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

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.

💡 Research Summary

This paper investigates a novel pathway for the formation and evolution of secondary atmospheres on tidally‑locked terrestrial exoplanets orbiting M‑dwarf stars. The authors focus on the interplay between atmospheric collapse—driven by CO₂ condensation on the permanently dark night‑side—and subsequent re‑inflation caused by stochastic meteorite impacts that vaporize the accumulated night‑side ice reservoir. Using a simple energy‑balance framework coupled with a two‑state atmospheric evolution model, they quantify the critical pressure at which collapse occurs, the night‑side temperature as a function of atmospheric opacity, and the minimum impactor size required to vaporize enough CO₂ to exceed this critical pressure.

Key physical ingredients include: (1) the CO₂ saturation curve (pressure‑dependent condensation temperature), (2) a thin‑radiator approximation for night‑side temperature (Tₙ ≈