A quick tour of the solar system
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Our trip from Mars to Jupiter takes at least two weeks, even at a million miles an hour. If the two planets don’t happen to be close together, the journey could take many more weeks still. Out here we are about five times as far from the Sun as Earth, and it can get very cold. We would probably need nuclear power to keep us warm – the solar panels that we can use on spacecraft nearer the Sun would not be much use.
As we get close to Jupiter, we realise that this is a giant planet. In fact it is the biggest planet in the solar system, and is a tenth of the diameter of the Sun itself. Like the Sun, but unlike the other planets we have visited so far, it is made only of gas – mostly hydrogen. But there is so much gas that you can’t see through it, and all we can ever see is the outer layers of the globe – there is no actual surface. We can see great swirls of gas, which change from week to week like a enormous flowing stream. If we were to try to descend into Jupiter, we would find ourself surrounded by gas that becomes denser and hotter, until our spaceship was crushed and fried. So we stay well away and admire the view. One of the storms on Jupiter has been raging for at least 300 years – a great whirlpool of red gas known as the Great Red Spot.
We can’t land on Jupiter, but fortunately it has dozens of moons which we can explore. The four largest are easily visible from Earth, even using binoculars. They are named Io, Europa, Ganymede and Callisto.
Io is just a little bigger than our own Moon, but it’s totally different. Instead of a grey, unchanging world the surface of Io is constantly in motion with constant volcanic eruptions and ever-changing seas of yellow molten lava. The orange colour is caused by sulphur, and this has turned the whole moon into a patchwork of oranges, reds and yellows – so it looks like a pizza! The heat to produce all this activity is caused by Io being the closest large moon to Jupiter. The gravitational pull of the giant planet on opposite sides of Io causes a great deal of friction, which heats up the moon’s interior.
The other three large moons are farther out, so they aren’t strained in the same way and are mostly ice. Europa, the next out, is covered by frozen oceans. Cracks in the surface have allowed different-coloured ice to ooze out, forming these amazing grooves. It seems that at some time in the past few million years the ice has melted and there have been ice floes floating around. Maybe the oceans are still liquid deeper down, and it’s just possible that there is some form of life below the surface. There are plans to send space probes to Europa to discover more.
Next out from Jupiter are Ganymede and Callisto, more icy worlds covered with craters as asteroids have rained down on them over millions of years. Because they are covered with craters, we know that their surfaces haven’t been altered much over a long period of time, otherwise the craters would have been smoothed out. Neither Io nor Europa have these craters.
Leaving Jupiter’s system of moons will take a lot of fuel because it is so massive. Jupiter has a great influence on the planets of the solar system, and in 1994 it even dragged a comet into it, which caused great black spots to form where the fragments plunged into the planet.
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Closeup of Jupiter’s Great Red Spot
Io seen against Jupiter
Lava flows on Io
Europa from space
Groove on Europa
Frozen floes on Europa
Marks from the comet crash on Jupiter