Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

Atmospheric escape is crucial to understanding the evolution of planets in and out of the solar system and to interpreting atmospheric observations. While hydrodynamic escape simulations have been actively developed incorporating detailed processes such as UV heating, chemical reactions, and radiative cooling, the radiative cooling by molecules has been treated as emission from selected lines or rotational/vibrational bands to reduce its numerical cost. However, ad hoc selections of radiative lines would risk estimating inaccurate cooling rates because important lines or wavelengths for atmospheric cooling depend on emitting conditions such as temperature and optical thickness. In this study, we apply the correlated-k distribution (CKD) method to cooling rate calculations for H₂-dominant transonic atmospheres containing H₂O or CO as radiative species, to investigate its numerical performance and the importance of considering all lines of the molecules. Our simulations demonstrate that the sum of weak lines, which provides only 1% of the line emission energy in total at optically thin conditions, can become the primary source of radiative cooling in optically thick regions, especially for H₂O-containing atmospheres. Also, in our hydrodynamic simulations, the CKD method with a wavelength resolution of 1000 is found to be effective, allowing the calculation of escape rate and temperature profiles with acceptable numerical cost. Our results show the importance of treating all radiative lines and the usefulness of the CKD method in hydrodynamic escape simulations. It is particularly practical for heavy-element-enriched atmospheres considered in small exoplanets, including super-Earths, without any prior selections for effective lines.

DOI： 10.3847/1538-4357/ad187f

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Other Link： https://iopscience.iop.org/article/10.3847/1538-4357/ad187f/pdf

Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

Abstract

Atmospheres play a crucial role in planetary habitability. 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. 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 late M dwarfs, N₂–O₂ atmospheres could escape significant thermal loss on timescales of gigayears. These results give new insights into the habitability of terrestrial exoplanets and the Earth’s climate history.

DOI： 10.3847/1538-4357/ac86ca

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Other Link： https://iopscience.iop.org/article/10.3847/1538-4357/ac86ca/pdf

Publishing type：Research paper (scientific journal)   Publisher：American Astronomical Society  

DOI： 10.3847/1538-3881/ac0fdc

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Other Link： https://iopscience.iop.org/article/10.3847/1538-3881/ac0fdc/pdf

Publishing type：Research paper (scientific journal)   Publisher：Oxford University Press (OUP)  

Terrestrial planets covered globally with thick oceans (termed ocean planets)
in the habitable zone were previously inferred to have extremely hot climates
in most cases. This is because ${\rm H_2O}$ high-pressure (HP) ice on the
seafloor prevents chemical weathering and, thus, removal of atmospheric CO$_2$.
Previous studies, however, ignored melting of the HP ice and horizontal
variation in heat flux from oceanic crusts. Here we examine whether high heat
fluxes near the mid-ocean ridge melts the HP ice and thereby removes
atmospheric ${\rm CO_2}$. We develop integrated climate models of an Earth-size
ocean planet with plate tectonics for different ocean masses, which include the
effects of HP ice melting, seafloor weathering, and the carbonate-silicate
geochemical carbon cycle. We find that the heat flux near the mid-ocean ridge
is high enough to melt the ice, enabling seafloor weathering. In contrast to
the previous theoretical prediction, we show that climates of terrestrial
planets with massive oceans lapse into extremely cold ones (or snowball states)
with CO$_2$-poor atmospheres. Such extremely cold climates are achieved mainly
because the HP ice melting fixes seafloor temperature at the melting
temperature, thereby keeping a high weathering flux regardless of surface
temperature. We estimate that ocean planets with oceans several tens of the
Earth's ocean mass no longer maintain temperate climates. These results suggest
that terrestrial planets with extremely cold climates exist even in the
habitable zone beyond the solar system, given the frequency of water-rich
planets predicted by planet formation theories.

DOI： 10.1093/mnras/stz1812

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Other Link： http://arxiv.org/pdf/1907.00827v3