

Distance or length/width

For indications of measurements, such as how distant something is or how long or wide it is, scientists use kilometres or metres. Units of the metric system (a kilometre is 1000 metres, a metre is little more than 3 feet in the old imperial unit system still in use in some regions). Kilometres is often shortened to km, and metres is often shortened to m. Kilometres and metres can also be spelled as Kilometers and Meters, but the International Bureau of Weights and Measurements uses the -re versions as the official spelling.

Since distances outside of the Earth get so vast, scientists have also invented new units of measurement to make it easier to measure large distances in space. They invented the Astronomical unit (㍳) which is equivalent to 149 597 871 kilometres. One Astronomical Unit is the approximate distance between the Earth and the Sun. The mean distance between the sun and Neptune (the farthest planet from the sun) is 30.1 ㍳, or 4.503 billion kilometres. This is why it's good to use ㍳ for a big distance like that: 30 distances from the Earth to the sun is easier to understand than four and a half billion kilometres. You might not realize there was something wrong if someone told you the distance from the sun to Neptune was 45.03 million kilometres, but if you thought of it as 0.301 ㍳, you would know it couldn't be right.

In astronomy, they have leveling-up scales similar to the metric (10 mm in 1 cm, 100 cm in 1 m) and customary (12 in. in 1 ft., 3 ft. in 1 yd.) scales. Usually, these distances are not used within the solar system, but they are important to know if you want to be an astronomer or astrophysicist.

1 Light-year (ly) = 63241.077 ㍳

1 Parsec (pc) = 3.26 ly

1 Kiloparsec (kpc) = 1000 pc

1 Megaparsec (mpc) = 1000 kpc

1 Gigaparsec (gpc) = 1000 mpc

To help you visualise just how big some of these are,

4.22 ly = The distance from Earth to the nearest star (Proxima Centauri) other than the sun

1.3 pc = The distance from Earth to Proxima Centauri

34 kpc = The length of the Milky Way

0.76 mpc = The distance from Earth to the nearest Galaxy, the Andromeda Galaxy

14 gpc = The radius of the observable universe

Mass

In order to measure how big something is, scientists measure the mass of an object in kilograms or grams. There are 1000 grams in a kilogram. Scientists do not use weight, because weight is a measurement that means how hard gravity is pulling on an object. An object’s mass is the same wherever you are in the solar system because it measures how much stuff, or matter, a thing is made up of. Your weight will change because the amount of gravity there is varies from place to place.

On Earth, mass and weight are the same. If you weigh 30kg (short for kilograms) on Earth, your mass is 30kg. If you are floating around in space, your weight if you try to stand on a set of scales will be 0kg, but your mass is still 30kg. You are still made up of the same amount of matter.

Temperature

Temperature is a numeric reference, on a scale of degrees, on how hot or cold something is in relation to a "constant" reference. There are several scales. In our everyday lives, we measure temperature in degrees Celsius, written °C for short (the little circle ° means "degrees"), or degrees Fahrenheit, written °F for short. But scientists, especially astronomers, use degrees Kelvin to measure temperature, written K for short (with no °). Don't use Celsius or Fahrenheit for temperatures in astronomy!

Some important temperatures to know in Kelvin:

• 0K (−273.15°C) is maximum coldness, also called absolute zero. This is a great thing about measuring temperature in Kelvin: that the number is always positive and tells you how much warmer things are than the coldest they could possibly be.
• The freezing point of water is 273.15K (0°C), and the boiling point of water is 373.15K (100°C).
• A sunny day of 30°C (86°F) would be 303.15K. This is 273.15 + 30, because degrees change equally in Kelvin and Celsius.

Introduction for Parents, Guardians, and Educators

The Solar System is a Wikijunior book written by a group of volunteers and made freely available to Internet users, printers, and distributors under the terms of its license. It is the result of cooperation between The Beck Foundation, The Wikimedia Foundation, and volunteer writers and editors.

The volunteer writers and contributors thank you for obtaining this book. By making it available to a young person, you complete the goal of the Wikijunior project, which is to encourage reading and literacy among young people.

The original text and graphics are available at http://www.wikibooks.org and printed versions may be available from many different entities under license.

Again, thank you, and enjoy.

Next Topic: Solar System

Solar System

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

The Hubble Space Telescope. This telescope is in space. It takes pictures of things that are too far away to be seen with a regular telescope.

Do you ever wonder about the things in the sky—the Sun, the Moon, the stars? People have been watching the sky for a long time, trying to figure out what is out there. We keep coming up with new ways to learn more about outer space.

Planets are big balls of rock or gas that move around stars. We live on one we call the Earth, which moves around a star we call the Sun. There are at least seven other planets moving around the Sun and a lot of other smaller things as well. All these things together are called a system. The Latin word for the Sun is Sol, so we call this system the Solar System. Far beyond our own Solar System are stars, bodies like the Sun but in some cases much bigger.

The eight planets of the Solar System, and the sun. Sizes are to scale, but distances are not.

Thousands of years ago, a man named Aristarchus said that the Solar System moved around the Sun. Some people thought he was right, but many people believed the opposite: that the Solar System moved around the Earth, including the Sun (and even the other stars). This seems sensible because the Earth doesn't feel as if it's moving, does it?About 500 years ago, another man named Copernicus said the same thing as Aristarchus: that all the planets moved around the Sun and the stars were fixed in space. This time, more people agreed, but there were still people who thought the opposite. Then, about 100 years later, a man called Galileo Galilei began looking at the sky with a new invention: the telescope. He showed that it was very likely that all the planets moved around the Sun. This time, even more people thought Galileo may be right and that the Earth really did move around the Sun. Soon, more and more people started using telescopes to study the sky. However, there were still some people who thought Galileo was wrong, and he was even arrested and taken to court for lying. All the people who believed him began to learn how the planets and the other things in the Solar System moved so they could prove that he was not lying. Thousands of years after Aristarchus, people finally said "Okay, the Earth does move around the Sun". Galileo couldn't be called a liar anymore.

We can use very large telescopes to see what has happened to other stars. We can compare pictures of distant stars with pictures of our own star, the Sun. We live in exciting times because, for the first time, we have sent people into space, and we also have telescopes in space. These telescopes in space take thousands of pictures of the planets, our sun, and the distant stars. On Earth, people use the photos to learn about all the different things in the Solar System and they try to explain how the Solar System began. We even have a robot on the red planet Mars that moves around, and people on Earth tell it where to go and what to photograph. We also want to know what will happen to the Earth and the Solar System in the future.

What is in the Solar System?

The Solar System, showing the Sun, inner planets, asteroid belt, outer planets, an outer dwarf planet, and a comet. (Not to scale!)

At the center of the Solar System is the Sun. It is a star, like the billions of other stars in the sky. The other stars are very, very far away, so they look tiny. The Sun is important to us because it gives us heat and energy that allows life. None of the life on Earth could exist without the Sun.

The rest of the things in the Solar System orbit (travel around) the Sun. The planets are the largest of these. Each planet is a little like the Earth. But the planets are also very different from each other.

Many of the planets have moons. A moon orbits a planet. Mercury has no moons, and neither does Venus. Earth has one. Saturn has more than 80!

The planets closest to the Sun are called the inner planets. These are Mercury, Venus, Earth, and Mars. Then comes a big ring of asteroids, chunks of rock much smaller than planets. This ring is called the asteroid belt. Within the asteroid belt, there is a dwarf planet (smaller than a normal planet) named Ceres. Then come the outer planets: Jupiter, Saturn, Uranus, and Neptune. Farther out there are two dwarf planets, Pluto and Eris.

The planets have the names of Roman gods that were worshipped by people thousands of years ago, though no one believes in them now. Did you know that some days of the week are also the names of ancient gods? Saturday means "Saturn Day". Thursday means "Thor Day". Thor was a Viking god and the son of Odin. Monday and Sunday simply mean "Moon Day" and "Sun Day". Some of the months are also named after Roman gods. The month of "March" is named after the Roman god "Mars"—he was the god of War!

Beyond the orbit of Neptune is another big ring of things like the asteroids, called the Kuiper belt. Kuiper (said "KYE-per") was the last name of the person who first wrote about it. Most of the things in the Kuiper belt are hard to see through telescopes.

After the Kuiper belt comes the Oort cloud. Scientists think this is where comets come from. It is very far away, many times farther away than Pluto is from the Sun (over a thousand times). It is near the edge of the Solar System. (Yes, "Oort" was the last name of the person who first wrote about it.)

Zodiacal light.

In between all the other things is dust. The pieces of dust are very far apart, but they shine in the light of the Sun. Before dawn, in September or October, they glow in the East. We call this the zodiacal glow or zodiacal light.

When pieces of space dust hit the Earth's atmosphere, they burn brightly. We call them shooting stars or meteors.

The Sun creates solar wind—a kind of gas that blows away from the Sun into space. This gas travels out past the planets into outer space. The edge, where the solar wind meets the wind from other stars, is called the heliopause. That is about 100 times as far from us as the Earth is from the Sun. Beyond that, there is a lot of empty space. The nearest star to our Sun is thousands of times farther away than the size of the entire solar system. The Universe is a really huge and empty place!

What holds it together?

Sir Isaac Newton, the discoverer of gravity. It is said that he came up with the concept of gravity when an apple fell on his head.

Why do all of the planets orbit the Sun? Why do moons orbit planets? Why doesn't the Sun move away and leave the planets behind? The answer to all of these questions has to do with gravity. Gravity is a force that is a property of mass. It pulls things together.

We don't notice the pull from the Sun because it also pulls on the Earth by the same amount. But the Sun's gravity is strong enough to keep the Earth from shooting away. Even though the Earth is going fast, it keeps turning to go around the Sun. It is like they were tied together with an invisible string. In the same way, moons orbit many of the planets. They are kept there by gravity. The Sun itself does not sit still in space. The entire Solar System orbits the center of our galaxy. The whole thing stays together because of the force of gravity.

About mass

Everything is made of matter. The amount of matter is called mass. Two apples have twice the mass of one apple. The more mass a thing has, the more gravity pulls it, and the more its gravity pulls other objects. We don't notice the pull from an apple because it is so much less than the pull from the Earth. If you stand on the ground and let go of an apple, gravity will pull it down towards the center of the Earth. It will hit the ground. If you could throw the apple hard enough at the right angle, it would go into orbit around the Earth. That is how rockets put astronauts into orbit. If you threw the apple really, REALLY hard in the right direction, it would fly away from Earth and never come back, but our arms are not that strong.

The force of gravity from anything is strongest when very close to that thing and weaker when further from it. Scientists use weight to mean how hard gravity pulls us. Astronauts weigh less on the moon because it has less mass. It does not pull as hard. We actually weigh a tiny bit less on top of a tall mountain than we do in a lower place. This is because we are farther from most of the Earth.

Who discovered the Solar System?

Anyone who looks up at the sky enough can see seven bright objects. These are the Sun, our Moon, Mercury, Venus, Mars, Jupiter, and Saturn. People have known about them for a very long time. Ancient people thought they were related to gods. In Babylon, they named the days of the week after them. Almost everyone was sure that all these things were orbiting the Earth. They did not know we lived in a Solar System.

In about 1500, Nicolaus Copernicus figured out that the planets orbit the Sun. Only the Moon orbits the Earth. But he was afraid to say so for most of his life and only published a full account of his ideas in 1543, the year of his death. Then Galileo Galilei pointed a telescope at the sky. He found moons orbiting Jupiter. He was certain Copernicus was right, and he got in trouble for saying so. It took seventy years to convince scientists that the planets orbit the Sun. Now, almost everyone on Earth understands that we live in a Solar System. People made better telescopes and found more things in the sky—moons, new planets, and asteroids. More things, like the dwarf planet Eris, are being found today.

How have we explored the Solar System?

The Voyager 2 spacecraft.

An artist's impressions of Spirit.

Before the telescope, people explored the sky with their eyes. They saw how the planets seemed to "wander" through the sky. They learned to predict where the Sun, the moon, and planets would be in the sky. They built some observatories—places for watching the sky. Observe is a more scientific word for watch. They observed the Sun and stars to tell the time of year. In China, they even knew when the moon would block the Sun. Most people thought that celestial bodies could cause war or peace on Earth.

After telescopes were first made in the early 17th century, people kept making them better. Astronomers saw that planets are not like stars. They are worlds, like the Earth. They could see that some planets have moons. They began to think about what these worlds were like. At first, some thought that the other planets and moons had people or animals living on them. They thought about how it would be to live on these other worlds. Then they made telescopes better and sent spacecraft into space, and found that there were no plants or animals on the Moon or on Mars.

Now, we can explore by going to some of the other worlds. Twelve Astronauts walked on the Moon about 35 years ago. They brought rocks and dirt back to Earth. Spacecraft flew by Venus, Mars, and the outer planets. The pictures they took showed us a lot of what we know about these worlds. Robots landed on Mars in 1971, 1976, and 1997. They took thousands of pictures of the planets. They send photos and movies back to Earth. They also check rocks to find out what they are made of.

So far, we have not found any life except on Earth. Maybe tiny, one-celled life once lived on Mars. Maybe there is life under the ice on Jupiter's moon Europa. New spacecraft are being planned to look for life on these worlds.

How was our Solar System formed?

Our Solar System is part of the Milky Way galaxy. Galaxies are big mixes of dust, gas, stars, and other things. Inside our Milky Way galaxy are clouds of dust and gas where stars are born. Our Solar System was created in this kind of cloud. A part of the cloud began to get smaller and less spread out. It formed a big, spinning disk of gas and tiny pieces of dust. This disk was thickest in the middle. The middle slowly collapsed until it became the Sun. We are still trying to learn how the planets were formed. Most scientists think that they were formed from leftover gas and dust.

An artist's illlustration of how the Solar System began.

This is how it could have happened. The rest of the disk continued to spin around the Sun. The tiny pieces of dust hit each other, and some of them stuck together. Next, the bits of dust slowly collected to form grains, which in turn joined to form lumps the size of gravel, then pebbles, and then rocks. The rocks crashed together into mountains. The mountains crashed together to make bigger things. These big things swept up most of the rest of the disk to form the planets, moons, and asteroids.

The Sun got hotter as it collapsed. It began to glow. The temperature at the center reached a million degrees Celsius. The Sun started to produce a lot of light and heat. This light and heat swept away most of the leftover dust and gas between the inner planets. This light and heat are the sunlight we see and feel every day on Earth.

What will happen to the Solar System?

In another five billion years, the Sun will use up most of its hydrogen fuel. It will enter the final stages of its life. The middle of the Sun will shrink down and become even hotter. The outer layer of the Sun will grow much bigger than it is now. It will form a red giant.

It will be so big that Mercury and Venus, probably Earth and maybe even Mars will be inside it. These planets will burn away. Which planets get destroyed will depend on how much mass the Sun has left. A strong solar wind will blow some of the outer layers of gas away from the Sun. The Sun will have less mass. The Sun’s gravity will be less. All of the planets will move further away from the Sun.

The massive, rapidly-aging star Eta Carinae throws off a giant cloud of gas, forming a planetary nebula.

After it has been a red giant for a while, the Sun will start to burn helium. It will shrink down and will not be a red giant any more. It will use the helium up in about a billion years. Then it will become a red giant once again. More gas will blow away for a few hundred thousand years.

A planetary nebula will form. The nebula could last for a few thousand to a few tens of thousands of years. It will glow in the light of the Sun.

At the center, the Sun might shrink into a tiny star called a white dwarf. That kind of star is about the size of Earth. It would take about 100 of these white dwarfs, stacked end to end, to be as wide as the Sun is today. The Sun will not have any more fuel to burn. It will have lots of heat left over and will keep getting cooler and dimmer. Then its light will go out in a hundred billion years from now.

Next Topic: The Sun

Mercury

Mercury from the MESSENGER spacecraft.

What is Mercury?

Mercury Facts

Mercury orbits around the Sun faster than any other planet. Mercury's surface temperature can vary from -180°C (-300°F) to 430°C (800°F). On Earth, the hottest temperature was recorded at 58°C (136°F). There may be ice on the top and bottom of Mercury. Mercury is the smallest planet in the solar system. The ancient Romans named a day of the week after the planet Mercury; even today, in French Wednesday is Mercredi, in Spanish Miércoles.

Mercury is the closest planet to the Sun. It is a terrestrial planet; that means a planet made from rock like Earth. It does not have a gas atmosphere, so there is no weather. For a long time, only one spacecraft, Mariner 10, had visited Mercury. In January 2008, the MESSENGER spacecraft went by Mercury. It has gone by Mercury two more times, and has started to go around the planet in 2011.

How big is Mercury?

Comparison of the size of Mercury to the Earth

Mercury is 4879 km across. Mercury's diameter is just less than half the diameter of the Earth. It is the smallest planet in the Solar System. Only dwarf planets like Pluto are smaller. Because Mercury is much closer to the Sun than the Earth, it can only be seen just after the sun goes down at night or shortly before it rises in the morning.

What is Mercury's surface like?

View of the surface of Mercury

Mercury has craters like those on the Earth's moon. The largest crater on Mercury is the Caloris Basin. It is about 1300 km wide. It was created by a huge asteroid hitting Mercury. The asteroid was probably 100 km wide, but it hit Mercury's surface so hard that it made a much bigger hole.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

The surface also has big cliffs called scarps. They were made long ago when Mercury cooled down. It shrank, causing the surface to get wrinkled in some places. This wrinkling created the scarps.

There may also be ice on the top and bottom of Mercury. Like the Earth, these areas (called poles) don't get much warmth from the Sun. Any ice there won't melt.

It is very hot during the day (over 400°C) because Mercury is so close to the Sun. At night it is very cold because Mercury loses almost all its heat since there is almost no atmosphere to keep the warmth there. The temperature can fall to almost -175°C.

How long is a day on this planet?

Mercury rotates (spins around) much more slowly than the Earth. It takes Mercury 58 days to spin once as viewed from a distant star. Because Mercury orbits the sun very quickly, a day on Mercury lasts longer than 58 days. If you were standing on Mercury at the equator and timed how long it took the sun to go from directly overhead to sunset to sunrise and then rise directly overhead again, it would take 176 Earth days. These long days and nights allow for the temperatures to rise as high, and fall as low, as they do.

How long is a year on Mercury?

Mercury from Mariner 10

Mercury has the shortest year in the Solar System. It is about 88 Earth days long.

It used to be believed that the same side of Mercury always faced the Sun. In order for this to be true, Mercury would have to take the same amount of time to rotate (spin around) as it does to circle the sun. From watching it carefully we now know that Mercury's rotation is somewhat faster than its orbit. Because of the way the orbit and rotation work together, on Mercury, a day (the time from one sunrise to the next) is actually almost twice as long as a year.

What is Mercury made of?

The center of Mercury is made of iron in partly-molten (liquid) form. We know that there is iron at the center because the planet generates a magnetic field. It contains more iron for its size than any other planet in the Solar System. The rest of Mercury, its thick crust, is made of a special type of rock called silicate rocks. There are craters near the poles that are constantly in shadow. Some of these craters contain ice. There is a huge crater on Mercury called Caloris Basin. It was formed when a comet hit the planet and lava or molten rock filled the impact crater. The round wall of this crater is over 2km tall.

How much would Mercury's gravity pull on me?

If you were on Mercury, it would pull you down less than half (38%)as much as the Earth. An item that weighs 100 pounds on Earth, would only weigh 38 pounds on Mercury.

Who discovered Mercury?

A computer-generated simulation of MESSENGER orbiting Mercury.

Nobody really knows who first discovered Mercury, but the first known recorded observations of it are from tablets in Assyria about three and a half thousand years ago, in the 14th century BCE. This was probably something like an informational article about Mercury's movements. In these tablets, Mercury is called (in translation) The Jumping Planet. Almost every ancient civilisation had their own written records and names for Mercury.

In 1639, an Italian astronomer named Giovanni Zupi observed that Mercury has phases, like the phases of the Moon, as it orbits around the Sun. This is evidence that it does orbit around the Sun, an idea that was fairly new at the time, suggested less than a hundred century earlier by Copernicus.

Before the twentieth century, it was a mystery to all the astronomers how long it took for Mercury to rotate. They solved this mystery in 1962, when some astronomers sent radar signals to Mercury, which then bounced back to Earth: it takes 59 days for Mercury to rotate.

