At present, we know of no worlds beyond our Earth where life exists. However, primitive organisms on our planet have evolved and adapted over billions of years, colonising the most inhospitable places.

Since life seems to gain a foothold in the most hostile environments, it seems distinctly possible that living organisms could exist in ice- covered oceans on worlds far from the Sun, according to Dr. David Rothery (Open University), who will be speaking today at the RAS National Astronomy Meeting in Milton Keynes.

Europa is the innermost of Jupiter's large icy satellites. It is slightly smaller than our own Moon, but its rocky interior is hidden beneath a 100 km blanket of ice. There has been much speculation as to whether the ice remains solid right down to the moon's rocky interior, or whether it consists of a thinner ice sheet floating on an ocean of water.

Data from NASA's Galileo spacecraft, which orbited Jupiter from 1995 until 2003, provided detailed insights into Europa's structure that will not be surpassed until the arrival of NASA's Jupiter Icy Moons Orbiter (which may not be until 2023).

The high-resolution Galileo images and other data revolutionised our knowledge of Europa's surface and interior structure, making it seem more likely that the ice is (at least at some times and in some places) relatively thin (much less than 10 km) and overlying a liquid water ocean. The images showed localized areas of 'melt-through' demonstrated by 'chaos' regions, where detached rafts of the icy shell can be seen dispersed in a now-refrozen matrix.

The cause of melt-through is likely to be tidal heating, which is especially strong within Europa because it orbits within the immense gravity of Jupiter and experiences competing tidal pulls from the large, neighbouring moons, Io and Ganymede. This process also powers the widespread volcanic eruptions on Io.

There may be occasional volcanic eruptions onto Europa's ocean floor - rather like a less active, ice-covered version of Io - or, more likely, hot springs where chemically-enriched water heated by passage through the rock re-emerges on the ocean floor.

This sort of environment is currently regarded as the most likely place for life on Earth to have begun, and life on Europa could have arisen in just the same way. If so, life could survive there today, supported by chemical energy in the same way that the 'hydrothermal vents' on Earth's ocean floors support ecosystems that do not depend on sunlight.

"Episodes of tidal heating in some of the Solar System's other icy bodies could equally well have given rise to life, even in such remote bodies as the newly discovered, remote planetoid Sedna if, as has been suggested, it has a satellite with which to interact tidally," said Dr. Rothery. "However, only in the case of Europa, and perhaps a few other satellites of the giant planets, does it seem plausible that life could flourish in the long term."