'The first age, the Phyllocian era, lasted from just after the planet's birth to about four billion years ago. Ancient rocks show the presence of clay-rich minerals - phyllosilicates - which to form would have required a water-abundant alkaline environment. Volcanoes erupt The second era emerged after a dramatic shift in the Martian climate. Now sulphate minerals dominated and the researchers have labelled this the Theiikian era, named after the Greek for sulphate. The team believes the change in mineral composition was caused by volcanic activity around four billion years ago.' BBC April 2006 So a likely target for forthcoming Exomars mission for micro-fossils would seem to be such delineated clay-rich mineral sites as detected by the Mars Express survey.