It isn't easy to sent a space probe from Earth to Mercury, because Mercury is much closer to the Sun and therefore orbits the Sun much faster than Earth does. So a space probe would have to burn a lot of fuel to match its speed with Mercury, in order to orbit—or land on—the planet. In 1973, a space probe Mariner 10 was sent to take measurements of Mercury and map its surface. Because orbiting Mercury would be so expensive, Mariner 10 orbited the sun instead, and would take pictures each time it got close enough to Mercury. Unfortunately, when Mariner 10 finally ran out of fuel, it had only mapped about 45% of the surface. However, it also discovered that Mercury has an iron-rich core and a magnetic field. 29 years later, in 2004, another probe was launched, named MESSENGER. MESSENGER stands for MErcury Surface, Space ENvironment, GEochemistry, and Ranging. MESSENGER followed a complicated path that would slowly match speeds with Mercury without using much fuel. It passed by Mercury three times, and finally settled into orbit around Mercury more than six years after it was launched. By March 2013, MESSENGER had mapped 100% of the surface of Mercury.

Who is this planet named after?

A statue of Mercury.

In Roman mythology, Mercury (Latin Mercurius) was the messenger of the gods. He wore a hat and sandals with wings on them, allowing him to travel around the world very quickly. The planet Mercury was named after him because it moves faster than any other planet in the Solar System. It moves nearly 48 km every second! (Of course, the Romans didn't know that, but they could see how fast it moved across the sky.)The Greeks called this god Hermes.

Next Topic: Venus

References

NASA's Solar System Exploration[1]
Arnett, Bill. The Nine Planets[2]
Worldbook Online [3]
Worldbook@NASA, "Mercury"[4]
NASA Planetary Fact Sheet [5]
Hamilton, Calvin J. . Solarviews.com, "Mercury" [6]
Encyclopedia Mythica, "Mercury" [7]
Col, Jeananda. Enchanted Learning/Zoom Astronomy [8] 1998-2005
Usborne Internet-Linked Science Encyclopedia, Usborne Publishing Ltd. ISBN 0794503314[9]
Dickinson, Terrence. The Universe and Beyond . Firefly Books ISBN 1552093611

Venus

Venus Facts:

• Venera 7, the first space probe to land on Venus, was destroyed by the hostile conditions on Venus after only 23 minutes.
• Almost all of the surface features on Venus are named after women.
• A day on Venus is equal to 117 days on Earth.
• It has been proposed that some microbes could live in Venus's clouds.

Venus is the second closest planet to the Sun. It is a terrestrial planet. This means that we think it was created in a similar way to our planet Earth and is made of rock.

Venus. You can't see the surface in this picture; the white is the clouds.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

How big is Venus?

Earth and Venus Comparison

Venus is only slightly smaller than the Earth. This is one reason why Venus is sometimes thought of as Earth's "twin". Venus has a diameter of about 12,100 km. It has also been visited by many probes.

What is the surface like on this planet?

Maat Mons on the Surface of Venus, by radar imaging

The surface of Venus is very different from the surface of the Earth. It is very dry and hot enough to melt lead. The pressure on the surface is very high. It is the same pressure as being 1 km (3,280 feet) below the surface of the sea on Earth.

Channels on the surface of Venus, looking like river channels on Earth

Channels that look like rivers have formed on Venus. Scientists think these channels are formed from erupting lava. The lava flows along as it cools down, creating the channels. One feature only Venus seems to have is unusual volcanoes called arachnoids. These are volcanoes that have formed differently from other volcanoes we have found in the Solar System. We don't know exactly how they were formed. Venus also has volcanoes like those on Earth.

Parts of the surface of Venus look something like continents. The largest of these areas is called Ishtar Terra (land of Ishtar, from the Babylonian goddess who was similar to Venus). Deep basins like those under the Earth's oceans have also been discovered. On Venus, though, they have no water. Features like mountain ranges and meteor craters have also been found on Venus. One of the highest mountains on Venus, Maxwell Montes, is about 11 km taller than Mount Everest, the highest mountain on Earth.

On the night side of the planet, there is a strange effect called Ashen Light. For some reason, the dark side of Venus has a subtle glow. There are various theories about this. One of the earliest theories — now disproved, of course — was that there were aliens on Venus and they were celebrating a new Venusian emperor. Currently, one of the more believed ones is this: There is a high concentration of carbon dioxide. When it is hit by the ultraviolet rays of the sun, they change into carbon monoxide and oxygen, and emit green light. The entire chemical procedure is CO₂ → CO + O.

The Venera 13 Lander as depicted on a stamp, which made scientific measurements and sent back pictures from the surface of Venus

How long is a day on Venus?

Venus rotates (spins around) even more slowly than Mercury. One full rotation of Venus takes about 243 Earth days. Venus also rotates in the opposite direction to most of the other planets in the Solar System. One day on Venus, from noon to noon, depends on the length of the year as well as the rotation time, and is about 117 earth days.

How long is a year on Venus?

One year on Venus is almost 225 Earth days long. This is less time than it takes Venus to rotate on its axis and less than two Venus days.

What is Venus made of?

The surface of Venus, its crust, is covered in nothing but rock. But, the core of Venus is made of nickel-iron. The atmosphere around Venus is very thick and is made of carbon dioxide, nitrogen, and poisonous gasses that create high pressure and trap in heat.

How much would Venus's gravity pull on me?

If you were on Venus, it would pull you down almost as strongly as Earth. The atmosphere exerts a pressure at the surface more than 90 times Earth's normal sea-level pressure.

Who discovered Venus?

Because Venus is closer to the sun than we are on Earth, we always see it close to the sun in the sky. So it only appears for us just before sunrise in the eastern sky, or just after sunset in the western sky. Many cultures thought Venus in the morning and Venus in the evening were two separate things. The ancient Romans called the evening object Venus (after the goddess of love) and the morning object Lucifer (which means light bearer — a servant who walked ahead of the sun's chariot with a torch, to light the way). Nobody knows who first thought the two were a single object. The first known written description of them as one object was the Venus Tablet of Ammisaduqa, from about three and a half thousand years ago — 1581 BCE.

A map of where the Veneras 9-14 and Vegas landed.

About three thousand years later, in 1610, Italian astronomer Galileo Galilei used a telescope to observe that Venus has phases, just as the moon does. Phases happen because only the side of Venus (or of the Moon) facing the Sun is lighted. The phases of Venus supported the theory of Copernicus that the planets go around the Sun. Then, a few years later in 1639, an English astronomer named Jeremiah Horrocks observed a transit of Venus. That's what it's called when Venus passes right between the Earth and the Sun, so that Venus is visible from Earth during the day as a tiny dot passing across the Sun. In 1761 a Russian astronomer, Mikhail Lomonsov, watching another transit of Venus, saw that Venus has an atmosphere.

Not much more was discovered about Venus until the 1920s. Then, a United States astronomer, Frank Ross, observed Venus using ultraviolet light — the light that causes sunburn — and for the first time saw the structure of the clouds on Venus.

However, there is only so much that can be learned about Venus by looking at it from Earth. The first successful pictures of Venus by a space probe were taken by Mariner 2 in 1962. Mariner 2 was the first space probe successfully sent to observe another planet. It showed two important things: Venus has practically no magnetic field, and Venus has temperatures of 490 to 590 K — that's as hot as the inside of a working oven on Earth!

The first written record of Venus. It describes Venus' movement over 21 years.

Who is this planet named after?

Venus is named after the Roman goddess of love. Sometimes it can be seen shining brightly just before dawn or just after sunset, when it is called the Morning Star or Evening Star. Some people, like the Aztecs and the Greeks, gave Venus two names – one for the morning and one for the evening.

Because Venus and the Earth are the same size, scientists call Venus "Earth's sister planet". For a long time most scientists thought that Venus had plants, animals, and possibly even people. However because Venus is so hot we now know that it is impossible for anything to live there.

How long would it take people to get there?

It could take around a year and a half to get there. But it is very unlikely someone would go to Venus.

References

• Volcano World, "Lava Flows"[10]
  
• Astronomy Picture of the Day [11]
  
• Alder Planetarium[12]
  
• Mount Everest [13]
  
• Windows to the Universe[14]
  
• NASA's Solar System Exploration[15]
  Arnett, Bill. The Nine Planets[16]
  
• Worldbook Online [17]
  
• Worldbook@NASA, "Mercury"[18]
  
• NASA Planetary Fact Sheet [19]
  
• Hamilton, Calvin J. . Solarviews.com, "Mercury" [20]
  
• Encyclopedia Mythica, "Mercury" [21]
  
• Col, Jeananda. Enchanted Learning/Zoom Astronomy [22] 1998-2005
  
• Usborne Internet-Linked Science Encyclopedia, Usborne Publishing Ltd. ISBN 0794503314[23]
  
• Dickinson, Terrence. The Universe and Beyond . Firefly Books ISBN 1552093611
  

Next Topic: Earth

Earth

Earth is the planet we live on. It is the only planet in the Solar System with liquid water on its surface. It is also the only planet we know to have life on it. Earth is also known as Terra.

Earth Facts

• The Earth is the only planet we know to have life on it (such as ourselves.)
• There is oxygen on Earth, and oxygen is necessary for life.
• The Earth is the third planet from the Sun.
• The Earth is the only planet we know that has liquid water on the surface, but scientists are trying to find others.
• The Earth's axis is tilted which is why we have four different seasons.
• The Earth is 4.6 billion years old.
• The Hoba meteorite in Namibia is the biggest known meteorite to crash on Earth in one piece.

A picture taken by Apollo 17, a space mission.

How big is the Earth?

The Earth is nearly 13,000 km wide. It's the largest terrestrial planet in the Solar System.

The Earth's mass is about 5,973,700,000,000,000,000,000,000 kg. That's a lot. But it is little compared with Jupiter (319 Earths) and tiny compared with the Sun (335,789 Earths) or other stars!

What is the Earth's surface like ?

The Earth's surface is made of rock. Most of it is underwater, but not all. Islands of rock rise up out of the water. The biggest islands are called continents, of which there are seven: North America, South America, Europe, Asia, Africa, Australia, and Antarctica. The largest bodies of water are called oceans, of which there are five: Pacific, Atlantic, Indian, Arctic and Antarctic or Southern.

Anawhata beach, west of Auckland, New Zealand

The Earth's surface is made up of huge plates. They are like huge jigsaw pieces made of rock. These plates move very, very slowly, carrying the continents with them. They can rub beside each other, push against each other, or even move away from each other. If there are gaps between them, hot molten rock can rise up and make volcanoes. Where the plates rub or push against each other, earthquakes may happen. When two plates push each other's rock upwards, mountains are formed.

Zabriske Point, Death Valley National Park, California

Earth has many kinds of environments. It is cold and icy in places like Antarctica. It is hot and dry in deserts like the Sahara in Africa and Death Valley in the United States. It is cold and dry in deserts like Siberia in Russia. Where it is warm and wet, rainforests grow.

Why is there life on Earth?

Wherever we have looked on Earth, we have found living things. They may be very small, like bacteria, but they are there. We have found bacteria where it is very cold, very hot, very deep, very high or very dark.

Galileo being deployed after being launched by the Space Shuttle Atlantis

What all living things on Earth seem to need is liquid water. Wherever you can find some water, there are almost always living things there too, even if you can't see them. If we find liquid water somewhere else in the Solar System, scientists think we might find some living things there too. If we don't, there is always the rest of the universe to explore!

There is another possibility. All the living things we know need liquid water. But maybe somewhere else there are living things that don't need water. Perhaps we will need to learn how to recognize them.

What about the Earth's moon?

Earth has one moon we call... the Moon!  Sometimes it is called by its name in Latin, Luna, so we don't get confused with other planets and their moons. The Moon has also been called Selene (pronounced "suh-LEE-nee") which is Greek for moon and was the name of the Greek moon goddess.

Recently we have also found some other objects that are said to go around the Earth. The largest one, called Cruithne (pronounced "cru-EE-nyuh"), is three miles wide. In fact, it orbits (goes around) the Sun in a way that makes it keep coming close to Earth.

There are various ideas about where the Moon came from (no-one was around to see it happen, after all), but the most widely held theory is that when Earth was young, a large body hit Earth and split off a section of the Earth that is now the Moon.

How long is a day on this planet?

A day on Earth is 24 hours long. That's daytime and night time. A 24 hour day is how long it takes the Earth to spin around once. On the half of the Earth that is facing the sun it is daytime and on the half of the Earth that is facing away from the sun it is nighttime.

The spin of the earth is also the reason why the sun appears to rise in the east and to set in the west. Although it looks like the sun is moving from the surface of the earth, it is really the surface of the earth that is moving. The reason we do not feel like we are spinning is because the earth is so big compared to the size of people.

Also, the Earth is tilted at about 23°, so there are times that the North or South pole is always facing or turned away from the sun. If you live at one of the poles of Earth it can be light or dark throughout the whole day!

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

How long is a year on this planet?

A year on Earth is about 365 and 1/4 days long. That's how long it takes the Earth to orbit the Sun once. Approximately every four years we have a leap year. A leap year contains an extra day in our calendar on February 29th in order to account for the 1/4 of a day left over each year.

What is the Earth made of?

When a planet is made of rock, we call its surface the crust. Below the Earth's crust is hot rock, some of which is molten. It is in a layer called the mantle. The hot molten rock is what comes out of volcanoes. It's then called lava.

Under the mantle is the core of the Earth. We think it is made from solid iron and nickel, surrounded by hot molten iron. The temperature there is very very hot!

The Earth's crust is very thin compared to the mantle and the core. But it is very thick to us. Nobody has drilled all the way through it yet. Even the deepest underground mines are far away from reaching its deepest base.

How much does the Earth's gravity pull on me?

It's easy to find your weight on Earth by using a scale. You have weight because the Earth's gravity pulls you towards its center. Normally, the ground or the floor get in the way, making you feel 'stuck' to them.

There are several kinds of scales:

A pan balance

1) Comparing of 2 masses (weights). You put the thing(s) you want to weigh on one pan (like some marbles), and then you put several "weights" on the other pan until the pointer shows that both pans have equal weights on them. Then you look at the pan with the known weights on it, and add them all up. The total is the mass of the thing(s) you want to weigh.

A spring balance

2) A spring balance usually has a hook on it, with a pan. You put the thing(s) you want to weigh on the pan, the spring is pulled, and the greater the weight, the further the spring is pulled. That distance, calibrated in pounds or kilogram (or whatever), is usually shown either on a dial or on a linear scale.

electronic balance

3) There are also electronic scales that give a properly calibrated reading—grocery stores, for example, use these.

NOTE: Gravity varies slightly depending on the location where you want to get the weight; spring balances and some electronic scales can, in theory, read slightly different weights at different places because of that, but usually in practice that difference is too small to be noticed. But, because the balance type of scales work differently from the spring or electronic types, they will always read the true, correct mass. They would even give the same mass on the Moon, where gravity is much less than on Earth.

Did you know? that Sir Isaac Newton was the first person to realize that the force pulling you down to the ground was the same force that keeps the planets going around the Sun? The story goes that he thought of this when he saw an apple fall from a tree.

Gravity is a very important force. As well as keeping you firmly stuck to the Earth, it keeps the Moon going round the Earth, The Earth going around the Sun and the Sun going around the center of the Milky Way galaxy. Gravity also makes stars and planets a nice round ball shape. In fact without gravity there wouldn't even be a Sun, Moon or Earth because the material that they are made of would just float away into space.

Who is it named after?

The Earth seen from the Moon

The word earth is used for both planet Earth and soil. Other names had been used for Earth such as Gaia, Tellus and Terra. Gaia is the Greek goddess (meaning Mother Earth) and Terra is the Roman name of the same goddess. Tellus is the Latin for "Earth", many scientific words dealing with Earth will be derived form Latin.

Next Topic: Moon

References

"It is also the only planet we know..." [24] [25]
"The Earth is nearly..." [26] [27]
"The Earth's mass..." [28]
"Earth's surface is made of..." [29] [30]

Earth/Moon

Moon Facts

When we look at the Moon from Earth, we always see roughly the same side. Until Luna 3 sent back photos in 1959, no-one knew what the other side looked like. The Moon is nearly twice as big as Pluto. The "Man in the Moon" isn't always seen as a man. People from India see an old woman with a spinning wheel. People from Mexico see a rabbit! The Moon isn't that small compared to the Earth - it's actually the largest moon in the solar system, relative to the size of its planet. Sometimes the Earth and Moon together are called a binary or double planet system. The Moon is the fifth largest moon in the solar system. In 2002, another object appearing to be an asteroid was observed orbiting around the Earth. Later it was found to be a rocket booster.

The Moon is our nearest neighbour in space.

How big is the moon?

Comparison of the size of the Moon and the Earth

Most of the planets in the Solar System are much bigger than their moons, but the Earth and the Moon are much closer in size. The Moon is just under 3,500 kilometers (km) wide. That's over a quarter of the size of the Earth (about 12,600 km wide) as you can see in the picture below. Because of this, the Earth and Moon together are sometimes called a binary or double planet system.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

What is the moon's surface like?

Astronaut Harrison Schmitt collecting rocks from the Moon during the Apollo XVII mission.

A map of the moon.

The Moon has no atmosphere. It also has no liquid water on its surface. During the day it becomes very hot, but at night it is icy cold. A person visiting the Moon needs an air supply and a special suit.

The Moon has many craters on its surface. The largest one is called the South Pole-Aitken Basin and is roughly 2500 km across.

We think nearly all the craters on moons or planets were made by huge rocks hitting them a long time ago. These collisions are called impacts.

Some of the craters on the Moon look as if they have rays coming out of them. These rays are rocks thrown across the Moon by the impacts that made the craters. Some of the craters around the poles of the Moon may have ice in them.

There are also darker areas called maria (said "MARR-ee-ah"). These are large pools of lava that cooled a long time ago. Most maria are on the side of the Moon we see from Earth. The lighter areas on the Moon are highlands.

How long is the Moon's revolution?

The Moon takes just over 27 Earth days to rotate (rotate means spin around) once.

How long is a year on the moon?

The Moon also takes just over 27 days to orbit (move around) the Earth. This is why we always see the same side of the Moon when we look from the Earth. We call this side the near side. The other side we call the far side. In 1959, a probe called Luna 3 sent back pictures of the far side. That was the first time anyone saw what it looked like.

What is the Moon made of?

The surface of the Moon is made of rocks and dust. The outer layer of the Moon is called the crust. The crust is about 70 km thick on the near side and 100 km thick on the far side. It is thinner under the maria and thicker under the highlands. There may be more maria on the near side because the crust is thinner. It was easier for lava to rise up to the surface.

We think the Moon has a small core (center) about 300 km across. The core is composed of solid iron. Because the core is solid, the Moon does not have its own magnetic field.

How much would the Moon's gravity pull on me?

If you were on the Moon, it would pull you down about a sixth as much as the Earth does, so you'd weigh a sixth as much. So would anything else. That's why it was much easier for the astronauts visiting the Moon to pick up rocks there.

Who is the moon named after?

The goddess Diane, the goddess of the Moon in Roman mythology

The names Moon and month both come from the ancient Greek name for the Moon, Mene. There have been other names for the Moon, like Selene and Luna. Selene was the Greek goddess of the Moon. Luna was the Roman goddess of the Moon. The Roman people also associated their goddess Diana with the Moon.

Who discovered the Moon?

Ancient Greece and Ancient China noted over 2000 years ago that the light from the Moon is reflected from the Sun. Also, Ancient Greece noted that the Moon causes tides on Earth. More recently, Luna 1 was the first spacecraft to perform a flyby of the moon. Luna 2 was the first spacecraft to land on the moon, and Luna 3 was the first to photograph the far side of the Moon which you can not see from Earth. Luna 1 through 3 all were launched in 1959. Surveyor 3, in 1967, was the first to examine the soil of the Moon. It dug to 17.5 cm. In 1969, Apollo 11 was the very first spacecraft to land people on the moon.

