Saturn's frozen moon has a mystery heat source beneath its south pole

The latest images of Enceladus, one of the innermost moons of Saturn, also reveal another distinguishing feature of this celestial body - it has an atmosphere.

Enceladus is only 500km (310 miles) in diameter, which means it could just fit on a map of England, and in theory such a small moon should not have a big enough gravitational pull to keep its atmosphere from disappearing into space. But scientists believe they have discovered why the atmosphere of Enceladus seems to be a permanent feature - it is being constantly regenerated by a mysterious source of heat buried deep beneath the moon's south pole.

Measurements taken by instruments on the Cassini space probe have shown that far from being the coldest spot on Enceladus, its south pole is in fact the warmest - warmer even than its sun-drenched equator.

By Earth standards, the temperatures are nothing to write home about. Cassini's instruments have measured a temperature of minus 188C at the moon's south pole, which is about five degrees warmer than its equator. Enceladus, which was discovered in 1789 by the English astronomer Sir William Herschel, is one of the coldest places in the Saturn system because its extremely bright, ice-capped surface reflects about 80 per cent of the sunlight that strikes the moon.

John Spencer, a Cassini scientist at the Southwest Research Institute in Boulder, Colorado, said the latest findings were unexpected. "We had quite a surprise. It was a bit like finding that Antarctica is warmer than the Earth's equatorial regions," he said. "It's that strange." The Cassini space probe, which is orbiting Saturn, flew past Enceladus on two occasions, and detected that the moon was unexpectedly distorting Saturn's magnetic field.

This led Professor Michele Dougherty of Imperial College London to suggest taking an even closer look, which Cassini did on 14 July with a fly-by that took it within 108 miles of the moon's surface. Images from the south pole revealed a series of "tiger stripes" that appeared to be deep fractures in the frozen ocean. An analysis of its atmosphere revealed it was about 91 per cent water vapour, 3 per cent carbon dioxide, 4 per cent nitrogen or carbon monoxide and about 2 per cent organic molecules.

Torrence Johnson of Nasa's Jet Propulsion Laboratory in Pasadena, California, said that a heat source beneath the moon's south pole appeared to be evaporating ice and other molecules to regenerate the atmosphere.

The heat may be caused by radioactive rocks beneath the moon's surface or by the tidal forces created as the moon orbits Saturn - although neither could generate enough heat to account for the atmosphere, Dr Johnson said.

Candy Hansen, a Nasa scientist working on Cassini's ultraviolet instruments, said that half a ton of material a second was ejected from Enceladus's south pole, which would, over the four billion-year lifetime of the solar system, amount to a loss of 5 per cent of the moon's original mass.