From Ancient Venus to the Present-Day: What Place for Habitability?

As Earth's closest sibling, Venus is a key to understand how a planet becomes or ceases to be habitable. Despite Venus' present-day inhospitable conditions, its history and origin remain enigmatic. There is no consensus about whether Venus was mostly dry or wet until very recently. Recent work suggests surface conditions on Venus could have been compatible with the presence of liquid water surface oceans and habitable conditions provided its atmosphere remained thin, until an event (or series of events) led to the build-up of the currently observed atmosphere. However, available measurements on Venus do not provide straightforward constraints on the evolution of the planet: all markers of past evolution depend on the interaction of multiple mechanisms. We investigate possible scenarios for the evolution of Venus and their consequences, highlighting the requirements and limitations of these evolutionary paths (and in particular that of the habitable Venus pathway) in order to lead to the observed present-day conditions. We investigate how Venus atmosphere, mantle and surface could have evolved in the past in light of the mechanisms affecting volatile exchanges. We use self-consistent numerical models of global thermochemical mantle convection coupled with both an atmospheric evolution model and a late accretion N-body delivery model. We identify mechanisms that affect the evolution of the planet and its surface conditions. We focus on the competing effects of outgassing and loss mechanisms (atmospheric escape, surface oxidation). We quantify possible volcanic input based on a range of realistic compositions and depletion states for the mantle, as well as taking into account the role of surface conditions and pressure on volatile release. In recent evolution, volatile exchanges are likely very limited, with low release of water in the atmosphere, especially. Escape mechanisms also appear to be unable to remove large amounts of water or oxygen from the atmosphere. Trapping oxygen on the surface through oxidation of newly emplaced volcanic material depends on the lava flows geometry and eruption style. Precise estimation of oxidation efficiency is difficult. It can explain the loss of at least a few mbar to bars of water, making it more efficient than escape. Higher loss may be explained by explosive volcanism.