Next Topic: Mars

References

"The Moon is just under..." [31] [32]
"So the Earth and the Moon together..." [33] [34]
"The Moon does not have any atmosphere." [35] [36]
"During the day it becomes..." [37] [38]
"The largest one is called..." [39] [40]
"These rays are rocks..." [41]
"Some of the craters around the bottom..." [42] [43]
"There are also darker areas..." [44] [45]
"The lighter areas..." [46]
"The Moon takes just over 27..." [47] [48]
"We call this side..." [49] [50] [51]
"The other side we call..." [52] [53]
"The surface of the Moon..." [54] [55] [56] [57]
"...it would pull you down..." [58] [59]
"The names "Moon" and..." [60] [61] [62]

Mars

Mars Facts:

• Mars is red because of rust in the surface rocks
• A volcano on Mars called Olympus Mons is the highest mountain in our Solar System.
• Mars has polar ice caps that look like the ones on Earth.
• Mars has ancient river beds where scientists think liquid water flowed millions or billions of years ago.
• The Tooting crater on Mars was named after a suburb in London because the discoverer "thought [his] mum and brother would get a kick out of having their home town paired with a land form on Mars".

Mars is the fourth planet from the Sun. It is called a terrestrial planet because its outer layers are made of rocky material like the Earth.

How big is the planet?

Comparison of the size of Mars and the Earth

Mars is the second smallest of the eight major planets in the Solar System. Only Mercury is smaller. It is nearly 7,000 kilometres (km) wide; just over half the width of the Earth. Its volume is about 15% of the Earth. Since a lot of the Earth is covered by water, the total surface area of the Mars is nearly as large as all of the land on the Earth. It is possible that its size may eventually permit human colonies.

What is its surface like?

A panorama view from the Mars rover Spirit.

The surface of Mars is a lot like a desert on Earth; it is very dry and dusty, but it is also very cold. There are a lot of loose rocks and dunes of fine sand. Crater impacts mark the surface, but these are not as common as on the Moon. One of the craters is the huge Hellas Planitia. It is about half the size of the continental United States. The southern half of the planet has more craters than in the north. The south is also higher in elevation.

An overhead view of Olympus Mons, the highest mountain in the Solar System.

There is an area on Mars called the Tharsis Bulge, which has four huge volcanoes. These volcanoes have not erupted for millions of years. The largest volcano is called Olympus Mons. It is 27 km tall, making it the highest mountain in the Solar System; more than three times higher than Mount Everest on Earth. It is 625 km across and takes up an area as big as the US state of Arizona. Mars also has a huge canyon called the Valles Marineris. It is much bigger than the Grand Canyon on Earth. It is 4000 km long, up to 7 km deep and up to 200 km wide. Scientists think that when the Tharsis Bulge was created, the surface of Mars cracked to form the Valles Marineris.

Like the Earth, Mars has ice caps at its poles. However, they are made from frozen carbon dioxide as well as ice. During the Martian winter at each pole, the cap grows as carbon dioxide from the atmosphere freezes. The cap shrinks again during the Martian summer. As on Earth, when it is winter at one pole it is summer at the other.

In some places, there are dry channels that look like they were made by running water. So, a long time ago Mars may have had lakes and streams made of water. Now all of the water is frozen into ice under the surface.

There is an atmosphere on Mars, but it is very thin. There is also much more carbon dioxide in it than oxygen. (Oxygen is the gas we need when we breathe in; carbon dioxide the gas we get rid of when we breathe out.) So, we would need spacesuits to visit Mars. The atmosphere helps protect the surface from smaller meteorites.

When Mars comes closest to the Sun, the atmosphere can stir up storms of dust. Some of these storms are gigantic; they can cover the entire planet in clouds of dust. Dust storms on Mars can last for hundreds of days, with wind speeds of up to 200 kilometres per hour. Huge storms like these have been seen from the Earth through telescopes.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

Mars

Phobos Deimos

How long is a day and year on this planet?

One day on Mars is only 39 minutes and 35 seconds longer than a day on Earth (1.026 Earth days). A year on Mars is almost two Earth years long (687 Earth days).

Much like the Earth, the axis of rotation of Mars is tilted at an angle. This tilt causes seasons on Mars as it travels around the Sun. Summer occurs on the half of the planet that is tilted toward the Sun, and winter on the other half. After half a Martian year has passed, the seasons are reversed. But these seasons are about twice as long as on Earth.

What is it made of?

visualization of the Martian interior.

The outer, rocky surface of Mars is called the crust. Most of the crust is made from basalt, a type of rock made when lava grows cold.

Like the Earth, Mars has a thick layer of rock below the crust called the mantle. The mantle is much hotter than the crust, and the mantle rock is partly molten. But the crust on Mars has grown thick, so the lava from the mantle no longer reaches the surface. There are volcanoes on Mars, but they are no longer active.

At the center of Mars is a core made of the metals iron and nickel. If Mars were the same size as the Earth, the core of Mars would be smaller than the Earth's core. So a larger amount of Mars is made out of rock. Because rock is lighter than the metals in the core, Mars has a lower density than the Earth.

How heavy would I be on Mars?

Detailed picture of Mars

If you were on Mars, you would be lighter, as Mars' gravity only has a force about two fifths as strong as the that of Earth's. You could lift objects that weigh almost three times as much compared to similar objects here on the Earth. You could jump up almost three times higher, and it would take much longer to fall to the ground from the same height.

Even though it looks as though you would be like a comic-book hero on Mars, there are some things you couldn't do. Although a big rock would weigh less and you could pick it up, it would still have the same mass. If you tried to catch it, it would knock you over, and if it landed on you it would crush you. A car on the surface of Mars would need the same amount of power to speed up, although going uphill would be less of a problem. It may, however, need more room to stop. Because of the reduced gravity a vehicle would not "grip" the ground on Mars as strongly, but the constant mass would keep the vehicle moving just as strongly, making it easy to go into a skid.

Who was it named after?

In Roman mythology, Mars was the god of war and agriculture. The planet Mars was named this because the planet looks red like blood, from rust in its surface rocks.

Who discovered Mars?

Nobody knows, but the earliest records we know of were by Ancient Egyptians, more than 4000 years ago, noting Mars' movement. On one pharaoh's tomb, named Seti I, Mars is drawn on the ceiling. The Babylonians (in the Middle East), Chinese, and Greeks also studied Mars more than 3000 years ago. The Greeks learned about Mars from the Babylonians, and since the Babylonians called it their god of war, named Nergal, the Greeks called it their own god of war, Ares. Exploration of Mars was first attempted in 1960, with Mars 1. It failed, along with several other missions by the Soviet Union in the 1960s. The first successful mission to Mars was in 1964, by Mariner 4, by the U.S. Most of the other Mariner missions to Mars were successful. The last Mariner mission to Mars, Mariner 9, got there in the midst of a dust storm, and orbited the planet for several months before it could get a good look at the surface. So far, all these missions were flybys or orbiters. The first spacecraft to land on Mars was Viking 1 in 1976. Viking 2 landed 19 days later. Together, they took many good pictures of Mars' surface.

Viking 1 lander site (February 11, 1978). Viking 1 lander site (1st color, July 21, 1976). Viking 2 lander site (1st color, September 5, 1976). Viking 2 lander site (September 25, 1977). Frost at Viking 2 site (May 18, 1979). Martian sunset over Chryse Planitia at Viking 1 site (August 20, 1976).

Next Topic: Asteroid belt

References

• Steven W. Squyres, Mars, World Book Online Reference Center, World Book, Inc., 2004. [63] [64]
• "a terrestrial planet" [65]
• "How big is the planet?" [66] [67]
• "How long is a day on this planet?" [68] [69]
• "What is it made of?" Steven W. Squyres, ibid.

Mars/Phobos

Phobos as seen from the Mars Reconnaissance Orbiter.

Phobos Facts:

• Phobos orbits Mars closer than any other moon in our Solar System.
• Over the next 50 million years it is expected the Martian gravity will shred Phobos into smaller pieces, possibly creating some spectacular impacts on Mars
• It takes less time for Phobos to orbit Mars than for Mars to turn round on its axis.
• Several craters on Phobos are named after characters in Gulliver's Travels.

How big is Phobos?

Mars

Phobos Deimos

Phobos is actually quite tiny compared to most other moons in the Solar System. It is not a round sphere-like object, but irregularly shaped, more like a lumpy potato. At the largest extreme, it is 26 km across, and the smallest is about 18 km across. Basically about the size of a large city here on the Earth.

Phobos in orbit above Mars, as captured by the Viking-2 orbiter, above the Martian volcano Ascraeus Mons

What is its surface like?

Phobos as seen by the Viking-1 Orbiter

From the basic appearance, it is very much as it would look on the Earth's Moon, except that the surface features are exaggerated. Just like the Earth's Moon, Phobos has no atmosphere. Also like the Earth's moon to the Earth, Phobos has one face that constantly points the same direction toward Mars, and is called tidally locked.

A photo closeup of Phobos by the Viking-1 Orbiter

One big difference you would notice is that there is almost no gravity at all on Phobos. Just by jumping with your own legs, you would be able to put yourself into "orbit" and "fly" around Phobos. The gravity is only 1/1000th as strong as it is on the Earth. This also affects "mountains" on Phobos, as there would appear to be huge cliffs and other features where on even the Earth's Moon they would have collapsed due to gravity pulling them down.

One of the most prominent features on Phobos is a giant crater named Stickney. The impact from this crater has a significant effect on the structure of the entire moon, and there are lines or "grooves" along the surface of Phobos that were formed as a result of this impact.

If you were standing on Phobos, Mars would be a significant feature in the sky, taking up almost 1/4 of the sky.

How long is a day on Phobos?

A day on Phobos is about 7 hours 40 minutes.

How long is its orbit around Mars?

Phobos is tidally locked to Mars. This means that a day on Phobos is precisely the same as the time it takes to orbit Mars. So, the same side of Phobos always faces Mars.

Phobos is very close to the surface of Mars. In fact, no other moon that has been discovered so far in the Solar System is closer to the surface of the planet that it orbits. This produces a very interesting experience to somebody on Mars, where Phobos rises from the west and sets in the east, as it travels faster than the Sun on a Martian day.

Solar eclipse by Phobos on Mars

Does Phobos cause a solar eclipse on Mars?

Just as on the Earth by the Earth's Moon, Phobos does eclipse the Sun on Mars. This is also called a transit, and produces many of the same effects that you see from a solar eclipse. If you were to see such an eclipse on Mars, it would significantly darken the Sun, but it would not go into totality as the Earth's Moon does on the Earth. This is because Phobos is too small to cover the Sun completely. Also, because the orbit of Phobos is so fast, the eclipse would happen very quickly, in just a few seconds instead of the several minutes you see an eclipse on the Earth.

Because Phobos orbits Mars so closely, an eclipse near the Martian equator will be much more noticeable than an eclipse further north or south, because Phobos is usually quite a bit closer to an observer at the equator.

Is there a future for people on Phobos?

Because Phobos is so close to Mars, and because of the very low gravity, Phobos may be a place where people and supplies are transferred before going to the surface of Mars and then going to the Earth, almost like a space station in orbit around the Earth. It is very likely that if people go to Mars as astronauts, they will be visiting Phobos as well. Phobos also has frozen water that could be useful to astronauts on Mars as drinking water and for extracting oxygen to breathe.

However, Phobos is a doomed world. In about 50 million years, it will not exist. Every year, it gets about two meters closer to the surface of Mars, and will eventually crash or be ripped apart, forming a ring around Mars which will eventually fall to the ground.

Who is it named after?

Phobos (Ancient Greek Φόβος) was named after the son of Mars, who in mythology was the god of "fear" or "fright", and one of the servants of Mars.

Asaph Hall, the discoverer of Phobos

How was it discovered?

Asaph Hall was an astronomer with the United States Naval Observatory, where he studied many of the planets and objects in the Solar System. In 1877 he discovered both Phobos and Deimos, and identified them as moons of Mars. The name for Phobos was suggested by Henry Madan, based on the book Iliad, a classical Greek book about mythology. His wife's maiden name, Stickney, gave the name of the giant crater.

How much will Phobos' gravity pull on me?

Phobos, because it is so small, has hardly any gravity at all. It has so little gravity that a 100-pound person on Phobos would only weigh 1/20 of a pound. This also means that you would be able to lift massive amounts of mass. A person who could carry 10 kg on Earth would be able to carry three elephants on Phobos.

Because Phobos' gravity is so small, it is easy to escape Phobos and float off into space. In fact, Phobos' gravity is so light that you could throw a tennis ball or a baseball and it would fly away from Phobos and become a new moon of Mars!

Mars/Deimos

Deimos Facts:

• Deimos is one of the smallest moons in the Solar System.
• Deimos has a satellite named after it, Deimos-1.
• Most people could sprint out of Deimos' gravitational zone.

Deimos as seen by the Mars Reconnaissance Orbiter

How big is Deimos?

Mars

Phobos Deimos

Deimos is one of the smallest moons in the solar system, at only 12 km in diameter, or about 1/2 the diameter of Phobos. Surprisingly, however, Deimos was spotted and identified as a moon before Phobos, partly because of its greater distance from Mars, which meant that it was not so hidden in the glare of Mars' light. The surface area of Deimos is roughly the size of a medium-sized city on Earth. It is also very irregular in shape due to its small size.

From the viewpoint of somebody standing on Mars, Deimos would appear almost as just a very bright star, and you would not be able to identify any surface features without a telescope.

What is its surface like?

A view of the surface of Deimos from only 30km away

The surface of Deimos has no atmosphere of any kind, and it is full of craters from meteors hitting the surface, just like the Earth's Moon. The surface of Deimos is made up of very dark rocks called carbonaceous chondrite, which includes a lot of the element carbon. There is also water ice on the surface of Deimos, as well as most of the interior.

Deimos has two named craters: Swift and Voltaire. Swift is named after Irishman Johnathan Swift (1667–1745), the author of Gulliver's Travels. Swift is the smaller out of the two named craters, having a 1000 m diameter, while Voltaire has a 1900 m diameter. The person Voltaire (1694–1778) was a French supporter of freedom of religion and freedom of expression.

Because Deimos is so small, the gravity on Deimos is very, very small. It is about 1/2500th of the force of gravity here on Earth. This is so little gravity that it would be very dangerous for people to walk on the surface without a tether or some other restraint to keep them from pushing themselves completely away from Deimos. If there were a structure on Deimos built for humans, it would resemble an orbital space station inside.

Because Deimos is tidally locked to Mars, it always keeps one side facing Mars during its entire orbit. This means that if you were on the side facing Mars, you would always see Mars in the same part of the sky all of the time. The other side would never see Mars in the sky. On the side facing Mars, the view of Mars itself takes up almost one eleventh of the sky, so Mars itself would be a very significant object to look at.

One of the reasons why scientists are interested in exploring Deimos is because it is believed to be an asteroid that was captured by Mars many millions of years ago. By studying Deimos and its brother moon Phobos, scientists hope to get a very close view of what other asteroids of a very similar size also look like elsewhere in the Solar System.

How long is a day on Deimos?

A "day" on Deimos lasts about 30.5 hours.

How long is its orbit around Mars?

Deimos is tidally locked to Mars, so a day on Deimos is precisely the same as the time it takes to orbit Mars. From the surface of Mars, Deimos still appears to rise from the east and set in the west like the Sun, planets, and everything else in the sky (except Phobos), but it lingers in the sky for a very long time, taking almost 3 sols (Martian days) before it sets in the western sky.

Deimos passing in front of the Sun, as seen by the Mars Rover Opportunity

Does Deimos cause a solar eclipse on Mars?

Just as the Earth's Moon can come between the Earth and the Sun, Deimos occasionally comes between Mars and the Sun. This is also called a transit, and in this case perhaps transit is a more appropriate term than eclipse.

Because Deimos is so tiny and relatively far away from Mars as well, the portion of the Sun that is covered by Deimos during an eclipse is very small, and somebody standing on the ground on Mars would hardly even notice it. It is similar to taking a flashlight and putting a small pebble on it. You can't see much difference in the light that comes from the flashlight.

Who is it named after?

Deimos (Ancient Greek Δεῖμος) is named after the Greek son of Ares (the Greek name for Mars), who is also called "panic" or even "dread". In Greek mythology, Deimos was the personification of terror. Deimos, together with Phobos and others, would often accompany Ares into battle with the gods.

Asaph Hall, the discoverer of Phobos

How was it discovered?

Asaph Hall was an astronomer with the United States Naval Observatory, where he studied many of the planets and objects in the Solar System. In 1877, he discovered both Deimos and, less than two hours later, Phobos and classified them as moons of Mars. The name for Deimos was suggested by Henry Madan based on the book Iliad, a classical Greek book about mythology.

Jupiter

Jupiter Facts:

• Due to its magnetic field trapping particles from the Sun, Jupiter is surrounded by very powerful radiation belts which would kill anyone who entered them.
• Jupiter's moon Europa is thought to have a giant ocean below its surface.
• Jupiter's moon Ganymede is the largest moon in the solar system.
• Jupiter is the third brightest object in the night sky, second being Venus and first being the Earth's moon.
• Jupiter has more than twice the mass of all the other planets in the solar system combined.
• Jupiter is considered an almost-star. The smallest red dwarf star was only 30% bigger than Jupiter.
• Jupiter's red spot and the Earth are close to the same size. That means that there is a storm going on in Jupiter that is as big as the Earth!
• Jovian in English means anything relating to Jupiter — the god or the planet. Sometimes the four outer planets are called the Jovian planets.

Jupiter is by far the largest planet in our Solar System: two and a half times larger than all of the other planets put together. It is the fifth planet from the Sun and one of the brightest planets as seen from Earth. Jupiter, along with Saturn, Uranus and Neptune, is sometimes called a "gas giant" because most of these planets are made up of liquid and gas.

How big is the planet?

Comparing the sizes of the Sun, Jupiter, Earth, and the Moon. Notice the red spot is about the size of the Earth!

Jupiter is 142,984 km or about 11 Earths in diameter at the equator. That makes it about one tenth as big in diameter as the Sun! You could fit about 1,400 Earths into the volume of Jupiter. It is 133,709 km or 10 Earths in diameter from pole to pole. Jupiter's rapid rotation (it spins around in under ten hours compared with 24 hours for Earth) makes it bulge out at the equator.

Jupiter's magnetic field is the largest single planetary thing in the Solar System. It is 26 million kilometers across, making it about 20 times bigger than the Sun. It has a tail that extends past Saturn's orbit. If it could be seen from Earth, it would appear to be five times the size of the full moon.

What is the surface like on Jupiter ?

Jupiter's atmosphere

The surface we see is not solid. This enormous planet has a relatively small solid and rocky core. Liquids and gases surround this core and blend with the atmosphere.

Jupiter is a cloudy, windy and stormy planet. It is always covered by a layer of clouds, and wind speeds of 600 km/h are not uncommon. The storms are visible as swirls, bands and spots. A particularly violent storm, about three times Earth's diameter, is known as the Great Red Spot. This storm has been in existence since at least 1831, and maybe since 1665. If the storm has existed since 1665, that would make it more than 300 years old!

The layer of clouds is divided into several bands. The lighter colored bands are called zones and the darker bands are called belts. The colors are caused by small changes in the temperature and chemistry. Each band rotates in the opposite direction from its neighbors. Along the edges where the bands meet, these winds collide and create swirling patterns.

The stormy atmosphere of Jupiter has flashes of lightning just like on Earth. However, while Earth's lightning strikes may be hotter than 50,000 °C, Jupiter's lightning strikes may go over 5,000,000 °C, which is a hundred times more than Earth lightning, and is more than the temperature of the sun's corona. The lightning is made by water near the tops of the clouds.

What are its rings like?

The rings of Jupiter as seen by the Voyager 2 spacecraft

Jupiter's rings are dark and hard to see. They are made of tiny particles that meteors knocked off Jupiter's small inner moons and debris left over from comets and other objects that came close to the surface of Jupiter. In fact, until the Voyager spacecraft arrived near Jupiter and took closeup pictures of the rings of Jupiter, scientists didn't even know that it even had rings at all. Two rings are clearly from material that can be associated with two sets of the inner moons of the planet.

