The Response of Planetary Atmospheres to the Impact of Icy Comets. I. Tidally Locked Exo-Earths

Impacts by rocky and icy bodies are thought to have played a key role in shaping the composition of solar system objects, including the Earth’s habitability. Hence, it is likely that they play a similar role in exoplanetary systems. We investigate how an icy cometary impact affects the atmospheric chemistry, climate, and composition of an Earth-like, tidally locked, terrestrial exoplanet, a prime target in the search for a habitable exoplanet beyond our solar system. We couple a cometary impact model, which includes thermal ablation and pressure driven breakup, with the 3D Earth system model WACCM6/CESM2 and use this model to investigate the effects of the water and thermal energy delivery associated with an R = 2.5 km pure water ice cometary impact on an Earth-like atmosphere. We find that water is the primary driver of longer timescale changes to the atmospheric chemistry and composition by acting as a source of opacity, cloud ice, and atmospheric hydrogen/oxygen. The water opacity drives heating at ∼5 × 10‾⁴ bar and cooling below, due to a decreased flux reaching the surface. The increase in atmospheric hydrogen and oxygen also drives an increase in the abundance of hydrogen/oxygen-rich molecules, with the exception of ozone, whose column density decreases by ∼10%. These atmospheric changes are potentially observable for ∼1–2 yr postimpact, particularly those associated with cloud ice scattering. They also persist, albeit at a much reduced level, to our quasi–steady state, suggesting that sustained bombardment or multiple large impacts have the potential to shape the composition and habitability of terrestrial exoplanets.