14:30 〜 14:45
[PAE17-10] Survival of Terrestrial N2-O2 Atmospheres in Violent XUV Environments by Efficient Atomic Line
Radiative Cooling
キーワード：系外地球型惑星、大気、大気散逸
Atmospheres have a crucial role in planetary climates and habitability.
Around M~dwarfs and young Sun-like stars, planets receiving the same insolation as the present-day Earth does are exposed to intense stellar X-ray and extreme-ultraviolet (XUV) radiation.
A fundamental question, then, arises whether the present Earth's atmosphere could survive in such harsh XUV environments.
Previous theoretical studies suggest that the stellar XUV irradiation is intense enough to remove atmospheres like the present-day Earth's one completely on short timescales.
In this study, we develop a new upper-atmosphere model and re-examine the thermal and hydrodynamic response of the thermospheric structure of an Earth-like N2-O2 atmosphere on an Earth-mass planet to increase in XUV irradiation.
The model includes newly the effects of radiative cooling via electronic transitions in the atoms (or atomic line cooling), in addition to thermo- and photo-chemistry, thermal and chemical diffusion,
adiabatic cooling, absorption of stellar IR radiation, and molecular IR radiative cooling, which previous models already took into account.
We demonstrate that the atomic line cooling dominates over the hydrodynamic effect at XUV irradiation levels higher than several times the present Earth's level.
As a consequence, the atmosphere's structure is kept almost hydrostatic, and its escape remains sluggish even at XUV irradiation levels even up to 5 hundred times the present Earth's one.
Our estimates for the Jeans escape rates of N2-O2 atmospheres suggest that the 1-bar atmospheres survive in early active phases of Sun-like stars except for rapid rotators.
Even around rapidly rotating Sun-like stars and M dwarfs, the N2-O2 atmospheres of several bars could
\new{avoid a significant thermal loss} on timescales of Gyr.
Those results give new insights into the habitability of terrestrial exoplanets and the Earth's climate history.
Around M~dwarfs and young Sun-like stars, planets receiving the same insolation as the present-day Earth does are exposed to intense stellar X-ray and extreme-ultraviolet (XUV) radiation.
A fundamental question, then, arises whether the present Earth's atmosphere could survive in such harsh XUV environments.
Previous theoretical studies suggest that the stellar XUV irradiation is intense enough to remove atmospheres like the present-day Earth's one completely on short timescales.
In this study, we develop a new upper-atmosphere model and re-examine the thermal and hydrodynamic response of the thermospheric structure of an Earth-like N2-O2 atmosphere on an Earth-mass planet to increase in XUV irradiation.
The model includes newly the effects of radiative cooling via electronic transitions in the atoms (or atomic line cooling), in addition to thermo- and photo-chemistry, thermal and chemical diffusion,
adiabatic cooling, absorption of stellar IR radiation, and molecular IR radiative cooling, which previous models already took into account.
We demonstrate that the atomic line cooling dominates over the hydrodynamic effect at XUV irradiation levels higher than several times the present Earth's level.
As a consequence, the atmosphere's structure is kept almost hydrostatic, and its escape remains sluggish even at XUV irradiation levels even up to 5 hundred times the present Earth's one.
Our estimates for the Jeans escape rates of N2-O2 atmospheres suggest that the 1-bar atmospheres survive in early active phases of Sun-like stars except for rapid rotators.
Even around rapidly rotating Sun-like stars and M dwarfs, the N2-O2 atmospheres of several bars could
\new{avoid a significant thermal loss} on timescales of Gyr.
Those results give new insights into the habitability of terrestrial exoplanets and the Earth's climate history.