Future prospects

Abstract

The 1960s premise in setting out to search for life on Mars was to ask, in essence, what kind of terrestrial life was best suited to the conditions on the surface of that planet (which were not well known) and then to presume that life there would be similar. At that time, the ‘dry valleys’ of Antarctica were the best Earthly analogue for Mars. Since then, many niches that had been considered sterile have been found to host ecosystems. In fact, extremophiles are metabolically diverse, and capable of utilising almost any chemical energy that is abundant. When Viking was designed, it was believed that all life ultimately derived its energy from sunlight, that metabolism involved gaseous exchange of carbon dioxide, and that wherever the biology package was placed on Earth it would detect life. However, it is clear in hindsight that it would not have detected many of the extremophiles. A genetic study has shown that the ‘common ancestor’ for terrestrial life was a hyperthermophyllic anaerobic autotroph that used hydrogen. The lesson from Earth would therefore seem to be that life is the direct result of chemical evolution, and developed as soon as it became possible for it to exist. As Bruce Jakosky, an astrobiologist at the University of Colorado, has put it, “the environmental prerequisites for life include only the presence of liquid water, access to the biogenic elements, and a source of energy that can drive chemical disequilibrium; these are not terribly stringent requirements”. When terrestrial life originated, there was intense volcanism, liquid water was on the surface and the air was predominantly carbon dioxide. If, as seems likely, early Mars was similar, then life may well have developed there, too.