A SMALL, bitterly cold moon orbiting Saturn may this week be identified as the most likely place in the solar system apart from Earth for liquid water to exist.

On Thursday, Nasa’s Cassini probe will pass just 109 miles above the surface of Enceladus, a 310-mile-wide chunk of rock and ice with surface temperatures that hover around -180C.

The new phase of exploration of Saturn comes as findings emerge from earlier stages of Cassini’s voyage. Researchers believe they have identified oxygen in at least one of Saturn’s rings, while an analysis of the surface of Titan, another of the planet’s moons, has found gravel made of chipped ice, and huge volcanoes spewing ice and frozen gases.

The new pictures of Enceladus are likely to be among the most spectacular yet. At the temperatures on its surface, water should be so deep frozen that it is as hard as rock, but researchers have now realised that the moon’s special character means liquids may be able to flow on it.

“The imaging we already have shows that the surface is active. The craters from meteor impacts are being removed. It is as if a liquid is regularly flowing over the surface,” said Philip Nicholson of Cornell University, a lead scientist on the mission.

The identity of the liquid is uncertain, but it is probably water mixed with ammonia, a chemical that can prevent water freezing even at temperatures below -100C. Nicholson and his colleagues believe that, below the moon’s crust, lies a vast reservoir of liquid that periodically erupts across the surface like an icy geyser or volcano.

The key question of where such warmth might come from has already been solved. Enceladus is too small to retain a molten core like Earth’s but it is locked into a gravitational tug of war with Dione, another of Saturn’s 30-plus moons.

Carl Murray, professor of mathematics and astronomy at Queen Mary, London University, said: “This generates huge forces on Enceladus which heat up its core.”

The Cassini team found powerful evidence for such processes earlier this year when it detected traces of an atmosphere around the moon. Enceladus is so small that any gases around it rapidly disappear into space so they must be constantly replenished.

Michele Dougherty, professor of space physics at Imperial College London and a senior member of the team that found Enceladus’s atmosphere, said “cryovolcanoes” (icy volcanoes) were the likely source.

If such findings are confirmed by Thursday’s fly-by, then Enceladus could become as important for planetary science as Titan, on which the European Huygens probe landed earlier this year.

Huygens produced spectacular pictures but the detailed analysis has taken many months and is only now about to be published. Among the findings is that Huygens landed not on slush, as previously announced, but on a “beach” of fine gravel made of ice.

John Zarnecki, professor of space science at the Open University, said: “On Titan the rain, rivers and lakes are made up of hydrocarbons such as methane. On Earth these hydrocarbons are gases, but Titan is so cold they stay as liquids. As they flow over the ice, it gets eroded just like rocks do on Earth.

“If you were to look in the lake edges you would find sandy beaches but the sand comprises tiny grains of ice rather than rock. That is what we think the Huygens probe landed on.”

Readings from a sonar probe released by Huygens suggest that just before it landed it floated over a small cliff, about 20ft high, thought to be the edge of a river channel or lake.

Another team has produced evidence for a cryovolcano 20 miles in diameter, thought to be heated by the same gravitational effect as on Enceladus.

The oxygen found in at least one of Saturn’s rings is thought to be formed as space radiation bombards ice particles.

“These results show the resurgence of planetary science in Britain,” said Zarnecki. “We are world players again.”

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