How much water on a habitable Venus? Constraints from modern atmospheric O2
Authors: Warren Alexandra , Kite Edwin 
Published: March 2022

&lt;p&gt;Understanding whether Venus may have been one of the first habitable planets in our Solar System is one of the primary drivers of the renewed interest in Venus exploration. Despite Venus&amp;#8217; current inhospitable state, recent climate models suggest habitable conditions may have been possible until as recently as the last global resurfacing event, and recent modelling of Venus&amp;#8217; coupled interior and atmosphere evolution starting from a magma ocean suggests that condensation of tens to hundreds of meters of liquid water on Venus&amp;#8217; surface may have been possible earlier in the planet&amp;#8217;s history.&lt;/p&gt;&lt;p&gt;There are 2 main constraints on the water inventory of a past habitable era on Venus: 1) Venus' modern atmosphere is dry and hydrous minerals are not stable on the planet&amp;#8217;s surface, so if water was added to the atmosphere, it must since have been removed by H loss to space. 2) Venus&amp;#8217; atmosphere is oxygen-poor, so O&lt;sub&gt;2&lt;/sub&gt; left behind by H&lt;sub&gt;2&lt;/sub&gt;O photolysis and H escape must be compensated by loss of O to space and other oxygen sinks, e.g. oxidation of Fe2+ to Fe3+ in crustal materials. We use these constraints and a mass balance model of the evolution of CO&lt;sub&gt;2&lt;/sub&gt;, H&lt;sub&gt;2&lt;/sub&gt;O, and O&lt;sub&gt;2&lt;/sub&gt; in Venus&amp;#8217; atmosphere to find an upper limit on the water inventory of a hypothetical past habitable period on Venus.&lt;/p&gt;&lt;p&gt;Our model considers initial inputs of CO&lt;sub&gt;2&lt;/sub&gt; and H&lt;sub&gt;2&lt;/sub&gt;O to Venus&amp;#8217; atmosphere at the end of a hypothetical past habitable era, loss of O and H to space, degassing of CO&lt;sub&gt;2&lt;/sub&gt; and H&lt;sub&gt;2&lt;/sub&gt;O through volcanism, and loss of O through oxidation of basaltic material. High atmospheric H&lt;sub&gt;2&lt;/sub&gt;O concentrations can lead to greenhouse warming strong enough that the temperature of Venus&amp;#8217; surface could have exceeded the melting point of basalt. So, we also consider oxidation of a basaltic surface melt layer in our models and compare this to model cases where surface melting does not occur.&lt;/p&gt;&lt;p&gt;Our model results show that without surface melting during a runaway greenhouse, the maximum water inventory on early Venus is 300m GEL. With surface melting, this increases to 700m GEL, as long as the habitable era ended around 4 Ga. For past habitable eras to be compatible with modern atmospheric constraints, our results suggest that high crustal production rates, and high extrusive volcanism fractions of volatile-poor melts are also required. We also compare our maximum water inventories to a viscous pore closure model for Venus&amp;#8217; crust with a range of crustal heat fluxes and initial porosities and show that all of Venus&amp;#8217; early water inventory could have been combined to the subsurface. This may have implications for Venus climate models that rely on exchangeable surface water inventories to maintain habitable conditions.&lt;/p&gt;&lt;p&gt;Upcoming Venus missions offer an opportunity to test the predictions made by our model. For example, noble gas measurements by DAVINCI+ will help constrain timing and volume of crust production, and the abundance of low Fe, high SiO2 rocks on Venus may be useful for determining whether high melt H&lt;sub&gt;2&lt;/sub&gt;O concentrations are typical of Venusian melts.&lt;/p&gt;