These are the names of the rings and their sizes:

Rings of Jupiter

Ring Name	Inner Radius	Outer Radius
Halo	100,000 km	122,000 km
Main	122,000 km	129,000 km
Gossamer (inner)	129,000 km	182,000 km
Gossamer (outer)	182,000 km	225,000 km

Gossamer means in English anything that is delicate, light, and flimsy.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

What is the planet's atmosphere made of?

Jupiter as seen by the space probe Cassini. This is the most detailed color portrait of Jupiter ever assembled.

In the outermost layer of Jupiter lie frozen Ammonia crystals. (Ammonia is a compound of hydrogen and nitrogen; its scientific designation is NH₃.) crystals.

Jupiter's atmosphere is mainly made of hydrogen(H₂). Near the surface, there is almost 90% hydrogen. Apart from this, the atmosphere has helium(He). Because of the high pressure, helium becomes a liquid further down in the planet. In addition, Jupiter has methane(CH₄)(0.3%), hydrogen deuteride(HD)(0.003%), ethane(C₂H₆)(0.0006%), and least of all, water(H₂O)(0.0004%).

Jupiter's temperature is very high. Because of this, scientists cannot tell all the materials the planet is made of. The outer core of Jupiter has hydrogen. The pressure present can make the gas solid. However, because of the very high temperature, the gas melts, and becomes liquid.

What are its moons like?

A simulation of the orbit paths of the moons of Jupiter.

Jupiter has 95 known moons. There are four major moons that were discovered by Galileo in 1610, the first moons ever discovered around another planet. Those moons are Io, Europa, Ganymede and Callisto; they are named from characters in mythology closely associated with Jupiter. They are called the Galilean moons. There are often eclipses on Jupiter's cloud tops by the Galilean moons.

Amalthea Group

There are four small moons orbiting inside Io's orbit. That group is called the Amalthea group because Amalthea is the largest one. They are all small and potato shaped. Amalthea is very red. The material of Jupiter's rings came from meteors knocking it off of those moons. Amalthea(Greek:Ἀμάλθεια) in Greek mythology is a foster mother of Zeus.

Io

The four largest moons of Jupiter. From left to right with increasing distance: Io, Europa, Ganymede, Callisto.

Io (pronounced EYE-oh) is Jupiter's closest major moon. It is 3643.2 km across, slightly larger than Earth's Moon. It has the most spectacular volcanoes in the solar system and molten sulfur lakes. Any craters formed by asteroids hitting the surface are quickly covered up by the volcanic activity. Io's core is made of molten iron and is surrounded by a rock shell. Unlike Jupiter's other moons, there is very little water on Io. Scientists think that when Jupiter was forming, it was hot enough to dry out Io, but not the other major moons. In Roman mythology, Io(Ancient Greek:Ἰώ) was a beautiful young nymph(Nymphs are divine spirits that animate nature and generally portrayed as beautiful young women.) that Zeus loved.

Europa

Europa (pronounced Eurṓpē) is 3,121.6 km across, about 10% smaller than Earth's Moon. It is made of silicates and has a layer of smooth water ice 10 to 30 km thick. The ice has long cracks in it and very few craters. It looks like the sea ice on Earth. The ice has slid around at the cracks. We believe there is liquid water under the ice as much as 100 km below the surface. There are also some large spots on the surface. In Roman mythology Europa was courted by Jupiter in the form of a bull. Europa (Ancient Greek:Ευρώπη) in Greek mythology was a Phoenician noblewoman who was abducted by Zeus to Crete (An island of Greece).

Ganymede

Jupiter and its four largest moons, as seen through a telescope.

Ganymede is 5262.4 km across, making it 380 km wider than Mercury. It is Jupiter's largest moon and the largest moon in the Solar System. It has plate tectonics like Earth. There are older, darker regions and newer areas with grooves where the plates have moved. Newer craters have bright rays around them from material thrown up by impacts. Older craters look flat and faded because the icy surface does not hold the shape of the crater as well as rock does over long periods of time. Ganymede may have an iron and sulfur core with a silicate mantle and an icy shell. It may be similar to Io except with a layer of ice on it. In Roman mythology Ganymede was a beautiful young man who Jupiter kidnapped and made cupbearer to the gods on Mt. Olympus.

Callisto

The orbit paths of the 4 largest moons.

Callisto is 4820.6 km across, about the same size as Mercury. It has many craters. Like craters on Ganymede, the older craters have faded. The largest crater is Valhalla. It has a bright center 600 km across with rings around it up to 3000 km across. Callisto is made of silicates and ice. There is a 200 km thick icy crust with a liquid water sea under it. In Roman mythology Callisto was turned into a bear by Jupiter's jealous wife Juno. Later Jupiter placed her in the stars as The Great Bear.

Other moons

The other moons are smaller in several groups outside the orbits of the major moons. There is also a small moon, Themisto and four groups of small moons that orbit very far from Jupiter.

How long is a day on this planet?

One Jupiter day is about 10 Earth hours long. You have to say "about" because different parts of Jupiter rotate about its axis at different speeds. This is caused by the fact that Jupiter is mostly gases that are in constant motion and sometimes going in different directions. Some efforts have been made to try and measure the rotation speed of the inner rocky core of Jupiter, but that has proved to be quite difficult to accomplish due to the magnetic fields that surround Jupiter and the very active radio energy that is generated by the atmosphere of Jupiter, which interferes with measuring techniques like radar that has been used to measure the surface of Venus and Mars.

How long is a year on Jupiter?

The Great Red Spot

Jupiter Eclipses

One year on Jupiter is 4,335 Earth days or 11.87 Earth years long.

A Jupiter year is about equal to four-tenths (or two-fifths) of a Saturn year. Thus after every two Saturn years, Jupiter has completed five full orbits about the Sun. So after 59 years, Saturn and Jupiter will be back in nearly the same position. When the orbits of two planets are simple ratios of each other like this, it is called a resonance.

How much would Jupiter's gravity pull on me?

If someone were floating close to the cloud tops of Jupiter, it would pull them down with a force about two and a half times as strong as the force of Earth's gravity.

Jupiter's rapid rotation causes the equator to bulge out. This would also cancel out about 10 percent of gravity's force on them if they were at the equator. The amount of this counteraction becomes lower the closer they get to the poles.

Who is it named after?

Statue of Zeus (Jupiter) in Olympia, Greece

Jupiter (Latin Iuppiter) is named after the king of the Roman gods, also called Zeus in ancient Greece. The god Jupiter was known for causing lightning strikes on Earth. He is associated with the eagle and the oak tree.

Next Topic: Saturn

Jupiter/Amalthea

Amalthea

Amalthea Facts:

• Amalthea is the reddest object in the Solar System, even redder than Mars.
• Before 1975, Amalthea was not considered the official name (though it had extensive informal use) of the moon. Prior to the current name, it was called Jupiter V.

A comparison between Io (Larger) and Amalthea (Smaller).

Amalthea is the third moon of Jupiter. Amalthea's oribt is very close to the Amalthea gossamer ring.

How big is Amalthea?

Amalthea is not spherical like a ball, instead it looks like an asteroid. In fact, there is an asteroid in the asteroid belt named 113 Amalthea. However, the asteroid is completely unrelated to the moon. This moon of Jupiter has an average diameter of 172 km. The entire moon is actually a bit smaller than Ireland.

What is its surface like?

The surface of Amalthea is bright crimson. The reddish colour may be from sulfur or some other non-ice material. Amalthea has multiple craters, but only two are named: Pan, named after the Greek god of the wild; and Gaia, goddess of the Earth. Amalthea also has two named mountains, both named after places in Crete, the largest island of Greece: Lyctos Facula, named after Lyctos; and Ida Facula, named after Mount Ida. Facula, in Latin, means a small torch.

How long is a day on Amalthea?

One day on Amalthea is equal to about half of an Earth day. One day is 11 hours, 57 minutes, and 23 seconds, barely more time than Jupiter takes to rotate.

How long is its orbit around Jupiter?

One orbit around Jupiter also takes the same time as a day on Amalthea — about half of an Earth day. Since these two times are the same, the same side of Amalthea always faces Jupiter. Earth's moon is the same way, which is why you can always see the Man in the Moon.

How would Amalthea's gravity pull on me?

If you stood on the surface of Amalthea, you would weigh only 1/555 of what you do on Earth. A one hundred pound person would weigh only 0.18 pounds on Amalthea, which is a little more than ten 50p, or 14 quarters! This means that if you could lift ten kilograms on earth, you could hold three cars on Amalthea. The strongest weightlifter would be able to lift a blue whale, the heaviest animal on Earth.

Who is it named after?

Amalthea (Greek Ἀμάλθεια) is named after a nymph in Greek myths who nursed Zeus, which is the Greek counterpart of the god Jupiter, with goat's milk.

How was it discovered?

Edward Emerson Barnard, the discoverer of Amalthea.

Amalthea was discovered on September 22, 1892 by Edward Emerson Barnard, using the 36 inch (91 cm) refractor telescope at Lick Observatory, in California. He was the first to discover a new moon of Jupiter since Galileo discovered Io and Europa in 1610.

Jupiter/Io

Io Facts:

• Io is the most volcanically active body in the Solar System.
• Io is one of the few moons to have an atmosphere although it is very thin.
• At any given time, there are approximately 9 volcanoes erupting on Io.
• Io is so close to Jupiter that Jupiter causes "tides" in its surface. These "tides" are what makes it so volcanically active.
• Io is one of only five of Jupiter's moons that can eclipse the Sun as seen from Jupiter.
• Io is the fourth-largest moon in the solar system.

A picture of Io taken by Galileo space probe in 1999

Io is the innermost moon to Jupiter. Be careful not to confuse Io the moon with 85 Io, an asteroid, or with I.O., an abbreviation that can stand for a number of different things.

How big is Io?

Jupiter has 66 moons, but only four are large ones comparable to our Moon. Io is one of these four, and the third largest. Io is 3642.6 km, or 0.28 Earths, wide. It only has a mass of about 1.4% of the mass of Earth. It is a similar size to our Moon (but a lot more exciting!).

What is its surface like?

Io size compared to Earth and Earth's Moon

Unlike most moons, Io has a "young" surface. Because there is so much volcanic activity, the surface is almost free of craters. Also, its volcanoes are quite unusual. Because of the low gravity of Io and the explosiveness of the volcanoes, sometimes they eject material as far as 500 km above the ground. 500 km is more than the distance from ground level on Earth to the International Space Station. Sometimes the volcanoes erupt more calmly, more like geysers on Earth. There are also shield volcanoes on Io, formed mostly from lava flows.

The temperature of Io varies wildly. At some spots, near volcanic eruptions, the temperature is very hot. But most of Io is very cold, as it is so far away from the sun. The average temperature of Io is -143°C. This is much colder than −90°C, the coldest it gets in Antarctica.

A map of Io surface

In addition to volcanoes, Io also has many mountains, lakes of molten sulfur, calderas, and flows of molten sulfur or silicate hundreds of kilometers long. The colour of the surface is due to sulfur being at different temperatures. When sulfur warms up and cools down, it changes colour. This is why the surface of Io has so many colours. Some people think it looks like a pizza!

Io has 10 named regions (regions on the Earth would be the Middle East, the Far East, the West, etc) which all end in the word Regio, which means in Latin Region. All of the regions are named after ancient kingdoms and empires of Earth, similarly to how most planets and stars are named after gods and other characters from mythology. For example, the largest regio, Colchis Regio correlates to Colchis Kingdom in 550-164 BC.

Io's volcanic features are all based on various beings in mythologies other than Greek mythology and Roman mythology. For example, Masubi is a volcano, but in Japanese mythology, he is the deity of fire who is said to have caused 8 volcanoes on earth. Another example would be another volcano named Ra. Ra in Egyptian mythology was the god of the sun.

Io's mountains are a mix of many mythologies, in the aspects of their names. They all end in several Latin words vaguely meaning mountain. Danube Planum is based on the Danube river, which runs from southwest Germany to the Black Sea, while Egypt Mons is based on Egypt. On the other hand, there are mountains such as Epaphus Mensa, named after Epaphus, son of Io and Zeus; and Ot Mons — in Mongolian mythology, Ot Ene (the final "e" is pronounced) is the goddess of marriage, who was born at the beginning of the world, when the sky and the earth separated.

How long is a day on Io?

A picture of Io taken by the Cassini space probe with Jupiter in the background

It takes 42 hours (1¾ Earth days) for Io to rotate on its axis. It also takes 42 hours for Io to complete one orbit around Jupiter. Since these numbers are the same, it means that the same side of Io always faces Jupiter. (Earth's moon is the same way, which is why you can always see the Man in the Moon.)

Who is it named after?

Io (Ancient Greek Ἰώ) — pronounced eye-oh — is named after one of Zeus' (Jupiter's) 1000 lovers. She was a nymph. Nymphs are spirits of nature, associated with water, forests, and grottoes. According to the legend, Io was transformed into a cow by Zeus in order to hide her.

How would Io's gravity pull on me?

If you stood on the surface of Io, you would weigh less than you do on Earth. A person who weighs 200 pounds (90kg) on Earth would weigh about 36 pounds (16kg) on Io. So the gravity, of course, pulls less on you.

How was it discovered?

Galileo, the first discoverer of the Galilean moons in 1610.

Io was discovered by both Simon Marius and Galileo Galilei. Galileo discovered it on January 7, 1610, and Marius discovered it around the same time.

Jupiter/Europa

Europa is one of the moons of Jupiter. Europa has an icy surface. Europa is special to scientists as they believe there is an ocean under the ice. Life may live in the ocean. These lifeforms, assuming they exist, would most likely be very different than the lifeforms on Earth, even the aquatic life here. The differences will probably be very random and very large. For example, the lifeforms may have not developed sight as we know it, but could see the radio waves instead, or the micro waves instead of the colour spectrum. They might not even be solid, but liquid or gas!

Europa Facts:

• Europa may have an ocean under its icy surface
• Some scientists think that extraterrestrial life may live in Europa's oceans
• Before this moon's current name, it was simply known as Jupiter II
• This shares a name with an asteroid, 52 Europa, in the asteroid belt

How big is Europa?

Europa is 3,122 km (0.246 Earths) across. It would take 125 Europas to equal the weight of one Earth, and almost 67 Europas could be fit into the volume that Earth occupies if they could be molded together like lumps of clay.

What is its surface like?

Picture of the surface of Europa taken by the Galileo spacecraft

The Europan surface is extremely smooth; few features more than a few hundred meters high have been seen. There are very few craters on Europa, and only three are more than 5 km wide. This would seem to indicate a young and active surface; based on estimates of the frequency of cometary bombardment Europa probably endures, the surface must be no more than 30 million years old. The smoothness and visible markings strongly resemble that of sea ice on Earth, and it is thought that under the surface there is a layer of liquid water kept warm by tidally generated heat. The temperature on the surface of Europa is far below freezing, even at the equator, so water ice is as hard as rock. The largest craters appear to be filled with flat, fresh ice; based on this and on the calculated amount of heat generated by Europan tides it is predicted that the outer crust of solid ice is approximately 10-30 kilometers thick, which could mean that the liquid ocean underneath may be as deep as 90 kilometers.

Cutaway of what we think the interior of Europa could look like

Europa's most striking surface feature is a series of dark streaks crisscrossing the entire globe. These streaks strongly resemble the cracks that form in sea ice on Earth, and close examination shows that the edges of Europa's crust on either side of the cracks have moved relative to each other. Another way to look at it is that Europa resembles a cracked eggshell. The larger bands are roughly 20 km across with a central band of lighter material that is thought to have been produced by a series of volcanic water eruptions or geysers as the Europan crust spread open to expose warmer layers beneath. The effect is similar to that seen in the Earth's oceanic ridges. These various fractures are thought to have been caused in large part by the tidal stresses exerted by Jupiter; Europa's surface is thought to rise and fall up to 30 meters between high and low tides. Since Europa is tidally locked to Jupiter- the same side always faces towards Jupiter - the stress patterns should form a distinctive and predictable pattern. However, only the youngest of Europa's fractures conform to the predicted pattern; other fractures appear to have occurred at increasingly different orientations the older they are. This can be explained if Europa's surface rotates slightly faster than its interior, which is possible due to the subsurface ocean separating the moon's surface from its rocky mantle.

It has been suggested that life may exist in this under-ice ocean. Scientists who suggest this point out that life can thrive in similarly harsh conditions on Earth: around deep-ocean hydrothermal vents or in the Antarctic Lake Vostok, which is also under a thick sheet of ice. There is currently no supporting evidence that life exists on Europa, but efforts have nevertheless been made to avoid any possibility of contamination. The Galileo mission was concluded by crashing the spacecraft into Jupiter—if simply abandoned, the unsterilized craft might have eventually crashed into Europa and contaminated it with microorganisms from Earth. This would have made it impossible to determine if Europa ever had its own native life, and could even destroy native organisms if they exist.

Europa's postulated ocean has excited tremendous interest in the scientific community. Just as the Russians have been planning to drill to Lake Vostok in order to sample its water and explore for life, there has been a great deal of speculation about ways to drill through Europa's surface ice to explore it's ocean. Finding life on Europa would be a watershed moment in the history of science.

How long is a day on Europa?

It takes 85 hours (3½ Earth days) for Europa to rotate on its axis. It also takes 85 hours for Europa to complete one orbit around Jupiter. Since these numbers are the same, it means that the same side of Europa always faces Jupiter. (Earth's moon is the same way, which is why you can always see the Man in the Moon.)

Who is it named after?

Europa and Zeus in the form of a white bull

Europa (Greek Ευρώπη) is named after one of Zeus' love interests. Europa was a Phoenician princess who was abducted by Zeus and brought to Crete.

How was it discovered?

Europa was discovered by two people: Simon Marius and Galileo Galilei. Galileo discovered it on January 7, 1610. Marius also observed it around the same time.

Jupiter/Ganymede

Ganymede from the Juno spacecraft.

Ganymede Facts:

• Ganymede is the only moon of Jupiter discovered by Galileo to be named after a male person.
• Ganymede, the Solar System's largest moon, is larger than either Mercury or Pluto.
• Ganymede is the only moon to have a magnetic field.
• Ganymede is the largest moon in the solar system.

How big is Ganymede?

Ganymede is 5262.4 km, or 0.413 Earths wide, making it a little larger than the Earth's moon. It is the largest of Jupiter's moons — in fact it is the largest moon in the solar system.

Comparison in size between Earth, the Moon and Ganymede

What is its surface like?

The Tros Crater of Ganymede.

The surface of Ganymede is divided into two regions: dark regions, and light regions. The dark regions are very old and highly cratered, while the light regions are younger (but still quite old), and marked by grooves and ridges. However, craters are found on both types of terrain, and are around 3 to 3¹/₂ billion years old.

Ganymede's crust is made of water ice. Like Earth's crust, it is broken up into plates, which can move. Along the fracture zones, mountain ranges have formed. Because the crust can flow, craters tend to be flat. Sometimes, old craters become palimpsests because of erosion casued by the crust.

Recently, the Hubble Space Telescope found evidence of an oxygen atmosphere on Ganymede. This does not mean that there is life on Ganymede; it is believed that the oxygen is produced when radiation splits the water ice (H₂O) into hydrogen and oxygen. The hydrogen is lost because it has a low mass, while the oxygen forms the atmosphere.

How long is a day on Ganymede?

One day on Ganymede is equal to about 7.15 days on Earth.

How long is its orbit around Jupiter?

It takes Ganymede 7.15 days to orbit Jupiter. Ganymede orbits Jupiter at the same speed that it spins.

Who is it named after?

Bust of Ganymede.

Ganymede (Greek Γανυμήδης) is named after a handsome Trojan boy who was made cupbearer to the gods by Jupiter (also known as Zeus).

How was it discovered?

Ganymede was discovered by Galileo Galilei and Simon Marius. Galileo found it on January 11, 1610, and Marius around the same time.

Jupiter/Callisto

Callisto from the Galileo space craft

How big is Callisto?

It is Jupiter's second largest moon, after Ganymede, and the third largest moon in the solar system. It is a little more than one third the size of the earth.

What is its surface like?

The Valhalla crater.

The surface of Callisto is one of the most cratered in the Solar System. In fact, impact craters are the only features found on the surface. Because Callisto's surface is icy, large craters and mountains are slowly erased. Callisto's crust is about 4 billion years old, dating back to the formation of the Solar System.

