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 (sulfate reduction, sulfur disproportionation sulfide oxidation) can produce unique S isotope signals that provide insight into biogeochemical S-cycling. The cycle is also relevant for extraterrestrial processes. On early Mars, existed different redox states was involved a large range of surface processes (e.g. volcanic, atmospheric, hydrothermal aqueous brines). compounds have been detected Europa's icy moon surface, the implicated ocean geochemistry. Given well-established utility providing record life Earth, valuable tool identifying biosignatures Mars Europa. Here, we review as biosignature, light two recent advances understanding both Europa: (1) measurements ? 34 situ at Gale Crater quadruple (QSI) Martian meteorites; (2) identification likely exogenous origin surface. We discuss important considerations unravelling QSI environments, considering high- low-sulfur atmospheric mass-independent fractionation (S-MIF) energy-limited niches. For Europa, describe potential probe biogeochemistry, identify key knowledge gaps be addressed unlock isotopic tools future detection efforts. resulting picture demonstrates how will sample return, missions focus search where signatures microbial greater chance being preserved. first step account composition various pools, recognize or rule out non-biologically mediated values, experimental examination from sources. Thematic collection: This article part Earth system collection available at: https://www.lyellcollection.org/cc/sulfur-in-the-earth-system