PLANETS: 3. The Icy Giants
As we move farther from the Sun, we leave behind the warm, rocky worlds of the Solar System. We enter a place where the Sun's heat and light is not so strong; where space is cold, and the planets are frozen. We enter the realm of the icy giants.
Jupiter – the Super-Planet
Within its huge body – 88,378 miles (142,200 kilometers) in diameter – Jupiter could hold more than 1,300 Earths. It outweighs, by 2½ times, all the others planets combined. And its powerful pull of gravity can be felt throughout the Sun's kingdom.
When we look at Jupiter, we do not see a firm surface. Instead we see the brown, yellow, and orange cloud-tops of an enormously thick atmosphere. Because Jupiter spins very quickly, these clouds are drawn out into dark and light bands. The spinning planet causes the cloud bands to swirl and to form raging whirlpools. The most fantastic of Jupiter's storms, the Great Red Spot, is a centuries-old cyclone big enough to swallow Earth.
If you can imagine a star that has been frozen, you'll have some idea of what Jupiter is like inside. It's made mostly of hydrogen – the lightest of all elements. It also contains some helium, and a dash of other chemicals such as ammonia, methane, and ice.
In the cloud-tops of Jupiter, the hydrogen takes the form of a gas. It's mixed with some rarer substances that give Jupiter's belts and spots their beautiful "fall" colors. Deeper down, layers of ammonia crystals, ice crystals, and water droplets appear. Hundreds of miles down, the gassy atmosphere of Jupiter changes to a liquid. This vast sea of liquid hydrogen is lit only by the glow of giant lightning bolts.
Most of Jupiter is filled by this gloomy underground hydrogen sea. At great depths, though, the hydrogen is so squashed that it may behave like a liquid metal. The center of Jupiter is probably a rocky ball, somewhat bigger than Earth. This solid core is wrapped in the blanket of the planet's immense atmosphere.
Like the Sun, Jupiter actually gives off more heat than it receives. At -240°F (-150°C), its cloud-tops are very cold, but the heat from within makes them warmer than they would otherwise be. Deep inside the atmosphere, the temperatures are much higher.
Saturn – the Beautiful Giant
In recent years scientists were surprised to discover that thin rings surround the planets Jupiter, Uranus, and Neptune. For a long time, only one planet was known to have rings. This was Saturn, the most beautiful of the icy giants.
Saturn, smaller and cooler than Jupiter, is 74,145 miles (119,300 kilometers) in diameter and 887 million miles (1.43 billion kilometers) from the Sun. Like Jupiter, it spins rapidly – once every 10 hours, 39 minutes compared with Jupiter's 9 hours, 55 minutes. And like Jupiter, this fast spin makes it a flattened, rather than a perfectly round, ball.
Saturn stands out among the planets because of the size and majesty of its rings. These stretch like a giant halo, 169,200 miles (273,300 kilometers) across, around Saturn's yellow, banded globe. Although the rings appear to be solid, they are in fact made of countless small, icy rocks that circle Saturn like tiny moons. The rings are also amazingly thin – less than a mile in depth!
Of all the planets, Saturn is the lightest for its size. Standing on Saturn's surface, you would weigh less than you do on Earth. And if the whole of Saturn could be dropped into a vast ocean of water, it would float!
Uranus and Neptune – the Distant Giants
Beyond Saturn lie the two smaller icy giants of the Solar System, Uranus and Neptune. Like monstrous twins, both are about 30,000 miles (48,000 kilometers) in diameter. However, Uranus, at a distance of 1.78 billion miles (2.87 billion kilometers) is more than a billion miles closer to the Sun. Both planets were visited in the late 1980s by the Voyager 2 probe, which earlier had flown past Jupiter and Saturn.
Uranus and Neptune, like Jupiter and Saturn, have small rocky cores surrounded by very thick layers of frozen gases. Because of their great distances from the Sun, though, they are even colder than their giant neighbors. Uranus's surface shivers at -350°F (-212°C). Neptune's stays at a frosty -360°F (-218°C).
Uranus is special because it's the only world "lying on its side." While the other planets spin like upright tops as they go around the Sun, Uranus spins like a top that has tipped over. For us this motion would make Uranus's year – lasting 165 Earth-years – and its seasons appear very strange, indeed.
In 1977, scientists learned that Uranus is surrounded by several rings. These are much narrower than those of Saturn and have bigger gaps between them. Neptune too has been shown to be a ringed world. Rings around planets, it seems, are not so rare as they were once thought to be.
Children of the Icy Giants
If scientists were surprised to discover new rings around Jupiter, Uranus, and Neptune, they were astonished to learn about the many moons of the icy giants. The various spacecraft that have flown to the outer Solar System have opened our eyes to the remarkable families of mini-worlds that circle these great planets.
Jupiter has several of the biggest moons in the Solar System. Two of them – Ganymede and Callisto – are even larger than Mercury.
Another of Jupiter's moons, Io, would surely take the prize as the strangest world in the Sun's kingdom. Its entire surface is covered by orange chemicals thrown out by volcanoes. In fact, during the Voyager missions, several volcanoes were seen shooting great fountains of molten material into Io's sky!
Callisto, and a fourth big satellite of Jupiter, Europa, show just how different two neighboring worlds can be. Callisto is battered and pockmarked by densely packed craters. Europa, on the other hand, has a strange, ice-cracked surface that, from a distance, looks as smooth as a billiard ball.
In addition to Ganymede, Callisto, Io, and Europa, Jupiter has 59 other satellites, mostly quite small. Saturn, too, has a big collection of moons – a total of 60 at the latest count.
Saturn's largest moon, Titan, is blanketed by a thick orange haze. At 3,200 miles (5,150 kilometers) in diameter, it is almost as big as Ganymede. What's more it is the only moon in the Solar System known to have a thick atmosphere.
Saturn's other moons are all fairly small, but still interesting. Iapetus has one side coated by ice and the other made of dark, bare rock. Mysterious Mimas has a crater that looks like a giant eye.
Uranus and Neptune also have families of moons. Neptune's big moon Triton is especially fascinating because it has ice volcanoes that shoot substances such as frozen nitrogen and methane several miles above the surface.