Geochemical bio-signatures in Martian analogue basaltic environments using laboratory experiments and thermochemical modelling

Abstract

The identification of geochemical bio-signatures is important for assessing whether life existed on early Mars. In this paper, experimental microbiology and thermochemical modelling were combined to identify potential inorganic bio-signatures for life detection on early Mars. An analogue mixed microbial community from an analogue terrestrial fluvio-lacustrine environment similar to an ancient lacustrine system at Gale Crater was used to study microbial dissolution of a basalt regolith simulant and the formation of bio-signatures over a short time frame (1°month) at 14°C, 2 bar. Microbial growth influenced element dissolution (Mg, Fe, Mn, Ca and K) and the formation of morphologies and Fe-Si amorphous layers on mineral surfaces. Thermochemical models were performed at 14°C, 2 bar; the results were compared with experimental data to predict bio-signatures that would occur over geological timescales. The pH was varied to simulate abiotic and biotic experimental conditions. Model results suggest that, at water to rock ratios of 100 to 38, a less complex secondary mineral assemblage forms during biotic dissolution compared to abiotic weathering. Carbonates, quartz, pyrite and hydroxyapatite form under biotic conditions, whereas in the abiotic system magnetite and phyllosilicates would also precipitate. These results could be used to distinguish between abiotic and biotic basalt weathering processes, aiding the interpretation of data from Mars exploration missions.

Geochemical biosignature formation in experimental Martian fluvio‐lacustrine and simulated evaporitic settings

“…Compared to previous studies that used less accurate fluid chemistries and regolith simulant compositions with respect to a Gale crater environment (Cogliati et al, 2022;Olsson-Francis et al, 2017), the models in this study do not predict the formation of high amount of hydrated Al-rich clays (kaolinite >50 wt%), carbonates (mainly siderite), or hydroxides (goethite) under biotic conditions. In this study, the mineralogical association was dominated by phyllosilicates of the smectite group (> 60 wt% nontronite-(CaO 0.5 ) 0.3 Fe 2 3+ (Si, Al) 4 O 10 (OH) 2 ÁnH 2 O) and of the kaolinite-group (kaolinite-Al 2 (Si 2 O 5 )(OH) 4 ).…”

“…In this study, the mineralogical association was dominated by phyllosilicates of the smectite group (> 60 wt% nontronite-(CaO 0.5 ) 0.3 Fe 2 3+ (Si, Al) 4 O 10 (OH) 2 ÁnH 2 O) and of the kaolinite-group (kaolinite-Al 2 (Si 2 O 5 )(OH) 4 ). Differences in secondary mineral associations and fluid chemistries between this and previous studies that investigated biosignatures formation through thermochemical modeling (Cogliati et al, 2022;Olsson-Francis et al, 2017) can be due to the variations of the input parameters used for modeling water-rock reaction pathways (fluid and rock chemistry, pH, initial oxidation state) and to the different (W/R) D ranges considered in each individual case (~10 6 to ~10 5 , Olsson-Francis et al, 2017;100 to 38, Cogliati et al, 2022;2000 to 278, this study).…”

“…The use of chemically accurate fluid and simulant regolith compositions to simulate the Rocknest environment at Gale crater increased the fidelity of the type of potential inorganic biosignatures that a selected microbial community could develop under specific Martian conditions. Following this, the results from previous studies that used a more generic Martian‐like conditions are still valid and should be examined for the exact conditions they are applicable to when searching for biosignatures, for example, to settings characterized by an altered basaltic composition (Olsson‐Francis et al., 2017) or more reducing conditions (Cogliati et al., 2022—iron was modeled as Fe 2+ ). All those conditions would be possible to find on Mars in ancient fluvio‐lacustrine environments, enabling the potential identification of putative biosignatures by current and future in situ life detection missions or in returned samples.…”

“…This unique, diverse, and complex combination of alteration assemblages and morphologies demonstrates that specific microbial metabolisms that was viable under chemically accurate Martian conditions can develop pronounced biosignatures that may be easily detectable in situ using the instruments on board of Martian rovers such as MAHLI (Mars Hand Lens Imager instrument) on the Curiosity rover (Allwood et al, 2020;Edgett et al, 2012), as well as PIXL (Planetary Instrument for X-Ray Lithochemistry), SHERLOC (Scanning Habitable Environments with Raman and Luminescence for Organics and Chemicals), and Wide Angle Topographic Sensor for Operations and eNgineering on Perseverance (Allwood et al, 2020). This is an important achievement considering that previous studies that investigated microbial weathering of a Martian simulant and biosignatures formation using individual microbial strains have shown only limited or rare evidence of alteration mineralogy and morphologies (Cogliati et al, 2022;Olsson-Francis et al, 2017).…”

“…Previous studies that combined laboratory experiments and thermochemical modeling to investigate inorganic biosignatures formation in simulated Martian lacustrine‐sedimentary systems identified mineralogical, microscopic, and geochemical changes that could be potentially used to distinguish between abiotic and biotic weathering processes that may have occurred on early Mars (Cogliati et al., 2022; Olsson‐Francis et al., 2017). These studies suggest also that, over geological time scales, a less complex secondary mineral assemblage forms during biotic dissolution compared to abiotic weathering (Cogliati et al., 2022; Olsson‐Francis et al., 2017). However, these experiments often used non‐ideal matches of fluid and rock chemistries (i.e., the presence of alteration phases in the simulant, differences in the Fe 2+ /Fe 3+ ratios, and elemental composition of the regolith) compared to Gale Crater's environment.…”