The largest crater on the surface is Valhalla. It measures 3000 km across. The second largest crater, Asgard, measures 1600 km across. Gipul Catena, a series of craters in a straight line, was probably caused by an object that was tidally disrupted as it passed close to Jupiter.

There is no oxygen in the atmosphere. Under the rocky crust of the surface there is a salty underground sea.

How long is a day on Callisto?

One day on Callisto is equal to approximately 16.7 days on Earth.

Callisto Facts:

• Callisto's crust dates back to the formation of the Solar System.
• A salty ocean may lay beneath the crust.
• It is the second largest of Jupiter's moons.

How long is its orbit around Jupiter?

One orbit around Jupiter is also equal to 16.7 days on Earth.

Who is it named after?

Callisto (Greek Καλλιστώ) is named after the Greek Mythological nymph named Kallisto, which meant "most beautiful" in Greek. Kallisto was one of Zeus' many lovers.

How was it discovered?

Callisto was discovered by Galileo Galilei and Simon Marius. Galileo discovered it on January 7, 1610, and Marius observed it around the same time.

Saturn

Saturn Facts:

• If you could find a bathtub big enough, Saturn would float in it.
• Some of Saturn's moons control the width of its rings. These are known as shepherd moons.
• Although it is made mostly of gases, scientists believe Saturn has a small rocky core.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

Saturn is the sixth planet from the sun, and is one of the gas giants. Although it is very large – almost ten times wider than Earth – it is mostly gas and liquid, and therefore very light. Its distinctive rings aren't solid, they're actually loose clusters of moving particles of rock and ice.

How big is the planet?

Comparison of the size of Saturn and the Earth

Saturn is 120,536 km or 9.449 Earths wide at the equator.

What is its surface like?

Saturn is mostly gas and liquid. Saturn may have a small core of rock and ice. The atmosphere has bands, but they are not as colorful as Jupiter's.

This is how Saturn appears in an amateur telescope. Even with a small telescope, it is possible to clearly see the rings.

What are its rings like?

Saturn's rings are composed of rock and ice particles ranging in size from specks of dust to the size of a house. Some particles might even be a few kilometers wide! The particles in the rings are actually spaced far apart. It would be easy to pass through the rings.

What are its moons like?

Map of the Saturn system (NASA)

Saturn has 146 moons, and many of them have names.The size of Saturn's moons and the size of the chunks of ice in its rings are similar, which means that we can never know the exact number of moons. New moons are still being discovered. Saturn's biggest moon is named Titan, and is large enough to be a planet in its own right!

Shepherd moons

There are small potato-shaped moons in or near Saturn's rings. They control the ring particles with their gravity. That is why they are called shepherd moons. Six of them are known, and there may be more.

Mimas

Mimas is mostly made of water ice with a small amount of rock. It has a large crater for its size called Herschel. It is 130 km across, making it about a third as big as Mimas. The crater makes Mimas look like the Death Star from the Star Wars movies.

Enceladus

Enceladus is made of ice. It is denser than the other icy moons. That suggests it also has some rock inside. It has smooth areas, cracks and some craters. The smooth areas are younger. Craters there have been erased within the past 100 million years. Water vapor was found over a smooth area around the south pole. The cracks and grooves suggest tectonics similar to Ganymede's. Some ridges similar to Europa's ridges were also found. Those suggest oceans like Europa's under some areas of Enceladus.Tidal forces from Dione could be powering some of this activity. It is because Enceladus orbits Saturn twice for every orbit by Dione. This makes Dione and Saturn tug on Enceladus. This is similar to how Europa and Ganymede's tidal forces on Io power Io's volcanoes.

Tethys

Tethys imaged by the Cassini spacecraft.

Tethys is an icy moon that has many craters, including the huge Odysseus. It is 400 km across, 1/5th as big as Tethys is. The crater had become flattened because the icy material does not hold its shape as well as rock would. There is also a large valley called Ithaca Chasma. It is 3 to 5 km deep, 100 km wide and 2000 km long, three fourths of the way around Tethys.

There are two moons, Telesto and Calypso, which share Tethys' orbit. Telesto is ahead of Tethys and Calypso is behind it.

Dione

Dione is made of lots of ice and there may be some rock in its core. It has lots of craters. The craters are flattened because the ice does not hold their shape as well as rock. One side has bright white lines that are fractures. Two moons share Dione's orbit. Helene is ahead of Dione and Polydeuces is behind it.

Rhea

Rhea is an icy moon similar to Dione with some rock in the core. It has many craters mostly on one side, and the other side has some bright white icy areas.

Titan

Titan imaged by the Cassini spacecraft.

Titan is the largest moon of Saturn and the second largest one in the solar system. It is the only moon in the Solar System that has a thick atmosphere. The atmosphere is made of nitrogen, argon, methane and various organic compounds. Its surface has light and dark areas and few craters. The Cassini probe discovered a huge crater 440 km wide.

Hyperion

Hyperion is made of water ice with a little rock. It is potato shaped. It wobbles instead of rotating in the same way other moons do.

Iapetus

Mosaic of Iapetus images taken by the Cassini spacecraft.

Iapetus is almost entirely ice. It has a light area called Roncevaux Terra that has craters. There is a big dark area called Cassini Regio that covers half of Iapetus. The dark material may be from Phoebe. Some of it is on the bottom of craters. Some huge craters and a ridge have been discovered in Cassini Regio by the Cassini probe. The ridge stretches 1300 km along the equator. It is up to 20 km high, which is over 2.26 times higher than Mount Everest. More huge craters were found in Roncevaux Terra when Cassini went by Iapetus again.

Phoebe

Phoebe is made of ice and rock. It looks dark because it has a layer of dark material on the outside. It also looks rough.

Other moons

There are two groups of small outer moons. Phoebe is part of the second outermost group.

How long is a day on this planet?

One day on Saturn is about 10 hours and 39 minutes in Earth time.

How long is a year on this planet?

One year on Saturn is about 29.46 Earth years long. That is 10,760 Earth days!

What is it made of?

Saturn has a rocky core. Around the core, there is ice. Above the ice is liquid metallic hydrogen. On top of that is gaseous hydrogen. There is no place where the hydrogen suddenly turns from a gas to a liquid.

The gaseous hydrogen makes up most of Saturn's atmosphere. Other gases there include helium and some other gases. There may be rain made of helium falling through the hydrogen. There is also ammonia on the surface.

How much would Saturn's gravity pull on me?

If you were floating close to the cloud tops of Saturn, it would pull you down with a force only a little stronger than the force of Earth's gravity. The effects of Saturn's large radius and its mass almost cancel each other out, making the force only a little bigger. So, if you weighed 100 lbs. on Earth, you would weigh 106 lbs. on Saturn.

Who is it named after?

Saturn is named after the most important Roman god of agriculture and harvest time. He taught people how to farm. He was the father of Jupiter. Saturday is named after him.

Saturn/Mimas

Mimas from Cassini

Mimas is one of the moons of Saturn.

How big is Mimas?

Mimas is 397.2 km wide (246 miles). It is egg shaped because Saturn and the other moons stretch it with their gravity.

What is its surface like?

The surface of Mimas is covered with craters, the largest being Herschel, which was named after William Herschel, the person who discovered Mimas. This crater is about one-third of Mimas' width. It is 5 km (3 miles) high, 10 km (6 miles) deep, and its peak is 6km (4 miles) above the crater floor. Mimas and Rhea, another of Saturn's moons, have been called the most heavily cratered bodies in the Solar System.

Mimas also has many chasmata (chasms), which are long, narrow, steep-sided depressions on the moon's surface.

How long is a day on Mimas?

One day on Mimas is equal to 23 hours.

How long is its orbit around Saturn?

Mimas takes 23 hours to make one orbit around Saturn. It has synchronous rotation, which means that it takes the same amount of time for Mimas to complete one orbit as it does for one rotation. The same side of Mimas is always facing toward Saturn.

What is it made of?

Mimas is made mostly of ice, with some rock. Mimas appears to be frozen solid despite its location to Saturn. This location should have made Mimas more susceptible to tidal heating from Saturn, thus causing partial thaw of the frozen ice.

How much would Mimas' gravity pull on me?

If you were on Mimas, the gravity would be 0.008 times as much as Earth's gravity.

Who is it named after?

Mimas is named after Mimas, the son of the Greek goddess Gaia. The named was suggested by John Herschel, William Herschel's son. Mimas is also called Saturn I.

How was it discovered?

Mimas was discovered in 1789 by William Herschel using a telescope. It was the second moon of Saturn to be discovered by Herschel

Saturn/Enceladus

Enceladus is a moon of Saturn.

Enceladus from Cassini.

How big is Enceladus?

Enceladus measures 498.82 km across.

What is its surface like?

Eruption plumes

The surface of Enceladus has a great deal of variety. Pictures taken by Voyager 2 revealed at least five different types of terrain, including several regions of cratered terrain, regions of smooth, young terrain, and lanes of ridged terrain that often border the smooth terrain. The picture also revealed cracks that crossed the cratered and smooth terrain.

The smooth plains have very few craters, indicating that they are very young. This means that there are cryovolcanoes on the surface, or something else that renews the surface.

When the Cassini spacecraft flew by on February 17 and March 9, 2005, it revealed even more detail about the surface of Enceladus. The smooth regions that Voyager 2 saw now had many small ridges and scarps, while the old cratered regions had many fractures.

How long is a day on Enceladus?

One day on Enceladus is equal to 1.37 Earth days, or one day, 8 hours, and 53 minutes. This is the same amount of time it takes to complete an orbit around Saturn. As a result the same side of Enceladus is always facing toward Saturn. When the orbit and the rotation times match like this, it is called synchronous rotation: Earth's Moon is another example.

How would Enceladus' gravity pull on me?

If you stood on the surface, you would only weigh about 1/10 of what you do on Earth. This is because, although you have the same mass in either place, Enceladus has much less mass than the Earth.

Who is it named after?

Enceladus is named after a Gigantes in Greek mythology. He was defeated by one of Zeus' thunderbolts, and placed under Mt. Etna. The fire from the mountains is said to be his breath, and the rumbling of the mountain is said to be from him rolling around inside.

The name, as well as the names of the other moons of Saturn, was suggested by John Herschel, son of William Herschel, in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope.

How was it discovered?

Enceladus was discovered in 1789 by the English astronomer William Herschel.

Saturn/Tethys

Tethys from Cassini

Tethys is a moon of Saturn.

How big is Tethys?

Tethys is about 1060 km wide.

What is its surface like?

Unusual red streaks mark some of Tethys.

The surface of Tethys has many craters, as well as faults caused by cracks on its icy surface.

There are two types of terrain on the surface. One is made of heavily cratered regions, and the other is made of a dark coloured, lightly cratered belt that extends across the surface of the moon. The light cratering of the region indicates that Tethys was once internally active, causing older terrain to resurface. The cause of the terrain's dark colour is unknown, but recent images of Ganymede and Callisto from the orbiter Galileo suggest that Tethys' surface might have been formed in a similar manner as these two moons, consisting of hazy polar caps of unresolved bright ice patches with a darker zone in between.

The largest crater on Tethys is called Odysseus. It is over 400 km wide, almost 2/5 of Tethys' surface! It is quite flat, due to the movement of Tethys' icy crust.

Another interesting feature of Tethys' surface is the Ithaca Chasma. It measures 100 km wide and 3 to 5 km deep. It runs 2000 km long, about 3/4 of the way around Tethys' circumference. Thwre are two theories as to how it formed. The first is that as Tethys' internal water solidified, the moon expanded and the surface cracked to accommodate the extra volume. The other theory is that as impact that created the crater Odysseus caused shockwaves that travelled through Tethys and cracked the ice on the other side.

How long is a day on Tethys?

One day on Tethys is equal to 1.89 Earth days, or just over 1 day and 21 hours. It takes the same amount of time to finish a single orbit around Saturn. This means that the same side of the moon is always facing toward Saturn

What is it made of?

Thethys is almost completely made of water-ice.

Who is it named after?

The Titan Tethys.

After they were discovered, Tethys, Dione, Rhea and Iapetus were named Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV. Astromomeers used to call them and Titan as Saturn I through Saturn V. (Tethys being Saturn III). Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII.

The name Tethys was suggested by John Herschel, son of astromomer William Herschel. In 1847, he published Results of Astronomical Observations made at the Cape of Good Hope, in which he suggested the names the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used.

In Greek mythology, Tethys was a Titaness and sea goddess who was both sister and wife of Oceanus. She was mother of the main rivers of the universe, such as the Nile, the Alpheus, the Maeander, and about three thousand daughters called the Oceanids.

How was it discovered?

Tethys was discovered by Giovanni Cassini on March 21, 1684.

Saturn/Dione

Dione from Cassini

Dione is a moon of the planet Saturn. It was first photographed in some detail when the Voyager 1 mission passed by Saturn in 1980. The Cassini spacecraft started taking more closeup pictures in 2004, and we now know much more about this moon.

How big is Dione?

Dione is 1118 km across. This is much smaller than our Moon, which is almost 3,500 km across. It is one-fifth as large as Titan, the largest moon in orbit around Saturn.

What is its surface like?

Photo of the surface of Dione from the Cassini spacecraft during a flyby.

This is a cold, icy world with a thin layer of oxygen for its atmosphere. The side of the moon that faces toward the direction of its orbit is covered from craters that were made by collisions with asteroids. On the back side are bright, wispy streaks across the surface. These are cliffs in the ice that were caused when the surface shifted about.

How long is a day on Dione?

It takes 2.74 days to complete an orbit around Saturn. This is just under 2 days and 18 hours. It also takes the same amount of time for the moon to rotate around the axis. This means that the same side of Dione is always facing toward Saturn, in much the same was as our Moon always keeps the same face toward us.

What is it made of?

Dione has a covering of ice surrounding a rocky interior. It has a higher portion of rock inside than any other moon of Saturn except Titan.

How much would Dione's gravity pull on me?

A person standing on the surface would weigh less than one-thirtieth as much as they would on Earth.

Who is it named after?

This moon is named for one of the titans from Greek mythology. She was an addition to the original twelve titans mentioned by the Greek poet Hesiod.

How was it discovered?

It was discovered by the Italian astronomer Giovanni Cassini in the year 1684. At the time he was the director of the Paris Observatory in France.

Saturn/Rhea

Image of Rhea from the Cassini spacecraft

How big is Rhea?

Rhea is 1530 km in diameter (the distance from one side to the other through the center). Rhea is relatively large when compared with the moons of other planets in the Solar System.

What is its surface like?

The unusual blue streaks of Rhea.

The surface of Rhea is heavily cratered and has bright wispy markings on its surface. The surface can be divided into two categories. The first area contains craters which are more than 40 km wide, while the second area contains craters smaller than this.

The leading hemisphere is heavily cratered and is the same brightness throughout. As on Callisto, the craters are eroded. On the trailing hemisphere there are bright swaths on a dark background and few visible craters. These bright swaths may be material from ice volcanoes early in Rhea's history when it was still liquid inside. However, recent observations of Dione, which has the same bright streaks, shows that these are in fact ice cliffs. Therefore, it can be assumed that the bright streaks on the Rhean surface are also ice cliffs. When Cassini does its flyby of Rhea on November 25, 2005, we will know for sure.

How long is a day on Rhea?

One day on Rhea is equal to four and a half days on Earth.

How long is its orbit around Saturn?

One orbit around Saturn also takes four and a half days. Rhea has synchronous rotation, meaning that it takes the same amount of time for it to complete one orbit as it does to complete one rotation on its axis. This also means that the same side of Rhea always faces Saturn.

What is it made of?

One third of Rhea's mass is rock, found at its core. The rest is water ice.

How much would Rhea's gravity pull on me?

If you stood on Rhea's surface, you would weigh less than you would on Pluto — which is about 6% of what you do on Earth.

Who is it named after?

Rhea is named after a Titaness in Greek mythology. The name was suggested by astromoner William Herschel in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope, in which he suggested the names of the Titans, sisters and brothers of Cronos (the Greek Saturn), be used. Before that, it, Tethys, Dione, and Iapetus were called Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV.

How was it discovered?

Rhea was discovered by Giovanni Cassini on December 23, 1672.

Saturn/Titan

Titan from the Cassini spacecraft

Titan is the largest moon of the planet Saturn. It is also the second largest moon in the solar system.

How big is Titan?

The moon Titan is 5150 km wide. It is the second largest moon in the Solar System. Only Ganymede, a moon of Jupiter, is larger. Titan is bigger than either the planet Mercury or dwarf planet Pluto. It is nearly one and a half times the size of the Earth's Moon, and has nearly twice the mass of the Moon. Titan is the only body besides Pluto, that we know of, that has a colder surface temperature because it has an atmosphere. The surface temperature of Titan is -180 degrees centigrade.

What is its surface like?

The surface of Titan. This photo was taken by a probe.

Titan is the only moon in the Solar System to have a thick atmosphere. Until the Cassini spacecraft arrived in orbit around Saturn, we did not know what the surface looked like. The ground is very cold on Titan, and all of the water is frozen. The surface looks like it is smooth, with areas of light and dark material. Most of the craters have been filled in.

It was thought that Titan may have seas of liquid methane, but it now looks like there may only be a few lakes. The Huygens probe was dropped into the atmosphere of Titan and slowly fell to the ground. This gave us our first close-up pictures of the surface. These showed what appear to be channels among hills that lead down to flat regions below. These may have been made by flows of liquid that left dark materials behind.

An atmosphere, complex molecules and the presence of methane on Titan mean that many people think there may be simple forms of life on or under Titan's surface.

How long is a day on Titan?

A day on Titan is the same as the time it takes for the moon to orbit around Saturn, about 15 days, 22 hours, and 41 minutes. The same side of Titan always faces toward Saturn, in much the same way as the same face of our Moon always points toward the Earth. This was caused by the gravity of Saturn creating a tidal force on Titan, which changed its rotation.

What is it made of?

Haze from the atmosphere of titan.

The atmosphere of Titan is mostly nitrogen, with small amounts of other gases. Many of these gases are called hydrocarbons because they include hydrogen and carbon. These hydrocarbons are created in the upper atmosphere from Ultraviolet sunlight. Ultraviolet is the same light that causes sunburn on your skin. The hydrocarbons create a thick orange smog that hides the surface.

The moon is made of about half ice and half rocks. Most of this rock may be in a core at the center of the moon. The core may still be hot, as is the core of our own world, and there could be some volcanic activity. The surface is mostly ice.

How much would Titan's gravity pull on me?

The gravitational pull on Titan is about one seventh the Earth's gravitational pull. So if you weighed 100 pounds here you would weigh 14 pounds on Titan.

Who is it named after?

Titan was named after the race of children borne by Gaea and Uranus in Greek mythology.

When was it discovered?

It was discovered in 1655 by the Dutch astronomer Christiaan Huygens.

Saturn/Hyperion

Hyperion

Hyperion is a moon of Saturn. It is completely covered in craters and spins at high speeds as it orbits Saturn.

How big is Hyperion?

Hyperion is 266 km wide.

What is its surface like?

The sponge like surface of Hyperion.

The surface of Hyperion is reddish in colour. Some interesting features include the Bond-Lassell Dorsum, the largest ridge on Hyperion (and the only one to have a name), and the largest crater, which is 120 km wide and 10 km deep. There are also many craters on the surface of Hyperion which make it look like a sponge.

How long is a day on Hyperion?

Hyperion has a chaotic orbit. The day usually lasts about five hours.

How long is its orbit around Saturn?

It takes Hyperion about 21.28 days to orbit once around Saturn.

What is it made of?

Hyperion is made of water ice, and a small amount of rock.

Who is it named after?

Hyperion is named after a Titan in Greek mythology. He was the son of Gaia (Earth) and Uranus (Sky). He was the father of Helios (Sun), Selene (Moon), and Eos (Dawn).

How was it discovered?

Hyperion was discovered by three people: William Cranch Bond, his son George Phillips Bond, and William Lassell in 1848. Lassell discovered Hyperion two days after the Bonds, and he beat them to publication. However, all three are given credit for its discovery.

Saturn/Iapetus

Iapetus from Cassini

Iapetus is a moon of Saturn.

How big is Iapetus?

