Was Venus Ever Habitable? Constraints from a Coupled Interior-Atmosphere-Redox Evolution Model (2111.00033v1)

Abstract: Venus's past climate evolution is uncertain. General circulation model simulations permit a habitable climate as late as ~0.7 Ga, and there is suggestive-albeit inconclusive-evidence for previous liquid water from surface geomorphology and mineralogy. However, it is unclear whether a habitable past can be reconciled with Venus's inferred atmospheric evolution. In particular, the lack of leftover atmospheric oxygen argues against recent water loss. Here, we apply a fully coupled model of Venus's atmospheric-interior-climate evolution from post-accretion magma ocean to present. The model self-consistently tracks C-, H-, and O-bearing volatiles and surface climate through the entirety of Venus's history. Atmospheric escape, mantle convection, melt production, outgassing, deep water cycling, and carbon cycling are explicitly coupled to climate and redox evolution. Plate tectonic and stagnant lid histories are considered. Using this coupled model, we conclude that both a habitable Venusian past and one where Venus never possessed liquid surface water can be reconciled with known constraints. Specifically, either scenario can reproduce bulk atmospheric composition, inferred surface heat flow, and observed $^{40}$Ar and $^{4}$He. Moreover, the model suggests that Venus could have been habitable with a ~100 m global ocean as late as 1 Ga, without violating any known constraints. In fact, if diffusion-limited water loss is throttled by a cool, CO$_2$-dominated upper atmosphere, then a habitable past is tentatively favored by our model. This escape throttling makes it difficult to simultaneously recover negligible water vapor and ~90 bar CO$_2$ in the modern atmosphere without temporarily sequestering carbon in the interior via silicate weathering to enhance H escape.

Insights into Venus’s Potential Past Habitability: Application of a Coupled Atmospheric–Interior Model

The paper "Was Venus Ever Habitable? Constraints from a Coupled Interior–Atmosphere–Redox Evolution Model" presents a comprehensive investigation into the historical climatology of Venus through the use of a complex coupled model. This model integrates Venus’s atmospheric, interior, and redox evolution to explore whether a habitable Venusian past aligns with observational constraints.

Methodological Approach

The research utilizes the PACMAN geochemical evolution model to simulate Venus’s development from a post-accretion magma ocean state to the present. Critical elements of the model involve tracking C-, H-, and O-bearing volatiles across billions of years, simulating atmospheric escape, mantle convection, outgassing, and various geochemical cycling processes. Two tectonic scenarios are considered: plate tectonics and a stagnant lid regime. Unique facets of the model include self-consistent treatment of volatile cycling and redox evolution. By altering key parameters such as early water inventory, solar XUV flux, and tectonic transitions, the authors explore a wide range of evolutionary paths for Venus.

Findings and Interpretations

The findings indicate two main evolutionary scenarios for Venus that are consistent with current atmospheric conditions: a planet that never hosted surface water and one that was once transiently habitable. The model suggests Venus could have supported a global ocean approximately 100 meters deep until 1 billion years ago. The possible presence of such an ocean hinges on cloud feedbacks and the cooling of the CO2-rich upper atmosphere, which may have slowed diffusion-limited water loss. This escape throttling plays a crucial role, favoring scenarios where carbon is temporarily sequestered in the lithosphere, only to be outgassed later, allowing the planet to dry out to its present state.

For a never-habitable past, the findings suggest this could also explain Venus’ current atmospheric state if the planet had extended epochs of runaway greenhouse effects, constantly losing water to space. Both scenarios are consistent with the measured atmospheric concentrations of $^40$Ar and $^4$He, surface heat flow estimates, and the negligible amounts of atmospheric water and molecular oxygen.

Implications and Future Directions

This research highlights the importance of integrating complex evolutionary pathways to understand planetary climates, both for the paper of Venus and for extrapolation to exo-Venusian landscapes. The paper presents a nuanced understanding that challenges a monolithic interpretation of Venus’s history, suggesting instead that its eventual climate could arise from several viable paths.

Future missions to Venus, such as DAVINCI and VERITAS, could gather crucial isotopic and mineralogical data, potentially verifying the presence of ancient oceans or refined atmospheric escape paths proposed in this paper. Enhanced modeling that includes photochemical dynamics and exotic atmospheric compositions could further strengthen conclusions concerning Venus's potential past habitability.

Ultimately, this work underscores the necessity for multi-faceted models in planetary climate studies, particularly for planets within habitable zones subjected to varying initial conditions and evolutionary pressures. This comprehensive approach not only aids in deciphering Venus’s past but also informs the paper of terrestrial exoplanets, deepening our understanding of how life-supporting conditions can arise and persist.