The dynamics of a rapidly escaping atmosphere: Applications to the evolution of Earth and Venus

Watson, Andrew J.; Donahue, T. M.; Walker, James C. G.

doi:10.1016/0019-1035(81)90101-9

Cited by 554 publications

(674 citation statements)

References 39 publications

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“…This assumption is more or less valid for massive and compact exoplanets (Erkaev et al 2007), such as HD 189733b with an average density ρ ∼ 0.95 g cm −3 , but will yield less accurate mass-loss rates for less compact objects with lower average densities, such as HD 209458b with 0.37 g cm −3 . This effect was first recognized by Watson et al (1981) and was more recently investigated in detail by Erkaev et al (2013Erkaev et al ( , 2014. R XUV can exceed the planetary radius R p quite substantially for a planetary body with a low average density when its atmosphere is exposed to high XUV fluxes.…”

Section: Fig 2 Top Panelmentioning

confidence: 89%

Heating efficiency in hydrogen-dominated upper atmospheres

Shematovich

Ionov

Lämmer

2014

A&A

118

104

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Context. The heating efficiency η hν is defined as the ratio of the net local gas-heating rate to the rate of stellar radiative energy absorption. It plays an important role in thermal-escape processes from the upper atmospheres of planets that are exposed to stellar soft X-rays and extreme ultraviolet radiation (XUV). Aims. We model the thermal-escape-related heating efficiency η hν of the stellar XUV radiation in the hydrogen-dominated upper atmosphere of the extrasolar gas giant HD 209458b. The model result is then compared with previous thermal-hydrogen-escape studies, which assumed η hν values between 10-100%. Methods. The photolytic and electron impact processes in the thermosphere were studied by solving the kinetic Boltzmann equation and applying a Direct Simulation Monte Carlo model. We calculated the energy deposition rates of the stellar XUV flux and that of the accompanying primary photoelectrons that are caused by electron impact processes in the H 2 → H transition region in the upper atmosphere. Results. The heating by XUV radiation of hydrogen-dominated upper atmospheres does not reach higher values than 20% above the main thermosphere altitude, if the participation of photoelectron impact processes is included. Conclusions. Hydrogen-escape studies from exoplanets that assume η hν values that are ≥20% probably overestimate the thermal escape or mass-loss rates, while those who assumed values that are <20% produce more realistic atmospheric-escape rates.

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Section: Fig 2 Top Panelmentioning

confidence: 89%

Heating efficiency in hydrogen-dominated upper atmospheres

Shematovich

Ionov

Lämmer

2014

A&A

118

104

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“…An upper limit on this escape flux is provided by the energy limited flux (Watson et al 1981) approximated by…”

Section: Atmospheric Escapementioning

confidence: 99%

3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability, and habitability

Leconte

Forget

Charnay

et al. 2013

A&A

238

318

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The inner edge of the classical habitable zone is often defined by the critical flux needed to trigger the runaway greenhouse instability. This 1D notion of a critical flux, however, may not be all that relevant for inhomogeneously irradiated planets, or when the water content is limited (land planets). Based on results from our 3D global climate model, we present general features of the climate and large-scale circulation on close-in terrestrial planets. We find that the circulation pattern can shift from super-rotation to stellar/anti stellar circulation when the equatorial Rossby deformation radius significantly exceeds the planetary radius, changing the redistribution properties of the atmosphere. Using analytical and numerical arguments, we also demonstrate the presence of systematic biases among mean surface temperatures and among temperature profiles predicted from either 1D or 3D simulations. After including a complete modeling of the water cycle, we further demonstrate that two stable climate regimes can exist for land planets closer than the inner edge of the classical habitable zone. One is the classical runaway state where all the water is vaporized, and the other is a collapsed state where water is captured in permanent cold traps. We identify this "moist" bistability as the result of a competition between the greenhouse effect of water vapor and its condensation on the night side or near the poles, highlighting the dynamical nature of the runaway greenhouse effect. We also present synthetic spectra showing the observable signature of these two states. Taking the example of two prototype planets in this regime, namely Gl 581 c and HD 85512 b, we argue that depending on the rate of water delivery and atmospheric escape during the life of these planets, they could accumulate a significant amount of water ice at their surface. If such a thick ice cap is present, various physical mechanisms observed on Earth (e.g., gravity driven ice flows, geothermal flux) should come into play to produce long-lived liquid water at the edge and/or bottom of the ice cap. Consequently, the habitability of planets at smaller orbital distance than the inner edge of the classical habitable zone cannot be ruled out. Transiting planets in this regime represent promising targets for upcoming exoplanet characterization observatories, such as EChO and JWST.

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“…We use the atmospheric mass loss model described by Erkaev et al (2007), which improves the standard model by Watson et al (1981). In the Watson et al picture, a layer in the atmosphere exists where absorption of high-energy photons heats the particles to a temperature that permits Jeans escape.…”

Section: Appendix B the Desiccation Timescalementioning

confidence: 99%

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

et al. 2013

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Traditionally, stellar radiation has been the only heat source considered capable of determining global climate on long timescales. Here, we show that terrestrial exoplanets orbiting low-mass stars may be tidally heated at highenough levels to induce a runaway greenhouse for a long-enough duration for all the hydrogen to escape. Without hydrogen, the planet no longer has water and cannot support life. We call these planets ''Tidal Venuses'' and the phenomenon a ''tidal greenhouse.'' Tidal effects also circularize the orbit, which decreases tidal heating. Hence, some planets may form with large eccentricity, with its accompanying large tidal heating, and lose their water, but eventually settle into nearly circular orbits (i.e., with negligible tidal heating) in the habitable zone (HZ). However, these planets are not habitable, as past tidal heating desiccated them, and hence should not be ranked highly for detailed follow-up observations aimed at detecting biosignatures. We simulated the evolution of hypothetical planetary systems in a quasi-continuous parameter distribution and found that we could constrain the history of the system by statistical arguments. Planets orbiting stars with masses < 0.3 M Sun may be in danger of desiccation via tidal heating. We have applied these concepts to Gl 667C c, a *4.5 M Earth planet orbiting a 0.3 M Sun star at 0.12 AU. We found that it probably did not lose its water via tidal heating, as orbital stability is unlikely for the high eccentricities required for the tidal greenhouse. As the inner edge of the HZ is defined by the onset of a runaway or moist greenhouse powered by radiation, our results represent a fundamental revision to the HZ for noncircular orbits. In the appendices we review (a) the moist and runaway greenhouses, (b) hydrogen escape, (c) stellar mass-radius and mass-luminosity relations, (d) terrestrial planet mass-radius relations, and (e) linear tidal theories.

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The dynamics of a rapidly escaping atmosphere: Applications to the evolution of Earth and Venus

Cited by 554 publications

References 39 publications

Heating efficiency in hydrogen-dominated upper atmospheres

Heating efficiency in hydrogen-dominated upper atmospheres

3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability, and habitability

Tidal Venuses: Triggering a Climate Catastrophe via Tidal Heating

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