The Response of Planetary Atmospheres to the Impact of Icy Comets. II. Exo-Earth Analogs

Abstract The orbital regime of a terrestrial planet plays a significant role in shaping its atmospheric dynamics, climate, and hence potential habitability. The orbit is also likely to play a role in shaping the response of a planetary atmosphere to the influx of material from an icy cometary impact. To investigate this response, we model the impact of an icy cometary body with an Earth-analog exoplanet (i.e., an Earth-like planet orbiting a Sun-like star with a diurnal cycle) using a cometary impact and breakup model coupled with the 3D Earth system model WACCM6/CESM2. To quantify the role that the atmospheric dynamics play in setting the response to a cometary impact, we compare our results with a previous study investigating an impact with a tidally locked terrestrial exoplanet. We find that the circulation regime of the planet plays a key role in shaping the response of the atmosphere to an icy cometary impact. The weak multicelled circulation structure that forms on Earth-like planets is efficient at mixing material horizontally but not vertically, limiting the transport of water from the deep breakup site to higher altitudes. In turn, this limits the rate of water photodissociation at low pressures, reducing the magnitude of postimpact changes to composition. It also reduces the potential observability of an impact due to weakened cloud ice formation, and hence scattering, at low pressures. Despite this, small changes to the overall composition of the planet persist to a quasi steady state, reinforcing the idea that ongoing bombardment may help to shape the composition/habitability of terrestrial worlds.