Morphological and mass spectrometric analysis of gypsum-permineralized microfossils and its implications for the search for life on Mars

Space exploration missions devoted to the detection of life have visited various celestial bodies in our Solar System, particularly Mars (1). Observations and analyses conducted by orbiters, landers and rovers provided insight into the characteristics of the Martian geology and paleoenvironment, suggesting a past aqueous process including both subsurface and surface water (ocean and lakes) (2). The water activity on Mars’ surface, billions of years ago, led to the precipitation of sulfate minerals such as kieserite, polyhydrated sulfates and hydrated gypsum (hydrous calcium sulfate, CaSO₄·2H₂O) (3), suggesting the evaporation of lakes and lacustrine systems. Once the surface of Mars started to dry out, acidic and hypersaline shallow surface waters and groundwater might have filled the ancient lake system (4).

On Earth, diverse prokaryotes live within, on the surface, or underneath the formed primary gypsum in hypersaline lakes as endolithic, epilithic, and hypolithic forms, respectively. The fast and early growth of gypsum in different geological systems allows for a rapid entombment of cells, good fossilization potential and exceptional biosignature preservation (5, 6), making it a promising target in the search for evidence of past life on Mars (7).

The kilometre thick primary lower gypsum deposits, accumulated in the Mediterranean Basin during the Messinian salinity crisis (5.97 – 5.33 Ma), revealed a well-preserved and prominent archive of diverse microbial life (8). This geological system was proposed to be a terrestrial analogue of what may have happened on Mars (9).

In this contribution, we present measurements conducted on a sample of Messinian Primary Lower Gypsum with preserved microfossils from the lower-Chelif marginal basin in northwest Algeria using our space-prototype laser ablation ionization mass spectrometer (LIMS). Previous measurements performed on microfossils preserved in ancient rock records showed that this instrument has the capability to detect biosignatures related to ancient life (10). In addition to the analyses conducted with the LIMS instrument, the sample was further investigated using optical and scanning electron microscopy. According to the observed morphology, the obtained 2D and chemical depth profiles, we established the biogenicity and syngenecity of the analysed sample material. This highlights the potential of gypsum as a possible target mineral for future in-situ space missions to search for signs of life on Mars.

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