On December 7, 1995, the Galileo spacecraft arrived at Jupiter after a six-year trip. Its job was to loop around Jupiter repeatedly, and to swing close to most of the moons to have a really good look. On one loop, it flew only a few hundred kilometres above the surface of Europa (the second moon of Jupiter).

Now the rocket that launched Galileo didn't have enough grunt to send the spacecraft directly out to Jupiter. So Galileo looped around Venus and Earth in a gravitational-slingshot manoeuvre, to pick up extra speed. When Galileo flew past Earth, it flew over Australia and the Antarctic. It took photographs of the ice in the Antarctic.

The surface of Europa looks just like this ice in the Antarctic! The surface of Europa seems to be a thin ice crust, covering either liquid water or frozen lush.

This is amazing stuff - the discovery of lots of frozen water elsewhere in our Solar System. This ice is genuine water ice, not some obscure hydrocarbon ice.

The Voyager Spacecraft took pictures of Europa from half-a-million kilometres, and showed us mysterious lines, but the Galileo pictures from a few hundred kilometres away showed us that the lines are ridges of ice. Some parts of Europa look like jumbled fields of icebergs. In some places, there are giant icebergs, eight kilometres across, that have been pulled apart and rotated. These rafts of ice look as though they've been broken off from bigger lumps of ice, and then tilted or spun, and then melted back into position. Some of the material on the surface looks like it's been spewed out of an ice volcano. And everywhere there is the amazingly complex system of criss-crossing ridges, as though the ice has been repeatedly pulled apart, or pushed together.

The scientists agree on two facts. First, they agree that the surface ice is water ice. Second, they agree that there was definitely liquid water on Europa in the past.

But is there liquid water underneath the ice today? Nobody knows, and Galileo's instruments are not the right ones to tell us. There are two pieces of evidence that suggest there might be liquid water underneath the ice.

Firstly, in some areas, there are virtually no craters on the surface of Europa. Jupiter has an enormous gravitational field, and like a cosmic vacuum cleaner, sucks in comets like crazy. All the other moons of Jupiter have pock marks from millions of comet impacts. But part of the surface of Europa has hardly any craters. Secondly, Europa is the smoothest body in the Solar System. The highest mountain on Europa is less than a kilometre high.

Perhaps the surface is slightly plastic and flows easily. Oceans of liquid water just under the ice are one obvious answer. After all, the core of a planet or moon is always hotter than the surface, and if you have ice on the surface, you could have liquid water underneath.

The Galileo findings were so exciting, that NASA has announced that in 2003 it will launch a spacecraft called the Europa Observer. It will have a sophisticated radar system that can look right through the ice, and find any liquid water underneath - if there is any. It will also probably have a laser instrument to look at the surface of Europa, to measure exactly how much the surface flexes up-and-down each day. And of course, it will have a few cameras.

Back in 1977, around when the Voyager Spacecraft were launched, strange new life forms were discovered on our ocean floor, a few kilometres down. There were giant three-metre long worms without a mouth or an anus, and giant white clams. These creatures were the first ever discovered that did not rely on photosynthesis. They did not need the energy of the Sun. Similar kinds of creatures could live off the tidal heating in the oceans of Europa.

Arthur C. Clarke wrote 2001: A Space Odyssey. In the follow up, 2010, a mysterious message arrives at Earth and warns us, "All these worlds are yours, except Europa. Attempt no landings there." But just recently, Clarke said that we have to explore this world that may have water on it. If there's liquid water, there is almost certainly life there - and that would be really exciting.