Sulfur isotopes as biosignatures for Mars and Europa exploration

Moreras-Marti, A.; Fox-Powell, M.; Cousins, C. R.; Macey, M. C. and Zerkle, A.L. (2022). Sulfur isotopes as biosignatures for Mars and Europa exploration. Journal of the Geological Society (Early access).



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:

Viewing alternatives

Download history


Public Attention

Altmetrics from Altmetric

Number of Citations

Citations from Dimensions

Item Actions