  The adaptability of extremophiles on Earth raises the question of what
strategies putative life might have used to adapt to the present conditions on
Mars. Here, we hypothesize that organisms might utilize a water-hydrogen
peroxide (H2O-H2O2) mixture rather than water as an intracellular liquid. This
adaptation would have the particular advantages in the martian environment of
providing a low freezing point, a source of oxygen, and hygroscopicity. The
findings by the Viking experiments are reinterpreted in the light of this
hypothesis. Our conclusion is that the hitherto mysterious oxidant in the
martian soil, which evolves oxygen when humidified, might be H2O2 of biological
origin. This interpretation has consequences for site selection for future
missions to search for life on Mars.
