Jupiter and Its Fascinating Moons


By Robert Hazen, Ph.D.George Mason University

The character of our solar system changes radically beyond the asteroid belt. In the realm of the gas giant planets, Jupiter, a ball of hydrogen, is the largest planet. The planet itself is 2.5 times as massive as all the other planets combined. Whenever you think of Jupiter, think of two things: “weather” and “amazing moons”.

A 3D illustration of Jupiter
Jupiter could have been a star if it had started with more mass. (Image: NASA images/Shutterstock)

Balls of Hydrogen with Different Masses

Any object that’s gravitationally bound just from gas and dust in space is going to be primarily hydrogen, with some helium. So all sorts of objects occur out there, from small ones to large ones. Think about what happens to balls of hydrogen in space.

If they begin ten times the mass of the Sun, they end up as exploding stars. If they start about the mass of the Sun, then they go through the hydrogen-burning phase; then they become a red giant, and then a white dwarf, and sort of dwindle away their life. If they begin a tenth of the mass of the Sun, they end up as brown dwarfs, stars that can burn for hundreds of billions of years. 

If they begin one-hundredth the mass of the Sun, you end up with a giant planet because nuclear fusion never gets started. You just have that big ball of hydrogen, and helium, and other atoms.

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Gigantic Storms on the Gas Giant

Jupiter is about five times the Earth-Sun distance; it orbits the Sun every 12 years. A day on Jupiter, though, is only ten hours long. Think about that: Jupiter is a much larger planet than Earth, yet it orbits once every ten hours. It’s spinning so fast that it bulges at the equator. 

It also causes intense and violent weather. Because when Jupiter rotates, it shears its atmosphere of hydrogen and helium into many, many bands and layers. You have intense vortices spinning where these atmospheric bands come in contact. 

You have giant storms, immense storms, bands, and swirls that you can see with the various close-up photographs that have been taken. The largest of these storms is the Great Red Spot; it would swallow 100 Earths without even missing them.

Jupiter has intense weather in its 1,000-kilometer-thick atmosphere. The Great Red Spot, for example, is a storm that has been raging for over 300 years. By the way, that red color is apparently due to carbon-based organic molecules, molecules that may have been created through lightning storms, which are also abundant in the upper atmosphere of Jupiter. 

The atmosphere of Jupiter appears to be about 79 percent hydrogen and 20 percent helium, plus maybe one percent of organic molecules that give it its bright colors.

Learn more about electromagnetism.

Magnetic Fields of Jupiter

Jupiter has a magnetic field, and that magnetic field may be ten times stronger than Earth’s.

An illustration of Jupiter and some of its moons
Jupiter’s moons are distinct and exciting, particularly the Galilean moons. (Image: Vadim Sadovski/Shutterstock)

It is thought that it might be generated in an interior region because of metallic hydrogen. At very high temperatures and high pressures, hydrogen—like all other materials – will become a metal, and electrical currents may flow in that metal and create a tremendous magnetic field.

Jupiter’s Moons

Astronomers have identified 16 different moons of Jupiter, ranging in size from asteroid-like bodies just 20 kilometers across to the giant planet-like Ganymede, which is 5,200 kilometers in diameter. It is like a small terrestrial planet.

The four of these large and exciting moons are called the Galilean Moons. Those are the moons that Galileo saw in his telescope in 1609; it includes Io, Europa, Ganymede, and Callisto. Galileo made drawings of their movements, and each of these objects looked similar in Galileo’s telescope. However, we now know that they differ in their size, composition, and their dynamics.

Learn more about extrasolar planetary systems.

Characteristics of Galilean Moons

Illustration of Europa, one of Jupiter’s moons.
Europa could have the most Earth-like environment in the solar system. (Image: Will hilton-kent/Shutterstock)

Io is the innermost of these large moons. It’s almost as large as Mercury and has its own atmosphere and extensive volcanism. In fact, Io is the most volcanically active place in the solar system. It’s so close to Jupiter, it turns out, that Io experiences severe tidal forces which stress and bend and break the moon, and that’s what pumps the energy into Io to keep it so volcanic. 

Europa is the second moon of Jupiter. It’s the smallest of the Galilean moons but by far the most interesting. Its surface is covered by what appears to be a cracked sheet of water ice, water that may cover a thick ocean. If tidal stresses from Jupiter keep this ocean warm, then Europa may actually harbor the most Earth-like environment anywhere in the solar system.

Then we have Ganymede, Jupiter’s third and largest moon, and Callisto, which both have worn, cratered surfaces. These suggest long periods of geological inactivity. Even so, there’s a possibility that Ganymede, too, may have a deep inner ocean, another exciting moon to watch.

Common Questions about Jupiter and Its Fascinating Moons

Q: Why are four of Jupiter’s moons called the Galilean moons?

Io, Europa, Ganymede, and Castillo are four of Jupiter’s moons that Galileo discovered in 1609. This is the reason why they are called the Galilean moons.

Q: What are the characteristics of Io?

Io is the innermost moon of Jupiter. It is extensively volcanic primarily because of its proximity to Jupiter, making it the most volcanically active place in the solar system. This moon also has its own atmosphere.

Q: Which of Jupiter’s moons is most Earth-like?

Europa is the candidate for having the most Earth-like environment; and not just compared to Jupiter’s moons, but also in the entire solar system.

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