Dark microbiome and extremely low organics in Atacama fossil delta unveil Mars life detection limits

Identifying unequivocal signs of life on Mars is one of the most important objectives for sending missions to the red planet. Here we report Red Stone, a 163-100 My alluvial fan–fan delta that formed under arid conditions in the Atacama Desert, rich in hematite and mudstones containing clays such as vermiculite and smectites, and therefore geologically analogous to Mars. We show that Red Stone samples display an important number of microorganisms with an unusual high rate of phylogenetic indeterminacy, what we refer to as “dark microbiome”, and a mix of biosignatures from extant and ancient microorganisms that can be barely detected with state-of-the-art laboratory equipment. Our analyses by testbed instruments that are on or will be sent to Mars unveil that although the mineralogy of Red Stone matches that detected by ground-based instruments on the red planet, similarly low levels of organics will be hard, if not impossible to detect in Martian rocks depending on the instrument and technique used. Our results stress the importance in returning samples to Earth for conclusively addressing whether life ever existed on Mars.
The research leading to these results is a contribution from the Project “MarsFirstWater”, funded by the European Research Council, Consolidator Grant no 818602 to AGF, and by the Human Frontiers Science Program grant n° RGY0066/2018 to A.A.-B. Additional funding provided was provided by MINECO grant PID2019-107442RB-C32 (O.P.-B. and A.M.), Grants-in-Aid for Scientific Research from the Japan Society for Promotion of Science grant numbers 17H06458 and 21H04515 (K.F.), grant numbers 17H06456, 17H06458, 20H00195, and 21H04515 (K.F. and Y.S.), Consejería de Educación e Investigación, Comunidad Autónoma de Madrid/European Social Fund program (MAFM), grant n° ESP2017-87690-C3-3-R (DC), Ramón y Cajal grant n° RYC2018-023943-I (L.S.-G.), AEI grant MDM-2017-0737 and MCIN/AEI grant PID2019-107442RB-C32 (V.M.-I.), MCIU/AEI (Spain) and FEDER (UE) grant n° PGC2018-094076-B-I00 (J.W. and C.A.), Italian Space Agency agreement 2017-48-H.0 (T.F., J.R.B. and G.P.), the Ministry of Science of Spain grant PID2019-107442RB-C31 (J.A.M., M.V., G.L.R., A.A. and F.R.), María Zambrano’ excellence grant program (CA3/RSUE/2021-00405), funded by the Spanish Ministry of Universities (MFM), NASA Mars Exploration Program contracts NNH13ZDA018O, NNH15AZ24I, NNH13ZDA018O and LANL Laboratory Directed Research and Development (LDRD) funding XX5V (A.M.O, R.C.W., A.R. and S.M.C.), NASA-GSFC grant NNX17AJ68G (M.M. and S.S.J.), NES focused on Sample Analysis at Mars of the Mars Science Laboratory mission, and Mars Organic Molecules Analyzer of the Exomars 2022 mission (O.M., C.S., and C.F.), and grants RTI2018-094368-B-I00 and MDM-2017-0737 Unidad de Excelencia “Maria de Maeztu”- Centro de Astrobiología (INTA-CSIC) by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/ 10.13039/501100011033 and by “ERDF A way of making Europe” (C.E., M.G.V., M.M.-P., and V.P.). R.C.W. thanks Dot Delapp for performing pre-processing of the LIBS data.
With funding from the Spanish government through the "Severo Ochoa Center of Excellence" accreditation MDM-2017-0737.