"If there's life on Mars today, I reckon it's a couple of kilometres underground living in melted permafrost - or it could be alive and well in pockets of water under the north polar ice cap," says David Wynn-Williams, a microbiologist with the Antarctic Astrobiology Project at the British Antarctic Survey, Cambridge, United Kingdom and a member of the Beagle 2 (the Mars Express lander) adjunct scientists group. His conclusion is based on similarities between habitats in Antarctica, where microbial life exists in rocks and at the bottom of extremely salty ice-covered lakes, and plausible Martian environments where primitive micro-organisms could be eking out a meagre living.

Are there underground aquifers on Mars?

The surface of Mars, however, is too dry, cold and corrosive for any form of life to exist there today. But that may not always have been the case. There's plenty of evidence that the elixir of life probably flowed copiously during the planet's youth. Early in its history, Mars, like Earth, may have been warm and wet. Indeed the conditions for life may have arisen on Mars first, as the planet is smaller than Earth and would have cooled down first to temperatures suitable for life.

If water did once flow and pool on Mars, where did it go? Did life evolve there? Does it still hang on by a thread in some protected niche? "The only way of answering such questions is to go to Mars and take a look," says Agustin Chicarro, Mars Express project scientist at ESA. "Mars Express, ESA's mission to the Red Planet due for launch in 2003, will be the first spacecraft equipped to search specifically for underground aquifers - and Beagle 2, the small lander it will carry, will search for signatures of past and present life," he adds.

Slightly smaller than our own Moon, Europa is one of Jupiter's 16 known moons. Its surface is covered entirely with water ice. Beneath could lie an ocean of liquid water - or so recent space-based observations suggest.

Fragmented chunks of ice seen on Jupiter's moon Europa (Credit: PIRL, University of Arizona)

If there's water could there be life? There's almost certainly carbon on Europa, deposited perhaps by meteorites or crashing comets, or originating in the interior. However, the sunlight penetrating the thick ice will be too weak to power the chemical processes needed for life. So the answer probably depends on whether Europa's interior is hot. If it is, then hot gases and molten rock, issuing from vents in the ocean floor, could create a chemical cocktail in the surrounding water just to the taste of some forms of life, as happens near such hydrothermal vents on the Earth's ocean floor.