James Farquhar

Evidence of atmospheric sulphur in the martian regolith from sulphur isotopes in meteorites.

Sulphur is abundant at the martian surface, yet its origin and evolution over time remain poorly constrained. This sulphur is likely to have originated in atmospheric chemical reactions, and so should provide records of the evolution of the martian atmosphere, the cycling of sulphur between the atmosphere and crust, and the mobility of sulphur in the martian regolith. Moreover, the atmospheric deposition of oxidized sulphur species could establish chemical potential gradients in the martian near-surface environment, and so provide a potential energy source for chemolithoautotrophic organisms. Here we present measurements of sulphur isotopes in oxidized and reduced phases from the SNC meteorites—the group of related achondrite meteorites believed to have originated on Mars—together with the results of laboratory photolysis studies of two important martian atmospheric sulphur species (SO2 and H2S). The photolysis experiments can account for the observed sulphur-isotope compositions in the SNC meteorites, and so identify a mechanism for producing large abiogenic 34S fractionations in the surface sulphur reservoirs. We conclude that the sulphur data from the SNC meteorites reflects deposition of oxidized sulphur species produced by atmospheric chemical reactions, followed by incorporation, reaction and mobilization of the sulphur within the regolith.

A 33S enrichment in ureilite meteorites: evidence for a nebular sulfur component

Abstract Acid volatile sulfur extracted from ureilite meteorites carries a small 33 S enrichment relative to carbonaceous chondrites, enstatite chondrites, ordinary chondrites, and troilite from iron meteorites: Δ 33 S (=δ 33 S − 1,000 × (1 δ 34 S/1,000) 0.515 − 1) = 0.042‰ ± 0.007‰ (standard error of 22 analyses). In situ production of sulfur by cosmic-ray spallation reactions involving Fe is unlikely to cause the enrichment because the ureilites have short cosmic-ray exposure ages, low Fe/S relative to the only documented phases that contain spallogenic sulfur (the metal phase in iron meteorites), and no corresponding 36 S enrichment. Sulfur derived from cosmic-ray spallation has been documented in the metal phase in iron meteorites, and it is characterized by Δ 36 S/Δ 33 S ∼ 8, inconsistent with present observations. We argue that this enrichment derives from heterogeneity in the presolar nebula. A 33 S enrichment in the presolar reservoir may derive from mixing among diverse nucleosynthetic sources or from mass-independent fractionations caused by gas-phase chemistry. In addition, several gas-phase reactions have been shown to produce mass-independent compositions for sulfur isotopes. One that both matches fractionations for all sulfur isotopes and is relevant to the presolar nebula has yet to be identified. An appropriate additive nucleosynthetic component has also not been identified.