THE ATMOSPHERES OF EARTHLIKE PLANETS AFTER GIANT IMPACT EVENTS

Abstract

It is now understood that the accretion of terrestrial planets naturally involves giant collisions, the moon-forming impact being a well-known example. In the aftermath of such collisions, the surface of the surviving planet is very hot and potentially detectable. Here we explore the atmospheric chemistry, photochemistry, and spectral signatures of post-giant-impact terrestrial planets enveloped by thick atmospheres consisting predominantly of CO[subscript 2] and H[subscript 2]O. The atmospheric chemistry and structure are computed self-consistently for atmospheres in equilibrium with hot surfaces with composition reflecting either the bulk silicate Earth (which includes the crust, mantle, atmosphere, and oceans) or Earth's continental crust. We account for all major molecular and atomic opacity sources including collision-induced absorption. We find that these atmospheres are dominated by H[subscript 2]O and CO[subscript 2], while the formation of CH[subscript 4] and NH[subscript 3] is quenched because of short dynamical timescales. Other important constituents are HF, HCl, NaCl, and SO[subscript 2]. These are apparent in the emerging spectra and can be indicative that an impact has occurred. The use of comprehensive opacities results in spectra that are a factor of two lower brightness temperature in the spectral windows than predicted by previous models. The estimated luminosities show that the hottest post-giant-impact planets will be detectable with near-infrared coronagraphs on the planned 30 m class telescopes. The 1-4 μm will be most favorable for such detections, offering bright features and better contrast between the planet and a potential debris disk. We derive cooling timescales on the order of 10[superscript 5-6] yr on the basis of the modeled effective temperatures. This leads to the possibility of discovering tens of such planets in future surveys.