J. Yang, J. Leconte, E. T. Wolf, T. Merlis, D. D. B. Koll, F. Forget, and D. S. Abbot. Simulations of Water Vapor and Clouds on Rapidly Rotating and Tidally Locked Planets: A 3D Model Intercomparison. Astrophysical Journal, 875:46, 2019. [ bib | DOI | PDF version | ADS link ]

Robustly modeling the inner edge of the habitable zone is essential for determining the most promising potentially habitable exoplanets for atmospheric characterization. Global climate models (GCMs) have become the standard tool for calculating this boundary, but divergent results have emerged among the various GCMs. In this study, we perform an intercomparison of standard GCMs used in the field on a rapidly rotating planet receiving a G-star spectral energy distribution and on a tidally locked planet receiving an M-star spectral energy distribution. Experiments both with and without clouds are examined. We find relatively small difference (within 8 K) in global-mean surface temperature simulation among the models in the G-star case with clouds. In contrast, the global-mean surface temperature simulation in the M-star case is highly divergent (2030 K). Moreover, even differences in the simulated surface temperature when clouds are turned off are significant. These differences are caused by differences in cloud simulation and/or radiative transfer, as well as complex interactions between atmospheric dynamics and these two processes. For example we find that an increase in atmospheric absorption of shortwave radiation can lead to higher relative humidity at high altitudes globally and, therefore, a significant decrease in planetary radiation emitted to space. This study emphasizes the importance of basing conclusions about planetary climate on simulations from a variety of GCMs and motivates the eventual comparison of GCM results with terrestrial exoplanet observations to improve their performance.

M. Turbet, E. Bolmont, J. Leconte, F. Forget, F. Selsis, G. Tobie, A. Caldas, J. Naar, and M. Gillon. Modeling climate diversity, tidal dynamics and the fate of volatiles on TRAPPIST-1 planets. Astronomy Astrophysics, 612:A86, 2018. [ bib | DOI | arXiv | PDF version | ADS link ]

TRAPPIST-1 planets are invaluable for the study of comparative planetary science outside our solar system and possibly habitability. Both transit timing variations (TTV) of the planets and the compact, resonant architecture of the system suggest that TRAPPIST-1 planets could be endowed with various volatiles today. First, we derived from N-body simulations possible planetary evolution scenarios, and show that all the planets are likely in synchronous rotation. We then used a versatile 3D global climate model (GCM) to explore the possible climates of cool planets around cool stars, with a focus on the TRAPPIST-1 system. We investigated the conditions required for cool planets to prevent possible volatile species to be lost permanently by surface condensation, irreversible burying or photochemical destruction. We also explored the resilience of the same volatiles (when in condensed phase) to a runaway greenhouse process. We find that background atmospheres made of N₂, CO, or O₂ are rather resistant to atmospheric collapse. However, even if TRAPPIST-1 planets were able to sustain a thick background atmosphere by surviving early X/EUV radiation and stellar wind atmospheric erosion, it is difficult for them to accumulate significant greenhouse gases like CO₂, CH₄, or NH₃. CO₂ can easily condense on the permanent nightside, forming CO₂ ice glaciers that would flow toward the substellar region. A complete CO₂ ice surface cover is theoretically possible on TRAPPIST-1g and h only, but CO₂ ices should be gravitationally unstable and get buried beneath the water ice shell in geologically short timescales. Given TRAPPIST-1 planets large EUV irradiation (at least 10³ × Titan's flux), CH₄ and NH₃ are photodissociated rapidly and are thus hard to accumulate in the atmosphere. Photochemical hazes could then sedimentate and form a surface layer of tholins that would progressively thicken over the age of the TRAPPIST-1 system. Regarding habitability, we confirm that few bars of CO₂ would suffice to warm the surface of TRAPPIST-1f and g above the melting point of water. We also show that TRAPPIST-1e is a remarkable candidate for surface habitability. If the planet is today synchronous and abundant in water, then it should very likely sustain surface liquid water at least in the substellar region, whatever the atmosphere considered.

