New images from the Hubble Space Telescope, released at a conference in the US last month, showed a cloud of water vapour - in effect a huge geyser - erupting from Europa's south pole. And new analysis of old data showed that the Europan surface is rich in organic compounds. According to Kevin Hand, a scientist at NASA's Jet Propulsion Laboratories in Pasadena, the ''drumbeat is getting louder'' for a mission to Europa. So why is Europa so exciting and why might it be home to life? Europa is about the same size as our moon. Before the Voyager probes arrived at Jupiter in the 1980s, it was thought that it would look like our moon as well. But it didn't. Instead the images showed a place that looked like a huge white billiard ball - gleaming, smooth and almost no craters at all. Analysis showed the surface was made of almost pure water ice and this fact, plus the lack of craters, started decades of speculation. Craters are used by astronomers as a dating tool. Everything big in the solar system gets hit from time to time. The number of craters we see is a good marker of how old the terrain we are looking at is. There are hardly any large, obvious craters on Earth's surface, not because we don't get hit, but because Earth's surface is, in geological terms, young. Earth itself is ancient, but its surface is constantly being modelled by plate tectonics and erosion. The big object that hit the Mexican coast 65 million years ago left a crater the size of Wales, but within a few tens of thousands of years it had been filled with mud and clay from the sea, its edges eroded. Today the Chicxulub crater is invisible from the surface.

The moon's surface, on the other hand, is ancient. Nothing much has happened there for 4 billion years, so the ancient impact craters dating from the moon's youth are still there. Mars has fewer craters than the moon but far more than Earth, suggesting that its surface lies between the two extremes in terms of age. Venus has a ''young'' surface, a hint that extensive volcanism may be constantly or episodically remodelling the crust. But Europa's surface looks like it was formed last week. Hal Levison, a scientist in the Southwestern Research Institute in Texas, has calculated that Europa should get hit by something big every million years or so. We see only a dozen craters on the surface, indicating that its surface is, on average, only 12 million years old, and in places no craters at all, meaning much younger again. How could this be, in a place with no air and where it is so cold (minus 160 degrees) that liquid water is an impossibility? Scientists eagerly awaited the arrival of the NASA Galileo space probe at the Jovian system. The pictures sent back in June 1996 astonished the scientists. Over the next couple of years a landscape of tumbled ice blocks, ''icebergs'' apparently frozen into the sea, grooves and enormous rifts poured into NASA's computers. Europa was an active world, whose frozen surface covered a huge secret. But how are these oceans possible? To melt ice that far from the sun, you need a source of heat, and on Europa there are two. One is volcanoes - Europa's neighbour, another (ice-free) moon called Io, is the most volcanically active object in the solar system. And another is tidal heating.

Europa is kneaded like putty by the gravitational forces of Jupiter and the other moons. This squeezing generates heat - maybe enough to melt the ice under the surface crust. Now scientists think that Europa is a ball of rock overlaid by a water ocean possibly a hundred miles deep, in turn covered by an active thin ice crust that periodically breaks up, new ice constantly forming as water gets close to the surface, much like the pack ice at Earth's poles. This would explain the lack of craters. The question is, how thin is the crust? The ''geysers'' spotted by Hubble indicate that in places the Europan ice may give way to open water. In 1998, one Galileo photo showed a tantalising glimpse of what may be a ''puddle'' - a completely smooth area of very thin ice. If the ice is tens of miles thick then the Europan ocean will probably be forever hidden to us. But if there are places where it is only a few centimetres to the water then a lander would be able to penetrate the crust and explore the depths below. In his novel 2010: Odyssey Two, a follow up to 2001: A Space Odyssey, Arthur C. Clarke speculated that Europa's oceans (technically one big ocean) could be home to life. If the ice is thin then enough sunlight could leak into the ocean's upper few feet for photosynthesis to be possible. But a more promising source of energy would be undersea volcanoes, which could spew nutrients into the sea and act as oases around which strange life forms could thrive. On Earth, this happens around deep-sea hydrothermal vents, where giant worms and strange yellow organisms inhabit a biosphere completely detached from the solar-driven one on the surface. Clarke speculated Europa's oceans could be home to large animals. Most scientists think this unlikely; there may not be enough energy available for anything more complex than microbes. But many scientists say we should send a follow-up mission to Europa to find out.

Ideally we would send a nuclear-powered lander equipped with a drill and a small robot submarine. There isn't the money for that right now, so the best hope lies with the ''Europa Clipper'', a NASA proposal to send a $2 billion probe to fly through any Europan geysers, analyse the water, and take detailed pictures of its surface. New Scientist magazine reported this month on a plan to send a small armada of tiny ''CubeSats'', perhaps privately funded, that could do a ''quick and dirty'' survey of Europa for a fraction of the cost of a big NASA mission. In 2010, humanity was instructed by Clarke's imagined god-like aliens to keep away from Europa: ''Attempt no landings there.'' In the real world the problem is not sinister monoliths but the realities of NASA budget cuts. If the privateers get there first, and find even hints of life, this still obscure little cue-ball world will become the most prized real estate in the solar system. Telegraph