The Response of Planetary Atmospheres to the Impact of Icy Comets. III. Impact Driven Atmospheric Escape

Abstract In an Earth-analogue atmosphere, water vapor is a key carrier of hydrogen in the lower atmosphere. The vertical transport of water to above the tropopause is one of the primary control valves on the atmospheric hydrogen escape rate. On the Earth, this escape is limited by transport through the tropospheric cold trap, where water vapor condenses. However, on a tidally locked exoplanet, the strong day-night temperature gradient drives a global-scale circulation. This circulation could rapidly transport water through the cold trap, potentially increasing hydrogen escape and impacting the composition of potentially habitable worlds. We couple cometary impact and planetary atmospheric models to simulate water-depositing impacts with both a tidally locked and Earth-analogue atmosphere and quantify how atmospheric circulations transport water from the impact site to high altitudes where it can potentially drive escape. The global nature of the atmospheric circulations on a tidally locked world enhances hydrogen escape, with both our unimpacted tidally locked and Earth-analogue atmospheres exhibiting similar mass loss rates despite the tidally locked atmosphere being both cooler and drier near the surface. When considering the effects of a cometary impact, we find a 1 order of magnitude difference in peak escape rates between impacts on the day-side (Φ escape = 1.33 × 10 10 mol mth −1 ) and night-side (Φ escape = 1.51 × 10 9 mol mth −1 ) of a tidally locked atmosphere, with the latter being of the same order of magnitude as the peak escape rate found for an impact with an Earth-analogue atmosphere (Φ escape = 2.7 × 10 9 mol mth −1 ). Our results show the importance of understanding the underlying atmospheric circulations when investigating processes, such as hydrogen escape, which depend upon the vertical advective mixing and transport.