The Children's Encyclopedia
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
At a distance of 484 million miles (778 million kilometers) from the Sun, we come
to the first of these giants. It is mighty 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
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
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.