Experimental Study of the Alteration of Basalt on the Surface of Venus

Venus is the second planet from the sun, distinguished by its thick atmosphere which prevents visible observations of the surface. Our understanding of its surface is therefore dominated by surface interactions with electromagnetic radiation (e.g. radar, visible-to-infrared, and X-ray wavelengths) which have been collected by a series of missions including the Soviet’s Venera missions, the National Aeronautics and Space Administration’s (NASA) Magellan mission, and the European Space Agency’s (ESA) Venus Express orbiter (Barsukov et al. 1982; Head et al. 1992; Smrekar et al. 2010). The atmosphere of Venus is composed almost entirely (~97%) of CO2 gas with trace amounts of SO2 gas (~150ppmv) (Zolotov 2018). The surface of Venus experiences temperatures of ~470˚C and pressures of ~92 bars, comparable to the terrestrial greenschist metamorphic facies, which has been used to suggest that rocks on the surface could interact with their atmosphere (Urey 1952, Lewis 1970, Barsukov et al. 1982; Fegley and Treiman 1992, Zolotov 2018). Most of the surface of Venus is covered by basaltic plains and volcanoes assumed to have been emplaced ~300–600 Ma (Nimmo and McKenzie 1998). However, several authors have proposed volcanism is currently active or has been within the last hundreds of thousands of years based on variations in thermal emissivity spectra and recent basalt and mineral oxidation experiments (Head et al. 1992; Smrekar et al. 2010; Stofan et al. 2016; D'Inecco et al. 2017; Filiberto et al. 2020). Should basaltic rock interact with the Venusian atmosphere, interpretations of spectral data rely on an understanding of the alteration products and their rates of formation. We used an experimental approach to model how basaltic glass interacts with a Venus-like atmosphere and quantified the changes in geochemical composition of the sample surfaces and at depth. Our results confirm that iron (Fe) oxides can form on the surfaces of basalts under Venus surface conditions in two weeks duration. Moreover, ii our results corroborate the findings of Cooper et al. (1996) that cation diffusion is the ratelimiting factor for basalt alteration and constrain the ages of basalts on Venus to be anywhere from 268 – 1,900 years old.