Fatty Acid Preservation in Modern and Relict Hot-Spring Deposits in Iceland, with Implications for Organics Detection on Mars

Abstract

Hydrothermal spring deposits host unique microbial ecosystems and have the capacity to preserve microbial communities as biosignatures within siliceous sinter layers. This quality makes terrestrial hot springs appealing natural laboratories to study the preservation of both organic and morphologic biosignatures. The discovery of hydrothermal deposits on Mars has called attention to these hot springs as Mars-analog environments, driving forward the study of biosignature preservation in these settings to help prepare future missions targeting the recovery of biosignatures from martian hot-spring deposits. This study quantifies the fatty acid load in three Icelandic hot-spring deposits ranging from modern and inactive to relict. Samples were collected from both the surface and 2-18 cm in depth to approximate the drilling capabilities of current and upcoming Mars rovers. To determine the preservation potential of organics in siliceous sinter deposits, fatty acid analyses were performed with pyrolysis-gas chromatography-mass spectrometry (GC-MS) utilizing thermochemolysis with tetramethylammonium hydroxide (TMAH). This technique is available on both current and upcoming Mars rovers. Results reveal that fatty acids are often degraded in the subsurface relative to surface samples but are preserved and detectable with the TMAH pyrolysis-GC-MS method. Hot-spring mid-to-distal aprons are often the best texturally and geomorphically definable feature in older, degraded terrestrial sinter systems and are therefore most readily detectable on Mars from orbital images. These findings have implications for the detection of organics in martian hydrothermal systems as they suggest that organics might be detectable on Mars in relatively recent hot-spring deposits, but preservation likely deteriorates over geological timescales. Rovers with thermochemolysis pyrolysis-GC-MS instrumentation may be able to detect fatty acids in hotspring deposits if the organics are relatively young; therefore, martian landing site and sample selection are of paramount importance in the search for organics on Mars.

An Overview of Lipid Biomarkers in Terrestrial Extreme Environments with Relevance for Mars Exploration

“…Hot spring environments present a truly contradictory situation. These are the best characterized settings on Earth when it comes to lipid biomarker studies (Robinson and Eglinton, 1990 ; Summons et al ., 1996 ; Jahnke et al ., 2001 , 2004; Zhang et al ., 2004 ; Pancost et al ., 2005 ; Sánchez-García et al ., 2019 , 2020a; Williams et al ., 2021 ), but they have been poorly prospected on Mars, with the Home Plate in Gusev being the only surveyed martian hydrothermal area (Cady et al , 2018 ; Ruff et al , 2020 ). Its silica sinter deposits, however, have not been examined in detail.…”

“…In other words, hydrothermal systems might have been a spot for life to emerge. Second, these environments are characterized by unprecedented rates of mineralization, which directly favors microbial entombment and subsequent fossilization, making such deposits rich storehouses of paleobiological information (Fernandez-Turiel et al , 2005 ; Parenteau et al , 2014 ; Campbell et al ., 2015 ; Williams et al , 2021 ). Third, the emergence of hydrothermal activity seems to be inextricably linked to planetary formation and evolution (Holm and Andersson, 2005 ).…”

“…The latter is what Williams et al . ( 2021 ) concluded after measuring low ACL in the compounds extracted from the vent of an active flowing spring in Hveravellir (Iceland), together with a growing ratio of branched-to-total fatty acids as temperature decreases away from the geyser vent, which related with an increasing gradient of mesophilic microorganisms.…”

The Ground-Based BIOMEX Experiment Verification Tests for Life Detection on Mars

“…A prime consideration for the detection of organic biosignatures on Mars is their longterm preservation on the surface, where the presence of UV and ionizing radiation is one of the main damaging factors. Although the preservation of complex organic molecules in the shallow subsurface (~10 cm) of Martian rocks drops 1000-fold due to ionizing radiation [67], there are recent indications that the rapid burial of organic molecules, short surface exposures thanks to slower weathering rates, and the characteristics of certain mineral depositions are key factors enhancing biosignature preservation in the subsurface of Mars [68,69]. Therefore, we exposed dried fungal colonies, once grown on Martian rock analogues, to increasing doses of Mars-relevant polychromatic UV irradiation (200-400 nm) up to 5.5 × 10 5 kJ/m 2 , which simulates 369.87 days on the Martian surface [30].…”