Nasa spacecraft reveals dance of Jupiter's moons finds water on Jupiter's moons

Just as in 2001: A Space Odyssey, a spacecraft built by humans has reached Jupiter, though this one does not have any people on board - nor, happily, a murderous computer.

Just as in 2001: A Space Odyssey, a spacecraft built by humans has reached Jupiter, though this one does not have any people on board - nor, happily, a murderous computer.

Instead, the latest images sent back by the Cassini spacecraft show an ancient orbital dance of Europa and Callisto, two of the moons of the largest planet in the solar system.

The moon on the lower left, Callisto, is roughly 1.8 million km (1.1m miles) above the tops of the clouds of the gas giant; while Europa, seen against the planet, is 600,000km (375,000 miles) above the clouds. Those clouds extend for hundreds of miles above the surface of the planet, which is enormously hot, and thought not to harbour life - although its moons, particularly Europa, may well have primordial organisms.

Europa is slightly smaller than our own Moon, and is one of the brightest objects in the solar system. Callisto is 50 per cent bigger - roughly the size of Saturn's largest satellite, Titan - and three times darker than Europa; its brightness had to be enhanced relative to that of Europa and Jupiter so that it would show up in this picture.

The two moons were formed by very different geologic processes, yet share some surprising similarities. Both have surfaces rich in water ice - the first requirement, many scientists think, for harbouring life like our own. But investigations of Callisto's magnetic field suggest that it has no internal structure, but is just a jumble of rock and water.

Europa has a rocky core and an outer layer of almost pure water ice 50km (30 miles) thick. As it orbits Jupiter, tidal effects "squash" this core, generating heat that melts the ice - creating a water ocean 130km (80 miles) deep beneath the surface, and hence some essential conditions for life to begin.

In April 1998, scientists learnt that there were organic chemicals beneath the ice: William Smythe, who analysed the data, said: "One of the absorption wavelengths indicated the presence of tholins - a sort of organic gunk. You get it left over from swamps, and it is also ... thought to have been around in Earth's primordial soup."

The Red Spot on Jupiter's surface is actually an everchanging atmospheric storm that has gone on for thousands - and probably millions or even billions - of years. It is large enough to swallow a hundred Earths, yet occupies only a tiny proportion of the entire planet's area.

The Cassini spacecraft took the pictures on 7 December, on the way to its final destination, Saturn, which it will not reach until 2004. Scientists at Nasa are excited about the results. "Everything has been working smoothly," said Bob Mitchell, Cassini programme manager at Nasa's Jet Propulsion Laboratory in Pasadena, California. Cassini had had problems with its reaction wheels, used to turn it during flight.

The spacecraft passed Jupiter at a distance of 9.7 million km (6 million miles) on Saturday, using the planet's gravity as a "slingshot" to boost its speed on its way to Saturn.