Narrow range of early habitable Venus scenarios permitted by modeling of oxygen loss and radiogenic argon degassing

Abstract

Whether Venus was ever habitable is a key question driving missions to Earth's sister planet in the next decade. Venus today has a dry, O2-poor atmosphere, but recent work has proposed that early Venus may have had liquid water [J. Krissansen-Totton, J. J. Fortney, F. Nimmo, Planet. Sci. J. 2, 216 (2021)] and reflective clouds that could have sustained habitable conditions until 0.7 Ga [J. Yang, G. Boué, D. C. Fabrycky, D. S. Abbot, Astrophys. J. 787, L2 (2014), M. J. Way, A. D. Del Genio, J. Geophys. Res.: Planets 125, e2019JE006276 (2020)]. Water present at the end of a habitable era must since have been lost by photodissociation and H escape, causing buildup of atmospheric oxygen [F. Tian, Earth Planet. Sci. Lett. 432, 126-132 (2015)]. We present a time-dependent model of Venus's atmospheric composition starting from the end of a hypothetical habitable era with surface liquid water. We find that O2 loss to space, oxidation of reduced atmospheric species, oxidation of lava, and oxidation of a surface magma layer formed in a runaway greenhouse climate can remove O2 from up to 500 m global equivalent layer (GEL) (30% of an Earth ocean), unless melts on Venus had a much lower oxygen fugacity than Mid Ocean Ridge melts on Earth, which increases the upper limit twofold. Volcanism is required to supply oxidizable fresh basalt and reduced gases to the atmosphere but also contributes 40Ar. Consistency with Venus's modern atmospheric composition occurs in less than 0.4% of runs, in a narrow parameter range where the reducing power introduced by O2 loss processes can balance O2 introduced by H escape. Our models favor hypothetical habitable eras ending before 3 Ga and very reduced melt oxygen fugacities three log units below the fayalite-magnetite-quartz buffer (fO2< FMQ-3), among other constraints.