火星古気候の同位体的証拠（Postcards from Ancient Mars: Isotopes Illuminate Early Martian Climate）

2025-10-29 カリフォルニア工科大学 (Caltech)

NASA’s Curiosity Mars rover took this selfie at a location nicknamed “Mary Anning” after a 19th century English paleontologist. Curiosity snagged three samples of drilled rock at this site on its way out of the Glen Torridon region, which scientists believe was a site where ancient conditions would have been favorable to supporting life, if it ever was present.Credit: NASA/JPL-Caltech/MSSS

＜関連情報＞

• https://www.caltech.edu/about/news/postcards-from-ancient-mars-isotopes-illuminate-early-martian-climate
• https://www.pnas.org/doi/10.1073/pnas.2511627122

酸素同位体による証拠は、火星のゲールクレーターにヘスペリアン初期の貯水池があり、それがかなりの蒸発を受けたことを示している Oxygen isotopic evidence that Gale crater, Mars, was home to an Early Hesperian water reservoir that underwent significant evaporation

Amy E. Hofmann, P. Douglas Archer Jr., Amy C. McAdam, +13 , and Ashwin R. Vasavada
Proceedings of the National Academy of Sciences  Published:October 20, 2025
DOI:https://doi.org/10.1073/pnas.2511627122

Significance

We present isotopic measurements of mineral-bound water from rocks sampled by the Curiosity rover. These hydrous minerals’ strong ¹⁸O enrichments stand apart from prior observations of ancient martian water reservoirs. Extreme deuterium enrichments record atmospheric hydrogen loss and reveal chemical weathering in early martian near-surface environments. The amplitude of ¹⁸O enrichment and its correlation with deuterium in related samples indicate formation in an ancient lacustrine setting that underwent extensive evaporation into a low-humidity atmosphere—an insight into the early martian hydrologic cycle that complements previous sedimentological, mineralogical, and stratigraphic evidence. These data provide the clearest view to date of a martian lake’s hydrology during a period when climate change, chemical weathering, and prebiotic chemistry were active on Mars.

Abstract

Simultaneous measurements of HDO, H₂¹⁸O, and H₂¹⁶O in water evolved during pyrolysis of powdered rock samples acquired by the Curiosity rover within Gale crater’s clay-bearing units indicate extreme and variable heavy-isotope enrichments averaging ~4.5 times the D/H ratio and ~1.03 times the ¹⁸O/¹⁶O ratio of terrestrial seawater. These enrichments are recorded in water desorbed from mineral surfaces and evolved from poorly crystalline phases, hydrated salts, jarosite, and clays. All evolved waters are deuterium-enriched relative to common terrestrial waters, reflecting hydrogen loss to space. Because oxygen in structurally bound hydroxyl groups is least likely to exchange with other sources over geologic timescales, we focus on oxygen in water evolved during dehydroxylation of smectite clays. Several samples have ¹⁸O/¹⁶O ratios commensurate with precipitation from, or near-complete equilibration with, water moderately ¹⁸O-enriched relative to terrestrial meteoric waters—consistent with other evidence that Mars’s hydrosphere is basically like Earth’s in terms of oxygen isotopes. Unlike hydrogen, oxygen atmospheric escape did not lead to extreme ¹⁸O enrichments on Mars. Locally, however, most Gale smectites’ ¹⁸O/¹⁶O values require a pronounced ¹⁸O-enrichment of their parental waters. On Earth, the most extreme ¹⁸O enrichments in surface waters are found in closed basins having undergone significant evaporative loss into a low-humidity atmosphere, and the ¹⁸O/¹⁶O of authigenic clay minerals formed in these environs reflect those enrichments. A similar process acting on the hydrologic reservoir local to Gale at the time of clay formation and early diagenesis is a plausible explanation for the distinctive oxygen isotopic compositions of these clays.