Of all the terrestrial planets in our Solar System, Venus is most similar to the Earth in size and, likely, composition. Understanding why Venus’ evolution diverged so dramatically from the Earth’s, and what implications this may have for habitable exoplanet candidates, is one of the primary drivers of the renewed interest in Venus exploration with missions like DAVINCI+ and VERITAS. Despite Venus’ current inhospitable state, recent climate models suggest habitable conditions may have been possible until as recently as the last global resurfacing event event with water inventories far less than 1 Earth ocean, but how a habitable era would fit into Venus’ atmospheric evolution over billion year timescales has yet to be investigated.
There are 2 main constraints on the likelihood of a habitable era on Venus: 1) Venus' present-day atmosphere is dry and hydrous minerals are not stable on the planet’s surface, so if large volumes of water were added to the atmosphere they must since have been removed by H loss to space. 2) Venus’ atmosphere is oxygen-poor, so O2 left behind by H2O 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 the crust. We present a new model that uses mass balance constraints on the evolution of CO2, H2O, and O2 in Venus’ atmosphere to determine areas of parameter space that enable a habitable era to have occurred on Venus.
Our model includes water vapor added to the atmosphere by the initial surface and groundwater inventory on Venus, thermal decomposition of carbonates formed during the early habitable era to account for varying fractions of the CO2 in Venus’ present day 93 bar atmosphere, loss of O and H to space, degassing of CO2 and H2O through volcanism, and loss of O through oxidation of basaltic material, including a runaway greenhouse triggered surface melt layer, pyroclastic material, and lava flows.
Our model results demonstrate that it is possible for Venus to have had an early habitable era and match present day observations of atmospheric composition under certain conditions. The six key parameters are: 1) timing of the end of the habitable era, 2) initial water inventory, 3) H2O and 4) CO2 concentration in typical basaltic melts erupted on Venus, 5) the fraction of Venus’ CO2 atmosphere contributed by volcanism, and 6) the fraction of extrusive melt on Venus that reaches the surface as either ash or lava flows.