Sulfur isotopes as biosignatures for Mars and Europa exploration

Sulfur (S) isotopes are used to trace metabolic pathways associated with biological S-cycling in past and present environments on Earth. These pathways (sulfate reduction, sulfur disproportionation, and sulfide oxidation) can produce unique S isotope signals that provide insight into biogeochemical S-cycling. The S cycle is also relevant for extraterrestrial environments and processes. On early Mars, sulfur existed in different redox states and was involved in a large range of surface processes (e.g., volcanic, atmospheric, hydrothermal, and aqueous brines). Sulfur compounds have also been detected on Europa's icy moon surface, with the S cycle implicated in Europa's surface and ocean geochemistry. Given the well-established utility of S isotopes in providing a record for past life on Earth, S isotopes are an valuable tool for identifying biosignatures on Mars and Europa. Here, we review S isotopes as a biosignature, in light of two recent advances in understanding the S cycle in both Mars and Europa: (i) the measurements of δ34S in situ at Gale Crater and quadruple S isotopes (QSI) in Martian meteorites, and (ii) the identification of a likely exogenous origin of sulfur on Europa's surface. We discuss important considerations for unravelling QSI biosignatures in Martian environments, considering high and low sulfur environments, atmospheric S-MIF signals, and metabolic energy-limited niches. For Europa, we describe the potential for S isotopes to probe biogeochemistry, and identify key knowledge gaps to be addressed in order to unlock S isotopic tools for future life detection efforts. The resulting picture demonstrates how S isotopes will be a valuable tool for Mars Sample Return, and how future missions can focus on the search for environments where QSI signatures of microbial S-cycling processes have a greater chance of being preserved. For Europa, the first step will be to account for the S isotope composition of the various S pools, to recognise or rule out non-biologically mediated S isotope values, with a focus on experimental examination of potential S isotope signatures from exogenous sulfur sources.Thematic collection: This article is part of the Sulfur in the Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-syste

Molecular biosignatures in planetary analogue salts: implications for transport of organics in sulfate-rich brines beyond Earth

Salts formed during evaporation or freezing of brines can potentially incorporate organic matter that can inform about past biological activity. We analysed the lipid fraction preserved within the contemporary Lost Hammer salt deposit (Canadian High Arctic) - an analogue to extraterrestrial salt systems - and paired this with space mission-relevant evolved gas analysis. Our findings show microbial organic matter (fatty acids and n-alkanes) is incorporated into Lost Hammer salts, which comprise polyhydrated sulfates and chlorides. We find a difference in the relative abundance of fatty acids vs. n-alkanes indicating how these biosignatures evolve across active and non-active parts of the spring. We also find differences between pristine salt-organic mixtures and deposits that may have been remobilised by subsequent dissolution and recrystallisation. In this system, n-alkanes have the highest preservation potential, surviving the likely dissolution and recrystallisation of hydrated salt phases. This is important for considering the fate of organic matter on icy moons such as Europa, where salts emplaced on the surface by briny extrusions may have undergone fractional crystallisation, or where subsurface salts are remobilised by localised melting. It is also relevant for once active brine systems on Mars, where cycles of groundwater recharge and/or deliquescence led to dissolution and re-precipitation of evaporitic salts

Volcanic controls on the microbial habitability of Mars‐analogue hydrothermal environments

Due to their potential to support chemolithotrophic life, relic hydrothermal systems on Mars are a key target for astrobiological exploration. We analysed water and sediments at six geothermal pools from the rhyolitic Kerlingarfjöll and basaltic Kverkfjöll volcanoes in Iceland, to investigate the localised controls on the habitability of these systems in terms of microbial community function. Our results show that host lithology plays a minor role in pool geochemistry and authigenic mineralogy, with the system geochemistry primarily controlled by deep volcanic processes. We find that by dictating pool water pH and redox conditions, deep volcanic processes are the primary control on microbial community structure and function, with water input from the proximal glacier acting as a secondary control by regulating pool temperatures. Kerlingarfjöll pools have reduced, circum-neutral CO2-rich waters with authigenic calcite-, pyrite- and kaolinite-bearing sediments. The dominant metabolisms inferred from community profiles obtained by 16S rRNA gene sequencing are methanogenesis, respiration of sulphate and sulphur (S0) oxidation. In contrast, Kverkfjöll pools have oxidised, acidic (pH 42- and high argillic alteration, resulting in Al-phyllosilicate-rich sediments. The prevailing metabolisms here are iron oxidation, sulphur oxidation and nitrification. Where analogous ice-fed hydrothermal systems existed on early Mars, similar volcanic processes would likely have controlled localised metabolic potential and thus habitability. Moreover, such systems offer several habitability advantages, including a localised source of metabolic redox pairs for chemolithotrophic microorganisms and accessible trace metals. Similar pools could have provided transient environments for life on Mars; when paired with surface or near-surface ice, these habitability niches could have persisted into the Amazonian. Additionally, they offer a confined site for biosignature formation and deposition that lends itself well to in situ robotic exploration

QSI results from Mars analogue systems

This data set includes the in-situ measurements of the field sites, the complete S isotope measurements, and the complete results of the mode

QSI results from Mars analogue systems

This data set includes the in-situ measurements of the field sites, the complete S isotope measurements, and the complete results of the mode

Dataset QSI biosignatures from terrestrial Mars analogue systems

In-situ measurements, quadruple sulfur isotope dataset, and complete data from the model

