On The Habitability Of An Impacted Young Earth

The formation of life on Earth is generally understood to have required the presence of liquid water, as well as an atmosphere within which the feedstock molecules — such as HCN — for more complex biomolecules are able to form. From the precipitation of these simple molecules, RNA can be built. The thermal profile and surface pressure of early Earth that was necessary for a liquid water cycle may have been created by a large impact, or series of larger impacts, following the formation of our Moon. Models which feature the consequences of very large impacts (e.g. Zahnle 2020) have dense, hydrogen-rich atmospheres that can be conductive to both the formation of HCN and a temperate surface temperature under the faint young Sun. In this work, we developed detailed self-consistent thermochemical equilibrium PT structures for post-large-impact atmospheres. We use a 1D radiative-convective equilibrium modelling code to obtain these thermal profiles and equilibrium chemistry. We found that the 5 optically thick cases for a dry atmosphere have a self-consistent surface temperature that is 742K on average; however, without the collisional opacity from H₂ molecules contributing to the radiative transfer, this self-consistent surface temperature is an average of 394K. For a wet atmosphere, these values are 842K and 568K, respectively. Our current results suggest that, in the work of Zahnle et al. (2020), early post-impact HCN yields were computed for atmospheres that are initially too hot for the necessary liquid surface water and too hot for these molecules to be stable.