The thermal consequences of giant impacts for planetary atmospheres

Despite their abundance, the formation mechanism for super-Earths and mini-Neptunes is not yet fully understood. Scenarios of in situ formation, formation at greater orbital distances followed by migration, and migration coupled with in situ gas accretion have been proposed. Multi-planet systems, such as Kepler-11 and Kepler-36, feature multiple close-in super-Earths with diverse densities; the formation mechanism must therefore be able to produce diverse outcomes without appealing to extremely different formation conditions.

 

Previous studies of photoevaporation and giant impacts have shown these processes can result in significant atmospheric mass loss, creating large differences in mean density between planets. Past work on the effect of giant impacts on the bulk densities of super-Earths focused only on the hydrodynamic shock caused by a large impactor and the subsequent atmospheric loss.

 

Here we extend this work and examine in detail the thermal consequences of these giant impacts and show that the resulting inflation of the gas envelope, and its subsequent thermal evolution, can produce significant atmospheric loss. We find that the final result is strongly dependent on the planet’s semimajor axis and envelope mass fraction. Planets with energy budgets dominated by their cores experience binary outcomes: either the envelope is retained or lost entirely. Impacts on planets with envelope-dominated energy budgets, however, can produce intermediate results. We show that close-in planets more easily generate the full range of outcomes, while planets with larger semimajor axes experience either very little or complete atmospheric loss.

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