Consequences for the early evolution of Venus from new simulations of atmosphere erosion by impacts.
Authors: Gillmann Cédric , Golabek Gregor , Kegerreis Jacob 
Published: July 2024

Overview:We investigate how both late accretion and long-term evolution of Venus are affected by early volatile exchanges (outgassing, loss, delivery), using a set of numerical models. In particular, we incorporate new scaling laws of large-impact erosion proposed by Kegerreis et al. (2020), and results of new atmosphere erosion simulations, using the open-source SPH code SWIFT. We assess the conditions and evolutionary pathways consistent with present-day observations of Venus and how to discriminate between late accretion scenarios.Motivation:Due to both the striking similarities and the obvious differences between Earth and Venus, understanding Venus might hold keys to how planets become &#8211; and cease to be &#8211; habitable. The question of the origin and persistence of water in the atmosphere/surface of Venus is directly linked to that of habitability. The divergence between Earth and Venus has been suggested to possibly occur during the first few hundred million years due to interaction between the interior of the planet, its atmosphere and escape mechanisms (e.g. Hamano et al., 2013).General approach:Late accretion impacts constitute both the tail end of the main delivery of material to the planet and, set after the magma ocean phase, the &#8220;initial conditions&#8221; for the planet&#8217;s long-term evolution. It could have important consequences for the distribution of elements, including volatiles. As no sample from Venus is available to constrain ancient history, we turn to modeling.The major constraints on Venus&#8217; volatile evolution come from the present-day state of its atmosphere and its bulk composition. We use different sets of late accretion scenarios and plausible volatile exchange mechanisms (sources and sinks). We compare the atmosphere composition of corresponding evolution models to the present-day state of Venus&#8217; atmosphere. Particularly, we investigate the loss mechanisms (sinks of volatiles) since they impose an upper limit on the amount of volatiles that can be injected into the system while still reaching present-day composition. Required initial conditions are also discussed.Models:Three separate models are used together to investigate volatile exchanges during and following late accretion. Late accretion scenarios are obtained from N-body simulations. They include sequences of collisions with rocky planets leading to scenarios producing Solar System-like configurations and are calibrated using Earth&#8217;s late accretion mass based on the siderophile elements content of Earth&#8217;s mantle (Rubie et al., 2016). We used previously generated scenarios (see Gillmann et al., 2020) to test the consequences of different mass-size distributions on volatile evolution. We include new high-resolution simulations from Joiret et al. (2024), which include tracking 1600 carbonaceous asteroids and 10,000 comets.Long-term evolution, interior evolution and atmosphere bulk composition are tracked using StagYY mantle dynamics models (see Gillmann et al., 2020). Important volatile-exchange mechanisms include volcanic outgassing, thermal and non-thermal atmosphere escape, and gas&#8211;surface chemical reactions through oxidation of fresh lava. Coupling between the interior and the atmosphere is obtained by tracking surface temperature evolution using a radiative convective grey atmosphere model. Impacts affect the atmosphere in three different ways: (i) collisions cause the mantle and surface to melt, releasing volatiles, (ii) impactors deliver volatiles to the atmosphere as they are vaporized depending on their composition, (iii) the impact process leads to atmospheric erosion through a variety of mechanisms.Previous results were obtained using (i) a geometric approach, through the tangent plane model or (ii) using small-impactor results from the SOVA hydrocode (Shuvalov et al., 2014). Here we investigate the importance of constraining impact erosion of the atmosphere by comparing those previous results to more recent larger-impactor scaling laws obtained by Kegerreis et al. (2020). We further include ongoing simulations specifically developed for Venus&#8217; atmosphere compositions and specific collisions defined by the late accretion impact scenarios described above.Results:The tiny amount of water in the present-day atmosphere of Venus limits water delivery from various mechanisms, even when considering water sinks throughout the history of the planet. The maximum amount of water that can be delivered, in turn, governs the estimated overall composition of late accretion impactors, imposing that the bulk of late accretion should be volatile poor.Non-thermal loss mechanisms can account for the loss in the range from 4 mbar up to a few bar of oxygen, depending on assumptions, and over >4 Gyr. Trapping oxygen on the surface through oxidation of newly emplaced volcanic material through solid&#8211;gas reactions appears inefficient (for a total loss similar to non-thermal escape), while recent oxidation of impact ejecta is a comparatively even smaller sink.On the first order, scaling laws extracted from simulations by Kegerreis et al. (2020) imply increased losses for high-energy collisions compared to previous estimates (Shuvalov et al., 2014), but use a different set of assumptions for the investigated atmospheres and consider larger bodies (figure 1). We investigate a series of possible parameterizations to reconcile those results.Figure 1: Loss rates for single impacts calculated from the two sets of simulations described in the text.Using lower atmosphere-erosion estimates leads to similar results regardless of the mass-size distribution of impactors, because the impact delivery of volatiles is the overall dominant effect. On the other hand, strong atmosphere erosion introduces divergences between late accretion scenarios depending on the distribution of impactor sizes, with smaller impactors having a net destructive effect while larger impactors contribute to increasing the atmosphere mass. This effect is not apparent in the water inventory but instead can be seen in the CO2 abundance evolution (figure 2). Therefore, we use CO2, N2 and Ar abundances to further discriminate between possible late accretion scenarios.Figure 2: Evolution of CO2 abundance in the atmosphere from simulations of two scenarios for late accretion. Scenario B includes a few large impactors, scenario D contains a larger number of smaller bodies.References:Kegerreis, J. A., et al. (2020), Astrophys. J. Let. 901.2.Hamano, K. et al. (2013), Nature 497, 607-610.Rubie, D. C. et al. (2016), Science 353, 1141&#8211;1144.Gillmann, C. et al. (2020), Nature Geoscience 13,&#160;265&#8211;269Joiret, S. et al., (2024), Icarus 414.Shuvalov, V. et al. (2014), Planet. Space Sci. 98, 120-127.