  We review the abundance and diversity of terrestrial rock hosted life, the
environments it inhabits, the evolution of its metabolisms, and its fossil
biomarkers to provide guidance in the search for the biomarkers of rock hosted
life on Mars. Key finds are metabolic pathways for chemolithotrophic
microorganisms evolved much earlier in Earth history than those of surface
dwelling phototrophic microorganisms,the emergence of the former occurred at a
time when Mars was habitable, whereas that of the latter occurred at a time
when the martian surface would have been uninhabitable, subsurface biomass do
not correlate with organic carbon and tends to be highest at interfaces where
chemical redox gradients are most pronounced, deep subsurface metabolic
activity does not rely upon the respiration of organic photosynthate but upon
the flux of inorganic energy and the abiotic and biotic recycling of metabolic
waste products, and the rock record reveals examples of subsurface life back to
3.45 Ga with several examples of good preservation potential in rock types that
are quite different from those preserving the photospheric supported
biosphere.These findings suggest that rock hosted life would have likely to
emerge and be preserved in a martian context. We thus propose a Mars
exploration strategy that scales spatially, focusing initially on identifying
rocks with evidence for groundwater flow and low temperature mineralization,
then identifying redox and permeability interfaces preserved within rock
outcrops, and finally focusing on finding minerals associated with redox
reactions and traces of carbon and diagnostic biosignatures. The lessons from
Earth show that ancient rock hosted life is preserved in the fossil record and
confirmable via a suite of morphologic, organic, mineralogical and isotopic
fingerprints and microscopic textures.
