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Rocky planets are mostly core and mantle.
They have a very, very thin crust and a very, very thin atmosphere. Gas giants are mostly atmosphere with very, very tiny cores compared to their size. Jupiter, Saturn, Uranus, and Neptune are big, swirling clouds of gas.
A cloudy atmosphere blocks our view of a gas giant, making it hard to learn more about the planet. It’s like studying an egg, but seeing only the shell. Scientists can only guess what’s under all those clouds.
Jupiter
▲ Jupiter has a cloudy atmosphere with raging storms and lightning. Its atmosphere is about 88 percent hydrogen and 11 percent helium. Those gases are invisible. Then why is Jupiter so colorful? It has pink, yellow, brown, white, red, and other shades. Along with hydrogen and helium, Jupiter’s atmosphere has tiny amounts of water droplets, ice crystals, ammonia crystals, and other chemicals. The chemicals reflect the Sun’s light differently, and that’s where the clouds get their colors.
◀ The Great Red Spot of Jupiter is a swirling storm big enough to cover two or three Earths! We’ve been watching it since 1655, but no one knows how long it’s been going. How long will it rage?
◀ Why does Jupiter have swirls and wiggly whirls? One reason is that parts of Jupiter (the equator, for example) spin faster than other parts (like the poles). Run a finger quickly through still water. Look for swirls, wiggles, and other patterns just like Jupiter’s!
Liquid hydrogen
Heat and pressure pack the hydrogen gas into a liquid.
Liquid metallic hydrogen
The liquid hydrogen is so thick, it acts like a molten metal.
Core
The core is very tiny compared to the gas layers of the atmosphere.
Cross Section of Jupiter
◀ Jupiter’s four biggest moons are known as Galilean moons. That name comes from the seventeenth-century scientist Galileo, who discovered them.
▲ Ganymede is the largest moon in the solar system.
▲ Callisto’s icy, dusty surface is coated with craters.
▲ Io looks like a pizza with “the works.” Its messy surface comes from many superactive volcanoes that cover the surface with fresh lava over and over.
▲ Europa’s icy surface is the smoothest body in the solar system.
Saturn
Saturn, Jupiter, Uranus, and Neptune have rings. But Saturn’s are the most amazing. They’re less than a mile thick and are made up of billions of pieces of ice, dust, and rocks. Some of these pieces are as small as a grain of sand, and others are as big as a house. ▶
Uranus
▲ Uranus is tilted on its side at an extreme angle. Sometimes the poles, and not the equator, face the Sun. Each pole gets 42 years of nonstop sunlight and then 42 years of total darkness.
Solar System Mystery
◀ Suppose an asteroid blasted apart a moon and all the pieces flew away from each other. But then they clumped back together, thanks to gravity. Like Frankenstein’s monster, Uranus’s moon Miranda looks like it was made from spare parts. It has scars, canyons, ridges, and cracks many times bigger than on Earth. Why? A “big blast” is one guess. Do you have another idea?
Neptune
▲ Like Uranus, Neptune is known as an “ice giant.” Along with hydrogen and helium, its atmosphere contains methane gas. That’s what gives the planet its blue color. Under the atmosphere is a thick, slushy layer of ice and water. Scientists say Neptune is one of the coldest places in the solar system. It has an average temperature of –390°F!
