Short and Long Term Effects of Meteoritic Impacts on the Evolution of Venus and Its Surface Conditions.

Abstract:

The influence of impacts on the history of terrestrial planets from the point of view of both internal dynamics and surface conditions is investigated in this study. We make use of our previous work on Venus’ long term evolution through a coupled atmosphere/mantle numerical code.

The solid part of the planet is simulated by the StagYY code (Armann and Tackley, 2012) and releases volatiles into the atmosphere through degassing. Coupling with the atmosphere is obtained by using surface temperature as a boundary condition.

The evolution of surface temperature is calculated from CO₂and water concentrations in the atmosphere with a gray radiative-convective atmospheric model. These concentrations vary due to degassing and escape mechanisms. We take into account hydrodynamic escape, which is dominant during the first hundred million years, and non-thermal processes as observed by the ASPERA instrument and modeled in various works.

Impacts can have different effects: they can (i) bring volatiles to the planet, (ii) erode its atmosphere and (iii) modify mantle dynamics due to the large amount of energy they release. A 2D distribution of the thermal anomaly due to the impact is used leading to melting and subjected to transport by the mantle convection. Volatile evolution is still strongly debated. We therefore test a wide range of impactor parameters (size, velocity, timing) and different assumptions related to impact erosion, from large eroding power to more moderate ones (Shuvalov, 2010).

Atmospheric erosion appears to have significant effects only for giant impacts and to be mitigated by volatiles brought by the impactor. While small (0-10 km) meteorites have a negligible effect on the global scale, medium ones (50-150 km) are able to bring volatiles to the planet and generate melt, leading to strong short term influence. However, only larger impacts (300+ km) have lasting effects. These can cause volcanic events both immediately after the impact and later on. Additionally, the amount of volatiles released is large enough to modify normal evolution and surface temperatures by tens of Kelvins. This is enough to modify mantle convection patterns. Depending on when such an impact occurs, the surface conditions history can be deeply modified. A key factor is thus the timing of the impact and how it interacts with other processes.