A Field Guide to Finding Fossils on Mars

Abstract

The Martian surface is cold, dry, exposed to biologically harmful radiation and apparently barren today. Nevertheless, there is clear geological evidence for warmer, wetter intervals in the past that could have supported life at or near the surface. This evidence has motivated National Aeronautics and Space Administration and European Space Agency to prioritize the search for any remains or traces of organisms from early Mars in forthcoming missions. Informed by (1) stratigraphic, mineralogical and geochemical data collected by previous and current missions, (2) Earth's fossil record, and (3) experimental studies of organic decay and preservation, we here consider whether, how, and where fossils and isotopic biosignatures could have been preserved in the depositional environments and mineralizing media thought to have been present in habitable settings on early Mars. We conclude that Noachian-Hesperian Fe-bearing clay-rich fluvio-lacustrine siliciclastic deposits, especially where enriched in silica, currently represent the most promising and best understood astropaleontological targets. Siliceous sinters would also be an excellent target, but their presence on Mars awaits confirmation. More work is needed to improve our understanding of fossil preservation in the context of other environments specific to Mars, particularly within evaporative salts and pore/fracture-filling subsurface minerals.

Keywords: Mars; astrobiology; fossils.

Figure 1

Terrestrial fossils that inform the search for life on Mars: (a) Calcified cyanobacterial sheaths (Girvanella) in limestone, upper Cambrian Campbell's Member, western Newfoundland. Image courtesy of S. Pruss, Smith College. (b) Stromatolites in chert, Archean Strelley Pool Formation, Western Australia. (c) Stromatolites in limestone, Paleoproterozoic Rocknest Formation, Wopmay Orogen, northwest Canada. (d) Stromatolites in sandstone, Neoproterozoic Witvlei Group, Namibia. (e) Filamentous and coccoidal microfossils in chert, Paleoproterozoic Gunflint Formation, Ontario, Canada. Image courtesy of A. H. Knoll, Harvard University. (f) Mat‐forming colonial coccoidal cyanobacteria in chert, Neoproterozoic Min'yar Formation. (g) Wrinkle structures in siltstone draped over conglomerate, middle Cambrian March Point Formation, western Newfoundland. Image courtesy of S. Pruss, Smith College. (h) Organically preserved cyanobacteria (Symplassosphaeridium sp.) macerated from shale, upper Mesoproterozoic Iqqittuq Formation, Arctic Canada. Image courtesy of H. Agić, University of California, Santa Barbara. Scale bar: (a) 200 μm, (e) 75 μm, (f) 625 μm, (g) 60 mm, and (h) 120 μm. The scale of Figures 1c and 1d is indicated by a Swiss army knife, hammer, and lens cap, respectively.

Figure 2

Photograph taken by the Curiosity rover of “Lamoose” target, a float block from the Murray Formation, Gale Crater, Mars. It is minimally dust covered (reddish tone) and sculpted by wind to reveal very fine lamination (here oriented upper left to lower right) and fine grain size. Wind‐induced surface striations trend obliquely to the primary depositional lamination. Scale bar: 1 cm.

Figure 3

Scanning electron micrographs of cells and precipitates in cultures of anoxygenic photosynthetic microbes enriched from the anoxic mud of Fayetteville Green Lake, NY. The cultures were enriched in a photosynthetic minimal medium with the concentrations of major ions that matched those in Fayetteville Green Lake. The biofilms grew on quartz sand in the presence of 1 mM Fe(II) under an atmosphere of 20% CO₂ and 80% N₂ at pH 7 on a 12‐h day:12‐h night cycle. Green sulfur bacteria (Chlorobium sp.) are the main photosynthetic organisms in these cultures; other strictly anaerobic microbes such as Geobacter, Acholeplasma, and Desulfomicrobium sp. are also present. (a) Cells heavily encrusted by microcrystalline minerals. (b) Close‐up of the nanometer‐scale minerals covering surfaces of cells. (c) Remnants of a rod‐shaped cell encased in minerals. (d) Energy dispersive X‐ray spectrum of precipitates marked by the white rectangle in Figure 3a shows the presence of Fe, Si, Ca, and O. The dissolved silica was not added to the culture medium: the silica in the precipitates around cells was derived from the quartz sand.

Figure 4

Scanning electron micrographs of photosynthetic cultures enriched from the anoxic mud of Fayetteville Green Lake, NY in the presence of methane (CH₄) and hydrogen (H₂). The cultures were enriched and grown on aragonite sand in a photosynthetic minimal medium with concentrations of major ions that matched those in Fayetteville Green Lake. The medium was reduced by 50‐mM Na₂S. All cultures were grown at pH 7, on a 12‐h day:12‐h night cycle. Cultures enriched in the presence of H₂ were grown under an atmosphere of 5% H₂, 15% CO₂, and 80% N₂, those enriched on CH₄ were grown under an atmosphere of 5% CH₄, 15% CO₂, and 80% N₂. The cultures contain anoxygenic photosynthetic microbes; cultures enriched on hydrogen also produce methane. (a) One‐month old culture enriched on CH₄. (b) One‐month old culture enriched on H₂. (c) Four‐month‐old H₂ culture. (d) Energy Dispersive X‐ray Spectroscopy spectrum (a) of the precipitates marked by the circle in Figure 4a. The cultures were grown in the absence of added clay minerals, but the Energy Dispersive X‐ray Spectroscopy spectrum (b) of the area marked by the circle in Figure 4b and X‐ray diffraction analyses suggests that clay minerals form in these cultures.