THE ATMOSPHERES OF EARTHLIKE PLANETS AFTER GIANT IMPACT EVENTS

Abstract

It is now understood that the accretion of terrestrial planets naturally\ninvolves giant collisions, the moon-forming impact being a well known example.\nIn the aftermath of such collisions the surface of the surviving planet is very\nhot and potentially detectable. Here we explore the atmospheric chemistry,\nphotochemistry, and spectral signatures of post-giant-impact terrestrial\nplanets enveloped by thick atmospheres consisting predominantly of CO2, and\nH2O. The atmospheric chemistry and structure are computed self-consistently for\natmospheres in equilibrium with hot surfaces with composition reflecting either\nthe bulk silicate Earth (which includes the crust, mantle, atmosphere and\noceans) or Earth's continental crust. We account for all major molecular and\natomic opacity sources including collision-induced absorption. We find that\nthese atmospheres are dominated by H2O and CO2, while the formation of CH4, and\nNH3 is quenched due to short dynamical timescales. Other important constituents\nare HF, HCl, NaCl, and SO2. These are apparent in the emerging spectra, and can\nbe indicative that an impact has occurred. The use of comprehensive opacities\nresults in spectra that are a factor of 2 lower in surface brightness in the\nspectral windows than predicted by previous models. The estimated luminosities\nshow that the hottest post-giant-impact planets will be detectable with\nnear-infrared coronagraphs on the planned 30m-class telescopes. The 1-4um\nregion will be most favorable for such detections, offering bright features and\nbetter contrast between the planet and a potential debris disk. We derive\ncooling timescales on the order of 10^5-10^6 Myrs, based on the modeled\neffective temperatures. This leads to the possibility of discovering tens of\nsuch planets in future surveys.\n