Here On Earth: A Natural History Of The Planet
Finding fossils preserved from early Mars might tell us that life once flourished on this planet. We can search for evidence of cells preserved in rocks, or at a much smaller scale: compounds called biosignatures are molecular fossils, specific compounds that give some indication of the organisms that created them. However, over hundreds of millions of years these molecular fossils on Mars are subject to being destroyed or transformed to the point where they may no longer be recognized as biosignatures. Future missions must either find surface regions where erosion from wind-blown sand has recently exposed very ancient material, or alternately samples must be obtained from a shielded region beneath the surface. This latter approach is being taken by the ExoMars rover under development where drilled samples taken from a depth of up to 2 meters will be analyzed.