Day: September 18, 2025

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

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Damanveer S. Grewala, Sujoy Mukhopadhayb

Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2025.09.014]
aDepartment of Earth and Planetary Sciences, Yale University, New Haven, CT 06511, USA
bSchool of Earth and Space Exploration, Arizona State University, Tempe, AZ 85281, USA
Copyright Elsevier

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 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. These findings suggest that 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.

Excess 40Ar in Chang’e-5 lunar soils suggests a possible origin from Earth wind

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Li Zhaoa,b,e, Liwu Lia,c, Chunhui Caoa,c, Qingyan Tangd, Xianbin Wanga

Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2025.116803]
aInstitut für Planetologie, Universität Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
bDepartment of Earth, Environmental and Planetary Sciences, Brown University, Providence, RI 02912, USA
aApollo 15 Commander, USA
Copyright Elsevier

The abundance of 40Ar in lunar soils is significantly higher than the expected values from solar wind implantation and 40K decay, a phenomenon known as 40Ar excess. Traditionally, this excess is attributed to 40Ar generated by the decay of 40K within the Moon. This radiogenic 40Ar degasses to the lunar surface, where it is ionized by solar radiation and subsequently captured by lunar soils. However, stepwise heating (200 °C–1300 °C) and degassing analyses of noble gas isotopes in Chang’e-5 lunar soils samples reveal the presence of two types of 40Ar: one unrelated to 36Ar, likely originating from in situ 40K decay in the soils, and another correlated with 36Ar, which may primarily derive from Earth wind. Earth wind, an ion flux formed by the escape of Earth’s atmosphere, is thought to be injected onto the lunar surface under the regulation of Earth’s magnetosphere. The study proposes that the excess 40Ar in lunar soils may primarily stem from the continuous escape of Earth’s atmosphere and be injected onto the lunar surface through both the inner and outer regions of Earth’s magnetosphere, offering a new perspective for understanding volatile exchange between the Earth-Moon system.

Sound velocities of Anorthite at high pressures and temperatures: Implications for estimating porosity in upper lunar crust

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Peng Chena et al. (>5)

Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2025.116808]
aHigh Pressure Science Experiment Center, College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
Copyright Elsevier

The porosity of the lunar crust serves as the key to deciphering crucial geological processes such as the Moon’s impact history, volcanic activity, space weathering, and the formation and evolution of its internal structure. In this study, we conducted ultrasonic interferometry experiments on polycrystalline anorthite samples under high pressures and temperatures (up to 5.7 GPa and 873 K) using synchrotron radiation technology. We obtained the relationships between the compressional wave velocity (VP) and shear wave velocity (VS) of anorthite with varying pressure and temperature. The elastic properties of anorthite were fitted as follows: . By integrating these results with the lunar velocity model derived from the Apollo missions, we constructed a porosity model for the upper lunar crust, using anorthite as a representative mineral. The results indicate that the thickness of the lunar regolith (weathered surface layer) is approximately 30 km. Within the top 1 km of the lunar surface, porosity ranges from 30 % to 90 %. At the depth of 1–30 km, porosity ranges from 0 % to 50 %. At the depth of 30–50 km, porosity is less than 10 %. This study provides constraints on the porosity of the lunar surface and offers scientific guidance for the safety and design of future lunar exploration missions.