Consequences of Impact Erosion and Volatile Loss Processes on the Evolution of Venus.

We model the long-term evolution of Venus through volatile exchanges and compare observed and simulated present-day states. This work focuses on quantifying the effect of different parameterizations for loss processes on the overall evolution.

Due to both the striking similarities and the obvious differences between Earth and Venus, understanding Venus might hold keys to how planets become -and cease to be- habitable. It has been suggested that the divergence between Earth and Venus could occur during the first few hundred million years due to interaction between the interior of the planet, its atmosphere and escape mechanisms. 

We develop coupled numerical simulations of the atmosphere and interior to test what evolutionary paths can reproduce the observed present-day state of Venus. They include modeling of mantle dynamics, core evolution, volcanism/outgassing, surface alteration, atmospheric escape (hydrodynamic and non-thermal), volatile deposition and loss through impacts. Impact histories representing different possible scenarios for late accretion are generated using n-bodies simulations.

Our previous efforts used hydrocode results to model impact erosion. A new parameterization has since been proposed by Kegerreis et al. (2020), with increased losses for high-energy collisions. We test if these results induce divergences between different impact histories (e.g., giant impact vs. small impactors) and combine these different parameterizations depending on impactor size.

Post-hydrodynamic escape, non-thermal loss mechanisms can remove low amounts of water and oxygen, from the surface/atmosphere (4 mbar to a few bar), making it quite difficult to accommodate large bodies of water, especially during Venus’ recent past. Trapping oxygen on the surface through oxidation of newly emplaced volcanic material through solid-gas reactions appears inefficient (totalling loses similar to non-thermal escape). Runaway greenhouse resulting in a molten surface could lead to the loss of multiple bars of oxygen but still leaves behind a significant atmospheric inventory. These results imply a maximum limit to water delivery by impacts.

Atmospheric delivery and erosion by impacts seem to be the largest source/sink of volatile species during evolution. The choice of parameterization for erosion can induce a large difference in total inventory (up to several 1-10 bar of H₂O and CO₂). However maximum delivery by impactors over Late Accretion are still limited by loss processes. Previously obtained upper limits for water content of the Late Accretion (95-98% dry enstatite chondrite, 2-5% of carbon chondrite) are revised upward to 5-10% Carbon chondrites for efficient atmospheric erosion models.