Iapetus is the third-biggest moon of Saturn, and is about 1436 km wide.

What is its surface like?

Iapetus features two hemispheres of differing colors.

From space, Ieaptus' two-tone colour is quite obvious. One hemisphere is a dark reddish-brown colour, while the other, while the other is bright and heavily cratered. The effect is like a ying-yang symbol.

Scientists don't know where the material covering the dark hemisphere comes from, but they think it might come from space or from inside Iapetus. If it came from space, it might have come from meteroites or the surface of comets, or it might have been knocked off from another moon of Saturn, Phoebe. If the material came from the inside of Iapetus, it might have come from cryovolcanoes or evaporation of water ice.

The equatorial ridge of Iapetus.

When Cassini flew by on Decembr 31, 2004, a ridge was discovered on Iapetus. It is 1300 km long, 20 km wide, and 13 km high. It follows Iapetus' equator almost perfectly, and is heavily cratered, and therefore ancient.

For now, scientists are not sure of the origins of the ridge. One theory is that the ridge is icy material that rose from inside the moon and solidified. The other is that Iapetus once grazed the outer edges of Saturn's rings.

How long is a day on Iapetus?

One day on Iapetus is equal to 79.32 days on Earth, or about 79 days, 7 hours and 41 minutes. It takes the same amount of time to finish an orbit around Saturn. This means that the same side of this moon is always facing Saturn.

What is it made of?

Iapetus is made of mostly ice, with a small amount of rock.

How much would Iapetus' gravity pull on me?

If you were standing on Iapetus, you would weigh about 1/40 of what you do on Earth.

Who is it named after?

When Iapetus, Tethys, Dione, and Rhea were discovered, they were named Sidera Lodoicea ("the stars of Louis") to honour king Louis XIV. However, astronomers called them and Titan Saturn I through Saturn V. Once Mimas and Enceladus were discovered, in 1789, the numbering scheme was extended to Saturn VII.

The name Iapetus was suggested by astronomer John Herschel (son of William Herschel, also an astronomer) in his 1847 publication Results of Astronomical Observations made at the Cape of Good Hope

Iapetus is a Titan from Greek mythology. He was son of Uranus and Gaia, and father (by an Oceanid named Clymene or Asia) of Atlas, Prometheus, Epimetheus, and Menoetius. Through Prometheus and Epimetheus and Atlas he is an ancestor of the human race.

How was it discovered?

Iapetus was discovered by Giovanni Cassini on October 25, 1671.

Saturn/Phoebe

Cassini image of Phoebe

Phoebe is a moon of Saturn.

How big is Phoebe?

Phoebe is about 220 km wide. However, because Phoebe is not very spherical in shape, it is wider in some places and narrower in others.

What is its surface like?

A large crater on Phoebe.

The surface of Phoebe is dark and heavily cratered. Craters are up to 80 km across, one of which has walls 16 kilometres high.

How long is a day on Pheobe?

One day on Phoebe is equal to 9 h 16 min 55.2 s on Earth. Unlike the other moons of Saturn, Phoebe does not have synchronous orbit. This means that the time it takes for Phoebe to rotate once is different from the amount of time it takes for Pheobe to orbit once around Saturn.

How long is its orbit around Saturn?

One orbit around Saturn is equal to 550.6 days, or 1 and a half years! Phoebe's orbit is retrograde, meaning it moves in the opposite direction of moons such as Rhea or Titan.

What is it made of?

Close up photo of Phoebe.

The dark colour of Phoebe led scientists to believe that it was a captured asteroid. However, recent pictures from Cassini show that Phoebe's craters vary greatly in brightnes. This means that underneath the dark surface, there is a large quantity of ice. In addition,quantities of carbon dioxide have been detected on the surface, something which has never been found on an asteroid. It is believed that Phoebe is around 50% rock, unlike the other moons of Saturn, which are about 35% rock. For there reasons, scientists now think that Phoebe is a captured Centaur, an icy planetoid from the Kuiper Belt that orbits the Sun between Jupiter and Neptune.

How much would Phoebe's gravity pull on me?

If you stood on Phoebe, you would weigh only 1/25th of what you do on Earth! For example, a 100 lb person on Earth would weigh only 4 lbs on Phoebe.

Who is it named after?

Pheobe is named after a Titan in Greek mythology. She was the grandmother of Apollo and Artemis, and mother of Leto and Asteria.

How was it discovered?

Phoebe was diacovered by William Henry Pickering on March 17, 1899 from photographic plates that had been taken starting on August 16, 1898, at Arequipa, Peru by DeLisle Stewart. It was the first satellite to be discovered photographically.

Uranus

Uranus, the seventh planet from the Sun, was discovered by William Herschel on March 13, 1781.

Uranus as seen from Voyager 2

	Uranus Facts:
Uranus's rings look white in pictures, but they are really made of asphalt-colored material. When it was first discovered, Uranus was mistaken for a star. It was named "34 Tauri". Uranus rotates on its side It is the only planet to do so.

How big is Uranus?

Comparison of the size of Uranus and the Earth

Uranus is 51,118 kilometers or about four Earths wide. It is the third widest and fourth heaviest planet in the Solar System.

What is the surface of Uranus like?

Uranus does not have a surface that you could stand on without going deep into the atmosphere. Under the atmosphere, there may be an even mixture of rock and ice.

What are the rings around Uranus like?

Uranus has eleven rings. They are dark in colour and very hard to see. They were discovered by accident in 1977. Scientists were studying a bright star near Uranus. However, the star's light was blocked before and after it disappeared behind Uranus. From this, they figured out that Uranus has a ring system.

What are its moons like?

Uranus's rings and moons

Uranus has 27 known moons, which places it third in the Solar System for number of moons. The five main ones are Miranda, Ariel, Umbriel, Titania and Oberon.

Miranda

Miranda is the smallest and closest of Uranus's major moons. It is mainly made of ice and rock. Miranda's surface has grooves, cliffs, and valleys. The moon was named after a character in The Tempest, a play by Shakespeare.

Ariel

Ariel is made of rock and ice. Ariel has many valleys, but not many craters. Ariel was named after a character in the poem The Rape of the Lock by Alexander Pope. Ariel is also a spirit in The Tempest by Shakespeare.

Umbriel

Umbriel is made of lots of ices and some rock. It is also the darkest of Uranus's major moons. Umbriel was named after a character in the poem The Rape of the Lock by Alexander Pope.

Titania

Titania is the largest moon of Uranus. It is mostly ice and rock. The surface is covered with canyons. It was named after the Queen of the Fairies in A Midsummer's Night Dream, a play by Shakespeare.

The Solar System

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Oberon

Oberon is the outermost of the major moons of Uranus. It is made of the same things as Titania. It has many craters. Some of them have white rays around them and dark crater floors. It was named after the King of the Fairies in A Midsummer's Night Dream.

Other Moons

There are 13 tiny moons known to be orbiting Uranus inside Miranda's orbit. Nine more tiny moons are known to be in big orbits beyond Oberon's orbit.

How long is a day on Uranus?

One day on Uranus is about 17.24 Earth hours long. Uranus spins on its side, maybe because of a big impact early in the history of the Solar System.

How long is a year on Uranus?

One year on Uranus would be 30,708 days or 84 years on Earth.

What is Uranus made of?

Unlike Jupiter and Saturn, Uranus is thought to be made mostly of rock and ice. The gases in its atmosphere are 83% hydrogen, 15% helium, and 2% methane. Other gases found in smaller amounts are ammonia, water, and methane. Uranus' blue color comes from methane clouds, which absorb red light and reflect blue light.

How much would Uranus's gravity pull on me?

If you were floating close to the cloud tops of Uranus, you would be pulled down with a force about 89% of Earth's gravity.

Who is Uranus named after?

Uranus was named after Ouranos, the Greek name for the sky. Ouranos was the ancient Greek deity of the heavens, the earliest supreme God. According to Greek mythology, Ouranos was the husband and son of Gaia, Mother Earth.

Next Topic: Neptune

Uranus/Miranda

Miranda as seen from Voyager 2

Miranda is a moon of the planet Uranus. It is the smallest and innermost of Uranus's five major moons. It is sometimes called "Uranus V".

The first pictures of this moon from a spacecraft were taken on January 24, 1986, when Voyager 2 passed by the planet. Only the southern half of Miranda has been viewed in detail, and the rest of the moon has yet to be seen.

How big is Miranda?

This moon has a diameter of only 472 km. This is slightly smaller than the asteroid Vesta, which is the second biggest object in the asteroid belt. Miranda would easily fit within the borders of the state of Arizona.

What is its surface like?

The grooves of Miranda.

The surface of Miranda seems to be made almost entirely of ice. There are huge, deep canyons, some reaching over 19 km (12 mi) deep, and a lot of rugged ground, with some large, strange parallel grooves in parts the surface. There are also some craters on Miranda. This moon does not have any atmosphere and it is bitterly cold.

How long is a day on Miranda?

This moon takes the same time to orbit Uranus as it does to rotate around its axis. This means that it has become tidally locked. The gravity from Uranus has caused it to gradually slow its rotation until now it always keeps the same face toward the planet. The length of one orbit, and the length of a day on Miranda, is 1.41 days, or about 1 day and just under 10 hours.

What is it made of?

This moon is made of frozen stuff such as water ice, and it has a core of rock on the interior.

How much would Miranda's gravity pull on me?

Newtons describe how hard gravity is pulling you down. On Earth, if you weigh 51 kilograms or 112 pounds, your weight is 500 newtons. We only use newtons for some special purposes, when we are really interested in how hard we push down on something because gravity is pulling us.

Your weight in newtons would be less on Miranda, only about 1/124 of what it is on Earth. To find your weight in newtons as you stand on Miranda, just take your weight on Earth, and if that weight is in kilograms, multiply it by 0.079. Iff that weight is in pounds, divide it by 28 to get newtons. But then you'll have to add in the weight of your spacesuit as well, because that is pressing down on the surface just like you are.

Who is it named after?

The English bard William Shakespeare wrote a play called "The Tempest". In this play there is a sorcerer named Prospero who has a daughter called Miranda. Both the sorcerer and his daughter were stranded on an island for twelve years in the Adriatic sea.

The name of Shakespeare's character Miranda comes from the Latin root "mira", which means "to wonder". In the play the daughter is often called "a wonder".

How was it discovered?

This moon was discovered by the astronomer Gerard Kuiper, who was born and educated in Holland, but later came to the United States. He found this moon on February 16, 1948. He also found Neptune's moon Neried just over a year later.

Uranus/Ariel

Ariel as seen by Voyager 2

Ariel is a moon of Uranus. The first and only observations of Ariel were made by Voyager 2 during its January 1986 flyby of Uranus. Only the Southern hemisphere was photographed, since the moon's south pole was pointed towards the Sun.

How big is Ariel?

Ariel is about 1157.8 km wide.

What is its surface like?

Canyons of Ariel.

The surface of Ariel does not have many craters. It has a huge network of fault canyons and liquid water outflows over its surface. It appears to shine because of its reflecting surface.

How long is a day on Ariel?

One day on Ariel is equal to 2.52 days on Earth.

How long is its orbit around Uranus?

One Orbit around Uranus is also equal to 2.52 days.

What is it made of?

Ariel is made of roughly 50% water ice, 30% silicate rock, and 20% methane ice, and it appears to have regions of fresh frost in places.

How much would Ariel's gravity pull on me?

Newtons describe how hard gravity is pulling you down. On Earth, if you weigh 51 kilograms or 112 pounds, your weight is 500 newtons. We only use newtons for some special purposes, when we are really interested in how hard we push down on something because gravity is pulling us.

Your weight in newtons would be less on Ariel, only about 0.27 times what it is on Earth. To find your weight in newtons as you stand on Ariel, just take your weight on Earth, and if that weight is in kilograms, multiply it by 2.65. If that weight is in pounds, multiply it by 1.2 to get newtons. But then you'll have to add in the weight of your spacesuit as well, because that is pressing down on the surface just like you are.

Who is it named after?

Ariel is named after a sylph, a creature made of air, in Alexander Pope's poem Rape of the Lock.

How was it discovered?

Ariel was discovered on October 24, 1851 by William Lassell. He also discovered Umbriel at the same time.

Uranus/Umbriel

Umbriel as seen from Voyager 2

Umbriel is a moon of Uranus

How big is Umbriel?

Umbriel is about 1169.4 km wide, or about 1/10th as wide as Earth.

What is its surface like?

The only close-up images we have of Umbriel's surface come from Voyager 2's flyby in January 1986. These images show that Umbriel has a dark surface, the darkest of any of Uranus' moons. Near the equator lies Wunda, a large ring of bright material. Wunda is assumed to be a crater, but nobody is sure what it is exactly. Near Wunda is the crater Skynd.

How long is a day on Umbriel?

One day on Umbriel is equal to about four Earth days.

How long is its orbit around Uranus?

One orbit around Uranus is also equal to about four Earth days.

What is it made of?

Umbriel is made mostly of water ice, followed by silicate rock and methane ice. Most of its methane ice is on its surface.

From guessing based on its mass and size, scientists have worked out that it is about 60% ice and 40% rock.

How would Umbriel's gravity pull on me?

If you stood on Umbriel, you would weigh about one-fortieth (0.023) of what you do on Earth. Someone who weighs 100 lbs on Earth would weigh only about 2.3 lbs on Umbriel.

Who is it named after?

Umbriel is named after the "dusky melancholy sprite" in Alexander Pope's The Rape of the Lock, and the name suggests the Latin umbra, meaning shadow.

How was it discovered?

Umbriel was discovered on October 24, 1851 by William Lassell. He also discovered the moon Ariel at the same time.

Uranus/Titania

Titania is the largest moon of the planet Uranus. The closest view we have of this moon came in 1986 when the Voyager 2 spacecraft flew by Uranus.

How big is Titania?

This moon is 1,578 km wide (981 miles). Titania is less than half the size of the Earth's moon. The total surface area of this moon is only a little larger than the area of Australia.

What is the surface like?

A picture of Titania taken by Voyager 2 in 1986

The surface of Titania is barren and there is no atmosphere. The ground on the surface is covered by dirty ice that has been mixed with rock. There are many craters which are formed when a rock from space collides with the moon, leaving a bright white patch surrounding the hole. Near the top of this moon is a large crater that has a double ring.

There are also long valleys on the surface that were formed when the interior heated up and expanded. This left what are called rifts in the surface. The longest of these valleys runs for a distance of about 1,000 km.

How long is a day on Titania?

It takes 8.71 Earth days for Titania to complete one full rotation. That is about 8 days and 17 hours.

How long is its orbit around Uranus?

Titania completes one full orbit around Uranus each day. That orbit takes 9991 Earth days to finish. Titania is tidally locked, meaning it always keeps the same side facing toward Uranus. This is very similar to how our Moon always keeps the same side facing the Earth.

What is it made of?

Nearly half of the moon is made of frozen water. The rest is rock and a mixture of frozen gas.

How much would Titania's gravity pull on me?

Newtons describe how hard gravity is pulling you down. On Earth, if you weigh 51 kilograms or 112 pounds, your weight is 500 newtons. We only use newtons for some special purposes, when we are really interested in how hard we push down on something because gravity is pulling us.

Your weight in newtons would be less on Titania, only about one twenty-sixth of what it is on Earth. To find your weight in newtons as you stand on Titania, just take your weight on Earth, and if that weight is in kilograms, multiply it by 3/8. If that weight is in pounds, divide it by 6 to get newtons. But then you'll have to add in the weight of your spacesuit as well, because that is pressing down on the surface just like you are.

Who is it named after?

The English poet and play author William Shakespeare once wrote a play called a Midsummer-Night's Dream. In this play there was a mythical woman named Titania who was the wife of Oberon and was also the Queen of the Faeries. Uranus also has a moon called Oberon which was named after Titania's husband.

The name of this moon is similar to Saturn's moon Titan. However Titan was named after a race of giants from Greek mythology.

How was it discovered?

This moon was discovered in 1787 by the English astronomer Sir William Herschel. This man also discovered Uranus and its moon Oberon. The name for this moon was suggested by Sir Herschel's son, John Herschel, in 1852.

Uranus/Oberon

Oberon as seen by Voyager 2

Oberon is a moon of the planet Uranus. It is the outermost of the major moons of Uranus. Oberon was first seen in detail when the spacecraft Voyager 2 passed by Uranus in 1986.

How big is Oberon?

This moon is 1523 km across. It is the second largest of the moons of Uranus, and is only slightly smaller than Titania. The total surface has an area that is less than the continent of Australia.

What is its surface like?

Only half of this moon has been seen from a close distance, so we only know what that surface is like. The ground looks like it is made of ice with some darker materials. The surface is covered by many craters that were formed when large rocks collided with the moon. It is a very, very cold place that does not have an atmosphere.

How long is a day on Oberon?

It takes 13 days and 11 hours for Oberon to complete an orbit around Uranus, just under two weeks.

What is it made of?

This moon is made of a mixture of rocks, ice, and other frozen materials.

How much would Oberon's gravity pull on me?

Newtons describe how hard gravity is pulling you down. On Earth, if you weigh 51 kilograms or 112 pounds, your weight is 500 newtons. We only use newtons for some special purposes, when we are really interested in how hard we push down on something because gravity is pulling us.

Your weight in newtons would be less on Oberon, only about one twenty-eighth what it is on Earth. To find your weight in newtons as you stand on Oberon, just take your weight on Earth, and if that weight is in kilograms, multiply it by 0.35. If that weight is in pounds, divide it by six to get newtons. But then you'll have to add in the weight of your spacesuit as well, because that is pressing down on the surface just like you are.

Who is it named after?

The name for this moon was suggested by John Herschel, the son of Sir William Hershel. It was named after the mythical King of the Faeries in William Shakespeare's play A Midsummer Night's Dream. This character was drawn from the legendary history of Britain, and is based on the legends of King Arthur and his times.

How was it discovered?

It was discovered by the English astronomer William Herschel on January 11, 1787.

Neptune

Neptune as seen from Voyager 2

	Neptune Facts:
Wind speeds on Neptune can reach 450 meters per second. Neptune was discovered because its gravitational field was affecting the orbit of Uranus. Neptune is sometimes farther from the Sun than Pluto.

How big is this planet ?

The Solar System

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Comparison of the size of Neptune and the Earth

Neptune is very similar to Uranus in size. Its diameter is only slightly smaller, at 49,528 km wide. It is almost as big as four Earths in length.

What is its surface like?

The atmosphere of Neptune has some dark blue spots. When the Voyager probe went by Neptune in 1989, it saw a large one called the Great Dark Spot. In 1994, it vanished, but later reappeared. There is also a large white cloud nicknamed "Scooter." It goes around Neptune every 16 hours. The winds of Neptune are very fast, blowing at up to 2000 km per hour(the fastest in the entire solar system). That is about four times faster than the fastest recorded tornado on Earth.

What are its rings like?

Neptune has some faint rings that are dark and hard to see. There are clumps in some parts of the rings where the material is denser.

What are its moons like?

Neptune has 14 moons. There could be more.

Inner Moons

There are five small potato-shaped moons orbiting close to Neptune.

Proteus

Proteus is a dark moon about 418 km across. It has an irregular shape. In Roman mythology Proteus was a sea-god who could change into any shape he wanted.

Triton

Triton is the largest moon of Neptune. Scientists think that it is a lot like Pluto. It is 2700 km across. It is made of rock and ice. It has a surface temperature of −235 °C Triton has a very thin atmosphere made up of nitrogen and a little methane.

There are volcanoes that have eruptions of liquid nitrogen, dust or methane compounds. The eruptions happen because of the seasons. There are few craters because the eruptions cover them up. There are ice caps that change sizes with the seasons. There are also ridges and valleys. They may have formed because of repeated freezing and thawing.

An interesting thing about Triton's orbit is that it goes around Neptune in the opposite direction that Neptune's rotates. Because of this, scientists think that Triton was captured by Neptune long ago. In Roman mythology, Triton was the son of Neptune.

Nereid

Nereid is an irregularly shaped moon about 340 km across. Its orbit is very eccentric or noncircular. It may have been captured by Neptune or moved into the eccentric orbit by Triton's gravity when Triton got captured. In Roman mythology Nereids were sea nymphs.

