THE BIOLOGICAL POTENTIAL OF MARS, THE EARLY EARTH, AND EUROPA

The potential biomass that could have existed on Mars is constrained b y the total amount of energy available to construct it. From an invent ory of the available geochemical sources of energy, we estimate that f rom the time of the onset of the visible geologic record 4 b.y. ago to the present, as much as 20 g cm(-2) of biota could have been created. This is the same amount that could have been constructed on the early Earth in only 100 million years. This indicates that there likely was sufficient energy available to support an origin of life on Mars but not sufficient energy to create a ubiquitous and lush biosphere. Simil ar calculations for Europa suggest that even less geochemical energy w ould have been available there.