Impact-atmosphere-interior interactions in terrestrial planets

Kurzfassung

We constructed a system of parameterized representations of impact-related processes such as crater formation, atmospheric erosion, and impact melt production in order to model how impactors of different types and a large range of sizes could affect CO2-H2O atmospheres and interiors of terrestrial planets similar to Mars, Venus, or the early Earth. Impactor-induced mass fluxes leading to e.g. atmospheric escape, delivery and outgassing are calculated assuming CO2-H2O atmospheres in order to assess under which conditions atmospheres and interiors could be depleted or enriched by processes related to impacts and associated melting or weathering. By combining parameterized models of single impacts with statistical information about the impactor flux such as the size-frequency distribution of impactors and the cratering chronology, one can deduce evolutionary paths of the volatile contents of the atmosphere and, within limits, of the interior. We consider rocky S-type and icy-rocky C-type asteroids as well as comets, covering a range of impactor-target density contrasts from about 1/6 to about 4/5 and a range of (absolute) impact velocities from a little less than 10 to almost 65 km/s. Impactor size ranges from 1 m to half the planetary radius. Atmospheric surface pressures cover almost five orders of magnitude, ranging from a few millibars (~modern Mars) up to 95 bar (~modern Venus). Mostly CO2-dominated atmospheric compositions representative for modern-day Mars and Venus were assumed; other gases were not included. With regard to atmospheric effects, there is a fundamental distinction to be made between blast-producing and crater-forming impacts; the boundary that separates these two regimes is mostly defined by the deceleration of the impactor and its resistance to breakup under the ram pressure during its traversal of the atmosphere. The direct effects of the former leave the interior essentially unaffected and interact only with the atmosphere. We use the formalism by Svetsov (2007) to assess the bulk mass transfer and balance resulting from mechanical erosion of the atmosphere and the disintegration of the impactor and estimate the balance for the individual volatiles from estimates of the impactor composition. In crater-forming impacts, there are additional effects that need to be included. Ejecta can contribute to the mechanical erosion of the atmosphere (e.g., Shuvalov et al., 2014) and also produce layers of porous material with a large, reactive surface that can absorb CO2 from the atmosphere by weathering in the long-term aftermath of an impact. Moreover, they produce craters which facilitate the interior-atmosphere mass exchange. A key process in this context is the production of impact melt, which can serve as a vehicle for volatiles between the atmosphere and the interior by either releasing or dissolving CO2 and water, depending mostly on the pressure conditions at the interface; generally outgassing is expected to be more common, but still the two volatiles may behave quite differently. Consistent with previous studies we find that CO2 is expelled from the melt much more easily than H2O and could therefore enter the atmosphere under all the conditions considered, whereas water may be retained in the melt at high atmospheric pressures.