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Survival of Terrestrial Atmospheres in Violent XUV Environments through Efficient Atomic Line Radiative Cooling

Presentation #629.04 in the session Habitability.

Published onApr 03, 2024
Survival of Terrestrial Atmospheres in Violent XUV Environments through Efficient Atomic Line Radiative Cooling

The atmosphere and planetary habitability in terrestrial exoplanets around M dwarfs are hotly debated. Atmospheres play a crucial role in planetary habitability because of their greenhouse and heat circulation effects. Around M dwarfs and young Sun-like stars, planets receiving the same insolation as the present-day Earth are exposed to intense stellar X-rays and extreme-ultraviolet (XUV) radiation which drives the atmospheric escape. This study explores the fundamental question of whether the atmosphere of present-day Earth could survive in such harsh XUV environments. Previous theoretical studies suggest that stellar XUV irradiation is sufficiently intense to remove such atmospheres completely on short timescales. In this study, we develop a new upper-atmospheric model and re-examine the thermal and hydrodynamic responses of the thermospheric structure of an Earth-like N2–O2 atmosphere, on an Earth-mass planet, to an increase in the XUV irradiation. Our model includes the effects of radiative cooling via electronic transitions of atoms and ions, known as atomic line cooling, in addition to the processes accounted for by previous models. We demonstrate that atomic line cooling dominates over the hydrodynamic effect at XUV irradiation levels greater than several times the present level of the Earth. Consequentially, the atmosphere’s structure is kept almost hydrostatic, and its escape remains sluggish even at XUV irradiation levels up to a thousand times that of the Earth at present. Our estimates for the Jeans escape rates of N2–O2 atmospheres suggest that these 1 bar atmospheres survive in early active phases of Sun-like stars. Even around active M dwarfs, N2–O2 atmospheres could escape significant thermal loss on timescales of gigayears. Thus, the atmosphere sufficient to maintain the warm climate would be sustained in a typical M dwarfs system. However, very low-temperature stars (e.g., TRAPPIST-1) would have very long active phases comparable to the geological timescales. Given planets gained volatile amounts similar to the Earth, no or a small amount of the atmosphere would remain in the geological timescales unless the degassing flux from the planetary interior is higher than the escape flux. These results give new insights into the habitability of terrestrial exoplanets and the Earth’s climate history.

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