20.11 Jupiter

"What Jupiter? Do not trifle. There is no Jupiter." — Aristophanes

The Romans named the largest planet for their most important god. They followed the tradition of the Greeks, who had similarly named the planet Zeus. They built a temple to Jupiter on the hill. Was Aristophanes denying the existence of the most important Roman god?

Characteristics

Jupiter is enormous, the largest object in the solar system besides the Sun. Although Jupiter is over 1,300 times Earth’s volume, it has only 318 times the mass of Earth. Like the other gas giants, it is much less dense than Earth.

Because Jupiter is so large, it reflects a lot of sunlight. Jupiter is extremely bright in the night sky; only the Moon and Venus are brighter (Figure below). This brightness is all the more impressive because Jupiter is quite far from the Earth — 5.20 AUs away. It takes Jupiter about 12 Earth years to orbit once around the Sun.

A Ball of Gas and Liquid

Astronauts trying to land a spaceship on the surface of Jupiter would find that there is no solid surface at all! Jupiter is made mostly of hydrogen, with some helium, and small amounts of other elements (Figure below). Jupiter's atmosphere is composed of hydrogen and helium. Deeper within the planet, pressure compresses the gases into a liquid. Some evidence suggests that Jupiter may have a small rocky core of heavier elements at its center.

A Stormy Atmosphere

The upper layer of Jupiter’s atmosphere contains clouds of ammonia (NH₃) in bands of different colors. These bands rotate around the planet, but also swirl around in turbulent storms. The Great Red Spot (Figure below) is an enormous, oval-shaped storm found south of Jupiter’s equator. This storm is more than three times as wide as the entire Earth. Clouds in the storm rotate in a counterclockwise direction, making one complete turn every six days or so. The Great Red Spot has been on Jupiter for at least 300 years, since astronomers could first see the storm through telescopes. Do you think the Great Red Spot is a permanent feature on Jupiter? How could you know?

Jupiter’s Moons and Rings

Jupiter has a very large number of moons — 63 have been discovered so far. Four are big enough and bright enough to be seen from Earth, using no more than a pair of binoculars. These moons — Io, Europa, Ganymede, and Callisto — were first discovered by Galileo in 1610, so they are sometimes referred to as the Galilean moons (Figure below). The Galilean moons are larger than the dwarf planets Pluto, Ceres, and Eris. Ganymede is not only the biggest moon in the solar system; it is even larger than the planet Mercury!

Scientists are particularly interested in Europa because it may be a place to find extraterrestrial life. What features might make a satellite so far from the Sun a candidate for life? Although the surface of Europa is a smooth layer of ice, there is evidence that there is an ocean of liquid water underneath (Figure below). Europa also has a continual source of energy — it is heated as it is stretched and squashed by tidal forces from Jupiter. Numerous missions have been planned to explore Europa, including plans to drill through the ice and send a probe into the ocean. However, no such mission has yet been attempted.

In 1979, two spacecraft — Voyager 1 and Voyager 2 — visited Jupiter and its moons. Photos from the Voyager missions showed that Jupiter has a ring system. This ring system is very faint, so it is difficult to observe from Earth.

Summary

• Jupiter is mostly hydrogen with some helium, and may contain a small rocky core.
• Jupiter has a thick atmosphere containing the Great Red Spot, a storm that has been going for at least 300 years.
• One of Jupiter's moons, Europa, appears to have a liquid ocean beneath the surface due to tidal forces from the massive planet; this ocean could be a place for life.

Review

1. Why is the Great Red Spot thought to be a storm?
2. What is Jupiter made of?
3. Why is Europa one of the few locations in the solar system where scientists think there could possibly be life?
4. What is the source of heat on Europa?