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3D Climate Modeling of Close-In Land Planets: Circulation Patterns, Climate Moist Bistability, and Habitability

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3D Climate Modeling of Close-In Land Planets: Circulation Patterns, Climate Moist Bistability, and Habitability A&A 554, A69 (2013) Astronomy DOI: 10.1051/0004-6361/201321042 & c ESO 2013 Astrophysics 3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability, and habitability J. Leconte1, F. Forget1, B. Charnay1, R. Wordsworth2, F. Selsis3;4, E. Millour1, and A. Spiga1 1 LMD, Institut Pierre-Simon Laplace, Université P. et M. Curie, BP99, 75005 Paris, France e-mail: [email protected] 2 Department of Geological Sciences, University of Chicago, 5734 S Ellis Avenue, Chicago, IL 60622, USA 3 Université de Bordeaux, Observatoire Aquitain des Sciences de l’Univers, BP 89, 33271 Floirac Cedex, France 4 CNRS, UMR 5804, Laboratoire d’Astrophysique de Bordeaux, BP 89, 33271 Floirac Cedex, France Received 4 January 2013 / Accepted 27 March 2013 ABSTRACT 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 redistribu- tion 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. Key words. planets and satellites: general – planets and satellites: atmospheres – planets and satellites: physical evolution – planet-star interactions 1. Beyond the runaway greenhouse limit equilibrium balance and is heated until the surface can radiate at optical wavelength around a temperature of 1400 K (Kasting Because of the bias of planet characterization observatories et al. 1984; Kasting 1988). All the water at the surface is va- toward strongly irradiated objects on short orbits, planets identi- porized. The planet is in a runaway greenhouse state. Roughly fied as near the inner edge of the habitable zone represent valu- speaking, this radiation limit stems from the fact that the surface able targets. This does, however, raise numerous questions con- must increase its temperature to radiate more, but the amount of cerning our understanding of the processes delimiting this inner water vapor in the troposphere, hence the opacity of the latter, edge. increases exponentially with this surface temperature (because For a planet like the Earth, which from the point of view of 1 of the Clausius-Clapeyron relation). Then the thermal emission climate can be considered as an aqua planet , a key constraint of the opaque atmosphere is emitted at the level at which the op- is provided by the positive feedback of water vapor on climate. tical depth is close to unity, a level whose temperature reaches an When the insolation of the planet is increased, more water vapor asymptotic value, hence the limited flux2 (Nakajima et al. 1992; is released in the atmosphere by the hotter surface, increasing the Pierrehumbert 2010). greenhouse effect. If on Earth today this effect is balanced by While large uncertainties remain regarding the radiative ef- other processes, in particular the increase in infrared emission fect of water vapor continuum and the nonlinear, 3D effect of by the surface with temperature, it has been shown that above dynamics and clouds, the critical value of the average absorbed a critical absorbed stellar flux, the planet cannot reach radiative 1 i.e. a planet whose surface is largely covered by connected oceans 2 A similar but slightly subtler limit stems from considering the radia- that have a planet-wide impact on the climate. Not to be confused with tive/thermodynamic equilibrium of the stratosphere (Kombayashi 1967; an ocean planet for which, in addition, water must represent a significant Ingersoll 1969; Nakajima et al. 1992), but it is less constraining in the fraction of the bulk mass (Léger et al. 2004). case considered. Article published by EDP Sciences A69, page 1 of 17 A&A 554, A69 (2013) stellar radiation, (1 − A¯)F?=4 (where F? is the stellar flux at the have performed simulations for two already detected planets. substellar point and A¯ is the planetary bond albedo), should con- We show that qualitatively robust features appear in our simu- servatively range between 240 W/m2 (the mean absorbed stellar lations and argue that the processes governing the climate dis- radiation on Earth) and 350 W/m2 (Abe et al. 2011). Terrestrial cussed here are quite general. planets emitting such (low) IR fluxes will be difficult to charac- In Sect.2 we introduce the various components of our nu- terize in the near future. merical climate model and present the basic climatology of our Fortunately, the runaway greenhouse limit discussed above model in a completely dry case (Sect.3). We show that the at- is not as fundamental as it may appear. Indeed, the theory out- mospheric circulation can settle into two different regimes – lined above assumes that a sufficient3 reservoir of liquid water is namely, super-rotation or stellar/anti-stellar circulation – and available everywhere on the surface (Selsis et al. 2007). While that the transition of one regime to the next occurs when the it would seem from 1D modeling that liquid water would disap- equatorial Rossby deformation radius significantly exceeds the pear at a lower surface temperature for a planet with a limited planetary radius. We also show that strong discrepancies can oc- water inventory, it goes the opposite way in 3D. Global climate cur between 1D and globally averaged 3D results. Then we show models show that such “land planets” can have a wider habitable that when water is present, the climate regime reached depends zone (Abe et al. 2011). Near the inner edge, this property stems on the initial conditions and can exhibit a bistability (Sect.4). from the fact that atmospheric transport of water from warm to We argue that this bistability is a dynamical manifestation of the cold regions of the planets is not counteracted by large-scale sur- interplay between the runaway greenhouse instability on the hot face runoff (Abe et al. 2005). Heavily irradiated regions are then hemisphere and the cold trapping arising either on the dark side drier and can emit more IR flux than the limit found for a satu- or near the poles. In Sect.5, we discuss the long-term stability of rated atmosphere. the different regimes found here and show that, if atmospheric In principle, it is thus possible to find planets that absorb and escape is efficient, it could provide a stabilizing feedback for re-emit more flux than the runaway greenhouse threshold and the climate. In Sect.6, we suggest that if the planet has been for which liquid or solid water can be thermodynamically stable able to accumulate a large amount of ice in its cold traps such at the surface, provided that they have accreted limited water that ice draining by glaciers is significant, liquid water could be supplies (or lost most of them). As shown hereafter, this could present at the edge of the ice cap where glaciers melt. We also be the case for Gl 581 c (Udry et al. 2007; Mayor et al. 2009; discuss the possible presence of subsurface water under a thick Forveille et al. 2011) and HD 85512 b (Pepe et al. 2011), the ice cap. Finally, in Sect.7 we present synthetic spectra and dis- only two confirmed planets having masses that are compatible cuss the observable signatures of the two climate regimes dis- with a terrestrial planet (M p ∼< 7−8 M⊕; although defining such cussed above. a mass limit is questionable) and that lie beyond the inner edge of the classical habitable zone but receive a moderate stellar flux, 2 F?=4 < 1000 W/m . These planets lie even closer to the star that 2. Method the inner edge of the limits set by Abe et al.(2011) for land planets. Our simulations were performed with an upgraded version of However, for those close-in planets whose rotation states the LMD generic global climate model (GCM) specifically de- have been strongly affected by tidal dissipation and which are veloped for the study of extrasolar planets (Wordsworth et al.
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