Sulfur isotopes as biosignatures for Mars and Europa exploration

Sulfur (S) isotopes are used to trace metabolic pathways associated with biological S-cycling in past and present environments on Earth. These pathways (sulfate reduction, sulfur disproportionation, and sulfide oxidation) can produce unique S isotope signals that provide insight into biogeochemical S-cycling. The S cycle is also relevant for extraterrestrial environments and processes. On early Mars, sulfur existed in different redox states and was involved in a large range of surface processes (e.g., volcanic, atmospheric, hydrothermal, and aqueous brines). Sulfur compounds have also been detected on Europa's icy moon surface, with the S cycle implicated in Europa's surface and ocean geochemistry. Given the well-established utility of S isotopes in providing a record for past life on Earth, S isotopes are an valuable tool for identifying biosignatures on Mars and Europa. Here, we review S isotopes as a biosignature, in light of two recent advances in understanding the S cycle in both Mars and Europa: (i) the measurements of δ34S in situ at Gale Crater and quadruple S isotopes (QSI) in Martian meteorites, and (ii) the identification of a likely exogenous origin of sulfur on Europa's surface. We discuss important considerations for unravelling QSI biosignatures in Martian environments, considering high and low sulfur environments, atmospheric S-MIF signals, and metabolic energy-limited niches. For Europa, we describe the potential for S isotopes to probe biogeochemistry, and identify key knowledge gaps to be addressed in order to unlock S isotopic tools for future life detection efforts. The resulting picture demonstrates how S isotopes will be a valuable tool for Mars Sample Return, and how future missions can focus on the search for environments where QSI signatures of microbial S-cycling processes have a greater chance of being preserved. For Europa, the first step will be to account for the S isotope composition of the various S pools, to recognise or rule out non-biologically mediated S isotope values, with a focus on experimental examination of potential S isotope signatures from exogenous sulfur sources

Quadruple sulfur isotope biosignatures from terrestrial Mars analogue systems

In this study, we present quadruple sulfur isotope values (QSI: 32S, 33S,34S,36S) measured in sediments from two sulfur-rich Mars analogue environments: i) the glacially-fed hydrothermal pools in Iceland (Kerlingarfjöll and Kverkfjöll), and ii) the Lost Hammer hypersaline spring from Axel Heiberg Island, Nunavut, Canada. The localities host different physical and geochemical characteristics, including aqueous geochemistry, volcanic input, temperature, pH and salinity. The δ34S values of sulfur compounds from the Lost Hammer hypersaline spring exhibit large fractionations typical of microbial sulfate reduction (MSR) with or without additional oxidative sulfur cycling and microbial sulfur disproportionation (MSD) (34εSO4-CRS from -49.5 to -43.5 ‰), contrary to the small S isotope fractionations reported for the Icelandic hydrothermal sites (34εSO4-CRS from –9.9 to -0.7 ‰). Lost Hammer minor S isotope values (Δ33S and Δ36S), interpreted within the context of a sulfur cycling box model, are consistent with a biogeochemical S cycle including both MSR and MSD. In contrast, the small range in δ34S values within the Iceland hydrothermal pools are consistent with a large volcanic H2S flux and minimal biological S cycling. The minor S isotope values recorded in the hydrothermal pools, however, indicate further biogeochemical sulfur cycling. Our results demonstrate that contrasting physical and chemical characteristics between sites support different microbial S cycling processes, as recorded in the QSI sedimentary values. The QSI data and the derived models support the strong potential for QSI values to be used as biosignatures in the search for life in Martian S-rich environments. These results also suggests that extreme, metabolic energy-limited environments with low abiotic sulfur fluxes could be more likely to produce unequivocal biological QSI signals than those with more moderate conditions or abundant available energy. This finding carries significant implications for targeting sites on Mars for in situ measurements or future sample return missions

Quadruple sulfur isotope biosignatures from terrestrial Mars analogue systems

In this study, we present quadruple sulfur isotope values (QSI: 32S,33S,34S,36S) measured in sediments from two sulfur-rich Mars analogue environments: i) the glacially-fed hydrothermal pools in Iceland (Kerlingarfjöll and Kverkfjöll), and ii) the Lost Hammer hypersaline spring from Axel Heiberg Island, Nunavut, Canada. The localities host different physical and geochemical characteristics, including aqueous geochemistry, volcanic input, temperature, pH and salinity. The δ34S values of sulfur compounds from the Lost Hammer hypersaline spring exhibit large fractionations typical of microbial sulfate reduction (MSR) with or without additional oxidative sulfur cycling and microbial sulfur disproportionation (MSD) (34εSO4-CRSfrom −49.5 to −43.5‰), contrary to the small S isotope fractionations reported for the Icelandic hydrothermal sites (34εSO4-CRS from −9.9 to −0.7‰). Lost Hammer minor S isotope values (Δ33S and Δ36S), interpreted within the context of a sulfur cycling box model, are consistent with a biogeochemical S cycle including both MSR and MSD. In contrast, the small range in δ34S values within the Iceland hydrothermal pools are consistent with a large volcanic H2S flux and minimal biological S cycling. The minor S isotope values recorded in the hydrothermal pools, however, indicate further biogeochemical sulfur cycling. Our results demonstrate that contrasting physical and chemical characteristics between sites support different microbial S cycling processes, as recorded in the QSI sedimentary values. The QSI data and the derived models support the strong potential for QSI values to be used as biosignatures in the search for life in Martian S-rich environments. These results also suggests that extreme, metabolic energy-limited environments with low abiotic sulfur fluxes could be more likely to produce unequivocal biological QSI signals than those with more moderate conditions or abundant available energy. This finding carries significant implications for targeting sites on Mars for in situ measurements or future sample return missions