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Paleo-Rock-Hosted Life on Earth and the Search on Mars: a Review and Strategy for Exploration

Onstott, T. C. and Ehlmann, B. L. and Sapers, H. and Coleman, M. and Ivarsson, M. and Marlow, J. J. and Neubeck, A. and Niles, P. (2019) Paleo-Rock-Hosted Life on Earth and the Search on Mars: a Review and Strategy for Exploration. Astrobiology, 19 (10). pp. 1230-1262. ISSN 1557-8070. PMCID PMC6786346. doi:10.1089/ast.2018.1960.

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Here we review published studies on 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 life on Mars. Key findings are (1) much terrestrial deep subsurface metabolic activity relies on abiotic energy-yielding fluxes and in situ abiotic and biotic recycling of metabolic waste products rather than on buried organic products of photosynthesis; (2) subsurface microbial cell concentrations are highest at interfaces with pronounced chemical redox gradients or permeability variations and do not correlate with bulk host rock organic carbon; (3) metabolic pathways for chemolithoautotrophic microorganisms evolved earlier in Earth's history than those of surface-dwelling phototrophic microorganisms; (4) the emergence of the former occurred at a time when Mars was habitable, whereas the emergence of the latter occurred at a time when the martian surface was not continually habitable; (5) the terrestrial rock record has biomarkers of subsurface life at least back hundreds of millions of years and likely to 3.45 Ga with several examples of excellent preservation in rock types that are quite different from those preserving the photosphere-supported biosphere. These findings suggest that rock-hosted life would have been more likely to emerge and be preserved in a martian context. Consequently, we outline a Mars exploration strategy that targets subsurface life and 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 associated traces of carbon and diagnostic chemical and isotopic biosignatures. Using this strategy on Earth yields ancient rock-hosted life, preserved in the fossil record and confirmable via a suite of morphologic, organic, mineralogical, and isotopic fingerprints at micrometer scale. We expect an emphasis on rock-hosted life and this scale-dependent strategy to be crucial in the search for life on Mars.

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Ehlmann, B. L.0000-0002-2745-3240
Additional Information:© 2019 T.C. Onstott et al. Published by Mary Ann Liebert, Inc. This Open Access article is distributed under the terms of the Creative Commons License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. Submitted 23 September 2018; Accepted 25 April 2019; Online Ahead of Print: June 25, 2019. This work has greatly benefited from the discussions and intellectual contributions of the 20 participants in the Rock Hosted Life Workshop, February 8–10, 2017, at Caltech as well as the >100 participants in the four pre-workshop telecons open to the community. Thanks to Mary Voytek and Michael Meyer at NASA Headquarters for workshop funding and to NASA Astrobiology Institute director Penny Boston and the NASA Ames Research Center meeting support team for providing web-hosting for the telecons and their recording; all the information can be found at We thank partial support of T.C.O. through a subcontract supported by NASA Exobiology Award NASA NNX17AK87G to Andrew Schuerger of the University of Florida and partial support to B.L.E. by the NASA Mars Data Analysis Program award 80NSSC17K0444. The contribution of M.C. was carried out partly at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA), and via Grant NNA13AA94A issued through the NASA Science Mission Directorate and supported by the NASA Astrobiology Institute. No competing financial interests exist.
Group:Astronomy Department
Funding AgencyGrant Number
Subject Keywords:Subsurface life; Microbial diversity; Biosignatures; Mars; Search for life
Issue or Number:10
PubMed Central ID:PMC6786346
Record Number:CaltechAUTHORS:20190402-084817695
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Official Citation:T.C. Onstott, B.L. Ehlmann, H. Sapers, M. Coleman, M. Ivarsson, J.J. Marlow, A. Neubeck, and P. Niles. Astrobiology. Oct 2019.1230-1262.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:94356
Deposited By: Tony Diaz
Deposited On:02 Apr 2019 17:14
Last Modified:16 Nov 2021 17:04

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