Atmospheric mass-loss due to giant impacts: the importance of the thermal component for hydrogen–helium envelopes

Abstract

ABSTRACT Systems of super-Earths and mini-Neptunes display striking variety in planetary bulk density and composition. Giant impacts are expected to play a role in the formation of many of these worlds. Previous works, focused on the mechanical shock caused by a giant impact, showed that these impacts can eject large fractions of the planetary envelope, offering a partial explanation for the observed compositional diversity. Here, we examine the thermal consequences of giant impacts, and show that the atmospheric loss caused by these effects can significantly exceed that caused by mechanical shocks for hydrogen–helium (H/He) envelopes. During a giant impact, part of the impact energy is converted into thermal energy, heating the rocky core and envelope. We find that the ensuing thermal expansion of the envelope can lead to a period of sustained, rapid mass-loss through a Parker wind, partly or completely eroding the H/He envelope. The degree of atmospheric loss depends on the planet’s orbital distance from its host star and its initial thermal state, and hence age. Close-in planets and younger planets are more susceptible to impact-triggered atmospheric loss. For planets where the heat capacity of the core is much greater than the envelope’s heat capacity (envelope mass fractions ≲4 per cent), the impactor mass required for significant atmospheric removal is Mimp/Mp ∼ μ/μc ∼ 0.1, approximately the ratio of the heat capacities of the envelope and core. Conversely, when the envelope dominates the planet’s heat capacity, complete loss occurs when the impactor mass is comparable to the envelope mass.