Modelling Oxygen Loss and 40Ar Degassing on Venus Suggests Slim Pickings for Early Habitable Scenarios Consistent with Modern Atmosphere

How much water could early Venus have had, and when could the planet have had it? With three new missions lined up for the next decade, understanding the history of Venus is more important than ever.

Oxygen is a by-product of water loss from terrestrial planets, so Venus' modern, oxygen-poor atmosphere is a clue to how much liquid water Venus could have had, if the planet was ever habitable. Low modern oxygen concentrations suggest either that Venus never had much water in its atmosphere, or, that Venus has lost oxygen through loss to space and oxidizing its surface over time.

Recent models have shown that, under a fainter young sun, condensation of surface liquid water and habitable conditions may have been possible on our sister planet (e.g. Krissansen-Totton et al. 2021, Way & Del Genio 2020). We use a mass balance model to track the evolution of Venus's atmosphere from the end of a hypothetical habitable era to find areas of parameter space that can reconcile a habitable era with Venus' modern atmosphere. We consider both the amount of oxygen in the atmosphere, and also 40Ar—a constraint on Venus' crustal production history—using an 40Ar degassing model based on O'Rourke & Korenaga (2015).

The combined oxygen sinks of loss to space, oxidation of lava, and oxidation of a surface magma layer formed during a runaway greenhouse climate can remove oxygen leftover from up to 500m Global Equivalent Layer (GEL) of water loss from the atmosphere. However, volcanism can add more water (and therefore oxygen) over time, and extensive crustal production—needed to remove oxygen from the atmosphere by oxidizing surface basalts—can degas more 40Ar than observed in Venus' atmosphere today.

When both oxygen and 40Ar abundance in Venus' modern atmosphere are taken into account, we find a 300m GEL upper limit on the amount of water at the end of an early habitable era on Venus, and find that the habitable era must have ended before 3 Ga. Overall, habitable eras that are consistent with modern Venus represent less than 0.6% of our model runs, suggesting that a habitable era only fits into Venus' history under a narrow range of conditions.