Paleo-Rock-Hosted Life on Earth and the Search on Mars: a Review and Strategy for Exploration
T.C. Onstott, B.L. Ehlmann, H. Sapers, M. Coleman, M. Ivarsson, J.J., Marlow, A. Neubeck, and P. Niles

TL;DR
This review discusses Earth's rock-hosted life, its evolution, and biomarkers to guide Mars exploration, emphasizing subsurface biosignatures and strategies for detecting ancient life on Mars.
Contribution
It provides a comprehensive synthesis of Earth's rock-hosted biosignatures and proposes a detailed exploration strategy for finding similar evidence on Mars.
Findings
Metabolic pathways for chemolithotrophs evolved early on Earth.
Subsurface biomass correlates with redox interfaces, not organic carbon.
Ancient rock record shows preserved subsurface life back to 3.45 Ga.
Abstract
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…
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