Atmospheric collapse and re-inflation through impacts for terrestrial planets around M dwarfs

TL;DR

This work addresses the question of whether terrestrial planets around M-dwarfs can host detectable atmospheres despite strong XUV-driven erosion by proposing a two-state, energy-balance model in which nightside volatile ice collapses are periodically reinflated by meteorite impacts. The authors couple a simple cooling/warming balance with stochastic impacts, using CO$_2$ as the primary atmospheric constituent and a conservative energy-budget for vaporization and plume heating. Their simulations show that intermediate impact rates ($\sim 1$–$100$ impacts per Gyr) can sustain long-lived transient CO$_2$ atmospheres on several JWST Rocky Worlds targets, with substantial fractions of the planets’ lifetimes spent in an inflated state (up to $\sim$75% in some cases) while also highlighting that collapse can protect volatiles during high-XUV phases. The results imply that atmospheric detectability around rocky M-dwarf planets is episodic and probabilistic, challenging monotonic evolution assumptions and informing observational strategies for JWST and future missions.

Abstract

Detection of an atmosphere around a terrestrial exoplanet will be a major milestone in the field, but our observational capacities are biased towards to tidally locked, close-in planets orbiting M-dwarf stars. The atmospheres of these planets are vulnerable to atmospheric erosion and collapse due to condensation of volatiles on the nightside. However, these collapsed volatiles accumulated as nightside ice constitute a stable reservoir that could be re-vaporised by meteorite impacts and re-establish the atmospheres. Through a simple energy balance model applied to atmospheric evolution simulations with stochastic impacts, we assess the viability and importance of this mechanism for CO$_2$ atmospheres. We find that moderate-sized impactors ($5-10 \rm{km}$ diameter) occurring at a frequency of $1-100 \rm{Gyr}^{-1}$ can regenerate observable transient atmospheres on previously airless planets. We focus on specific targets from the JWST DDT Rocky Worlds programme, and compute the fraction of their evolution spent with a transient CO$_2$ atmosphere generated through this mechanism. We find this fraction can reach $70\%$ for GJ 3929 b, $50\%$ for LTT 1445 Ac, $80\%$ for LTT 1445 Ab, at high impact rates and strong CO$_2$ outgassing over the planet's lifetime. We also show that atmospheric collapse can shield volatiles from escape, particularly in the early, high-XUV phase of M-dwarf evolution. Overall, our work suggests that terrestrial planet atmospheres may not evolve monotonically but instead may be shaped by episodic external forcings.

Figures (5)

• Figure 1: Schematic of episodic atmospheric collapse and regeneration through impacts for a tidally locked planet. A) The planet has a volatile rich atmosphere which redistributes heat from the day- to the nightside. B) Atmospheric escape thins the atmosphere, heat redistribution becomes less efficient, nightside temperatures drop. C) Nightside temperatures have reached the volatile condensation temperature, the atmosphere collapses. D) Volatiles outgassed through volcanism or magma ocean pockets accumulate on the nightside as ice. E) An impactor hits the nightside and vaporises ice and rock. Hot vapour, ejecta, and silicate rain further vaporise the nightside ice sheets (see Section \ref{['subsec:th_reinf']} for details). An atmosphere is regenerated.
• Figure 2: Minimum diameter of an impactor necessary to trigger reinflation on an Earth-like planet for different values of $\epsilon$ and $v_{imp}$, assuming $\rho_{imp} = 3.0~\rm{g~cm^{-1}}$.
• Figure 3: Evolution of an Earth-like planet around an M0-type star at different orbital distances, with inflated (turquoise) and collapsed (brown) states. The atmospheric evolution is uniquely determined by escape ($\eta = 0.01$), outgassing (current Earth CO$_2$ outgassing rates), collapse, and reinflation triggered by impacts (signified by the red dashed lines, impact rate $10^{-9}~\rm{yr}^{-1}$), as shown in Equation \ref{['eq:evolution']}.
• Figure 4: Fraction of time spent with transient CO$_2$ atmospheres generated by impacts between 2.2 and 12 Gyr of evolution.
• Figure 5: Comparison of CO$_2$ escape fluxes as modeled using energy-limited escape with an efficiency $\eta=0.1$ (green line), carbon thermal escape fluxes for different super-Earths Tian2009, and stellar-wind driven ion escape for the TRAPPIST-1 planets Dong2018.