Outer Moons

There are five other known moons. They are small potato-shaped moons far from Neptune. There might be more we haven't seen yet.

What is the Mass?

A day on Neptune lasts 16 hours and 7 minutes.

How long is a year on this planet?

One year on Neptune is about 165 Earth years, or 60,265 days.

What is it made of?

Neptune is made of rock and metal in the core. The core is probably bigger than Uranus's because Neptune weighs more, but is the same size. Around the core is rock, water, ammonia and methane. The atmosphere is made of hydrogen and helium. Lower down in the atmosphere, there is methane and ammonia too. The methane makes Neptune look blue-green.

Artistic impression of the mythological Neptune

How much would Neptune's gravity pull on me?

If you were floating close to the cloud tops of Neptune, it would pull you down with a force only a little stronger than the force of Earth's gravity. The effects of Neptune's larger radius and its mass almost cancel out, making the force only a little bigger.

Who is it named after?

Neptune is named after the Roman god of the seas, also known as Poseidon in ancient Greece.

Next Topic: Pluto

Neptune/Proteus

Proteus from Voyager 2 spacecraft

Proteus is a moon of the planet Neptune. It is sometimes called Neptune VIII. We still do not know a lot about this moon, as only a few pictures have been taken when the Voyager 2 flew by in 1989.

How big is it?

This moon is uneven in shape and has an average size of just over 400 km across. This moon is nearly as a big as a satellite can be without it being pulled into a sphere by its own gravity. Even though it is a small moon, it is the second largest moon of Neptune after Triton.

What is its surface like?

The surface of this moon is covered in craters and some grooves. It is also a very dark moon, with a ground that is as dark as soot. It is one of the darkest objects in the Solar System.

How long is Proteus's revolution?

It takes 1 day, 2 hours, and 56 minutes for this moon to orbit the planet Neptune.

Who is it named after?

This moon is named for a sea god from ancient Greek mythology. In some tales he was the first son of Poseidon, the ancient Greek god of the sea. The Romans later called him Neptune, so Proteus was the first son of Neptune.

In Greek mythology, Proteus was able to foretell the future. He was also able to change his shape, and he used this ability to avoid being asked about what is to come.

How was it discovered?

Proteus was discovered in 1989 when the Voyager 2 spacecraft flew by Neptune.

Neptune/Triton

Triton as seen from Voyager 2

Triton is the largest moon of the planet Neptune, and the seventh largest in the Solar System. The best view we have had of Triton was in 1989 when the Voyager 2 spacecraft passed close by Neptune.

How big is Triton?

Triton has an average size of 2700 km across, which is larger than Pluto and about the same size as Europa. The moon is about half the width of the United States.

What is its surface like?

What is Triton like? Scientists are still researching what makes Triton so unique.

The surface is icy and the temperature on the surface is bitterly cold. The southern half of the planet is covered by an ice cap. The moon does have a very thin atmosphere.

Triton is one of the few places in the Solar System where there are volcanoes. These are caused by the distant heat from the Sun. The volcanoes spew material from below the surface, mostly very cold dust and gas. These geysers are slowly reforming the surface, and have wiped out any craters.

How long is a day on Triton?

One day on Triton lasts for 5.877 days on Earth, around 5 days and 21 hours.

How long is its orbit around Neptune?

Triton takes 5.877 days to complete an orbit around Neptune, the same time as the length of one day. This means it always keeps the same side facing toward Neptune.

This moon is unusual because of the direction of its orbit around Neptune. Normally a moon orbits in the same direction that the main planet rotates. However Triton has a retrograde orbit, which means it orbits in the opposite direction. It is the only large moon in the Solar System to have a retrograde orbit.

Because of this backward orbit, Triton is gradually being dragged closer and closer to Neptune. Millions of years from now it will come too close to the giant planet and break apart. When it does so, the debris will form a large ring around Neptune that is similar to the ring around Saturn.

What is it made of?

This moon is mostly made of rocky material with about a quarter made of ice. It is very similar to Pluto temperature wise, but it is more of an icy texture and composition.

How much would Triton's gravity pull on me?

If you were on Triton, it would pull you down with a force less than one-twelfth as strong as the force of Earth's gravity.

Who is it named after?

Triton was the son of Poseidon in Greek mythology. In Roman Mythology, Poseidon was called Neptune.

How was it discovered?

Triton was discovered by William Lassell in 1846, shortly after the planet Neptune was discovered. However the moon was named by Camille Flammarion in 1880. The name of this moon did not come into general use until the middle of the twentieth century.

Neptune/Nereid

Nereid from Voyager 2 spacecraft

Nereid is a moon of the planet Neptune. It is sometimes called "Neptune II". We still do not know much about this moon. Because it has an unusual orbit, scientists think that it may have been an asteroid that was captured by Neptune's gravity.

How big is Nereid?

This is the third largest of the moons of Neptune, but it is only 340 km across

How long is a day on Nereid?

A day on Nereid is 11 and a half hours.

How much would Nereid's gravity pull on me?

Nereid is only a small moon that has very low gravity.

Newtons describe how hard gravity is pulling you down. On Earth, if you weigh 51 kilograms or 112 pounds, your weight is 500 newtons. We only use newtons for some special purposes, when we are really interested in how hard we push down on something because gravity is pulling us.

Your weight in newtons would be much less on Nereid, only about 1/137 what it is on Earth. A typical adult would weigh about half a newton. But then you'll have to add in the weight of your spacesuit as well, because that is pressing down on the surface just like you are.

Who is it named after?

This moon was named for the sea-nymphs from Greek mythology. A nymph is a nature spirit that looks like a beautiful woman.

How was it discovered?

It was discovered in 1949 by the astronomer Gerard P. Kuiper.

Pluto

Pluto is a dwarf planet that was discovered by the astronomer Clyde W. Tombaugh in Arizona on February 18, 1930.

Pluto, as seen from the New Horizons spacecraft.

Pluto Facts:

• Pluto is a dwarf planet, smaller even than Earth's Moon.
• Pluto sometimes moves closer to the sun than Neptune.

How big is Pluto?

Comparison of the sizes of Pluto, Charon, the Moon and Earth

Pluto's mass is about 12,500,000,000,000,000,000,000 kilograms. While this may seem large, it's only about 1/500th of the Earth's mass. Pluto is between 2200 and 2400 kilometers across. Its surface area is about 17,950,000 square kilometers (or 1/30th of the Earth's). Its volume is 7,150,000,000 km³ (or 1/150th of the Earth's).

The Solar System

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What is its surface like?

The surface of Pluto.

Pluto's surface is covered with ice. It is very cold, at about -230 °C. Pluto also has a very thin atmosphere which freezes when Pluto moves far away from the Sun.

The image above on the right shows Pluto's color.

What are Pluto's moons like?

Pluto has three known moons. The largest is called Charon. Charon is about half as wide as Pluto. Because Pluto and Charon are so close in size, they are sometimes called a "double planet". Charon's surface is covered in water ice. Two other moons were discovered in 2005. They have been named Nix and Hydra.

How long is a day on Pluto?

One day on Pluto is about 6.487 Earth days long. Like Uranus, Pluto also spins on its side.

How long is a year on Pluto?

One year on Pluto would be about 90,613 days or 248 years on Earth!

What is it made of?

Scientists believe Pluto is made mostly of rock and ice, but they will not be sure until more research is done. The discovery of Charon helped scientists estimate the density of Pluto. The information collected told them what Pluto was and was not made out of. If Pluto were made out of heavy solids, it would have a very high density. If it were made of gases, it would have a low density. Pluto is somewhere in between, so it is probably made of rock and ice.

How much would Pluto's gravity pull on me?

If you were on Pluto, gravity would be only 0.06 times as strong as it is on Earth. This means you could do really high jumps—even more than people could on the Moon!

Who is Pluto named after?

Pluto was named after the Roman god of the underworld. In Roman mythology, he kidnapped Proserpina (Persephone) so he could marry her. This made her mother, Ceres, the goddess of agriculture, very sad, causing winter. To end winter, Jupiter, the king of the gods and her brother, decreed that Proserpina could return to the surface as long as she hadn't eaten any food of the Underworld. However, she had eaten six pomegranate seeds, so Jupiter decided she had to spend six months in the underworld each year. This is the Roman myth of winter. When she goes to the Underworld, everything stops growing. When she comes back, her mother is happy again, and life returns.

In Roman mythology, Charon took dead souls across the river Acheron to the land of the dead.

Is Pluto a planet?

Pluto has been officially classified as a dwarf planet, which is different from a regular planet. One reason is its small size - although it is the tenth largest known object that revolves around the sun, it is smaller than many moons, including Earth's moon. Scientists used to think that Pluto was a lot larger than it actually is, and it was thought of as the ninth planet for many years.

Another key reason is that Pluto is part of a large group of objects called the Kuiper Belt, which all revolve around the Sun in the area beyond Neptune. In January 2005 another object "Eris" was discovered in the Kuiper Belt. Eris is larger than Pluto. Scientists think there are even more Pluto-sized objects in this part of the solar system, as well as millions of smaller objects. Because of this, the International Astronomical Union (IAU) defined the term 'planet' for the first time. Under the definition, both Eris and Pluto (along with Ceres, Haumea and Makemake) are dwarf planets.

In spite of this, some people continue to hold on to the idea that Pluto is a regular planet because of tradition. Also, some textbooks and references are not up to date, and still list Pluto as the ninth planet.

Next Topic: Comets

Pluto/Charon

Charon as seen by the New Horizons spacecraft.

Charon is one of the three moons of Pluto.

How big is Charon?

Charon measures about 1,200 km across, about half the diameter of Pluto. Because the size difference between Pluto and Charon is so small, they are sometimes considered to be double planets. They are also sometimes thought of as the first two trans-Neptunian objects.

What is its surface like?

This picture taken by the Hubble Space Telescope shows Pluto and Charon. The small moon on the right is Charon. We can't see it in detail without probes because it is too far away.

The surface of Charon is covered in water ice.

How long is a day on Charon?

One day on Charon is equal to 6 days, 9 hours, and 18 minutes on Earth.

How long is its orbit with Pluto?

Charon and Pluto share an orbit around a point that is directly between them, this orbit takes roughly six and a half days

Who is it named after?

Charon is named after a figure in Greek mythology. Charon ferried the dead across the river Acheron to Hades if they could pay him.

How was it discovered?

Charon was discovered by James Christy on June 22, 1978. He was examining highly magnified images of Pluto and noticed a small bulge sometimes appeared in the pictures. The bulge was later confirmed in older images and was given the temporary name of S/1978 P 1.

Comets

	Comet Facts
Comets are often described as giant "dirty snowballs" because they are mostly made up of ice and some dirt. Comets have two "tails", one made up mostly of rocks and dust, the other mostly made of gas. Comet tails always point away from the Sun.

What is a Comet?

The Hale Bopp comet

Think of a comet as a big, dirty, gassy snowball. Comets are formed in the ring of rocks, dust, and ice that orbits the Sun beyond Pluto called the Kuiper Belt. Comets form when rocks, dust, and ice condense – that is, join together. As a comet grows larger, it starts to be pulled towards and around the Sun. Comets in our Solar System usually take many years to go around the Sun – from a few dozen years to many thousands of years. This is because they start to orbit the sun from very far away. They make long, egg-shaped orbits around the Sun instead of almost circular ones like the planets.

What does a Comet look like?

The Solar System

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The strange surface of Churyumov-Gerasimenko, a comet. Photograph taken by the Rosetta space craft.

The comets that can be seen in the sky without telescopes are unusual because they are the biggest and brightest comets of all. You might have the chance to see one once or twice during your life. Most comets can only be seen with a telescope. The few that can be seen with human eyes are usually just hazy streaks or faint smudges in the night sky.

When comets are very far away from the Sun, they are covered in a coating of icy, black rocks and dust. As a comet approaches the Sun, however, the ice starts to melt. This creates large amounts of water and gas that break through the coating, freeing some of the dust and rocks. Sometimes this water, gas, rocks, and dust can be seen from the Earth as one or two tails streaming away from the comet. Even when only one tail can be seen, there are two, one made from the lighter gas and water, and the other from the rocks, dust, and chunks of ice.

Comets themselves are usually between a few kilometres and several hundred kilometres in size, but their tails can be several million kilometres long.

Seeing Comets in the Sky

The "great comets" that produce particularly spectacular tails are some of the rarest objects in our solar system. Usually they can only be seen about once every hundred years, so it is very rare to see one of these comets. The last great comet appeared in 1910, but it may still be another hundred years before another one comes near the Earth. Astronomers can't predict exactly how or when they will appear as there are still things about our Solar System that they don't understand. If you hear about a comet coming into the sky soon, follow the instructions below to watch it!

1. Find out if the comet is going to be in your area of the sky.
2. Get a telescope or binoculars and some chairs to view it. Many of the largest comets have never even needed a telescope to see them.
3. Ask your parents to take you to a park, the woods or another dark place away from city lights.
4. Look up into the sky and enjoy this amazing sight.

Usually the dust making up the comet's tail is so faint you can't see it. However, when the Earth's orbit takes it through one of these tails, the dust hits the Earth's atmosphere and burns up. These are the recurring meteor showers that happen from time to time, and most major meteor showers have now been identified either with an existing comet or the remains of a comet that was observed earlier, usually in previous centuries. When the Earth travels through this "swarm" of dust left behind, you can see shooting stars or meteors at night.

How many Comets are there?

No one really knows. All comets spend most of their orbit so far away from the sun that they can't be seen -- even with a telescope. However, every year amateur astronomers[70] discover over 100 never-seen-before comets that have come close enough to be discovered[71]. As of November 2005, astronomers have discovered 2 857 comets[72]. Most of the comets we see either crash into the Sun, or leave the Solar System entirely. There might be millions of these comets that sooner or later will come within range of our telescopes.

Of all the comets that have ever been seen, astronomers only expect 253 comets to ever return[73].

How is a Comet named?

Edmond Halley

A comet is usually named after the astronomer who first discovered it. When several people are involved in its discovery, sometimes you will see multiple names on a comet, like Comet Hale-Bopp, or Comet Shoemaker-Levy. It is generally considered to be a great honor to have a comet named after you.

What are some famous Comets in history?

This diagram shows the orbit of Halley's comet around the Sun. There are a few things to note about this orbit.
*It is much more elongated than a planet's orbit.
*It is not in the same plane as the planets.
*It goes round its orbit in the opposite direction. This is called retrograde motion.

• Halley's Comet - Perhaps the most famous of all comets, and this was the first comet to be identified as a recurring comet.
• Comet Encke - The second comet to be identified as a recurring comet.
• Comet Shoemaker-Levy 9 - This was the first comet to have been observed hitting another body in the Solar System. In this case, it smashed into the planet Jupiter in what was possibly the most studied astronomical event in history.

Do comets bring bad luck?

In ancient times people didn't have a very good understanding of what comets really were or where they came from. They were seen as very unusual objects in the sky, and very temporary in nature as well. In some societies it was often a sign of bad events in the future when a comet arrived, associated with the death of a king or a significant military defeat. In other countries comets were considered to bring good luck, bringing increased fertility and more food. The ancient Chinese astronomers seem to have done the best job of actually recording when comets appeared in the sky, and left detailed descriptions of what they looked like and approximately where in the sky each comet was seen.

Even as recently as the 1910 appearance of Halley's Comet there was widespread panic when it was discovered that the Earth might pass through the tail of that comet. The panic was over the possibility of gases from the comet flooding the atmosphere of the Earth with poison. The reality was that there is so little gas in a comet tail that there is no measurable effect in the content of the Earth's atmosphere when an event like this occurs. Next Topic: Kuiper Belt

References

• "What is a comet?" [74]
• "What does a comet look like?" [75] [76] [77] [78]
• "Seeing comets in the sky" [79] [80] [81]
• "How many comets are there?"
• "How is a comet named?" [82] [83]
• "Do comets bring bad luck?" [84]

Kuiper Belt

Kuiper Belt Facts:

• The Kuiper belt is the most recently discovered section of the Solar System.

Beyond the orbit of Neptune lies the Kuiper belt. It extends outward an additional three billion kilometers away from the Sun. The belt contains different-sized lumps of icy mixtures. These lumps are called Kuiper belt objects. The biggest are called minor planets or dwarf planets. The Kuiper belt may have formed when the gravity of the young planet Jupiter hurled the objects out to where they are now. The Kuiper belt is named after Gerard Kuiper (rhymes with "viper"), one of several astronomers who hypothesized about a field of small objects beyond Neptune.

What are Kuiper belt objects?

The objects in the Kuiper belt are frozen mixtures of dirt, ice and organic compounds. They are a lot like comets. Some of the objects have a reddish color and others are gray.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

How big are the Kuiper belt objects?

Scientists consider Pluto to be one of the largest Kuiper belt objects. It is 2390 km across and is a dwarf planet. The next largest known Kuiper belt objects are Orcus, 2003 EL61 and 2005 FY9. Orcus is about 1600 km (1,000 miles) across; 2003 EL61 is 70% the size of Pluto and 2005 FY9 is about 50% to 70% of Pluto's size.

Recently, scientists found another dwarf planet named Eris that is even bigger than Pluto. The scientists don't know its exact size, but they think it is about 20% larger than Pluto. At the time it was found, it was almost 100 times further away from the Sun than the Earth. It can come about as close to the Sun as Pluto. Eris has a moon named Dysnomia. The orbit of Eris is tilted almost 45 degrees compared to Earth's orbit. Pluto's orbit is only tilted by 17 degrees.

Other large Kuiper belt objects about or over 1000 km across are Pluto's moon Charon, Quaoar, Varuna, Ixion, 1996 TL66, 2002 TX300, 2002 TC302, 2002 UX25 and 2002 AW197. Ceres, the largest asteroid in the asteroid belt, is about 950 km across.

There are many other Kuiper belt objects that are only a few kilometers or tens of kilometers across.

How many Kuiper belt objects are there?

Over a thousand Kuiper belt objects had been found by astronomers. Scientists think that there might be more than seventy thousand large objects in the Kuiper belt. Even though there are so many objects in the Kuiper belt, it is very light, weighing between ¹/₂₅ and ¹/₃₀ of Earth's mass.

What is it named after?

After the first object in the belt other than Pluto and its moon Charon was spotted from the Mauna Kea Observatory in Hawaii in 1992, the belt was named after the astronomer Gerard Kuiper. Back in 1951 this scientist wrote that he thought this belt might exist, but there was no proof at that time. Other astronomers, including Frederick Leonard, Kenneth Edgeworth, and Julio Fernandez, also thought that the belt existed. For this reason some astronomers call it the Edgeworth-Kuiper belt.

What are the Kuiper belt objects named after?

When an object is discovered in space, it is given a temporary name called a "provisional designation". This temporary name begins with the year the object was discovered, followed by some letters and numbers that tell in what month and in what order it was discovered. Later on, important objects are given formal names, often taken from mythology.

The Kuiper belt objects Orcus, Charon, and Varuna were all named after mythological gods of the underworld. Ixion was named after a mythological person in the underworld. Quaoar was named after a creation god of the Native American Tongva people.

Next Topic: Oort Cloud

Oort Cloud

	Oort cloud Facts:
The Oort cloud is as far from the Sun as you can go without leaving the Solar System. It is believed that most comets originate in the Oort cloud before "falling" toward the Sun.

The Oort cloud is a huge halo made of millions of comets that lies at the outermost edge of the Solar System.

What is the Oort Cloud?

Scientists say there is a distant group of objects, made of rock and ice, that forms a cloud-like region surrounding our Solar System. It is a cloud of comet-like objects orbiting far away from the Sun. Even though the comets are very widely scattered from each other, there many millions of them. The total mass of all these comets may be up to 100 times the mass of the Earth. The Oort Cloud is named after a Dutch astronomer Jan Oort who took the original idea, improved upon it and made it widely known.

This diagram shows about how far away the Oort cloud might be compared to the planets of the Solar System. Start in the upper left frame, then follow the pictures clockwise. Each picture shows a bigger volume of space.

As a comet makes several passes through the solar system, the Sun slowly melts and vaporizes the ice and only little bits of solid debris are left behind. But if the comets are all destroyed when they pass through the system, then new comets will need to appear. Otherwise we would not see any more comets. Jan Oort used the idea of the Oort cloud to explain why new comets keep appearing.

