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

TL;DR

The paper investigates how icy comet impacts alter hydrogen escape from Earth-analogue and tidally-locked exoplanet atmospheres by focusing on water transport through the cold trap and into high-altitude regions where photodissociation products contribute to diffusion-limited escape. It couples a parametrized impact model with a high-top CESM2/WACCM6 climate framework, applied to TRAPPIST-1e-like and Earth-like atmospheric regimes, to quantify how global circulation governs vertical mixing and escape rates. Key findings show day-side impacts on tidally-locked planets yield the strongest but shortest-lived enhancements, while exo-Earth-analogue cases exhibit delayed but longer-lasting high-altitude enrichment, with diffusion-limited escape remaining the dominant hydrogen-loss mechanism and strong implications for atmospheric oxygenation and metallicity. The work underscores the necessity of global, coupled atmospheric models to accurately capture transport and loss processes after external material delivery in diverse planetary environments.

Abstract

In an Earth-analogue atmosphere, water vapour is a key carrier of hydrogen in the lower atmosphere with its transport above the tropopause controlling the atmospheric hydrogen escape rate. On the Earth, this escape is limited by transport though the tropospheric cold trap where water vapour 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 bpth cooler and drier near the surface. When considering the effects of a cometary impact, we find an order of magnitude difference in peak escape rates between impacts on the day-side ($Φ_{\mathrm{escape}}=1.33\times10^{10}\,\mathrm{mol\,mth^{-1}}$) and night-side ($Φ_{\mathrm{escape}}=1.51\times10^{9}\,\mathrm{mol\,mth^{-1}}$) 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 ($Φ_{\mathrm{escape}}=2.7\times10^{9}\,\mathrm{mol\,mth^{-1}}$). 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.

Figures (6)

• Figure 1: The mean surface temperature of our reference tidally-locked planet as a function of latitude and longitude. Here we show the boundaries of our model's Earth-like land-ocean distribution using white contours and the impact locations of each of our six cometary impacts as labelled, coloured, points.
• Figure 2: Monthly and global mean total hydrogen mixing ratio profiles ($\mathrm{f}(\mathrm{H}_\mathrm{tot})$) showing the enhancement in the high altitude abundance of hydrogen carrying molecules for an impact with the sub-stellar point of a tidally-locked atmosphere (top row) and over the Pacific Ocean of an Earth-analogue atmosphere (bottom row). Here we can see the rapid post-impact evolution of our tidally-locked atmosphere over the first 10 months post-impact (left/green) and the delayed enrichment of high-altitude hydrogen as well as the slow, but steady, settling of the atmosphere towards a quasi-steady-state (right/blue) reminiscent of the unimpacted reference state (grey dashed).
• Figure 3: Snapshots of the total hydrogen mixing ratio ($\mathrm{f}(\mathrm{H}_\mathrm{tot})$) and the mixing ratios of its consituents ($\mathrm{f}(\mathrm{H})$, $\mathrm{f}(\mathrm{\ce{H2O}})$, $\mathrm{f}(\mathrm{\ce{H2}})$, $\mathrm{f}(\mathrm{\ce{CH4}})$ and $\mathrm{f}(\mathrm{OH})$) for a comet impacting at the sub-stellar point of our tidally-locked, TRAPPIST-1e-like (top) atmosphere and over the Pacific Ocean in our Earth-analogue atmosphere (bottom). Note that we have selected these two points in time (one year post impact - left - and three years post impact - right) due to differences in the strength of vertical advective transport between tidally-locked and diurnally-rotating atmospheres.
• Figure 4: Select temporally averaged meridional mass streamfunctions taken from both tidally-locked (top row and bottom right) and exo-Earth-analogue (bottom left) atmospheric models. Panels in the left column show the (global) zonal mean of the circulation, whilst panels in the right column show the circulation calculated over a narrow ($15^\circ$ degree) zonal mean at the sub-stellar and anti-stellar points of our tidally-locked atmosphere respectively. Note that the meridional circulation is plotted on a log-scale with clockwise circulations shown in red and anti-clockwise circulations shown in blue. Together these circulations can combine to drive a net flow.
• Figure 5: Time evolution of the annual mean (dashed lines) and monthly mean (fainter solid lines) total hydrogen escape rate ($\Phi_{\mathrm{escape}}$ [mol mth$^{-1}$]) for each of our seven different models of a cometary impact models. The top panel shows a comparison of the six difference models of a cometary impact with a tidally locked atmosphere whilst the bottom panel compares an impact with the sub-stellar point of a tidally-locked atmosphere with an impact over the Pacific Ocean of an exo-Earth-analogue atmosphere. We also plot the escape rate from our unimpacted tidally-locked (grey) and exo-Earth-analogue (red) reference states. Note that the oscillations in the exo-Earth-analogue cases are driven by the inherent seasonality of the model.