Short‐Term Effect of Mars‐Like Radiation on Carbon and Oxygen Isotope Compositions of Carbonates

Abstract The stable isotope composition of carbonates records the environmental formation conditions and can indicate potential biosignatures if formed biologically. Martian meteorite carbonates display unusually high δ 13 C values, not explained using known terrestrial processes. Carbonates have been detected across the martian surface, including observations by the Perseverance Rover in Jezero Crater. However, the stable isotope effects of surface irradiation in carbonate minerals remain poorly constrained. We investigated the influence of ultraviolet (UV) and non‐UV radiation on δ 13 C and δ 18 O values of carbonates under both Low Earth Orbit (LEO) and laboratory conditions. Three natural carbonates, two calcites (Iceland spar, Bolivian stromatolite) and one aragonite (French microbialite), were exposed to LEO UV and non‐UV radiation for 29 ± 4 hr. Laboratory experiments employed reagent‐grade calcite and natural aragonite exposed to UV under N 2 atmospheres at 6 mbar (4.5 torr) or 1.73 mbar (1.3 torr) for 24–70 hr in the McMaster Planetary Simulator (MPS). Most carbonates showed no significant radiation‐induced changes in their δ 13 C and δ 18 O values. However, three cases exhibited minor stable isotope shifts, including carbon and oxygen isotope effects in LEO and MPS carbonates, an oxygen isotope effect in the LEO‐UV‐exposed French microbialite, and carbon isotope effects in two UV‐exposed MPS calcites. These results suggest that short‐term radiation cannot explain the 13 C enrichment in martian meteorites but may initiate subtle stable isotope effects. This study establishes a baseline for radiation‐induced stable isotope effects in carbonates, informing interpretations of martian carbonate isotopes, biosignature preservation potential, and analyses planned for Mars Sample Return.