2nd International Mars Sample Return 2018 (LPI Contrib. No. 2071)
6078.pdf
HOW CAN WE LOOK FOR FOSSILS ON MARS? S. McMahon1, T. Bosak2, J. P. Grotzinger3, D. E. G. Briggs4, J. Hurowitz5, N. Tosca6, A. Petroff7, R. E. Summons2, B. P. Weiss2 1UK Centre for Astrobiology, University of Edinburgh, UK,
[email protected], 2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA, 3Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, 4Department of Geology and Geophysics, Yale University, New Haven, CT 06520, 5Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, 6 Department of Earth Sciences, Oxford University, Oxford, UK, 7Department of Physics, Clark University, Worchester, MA 01610. Introduction: The Mars 2020 mission seeks to collect samples with a high potential to preserve signatures of past life. The emerging field of comparative planetary taphonomy aims to guide this search and identify optimal targets. Insights from Earth show that environments with fine-grained sediments are likely to yield fossils. In contrast, other settings, such as serpentinizing environments or deep aquifers on Earth, have a poor potential to preserve fossils and would require very large samples to reliably yield any biosignatures. Recent discoveries by Mars missions have revealed paleoredox boundaries, precipitated Fe-Mg clay minerals, Fe oxides and Fe-Mg carbonates in environments with much greater potential to harbor and preserve abundant life [1]. Notable is the high abundance of oxychlorine compounds unique to Mars that complicate the detection of organic compounds. Experiments that integrate insights from fossil preservation on Earth with chemical conditions relevant for Mars can guide the identification of sampling targets and improve detection techniques as well as our understanding of processes that yield false positive biosignatures. Exceptional preservation: Exceptionally preserved fossils in iron-, silica- and sulfur-rich sediments on Earth provide the framework for comparative planetary taphonomy. Regardless of environmental context, organisms are fossilized when the original organic material resists decay, or when early precipitation of authigenic minerals replicates morphological details. This does not happen often: most bacteria, archaea and organic-walled unicellular eukaryotes, and about 60% of macroscopic marine animals, are soft-bodied and decay within weeks, before they can become fossilized. Thus, Mars 2020 should target environments that may have harbored abundant life and experienced rapid mineral precipitation. Fossilization and organic preservation on Mars: Laboratory experimental studies provide processoriented understanding of fossilization and can address planet-specific differences. Environments where secondary minerals form rapidly during authigenesis and early diagenesis, such as those recognized in Gale crater [2], are most likely to preserve potential biosignatures on both early Earth and Mars. Modeling constrained by rover-based observations can provide better
constraints on rates of sediment transport and authigenic mineral formation on Mars. Oxychlorine compounds: A major gap in our understanding of organic preservation on Mars arises from the high abundance of oxychlorine compounds which obstruct the detection of organic compounds by the Sample Analysis at Mars (SAM) instrument’s pyrolysis approach. These compounds appear to be a ubiquitous component of rocks at Gale crater, present at ~1 wt.% concentrations in most sedimentary rocks examined by Curiosity [3]. We do not know how soluble oxychlorine compounds were incorporated into Martian sedimentary rocks, why they are distributed uniformly within Gale crater, and what implications they have for Martian taphonomy. We hypothesize that: 1) these compounds were incorporated by adsorption and/or co-precipitation with early authigenic and early diagenetic mineral phases; and 2) their presence may have affected fossil and organic preservation. Taphonomic experiments can test these hypotheses by characterizing the distribution of soluble oxychlorine compounds in organic matter and authigenic mineral phases. The results will have direct implications for fossilization on Mars and could lead to new approaches for in situ characterization of Martian organic matter. False positives: Abiotic water-rock interactions, redox gradients and diagenetic processes can form silica sinter on altered volcanic rocks [4, 5], hydrothermal textures in sediments, and accompanying elemental distributions that mimic traces of life. Thus, the search for biosignatures on Mars must: 1) recognize abiotic processes that produce silica- and iron-rich structures with morphologies similar to examples on Earth and Mars; and 2) rigorously estimate the probability that a given sample is the result of an abiotic process. We address this by developing experimental systems that mimic hot springs and other hydrothermal settings where silica-saturated solutions evaporate quickly in a lowpressure atmosphere similar to that on Mars (
