Dr Felix Sainsbury-Martinez (University of Leeds) – The Response and Observability of Exo-Earth Climates to Cometary Impacts

Impacts by icy bodies likely played a key role in shaping the composition of solar systems objects, including the Earth's habitability. Hence, it is likely that they play a similar role in exoplanetary systems. Here I discuss how an impact from a comet affects the atmospheric chemistry, climate, and composition of two Earth-like terrestrial exoplanets with differing orbital configurations: a short (6 days), tidally-locked, orbit and an Earth-analogue orbit with a diurnal cycle.

To investigate this, I coupled a cometary impact model, which includes thermal ablation and pressure-drive breakup, with the 3D Earth-System-Model WACCM6/CESM2, quantifying the impact of a 2.5 km radius pure water ice comet. This revealed how both the impact-delivered water and thermal energy together affect the planetary atmosphere, including changing i) the cloud greenhouse effect, ii) the planetary albedo, iii) and the overall atmospheric composition. For the latter, we generally find an increase in the abundance of oxygen-bearing molecules with one key exception: ozone, the abundance of which is highly sensitive to products of the photodissociation of water, e.g OH.

My models also revealed how the response of the planetary atmosphere to the impact is shaped by the orbital configuration, and hence circulation regime of the planet. I find that the global atmospheric circulation may play a key role in setting the potential observability of individual massive impacts in future observations of exo-Earths. On the other hand, longer term changes to atmospheric composition appear less sensitive to orbital configuration, suggesting that sustained bombardment, or multiple large impacts, have the potential to measurably change the composition, and hence the habitability, of terrestrial exo-Earths.

In summary we find that cometary impacts may play an important role in shaping terrestrial exoplanetary atmospheres, and that to fully understand their impact we must also understand the underlying atmosphere they interact with.