Using carbon isotopes to trace the origin of volatiles on Earth and Mars

Abstract

The distinct accretionary histories of Earth and Mars – with Earth experiencing protracted growth and small contributions from outer solar system (carbonaceous, CC) materials, and Mars undergoing rapid growth with building materials drawn almost exclusively from the inner solar system (non-carbonaceous, NC) – highlight key differences in planetary formation. These contrasts underscore the importance of a comparative planetology framework for understanding the origin of volatiles in terrestrial planets. In this study, we examined the relationship between the carbon (C) isotopic compositions of planetary and planetesimal reservoirs to trace the origin of volatiles on Earth and Mars. The mean δ¹³C value of magmatic C in Martian meteorites (−20 ‰) is significantly lower than that of the bulk silicate Earth (BSE), with a canonical value of −5 ‰. While basaltic achondrites, magmatic iron meteorites, and ordinary chondrites from the NC reservoir display δ¹³C values similar to Martian meteorites, the BSE δ¹³C value is comparable to volatile-rich CC chondrites such as CI, CM, and CR, as well as with enstatite chondrites and ureilites from the NC reservoir. If Martian magmas underwent minimal C isotopic fractionation during degassing or degassed under kinetic conditions, then the δ¹³C value of the Martian mantle likely reflects accretion from thermally processed undifferentiated (ordinary chondrite-like) and differentiated NC materials. In contrast, if extensive degassing occurred via Rayleigh fractionation under equilibrium conditions, the δ¹³C value of the Martian mantle would have a higher δ¹³C value (−12 to −10 ‰) than that recorded in Martian meteorites – though still lighter than that of the canonical BSE δ¹³C. This implies a contribution from relatively ¹³C-rich NC materials, potentially similar to enstatite chondrites. For BSE, although the canonical δ¹³C value of –5 ‰ overlaps with those of enstatite chondrites and ureilites, the late-stage delivery of volatile-rich CC materials during the main phase of Earth’s growth, which was critical for establishing its water and nitrogen inventories, likely biased its C isotopic composition towards a CC-like signature. However, a lower mean δ¹³C value of −8.4 ‰ of the MORB mantle, as proposed by recent studies, could mean that Earth’s mantle still preserves the primordial signature of ¹³C-poor, thermally processed NC materials accreted during the early stages of the planet’s growth. The observed heterogeneity in mantle C isotopic compositions, similar to that seen in H and N isotopes, could therefore reflect a mixed contribution from both NC and CC materials. Thus, the δ¹³C value of the BSE could be lower than the canonical estimate and may align more closely with the proposed value for the MORB mantle. Taken together, these findings suggest that the contrasting accretionary histories of Earth and Mars led to fundamentally different pathways for volatile acquisition. These divergent pathways likely shaped the long-term geochemical evolution of each planet and influenced their potential for habitability.