The recent discovery of abundant water on Mars, albeit in the form of permafrost, has raised hopes for finding traces of life there. The Red Planet has long been a favourite location for those speculating about extraterrestrial life, especially since the 1890s, when H. G. Wells wrote The War of the Worlds and the American astronomer Percival Lowell claimed that he could see artificial canals etched into the planet's parched surface.

Today, of course, scientists expect to find no more than simple bacteria dwelling deep underground, if even that. Still, the discovery of just a single bacterium somewhere beyond Earth would force us to revise our understanding of who we are and where we fit into the cosmic scheme of things, throwing us into a deep spiritual identity crisis that would be every bit as dramatic as the one Copernicus brought about in the early 1500s, when he asserted that Earth was not at the centre of the universe.

Whether or not we are alone is one of the great existential questions that confront us today. Probably because of the high emotional stakes, the search for life beyond Earth is deeply fascinating to the public. Opinion polls and Web-site hits indicate strong support for and interest in space missions that are linked even obliquely to this search. Perceiving the public's interest, NASA has reconfigured its research strategy and founded the NASA Astrobiology Institute, dedicated to the study of life in the cosmos. At the top of the agenda, naturally, is the race to find life elsewhere in the solar system...