2026ApJ...996L..26A

Detection of an atmosphere around a terrestrial exoplanet will be a major milestone in the field, but our observational capacities are biased towards 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 condensed volatiles constitute a stable reservoir that could be revaporized by meteorite impacts and reestablish 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 CO₂ atmospheres. We find that moderate-sized impactors (5-10 km diameter) occurring at a frequency of 1-100 Gyr^–1 can regenerate observable transient atmospheres on previously airless planets. We focus on specific targets from the James Webb Space Telescope Director's Discretionary Time Rocky Worlds programme and compute the fraction of their evolution spent with a transient CO₂ atmosphere generated through this mechanism. We find this fraction can reach 70% for GJ 3929 b, 50% for LTT 1445 Ac, and 80% for LTT 1445 Ab at high impact rates and strong CO₂ outgassing over the planet's lifetime. We also show that atmospheric collapse can shield volatiles from escape, particularly in the early, high X-ray and ultraviolet 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.