Losing Oceans: The Effects of Composition on the Thermal Component of Impact-driven Atmospheric Loss

Abstract: The formation of the solar system's terrestrial planets concluded with a period of giant impacts. Previous works examining the volatile loss caused by the impact shock in the moon-forming impact find atmospheric losses of at most 20-30 per cent and essentially no loss of oceans. However, giant impacts also result in thermal heating, which can lead to significant atmospheric escape via a Parker-type wind. Here we show that H2O and other high-mean molecular weight outgassed species can be efficiently lost through this thermal wind if present in a hydrogen-dominated atmosphere, substantially altering the final volatile inventory of terrestrial planets. Specifically, we demonstrate that a giant impact of a Mars-sized embryo with a proto-Earth can remove several Earth oceans' worth of H2O, and other heavier volatile species, together with a primordial hydrogen-dominated atmosphere. These results may offer an explanation for the observed depletion in Earth's light noble gas budget and for its depleted xenon inventory, which suggest that Earth underwent significant atmospheric loss by the end of its accretion. Because planetary embryos are massive enough to accrete primordial hydrogen envelopes and because giant impacts are stochastic and occur concurrently with other early atmospheric evolutionary processes, our results suggest a wide diversity in terrestrial planet volatile budgets.

• The paper shows that hydrogen-dominated atmospheres are efficiently stripped following low-mass impacts using a two-layer thermal wind model.
• The paper finds that secondary steam atmospheres resist thermal loss unless impacted by exceptionally massive bodies.
• The paper reveals that mixed atmospheres entrain heavy molecules in escaping hydrogen, offering insights into terrestrial volatile depletion.

Summary of "Losing Oceans: The Effects of Composition on the Thermal Component of Impact-driven Atmospheric Loss"

This paper by J. B. Biersteker and H. E. Schlichting addresses the influences of planetary atmospheric composition on the thermal component of atmospheric loss driven by giant impacts during terrestrial planet formation. The research explores the potential for significant volatile loss via thermal winds, offering crucial insights into the evolution of terrestrial planet atmospheres and their volatile inventories.

Theoretical Foundation and Context

The study builds on the established understanding that the process of forming terrestrial planets is marked by a period of giant impacts among growing planetary embryos. These embryonic collisions lead to substantial heating, driving atmospheric escape, commonly modeled as a Parker wind when the atmosphere is subjected to intense thermal pressure. The authors critically examine the loss of high mean molecular weight species like $H_2O$ from hydrogen-dominated atmospheres, contrasting with previous paradigms focusing predominantly on impact shocks.

Atmospheric Composition and Impact-driven Loss

The core contribution of the paper is its insight into how atmospheric composition, particularly the presence of hydrogen-dominated versus secondary atmospheres, significantly affects the magnitude of atmospheric loss post-impact. The authors employ a two-layer atmospheric model to simulate various compositional scenarios:

1. Hydrogen-dominated Atmospheres: The study finds that primary $H/He$ envelopes are readily lost following impacts with relatively low mass impacts (impactor-target mass ratio as low as 0.05). This efficient stripping has substantial implications for the volatile retention on evolving terrestrial planets.
2. Secondary Atmospheres: Purely secondary, outgassed steam atmospheres show resilience to thermal loss unless impacted by extraordinarily massive bodies, highlighting the minimal effect of post-impact thermal expansion for heavier atmospheric species.
3. Mixed Atmospheres: The research further explores mixed atmospheres, where secondary volatiles coexist with hydrogen-dominated components. This composition allows the entrainment of heavy molecules in the escaping hydrogen flux, leading to relatively efficient depletion of volatiles despite their higher molecular weight.

Results and Implications

Numerical results in the study illustrate a pronounced loss for mixed atmospheres due to hydrodynamic escape mechanisms. Specifically, compositions including hydrogen augmented by significant amounts of outgassed volatiles still undergo substantial escape. This distinction between mixed and purely secondary atmospheres potentially elucidates Earth's and Mars's observed noble gas budgets, reflecting significant early atmospheric loss that is consistent with terrestrial volatile depletion hypotheses.

Broader Implications

In examining atmospheric loss dynamics, the authors emphasize the stochastic nature of impact timing and its critical role in shaping planetary volatile budgets. The variability in impactor mass and the timing of impacts during the giant impact phase suggest diverse evolutionary pathways for planetary atmospheres, potentially explaining observed differences among terrestrial planets.

Conclusion

Biersteker and Schlichting's exploration of atmospheric composition effects on impact-driven atmospheric loss offers valuable understanding necessary to appreciate the complex evolutionary scenarios of terrestrial planets. It sets the stage for further investigations into the diversity of planetary atmospheres, with implications for the study of exoplanetary systems where similar processes may be at play. Future studies could enhance these findings by incorporating additional atmospheric processes, such as chemical interactions and broader stellar environments, to better constrain the final stage of planetary volatile evolution.