¹Alison R. Santos, ²Mikhail Yu. Zolotov, ¹Martha S. Gilmore, ³Craig Motil, ³Kyle Phillips, ⁴Valerie Tu
Journal of Geophysical Research: Planets, 131, e2025JE009457
Open Access Link to Article [DOI: 10.1029/2025JE009457]
¹Department of Earth and Environmental Sciences, Wesleyan University, Middletown, CT, USA
²School of Earth and Space Exploration, Arizona State University, Tempe, AZ, USA
³NASA Glenn Research Center, Cleveland, OH, USA
⁴Texas State University, Amentum JETSII Contract at NASA Johnson Space Center, Houston, TX, USA

Published by arrangement with John Wiley & Sons

Investigations of the venusian surface suggest chemical alteration (weathering) of rocks in contact with a hot, high-pressure atmosphere. This alteration was anticipated based on mineral stability considerations and has been supported by laboratory experiments conducted under Venus surface conditions. Here, we exposed iron-bearing samples (oxides, sulfides, iron-metal) to a simulated venusian environment (93 bars, 733 K, and a 9-gas mixture) for 60 days at the NASA Glenn Extreme Environments Rig (GEER). Iron metal, pyrrhotite, and troilite were altered to pyrite and magnetite. Some magnetite formed at the expense of hematite. Neither magnetite nor pyrite was altered. Measured concentrations of CO₂, SO₂, and OCS during the run suggest rapid OCS-SO₂ equilibration and drawdown of sulfur-bearing gases through reactions with the vessel and sample materials. The observed alteration assemblages and the gas composition in the vessel suggest experimental gas-phase fugacities (fO₂, fS₂, fSO₂, fOCS, fCO, fCO₂) in the vicinity of the magnetite-pyrite phase boundary and within the uncertainty of the magnetite-hematite boundary. Fugacities (log₁₀fO₂ of −22.5 to −21.5; log₁₀fS₂ of −5.4 to −4.6) and mineralogy in the Fe-O-S system are consistent with observation-based models for the near-surface conditions on Venus. This experiment supports both gas-phase chemical equilibrium and equilibration between gases and minerals in the Fe-O-S system on the surface of Venus, as proposed six decades ago.