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Jupiter and Its Moons

  • Grades: 1–2, 3–5

The following questions were answered by astronomer Dr. Cathy Imhoff of the Space Telescope Science Institute.

Why is Jupiter called Jupiter?
Jupiter got its name about two thousand years ago. It is named for the Roman god Jupiter, who was the chief god of the many gods that the Romans believed in. All the planets have names that come from the Romans.

How far is Jupiter from Earth? How long would it take to go to Jupiter and back?
Jupiter is about five times farther from Earth as Earth is from the sun. Let's see the distance between Earth and the sun is about 93 million miles, so the distance to Jupiter is about 500 million miles!

How long it takes you to get there depends on how you go. If you could drive a car going 60 miles an hour to Jupiter, it would take you about 950 YEARS to get there! Fortunately our spacecraft like Voyager and Galileo go much faster. The Galileo spacecraft took six years to get to Jupiter, and it was going thousands of miles per hour. The solar system is a BIG place!

How long does it take for Jupiter to go around the sun?
Almost 12 years (Earth years that is).

What are some of the gases on Jupiter?
The gases include nitrogen, hydrogen, helium, methane, and ammonia. There is even a little water. Not all of these gases are poisonous. Most of Earth's atmosphere is nitrogen, and there is a little helium too. There is no oxygen on Jupiter like there is on Earth. The plants on Earth have made the oxygen that we breathe.

Since Jupiter is made of gases, is it possible to walk on Jupiter or would you just fall through it?
The gases are pretty thick but not enough to stand on. I think it would be almost like being in the ocean, so I guess you would want to have a special submarine to explore Jupiter!

Why doesn't Jupiter blow up if it's made out of gas and on fire?
Jupiter is made up of gas, mostly hydrogen, but it isn't on fire. To burn the hydrogen gas, you would also need oxygen, but there is very little oxygen on Jupiter. Jupiter has lightning storms much like Earth. But they don't start any fires because there is no oxygen.

How does the gravity of Jupiter compare with Earth?
Jupiter is much larger than Earth, so its gravity is stronger. If you could stand on the surface of Jupiter, you would weigh 2.6 times as much as on Earth. So if on Earth you weigh 100 pounds, on Jupiter you would weigh 260 pounds.

Does Jupiter have rings?
Yes, Jupiter has rings. They are not as big and bright as Saturn's, so they weren't discovered until Voyager passed by in 1979. This was a big surprise. Everyone thought that rings would be a very unusual occurrence. Later some parts of rings, called ring arcs, were found around Uranus. So now we understand that rings around planets are not so unusual after all.

Does Jupiter have an outer crust?
No. Jupiter (also Saturn, Uranus, and Neptune) is made up of thick, cold gas. The gas is mostly hydrogen, with some helium, ammonia, methane, nitrogen, and water vapor. We call those planets gas giants. The inner planets (Mercury, Venus, and Mars) are more like Earth. They are rocky, and probably have an outer crust, mantle, and core like Earth.

Have we ever found out how many moons Jupiter really has? What about Io? Has anyone found out anything new or interesting about it?
Well, so far we have found 16 moons around Jupiter. We always find the biggest ones first, but the little ones with odd orbits are harder to find. It wouldn't surprise me if a few more little moons were found in the future! As you may know the Galileo spacecraft is currently orbiting Jupiter and studying the four biggest moons, including Io. We now have images of volcanoes going off on Io, spewing out mostly sulfur dioxide. Recently a large dark area the size of Arizona was seen, perhaps the result of a lava flow!

Why does Jupiter have a red spot?
The red spot on Jupiter appears to be a huge storm, like a gigantic hurricane. The clouds of the red spot actually rise a little higher above the atmosphere than the surrounding clouds, like hurricane clouds. For some reason it has persisted for over 300 years! Some people think that deep down inside Jupiter there may be a mountain or something that helps stir up the red spot storms. The red color comes from the slightly different composition of atmospheric gases that are in the spot.

How big is the red spot on Jupiter?
You may be surprised to learn that the spot varies in size. Sometimes it is only 12,000 miles long, and sometimes it is 24,000 miles long. It is an oval shape, and the short distance across the oval is about half the length (6,000 to 12,000 miles across). Earth is about 8,000 miles across, so that makes it smaller than the red spot.

How does the storm stay on Jupiter?
The red spot has lasted for a long time, at least ever since people have looked at Jupiter with telescopes. But it may not be there forever. We aren't sure why it stays there so long. One idea is that below the surface there is some feature, like a giant mountain, that causes the red spot.

Are Neptune's two dark spots the same as Jupiter's red spot?
They are similar. Neptune's dark spots and Jupiter's red spot are all big storms, with winds blowing at a thousand miles per hour. They also spin, like a hurricane.

Is there really a planet about the size of Earth inside of Jupiter?
The rocky planetesimals that formed the core of Jupiter would be sort of like a planet like Earth. So it's not really another planet, just the core of Jupiter.

If you could get under the gas on Jupiter, what would the surface look like?
We're not completely sure, but here is what some calculations tell us. The gases become thick and dense they become liquid. This is primarily liquid hydrogen. Inside this the liquid hydrogen becomes like a metal, because it is under so much pressure from the weight of Jupiter. There is probably a solid core deep inside Jupiter, consisting of all the other elements that sank to the center of the planet. We are hoping that the comet hitting Jupiter will be like an earthquake scientists can use the vibrations to tell us about the center of the earth.

Could a meteorite hit Jupiter or would it go through it?
A meteor can hit Jupiter I'm sure that meteors hit it all the time. But it can't go through the planet. We describe Jupiter as a gas giant, but that doesn't mean that it is light and fluffy like a cloud. The gases are so thick that they are more like an ocean.

Did you hear about Comet Shoemaker-Levy last summer? It was a comet that hit Jupiter. The pieces hit the clouds of Jupiter so fast and hard that they exploded and caused dark spots on Jupiter.

How long did it take for Galileo to get to Jupiter?
Galileo was launched from the space shuttle Atlantis on October 18, 1989. It took a long route, going by Venus and the Earth to use their gravity to give it an extra boost so it could go all the way to Jupiter. Six years that is really the LONG way!

What damage did the comet that struck Jupiter do?
As it turns out, not much. It seems to have exploded just above Jupiter's atmosphere of cold gases. The regions have turned dark, so there have been some long-term effects, but we expect them to disappear with time.

What is the relationship between the amount of energy absorbed by Jupiter and the amount of energy radiated from Jupiter?
In general, we think of planets shining only by the reflected light of a star. The planet itself doesn't have its own source of energy. But Jupiter is different. Scientists were surprised to discover that Jupiter radiates TWICE as much energy as it receives from the sun. Therefore it must have its own source of heat deep inside the planet. It might be heat that has been trapped inside the planet from when it formed, and now is slowly leaking out. Or it might be that the planet is slowly contracting that would provide gravitational energy that gets converted to heat. Maybe both explanations are correct. One result of this extra source of heat inside Jupiter is its beautiful clouds. The heat rising from inside the planet causes convection the rising and falling of gases or liquids that are being heated. You can see this in a pan of boiling water the hot water rises, bubbles and cools, then sinks to the bottom of the pan to be warmed again by the burner on the stove. We also see convection in our own clouds on a hot day the ground warms the air, the air rises and cools, forming clouds, then the cool air falls back to the ground to be warmed again.

  • Subjects:
    Astronomy and Space, Planets, Moons, Solar Systems, Real-World Science
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