Venus today represents an uninhabitable terrestrial planet within the Solar System, with surface temperatures approaching 600K, no surface liquid water, and a thick, dry, CO2-dominated atmosphere. However, in recent years it has been suggested that early in its history, when the Sun was younger and dimmer, Venus might have had habitable conditions (e.g. Way et al., 2016; Kane et al., 2019; Way & Del Genio, 2020). Based on the models of Way & Del Genio (2020), early habitable climates on Venus likely required an atmosphere thinner than the present-day 93 bar atmosphere. A natural question resulting from this is where did the rest of Venus’ atmosphere come from? The two possible major contributors to Venus' atmospheric CO2 are:
Volcanic degassing: Delivery of melt to Venus' crust and surface brings magmatic volatiles which are released into the atmosphere.
Metamorphic decarbonation: Any carbonates formed during the early habitable period are heated by a combination of burial and increasing surface temperatures until the point of thermal decomposition, releasing CO2 back into the atmosphere.
In both cases, CO2 is not the only volatile introduced into the atmosphere. In volcanic degassing, unless Venus’ mantle is desiccated/volatile depleted, water vapor, nitrogen, and sulphur species may degas during volcanic eruptions. In the case of metamorphic degassing, the formation of carbonates, and maintaining habitable conditions requires some amount of surface liquid water. In order to maintain surface liquid water, groundwater is necessary, otherwise, the surface water would simply infiltrate into the deep crust. Therefore, as temperatures on Venus rose due to the Sun's evolution and/or changes in atmospheric composition, at least the volume of water stored in the deep crust would have evaporated, entering Venus’ atmosphere. If Venus had an earlier, thinner atmosphere, sources of water to Venus' atmosphere during subsequent CO2 degassing are important because Venus' present-day atmosphere is very dry, with only 200–300 ppm water vapor (Johnson & Fegley, 2000). If water is added to Venus' atmosphere by degassing, evaporation of groundwater, or dehydration of hydrous minerals, it must since have been removed. Additionally, photodisocciation of water in Venus' upper atmosphere and preferential loss of the resulting H to space would have led to an accumulation of oxygen in Venus' atmosphere (Gillmann et al., 2020; Gillmann et al., 2016). This is not consistent with Venus’ observed atmospheric composition, so the combined loss rates of H and O over time can be used to constrain how much water could have been added to the Venus atmosphere since a potential early habitable period.
Each model begins at the end of the early habitable era, between 0.1 and 3.8 Gyr after CAIs. Metamorphic decarbonation of any carbonates and evaporation of any groundwater are assumed to occur instantaneously, so the atmosphere begins with some initial CO2 and H2O inventory that is dependent upon the ratio of metamorphic to volcanic CO2 degassing, and the crustal heatflow during the early habitable period, both of which are left as free parameters. Subsequent volcanic degassing depends on the concentrations of CO2 and H2O in melts erupted on Venus. Chemical analyses of the surface at the Venera 14 and VEGA 2 landing sites are consistent with tholeiitic basalts, so we consider a range of melt volatile concentrations based on those observed in terrestrial mid ocean ridge basalts (MORBs). We use the open-source degassing Python program VolcGasses (Wogan et al., 2020) to calculate the atmospheric pressure dependent H2O and CO2 solubility in basaltic melt as atmospheric pressure evolves throughout the model. This program closely reproduces the degassing trends in a more complex C-O-H-S-N system, with degassing at pressures up to around 10 bar dominated by water vapor, and degassing at higher pressures dominated by CO2 (Gaillard & Scaillet, 2014). Present day eruption rates on Venus are estimated to be 0.1–0.2 km3 yr-1, but the eruption history of Venus is unknown. We assume that the eruption rate has decreased exponentially since the end of the early habitable era, and leave the e-folding timescale of the eruption rate as a free parameter, keeping only the total erupted mass of CO2 constant.
For volatile loss from Venus’ atmosphere, we assume that all H2O is photodissociated by UV photons, and that the resulting H undergoes hydrodynamic escape. At high H2O mixing ratios, this escape is limited by incoming solar XUV radiation, which varies over time (Ribas et al., 2005). At lower H2O mixing ratios, the escape is limited by the ability of H to diffuse through the surrounding mixture of heavier atmospheric species (Tian, 2015). Oxygen is lost from the atmosphere by early hydrodynamic escape, and on-going non-thermal escape through processes such as photochemical reactions, sputtering, and plasma instabilities. Modelling these processes explicitly is beyond the scope of our model, so we implement the loss rates of oxygen as a function of time calculated by Gillmann et al. (2020). Oxygen can also be lost through oxidation of volcanic products. Hematite is stable under present-day Venus conditions, and experimental studies suggest that exposed basaltic olivine and glasses should oxidise under present-day Venus conditions within weeks to months (e.g. Cutler et al., 2020; Berger et al., 2019). However, diffusion in basalt is slow, so oxidation of lava flows may only be in a thin surface rind, but explosive basaltic volcanism on Earth produces fine-grained ash and scoria which could fully oxidize more quickly. We use volumes of degassed CO2 and H2O at each timestep to calculate whether the gas:magma ratio exceeds 3:1, which is a potential predictor of whether explosive volcanism can occur (Sparks, 1978). When the gas:magma ratio exceeds 3:1, we assume complete oxidation of the basaltic volcanic products.
In the end-member case where all the CO2 in Venus' present-day atmosphere is volcanically derived, only atmospheres resulting from low melt H2O concentrations (below 0.5 wt%) are able to lose all of their oxygen. High water concentrations lead to rapid accumulation of a thick, water vapor-rich atmosphere. As H is lost to space, more O accumulates in the atmosphere than can be lost to space or oxidized, even by explosive volcanic products such as ash. In the end-member case where all the CO2 is from metamorphic degassing, the early habitable era must end before 0.2 Gyr after CAIs. This is because evaporation of groundwater adds of order 102 m global equivalent layer (GEL) of H2O to the atmosphere, which requires the high O and H loss rates early in Venus' history to get rid of by 4.5 Gyr after CAIs.
There are many unknowns in Venus' atmospheric history, including the ratio of metamorphic:volcanic atmospheric CO2, the intrusive:extrusive ratio of volcanism on Venus, and the end time of any potential early habitable period. We will take a probabilistic approach to find the combination(s) of parameters most consistent with an early habitable period on Venus while still meeting present-day constraints on atmospheric O2 and H2O. Constraining the likelihood of this early habitable period may be useful for determining whether young exo-Venuses are good potential targets in the search for habitable worlds beyond our Solar System.
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