Where is the Oort Cloud?

If you can imagine the distance from the Earth to the Sun, then the comets in the Oort cloud are 50,000 to 100,000 times further away! That is 1,000 times further away from the Sun than is Pluto, and about one fourth the distance to the nearest neighboring star—Proxima Centauri. Light takes a year to travel from the Sun to the outer edge of the Oort Cloud.

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

How did the Oort Cloud start?

The Oort cloud objects may have started closer to the Sun during the Solar System's formation. When they passed near the gas giants, the gravity of those planets hurled the objects into very distant orbits. The Oort cloud objects were sent in all directions, making the Oort cloud ball-shaped instead of disk-shaped. The gravity of passing stars also made the orbits of these objects more circular, and pulled them further from the Sun. But sometimes the gravity of other far away stars can send the objects hurtling back toward the Sun. These become the comets.

What objects are in the Oort Cloud?

An object named Sedna has been discovered that may belong to the Oort Cloud (although it is actually between the Kuiper Belt and the Oort Cloud.) It is from 1,180 to 1,800 km across. Its orbit stretches from 76 to 928 times further from the Sun than does the Earth's. Sedna orbits the Sun about once every 11,250 Earth years. The last time Sedna was where it is now in its orbit, Earth's last Ice Age was ending! Some scientists think that Sedna should be included in the Kuiper Belt, making the belt bigger.

Next Page: Glossary

Glossary

The Solar System

Introduction Our Solar System The Sun Mercury Venus Earth Moon Mars Asteroid belt Jupiter Saturn Uranus Neptune Pluto Comets Kuiper Belt Oort Cloud Glossary Test

A glossary of words used in this book:

• Antimatter: the opposite of normal matter. Not usually found outside of a laboratory. When mixed with matter they cancel each other out and release lots of energy.

• Arachnoid: a scientific term for something shaped like a spider, like the legend of the weaving contest.

• Asteroid: a large rocky object that orbits a star, but is too small to be a planet. It is found in space.

• Astronomer: a person who studies stars and planets. Also a person who explores new planets and solar systems.

• Astronaut: a person who travels beyond the atmosphere of the Earth.

• Atmosphere: a layer of gases around a planet.

• Atom: a very tiny particle that is the basic building block of matter. It is the tiniest thing on Earth.

• Basalt lava: molten basalt, a kind of rock from a volcano.

• Belt: A name used for bands of dark-colored cloud layers on Jupiter.

• Binoculars: a folding pair of small telescopes with an eyepiece for each eye.

• Carbon dioxide: a gas that animals breathe out and plants take in.

• Carbonaceous chondrite: A type of meteorite that contains a lot of water and organic compounds.

• Centaur: an icy planetoid that orbits the Sun between Jupiter and Neptune.

• Channel: a groove in the surface of something.

• Comet: a small icy object orbiting a star.

• Conjunction: when two objects orbiting the same body come closest together.

• Continent: a huge landmass on a planet, usually made of tectonic plates that have locked together.

• Convection: a type of movement in a gas or liquid that carries heat toward a cooler location. When the gas or liquid cools, it sinks back down again.

• Core: the center of a planet or star.

• Corona: a region of very hot gas that surrounds the photosphere of a star.

• Crater: a dent in a planet's surface made by a meteorite falling on it.

• Crust: the outermost layer of a planet's surface.

• Dwarf planet: a rounded body that is in orbit around the Sun. It is not a moon and it is not big enough to sweep up the other objects along its orbit.

• Eclipse: the shadow made when one object comes between another object and the Sun.

• Energy: what you use to do work.

• Environment: the conditions on a planet.

• Equator: an imaginary line around a planet, perpendicular to the axis of rotation.

• Erosion: the slow wearing away of a surface, usually from wind, water, and temperature changes.

• Galaxy: a huge mix of gas, dust, stars, planets and other objects that are held together by their own gravity.

• Gas giant: one of the four outer planets made out of giant balls of gas.

• Gravity: the force that pulls on anything with mass (see the About gravity, mass, and weight section).

• Hemisphere: one half of a planet's surface.

• Ice cap: A huge body of ice at the pole of a planet.

• Lagrange point: the places where the gravity from two orbiting objects balance each other.

• Lava: molten rock above a planet's surface.

• Latin: the language of the Roman Empire that was later used by scientists to name things.

• Mantle: a layer of molten rock below a planet's crust.

• Maria: a large sea of magma that has cooled into solid rock.

• Mass: the amount of matter that something is made of (see the About gravity, mass, and weight section).

• Matter: a scientific word for 'stuff'.

• Meteor: a small or medium-size rock from space that has entered a planet's atmosphere but has not reached the ground.

• Meteor shower: a large number of meteors that enter a planet's atmosphere at about the same time.

• Meteorite: A meteorite that made it through a planet's atmosphere and landed on the ground.

• Methane: a gas that makes up most of the gas giants.

• Near Earth asteroid: an asteroid that has an orbit that brings it very close to the earth.

• Newton: a unit of measurement the describes how hard gravity is pulling you down (see the About gravity, mass, and weight section).

• Observatory: A special building where astronomers keep their telescopes ready for use.

• Orbit: the path that an object takes around a larger object.

• Orbit System: a planet and its moons rotating around a star.

• Organic compounds: compounds (collections of atoms) containing carbon.

• Phase: how a planet or moon looks to us at some part of its orbit, when it is lit by the Sun.

• Planet: the celestial body that has a greater mass than all other objects of the same orbit system together and that describes a well-defined, special orbit around a star.

• Planetary nebula: a great cloud of gas that was blown off by an old star.

• Photosphere: the layer of a star that releases light and other energy into space.

• Prominence: an eruption of hot gas at the surface of the Sun.

• Provisional designation: a temporary name given to a newly-found object. Later a permanent name may be picked.

• Radar: radio waves used to find distances to and make maps of things.

• Regolith: loose soil on the Moon created by rocks hitting the surface at very high speed.

• Retrograde motion: a rotation that is the opposite way from the rotation of most of the Solar System.

• Retrograde orbit: an orbit that is the opposite way from the orbit of most of the planets and moons in the Solar System.

• Ring: A flat, circular band of many small, loose objects that orbit a planet.

• Rotate: to spin around on an axis.

• Satellite: an object in a stable orbit around a much larger object.

• Scarp: a type of cliff.

• Sidereal day: the time for a planet or moon to rotate so that a distant star overhead is again overhead.

• Silicate: an object composed mostly of the element silicon, which makes rocks.

• Shooting star: another name for a meteor.

• Solar day: the time for a planet or moon to rotate so that the Sun is again overhead.

• Solar wind: a very hot gas that is being blown away from the Sun at a high speed.

• Spacesuit: A special sealed suit that protects an astronaut. It has its own air supply so the astronaut can breath, and is insulated against the cold of space.

• Spectrum: the colored band of light made when white light passes through a prism.

• Star: a huge ball of gas that is so heavy that it causes nuclear reactions inside itself. This produces heat and light.

• Sulfuric acid: a strong type of acid that is used in car batteries, and contains the element sulphur.

• Supergiant: a star near the end of its life that puffs out into a huge body many times larger than a normal star.

• Surface area: the area on the outside of something.

• Tectonic Plate: a solid part of the crust that very slowly moves across the surface of a planet

• Telescope: a system of lenses or mirrors that are used to see distant objects.

• Terrestrial planets: the four planets closest to the Sun.

• Tether: A cord that is used to keep two things attached to each other, such as an astronaut to a spaceship.

• Tide: the rise in the surface caused by gravity from another object, such as the Moon or Sun.

• Tidal lock: when tides have slowed rotation so that a moon or planet is always facing the same side toward the planet or star.

• Transit: When astronomers observe one object pass in front of a larger object.

• Trojan asteroid: an asteroid in the same orbit as a planet or moon that always stays the same distance ahead or behind.

• Volcanic: something that relates to volcanoes.

• Volume: the size of a three-dimensional object.

• White dwarf: a star that has run out of fuel to burn and is slowly cooling off.

• Zone: A name used for bands of light-colored cloud layers on Jupiter.

About gravity, mass, and weight

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Please see About weight and gravity for an alternate version of this module.

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Do you know your weight in kilograms or pounds? On Earth, your weight is a number. If you are heavier, then this number is larger. If you go to the moon, or to a space station, is this number the same? Some people say that it is the same, and some people say that it is different. To understand why, you need to know about gravity, mass, and weight.

Mass

Like many words, the word weight can have several different meanings. One meaning of weight is called mass. The word "mass" is used in astronomy and other parts of science.

The mass of an object is simply the amount of material the object is made of. The more material the object is made of, the more mass it has. Things that have a big mass are harder to move and harder to stop than objects with just a little bit of mass. So an empty box (with only air inside) is easier to move than a box filled with books. The box with books has more material, and so more mass, than the empty box.

Our mass is a number for how much stuff there is in our body. That is, if we grow, adding more cells to our body, or retaining more body fat or increasing how big our muscles are, our mass will increase; the number for your mass becomes larger. If we start a diet, and reduce how much fat is kept by our body, the number for our mass becomes smaller. In countries that use the metric system, also called the "Le Système International d'Unités" or SI, the units of mass used for the weight of people is often kilograms (kg).

Suppose that a child has a mass of 40 kg. Now suppose the child goes to the moon or to a space station, but does not change in body composition. Then the number for how much stuff the body has does not change. The child's mass on the moon or at a space station is 40 kg.

Our mass on any planet on the solar system (Jupiter, Venus, Earth, or anywhere else) is the same. But if we step on a scale, the reading will be different on all of these planets, because a scale measures weight, not mass, and weight depends on gravity as well as on mass.

Gravity

There is another meaning of weight, called "force of gravity". The word "weight" can mean one of two things, "mass" or "force of gravity". But what is "gravity"?

Suppose we jump into the air. We cannot fly, but instead we fall and land on the ground. There is a force which pulls us to the ground. This force is called gravity. The Earth makes gravity, so every time that we jump, we will land on Earth again, because the Earth's gravity pulls us, and we are not strong enough to jump fast enough to escape that gravity.

Which objects make gravity? To make significant gravity, an object must have a very large number for mass. A child may only be 40 kg. The Earth's mass is about 6 million billion billion kg. (That's six followed by twenty-four zeros). The Earth has enough mass to make us fall, as quickly as we do, when we jump.

All things with mass make gravity and attract one another. The more mass an object has, the more it attracts other objects toward it. So while the Earth pulls on a child, the child also pulls on the Earth. The force on the child is the same as the force on the earth. Because the earth is so massive, that force moves it very little. If we could look very closely, though, we would see that when the child jumps, the Earth is pushed away by the force of the child's legs, a very small distance, mostly it's the child who moves. And then, when the child falls, the Earth also "falls" just a little bit toward the child.

Everything with mass makes gravity. When we are on Earth, the moon, the other planets, and the sun are far away, and the force of gravity gets smaller with distance, so we land on Earth again. (Gravity pulls us towards the center of any massive object. Because we are so close to the center of the Earth, compared to the center of other planets and the Sun, we are mostly pulled toward the center of the Earth when we are on the Earth. Gravity would pull us towards the center of the Venus, or any planet, if we were on the Venus or another planet.)

Suppose we went to the moon. Now the Earth is too far away to have much effect on how we would fall. If we jump from the moon, we will land on the moon again. The mass of the moon is about 70 thousand billion billion kg. This is much less than the mass of the Earth, 81 Moons would have as much mass as the Earth.

The force of gravity varies directly with the two masses that we consider, multiplied together. If we are jumping on the moon, our mass would be the same as on the Earth, but the mass of the Earth is 81 times that of the Moon, so the attractive force, if we were at the same distance from the center of the Earth or Moon, would be 1/81 as great for the Moon as for the Earth. Because the Moon is smaller, though, we are closer to the center, and the gravity on the Moon's surface is about one sixth that of the gravity on the surface of the Earth.

To describe how gravity changes with distance is a little more complicated. If we divide the distance by two, we will increase the force by two times two, or four. That's said this way: "The force of gravity varies with the inverse square of the distance." When the distance is large, a little change in distance makes almost no change in the force of gravity, but the change in gravity from moving from a beach near the ocean to the top of a mountain can be measured. It's not enough to notice just from how it feels to jump!

Force

Gravity exerts a force and that force may be measured in units called "newtons." The force on a mass exerted by gravity will vary with the mass and how strong the gravity is where the mass is located. If we double the mass we will double the force.

However, when we push on an object that is free to move, that object will start to move in the direction we push. How fast it moves depends on three things: how hard we push, the mass of the object, and how long we push.

The more massive the object, the slower will be its response to our pushing. The result is that, although the force of gravity increases with mass, the motion decreases with mass, and these two effects cancel each other out.

So how fast things fall doesn't change with the mass. However, other things may change it, especially friction or air resistance.

A cannonball is pulled more strongly by gravity than a ball-bearing, but it has more mass and takes more effort to start moving. A ball-bearing has less mass, but takes less effort to start moving. Both then take the same amount of time to roll down a ramp, and perhaps you have seen or will see a demonstration of something like this.

Why, then, does something like a feather or piece of paper take so much longer to fall to the ground than a cannonball? This is because the resistance from the air in which they are falling is much greater for the feather or paper than for the cannonball. The resistance is a force opposite in direction to the motion, so it reduces the net force on the object.

(Gravity is always attracting us to the center of the Earth. Even when we are standing on the ground. The ground resists the force of gravity, pushing back up with an equal and opposite force, so we stay put.)

What happens, then, if there is no air? In 1971, astronaut David Scott visited the Moon, where there is no air. He held a feather and hammer, each in one hand, and then dropped them at the same time. They both hit the surface of the moon, also at the same time.

A table showing how gravity changes what happens elsewhere

If we were able to travel to another world, like the astronauts of the Apollo lunar exploration crews, there are a number of things that we would notice that are different from what you would experience on the Earth. There are also some things that would be just the same.

The following is a table regarding what kinds of experiences, if we weigh 40 kg. on Earth, would have if we visited different planets or moons in the Solar System:

	Earth	Moon	Mercury	Venus	Mars	Phobos
Surface Gravity (compared to Earth = 1000 milli-g)	1000	170	380	900	380	5 (average)
Our weight (mass)	40 kg	40 kg	40 kg	40 kg	40 kg	40 kg
How much we could lift	10 kg	59 kg	26 kg	11 kg	26 kg	2000 kg
How high we could jump	20 cm	120 cm	53 cm	22 cm	53 cm	400 m
Time to fall back to ground (seconds)	0.4	2.4	1.1	0.4	1.1	380
How far we could kick a ball	20 m	120 m	53 m	22m	53 m	(into Martian orbit)

In a pressurized chamber like a huge domed city, on the Moon, we would be able to put on wings and flap our arms to fly like birds do here on the Earth. Human powered flight is almost impossible here on the Earth because humans weigh too much here.

Phobos is one of the moons of Mars, and is so tiny that the gravity is very low. For example, if we kicked a ball really hard it could leave Phobos completely and go into orbit as a separate object orbiting Mars. Jumping up would take several minutes before the gravity would pull us back down, so we could jump over a mountain on that moon of Mars if we wanted to.

Of all the objects in the Solar System with a "solid" surface that we could walk on, the Earth has the strongest gravity. Jupiter and Saturn may have stronger gravity, but there is nothing we can say is a "solid" surface to walk on. There may be planets that are larger than the Earth with a solid surface, but they are not found in our Solar System. (However, if there were a floating platform on Jupiter or Saturn, we could walk on it, but it would be difficult, we'd weigh so much.)

Space exploration

Space exploration — A long dream

An illustration of a spacecraft in From the Earth to the Moon

Going into space was always one of the biggest dreams people had, even thousands of years ago. Many science fiction authors wrote about traveling in space even before the first airplane flight in 1903. One of the most famous science fiction books on space travel is From the Earth to the Moon by Jules Verne — it was written in 1865, more than one hundred years before the first person walked on the moon. Jules Verne's idea was to use a giant cannon! That might seem silly today, but it shows how much our ideas on space travel have changed.

The first exploration of space

Laika: the first space traveller

Space begins about 100 km or 62 miles above the earth. A more realistic way to travel in space is with a rocket. Within a rocket is a controlled explosion. However, exhaust (things left over after burning) is only allowed to leave the rocket in one direction. As a result, the rocket is pushed in the other direction. In 1942, the German rocket A-4 became the first to reach that height, but it wasn't meant to do anything but fall straight down again and so wasn't terribly useful. Still, it was an advance in rocket technology.

The Soviets were the first to put anything in space that would stay up: they launched the Sputnik 1 satellite on October 4, 1957. Within a month, the Soviets launched Sputnik 2, and in that spacecraft was the first space traveler: a dog called Laika.

The launch of the Sputnik started the Space Race, a competition between the United States and the Soviet Union to obtain more and more advanced space technology. Americans were very surprised that the Soviets could have launched 'Sputnik', and began to design rockets and satellites of their own. The race would last for the next few decades.

A man in Space

Yuri Gagarin

On April 12, 1961, the first person was sent into space: Yuri Gagarin, a Soviet, riding in the spacecraft Vostok 1. The Soviets would send more people into space over the next few decades, and so would the Americans, but it wouldn't be until 2003 that a different country would launch their own spacecraft with a person in it: China, with the Shenzhou 5.

The race to the moon

An Astronaut on the Moon.

At the beginning of the 1960s, American president John F. Kennedy made a famous speech in which he said that the U.S. was going to send people to the moon within the next 10 years. And that's what happened: in July of 1969, Neil Armstrong stepped off the there spacecraft and said "One small step for [a] man and one giant leap for mankind". He and Buzz Aldrin walked on the moon where they put an American flag. Their footprints are still there because there is no wind or water on the Moon to wash them away.

The Space Shuttle

Launch of a space shuttle.

After the Apollo program that sent people to the Moon, the U.S. built the Space Shuttle, that is like a jet-plane that can go to space and return (with the help of rockets of course)! The Space Shuttle helped to construct the ISS (International Space Station) among another things.

The last space shuttle mission was on June 28, 2011, but it will be replaced with new vehicles that will take mankind to the Moon, Mars, and beyond!

Spaceships of the future

Right now, spaceships are not very efficient. The Saturn V rocket was 363 feet or nearly 111 meters tall, and it could only take people to the moon! To get people further, better rockets must be invented. One of the most popular ideas for a rocket is the antimatter rocket. This type of rocket collides a small amount of antimatter with an equal amount of normal matter to create a large amount of energy!

Other ideas for going into space, that do not need rockets, have been thought of by scientists and astronomers. One of these is a space elevator. A space elevator is basically a big lift into space. It will cost a lot less to get things up into space if a space elevator is built.

Is this what space travel will look like in the future?

Another idea, a bit like the Jules Verne idea, is an electromagnetic catapult. This catapult works by accelerating the spaceship along a rail, similar to a maglev train. Unfortunately, the air on Earth would set spaceships on fire as they launched, but scientists aren't thinking of putting one there: one could go on the Moon! The catapult on the moon could send metal and other resources to Earth's orbit, where a space station could collect them.

Exploration beyond the Solar System

Many people dream of the day when humans can travel to another star and explore other worlds, some people wonder what's out there some belive that aliens or other life may live on another plant. But, if this ever does happen probably won't happen for a very long time. The stars are so spread out that there are trillions of miles between stars that are "neighbors". Maybe one day, your great grandchildren will be standing atop an alien world wondering about their ancient ancestors?

The eye beyond Earth

The Hubble Telescope as seen by the space shuttle.

Many people say the very best invention ever (not just in space technology) was the Hubble space telescope (HST). Others say it's just the space station being selfish having the best technology in the world.

The Hubble Space telescope is a giant telescope that is in orbit around the Earth. Because there is no atmosphere, the Hubble Space Telescope has a clear view of even distant galaxies. One of the pictures the Hubble space telescope has made is called the 'Hubble Deep Field'. The Hubble Deep Field is a picture of some of the most distant galaxies, and it gives a snapshot of what the universe looked like when it was younger.

A futuristic space telescope planned for the year 2021.

Even bigger telescopes are also in the planning, so we might be able to see right to the edge of the universe some day soon.