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Abstract To elucidate the relationship between oxygen fugacities (f_O2) recorded in martian basalts and redox processes in the martian interior, superliquidus 100‐kPa furnace experiments on a composition similar to Humphrey (Adirondack basalt) were conducted at variablef_O2and temperature. Quenched glasses were analyzed by EPMA, Mössbauer spectroscopy, colorimetric wet chemistry, and microbeam X‐ray absorption near edge structure (XANES) spectroscopy. The experiments reveal Mössbauer and wet chemical determinations of silicate glass Fe³⁺/Fe^Tagreeing within uncertainty, supporting the accuracy of extended‐Voigt‐based fitting of Mössbauer spectra when recoil‐free fraction is considered. Fe³⁺/Fe^Tratios determined from Mössbauer spectroscopy from Humphrey and previously studied martian‐relevant glass compositions are combined to calibrate models that characterize the relationship between Fe³⁺/Fe^T,f_O2, temperature, and composition in martian silicate liquids. The models demonstrate, similar to previously investigated silicate liquids, that the correlation between and logf_O2in martian magmas has a slope less than the value (0.25) expected if ferric and ferrous iron oxide mixed ideally. Martian magma Fe³⁺/Fe^Tratios are more temperature‐sensitive compared to non‐martian compositions, suggesting that temperature variations may contribute to comparatively largef_O2variations in martian basalt. The models are applied to demonstrate that the Fe³⁺/Fe^Tincreases required to explain multiple‐log unit changes inf_O2in shergottite magma would not increase terrestrial magmaf_O2as effectively. To aid in future investigations of martian magma redox, a XANES technique that allows for non‐destructive, microanalytical characterization of Fe³⁺/Fe^Tin natural martian materials and martian‐relevant experiments is introduced. 

Carbon is an essential element for life, but its behavior during Earth’s accretion is not well understood. Carbonaceous grains in meteoritic and cometary materials suggest that irreversible sublimation, and not condensation, governs carbon acquisition by terrestrial worlds. Through astronomical observations and modeling, we show that the sublimation front of carbon carriers in the solar nebula, or the soot line, moved inward quickly so that carbon-rich ingredients would be available for accretion at 1 astronomical unit after the first million years. On the other hand, geological constraints firmly establish a severe carbon deficit in Earth, requiring the destruction of inherited carbonaceous organics in the majority of its building blocks. The carbon-poor nature of Earth thus implies carbon loss in its precursor material through sublimation within the first million years.