M. Turbet, F. Forget, J. Leconte, B. Charnay, and G. Tobie. CO₂ condensation is a serious limit to the deglaciation of Earth-like planets. Earth and Planetary Science Letters, 476:11-21, 2017. [ bib | DOI | arXiv | PDF version | ADS link ]

It is widely believed that the carbonate-silicate cycle is the main agent, through volcanism, to trigger deglaciations by CO₂ greenhouse warming on Earth and on Earth-like planets when they get in a frozen state. Here we use a 3D Global Climate Model to simulate the ability of planets initially completely frozen to escape from glaciation episodes by accumulating enough gaseous CO₂. The model includes CO₂ condensation and sublimation processes and the water cycle. We find that planets with Earth-like characteristics (size, mass, obliquity, rotation rate, etc.) orbiting a Sun-like star may never be able to escape from a glaciation era, if their orbital distance is greater than ~1.27 Astronomical Units (Flux 847 Wm^-2 or 62% of the Solar constant), because CO₂ would condense at the poles - here the cold traps - forming permanent CO₂ ice caps. This limits the amount of CO₂ in the atmosphere and thus its greenhouse effect. Furthermore, our results indicate that for (1) high rotation rates (P_rot 24 h), (2) low obliquity (obliquity 23.5deg), (3) low background gas partial pressures (1 bar), and (4) high water ice albedo (H₂O albedo 0.6), this critical limit could occur at a significantly lower equivalent distance (or higher insolation). For each possible configuration, we show that the amount of CO₂ that can be trapped in the polar caps depends on the efficiency of CO₂ ice to flow laterally as well as its gravitational stability relative to subsurface water ice. We find that a frozen Earth-like planet located at 1.30 AU of a Sun-like star could store as much as 1.5, 4.5 and 15 bars of dry ice at the poles, for internal heat fluxes of 100, 30 and 10 mW m^-2, respectively. But these amounts are in fact lower limits. For planets with a significant water ice cover, we show that CO₂ ice deposits should be gravitationally unstable. They get buried beneath the water ice cover in geologically short timescales of ~10⁴ yrs, mainly controlled by the viscosity of water ice. CO₂ would be permanently sequestered underneath the water ice cover, in the form of CO₂ liquids, CO₂ clathrate hydrates and/or dissolved in subglacial water reservoirs (if any). This would considerably increase the amount of CO₂ trapped and further reduce the probability of deglaciation.

F. Forget, T. Bertrand, M. Vangvichith, J. Leconte, E. Millour, and E. Lellouch. A post-new horizons global climate model of Pluto including the N₂, CH₄ and CO cycles. Icarus, 287:54-71, 2017. [ bib | DOI | PDF version | ADS link ]

We have built a new 3D Global Climate Model (GCM) to simulate Pluto as observed by New Horizons in 2015. All key processes are parametrized on the basis of theoretical equations, including atmospheric dynamics and transport, turbulence, radiative transfer, molecular conduction, as well as phases changes for N₂, CH₂ and CO. Pluto's climate and ice cycles are found to be very sensitive to model parameters and initial states. Nevertheless, a reference simulation is designed by running a fast, reduced version of the GCM with simplified atmospheric transport for 40,000 Earth years to initialize the surface ice distribution and sub-surface temperatures, from which a 28-Earth-year full GCM simulation is performed. Assuming a topographic depression in a Sputnik-planum (SP)-like crater on the anti-Charon hemisphere, a realistic Pluto is obtained, with most N₂ and CO ices accumulated in the crater, methane frost covering both hemispheres except for the equatorial regions, and a surface pressure near 1.1 Pa in 2015 with an increase between 1988 and 2015, as reported from stellar occultations. Temperature profiles are in qualitative agreement with the observations. In particular, a cold atmospheric layer is obtained in the lowest kilometers above Sputnik Planum, as observed by New Horizons's REX experiment. It is shown to result from the combined effect of the topographic depression and N₂ daytime sublimation. In the reference simulation with surface N₂ ice exclusively present in Sputnik Planum, the global circulation is only forced by radiative heating gradients and remains relatively weak. Surface winds are locally induced by topography slopes and by N₂ condensation and sublimation around Sputnik Planum. However, the circulation can be more intense depending on the exact distribution of surface N₂ frost. This is illustrated in an alternative simulation with N₂ condensing in the South Polar regions and N₂ frost covering latitudes between 35degN and 48degN. A global condensation flow is then created, inducing strong surface winds everywhere, a prograde jet in the southern high latitudes, and an equatorial superrotation likely forced by barotropic instabilities in the southern jet. Using realistic parameters, the GCM predict atmospheric concentrations of CO and CH₄ in good agreement with the observations. N₂ and CO do not condense in the atmosphere, but CH₄ ice clouds can form during daytime at low altitude near the regions covered by N₂ ice (assuming that nucleation is efficient enough). This global climate model can be used to study many aspects of the Pluto environment. For instance, organic hazes are included in the GCM and analysed in a companion paper (Bertrand and Forget, Icarus, this issue).

M. Turbet, J. Leconte, F. Selsis, E. Bolmont, F. Forget, I. Ribas, S. N. Raymond, and G. Anglada-Escudé. The habitability of Proxima Centauri b. II. Possible climates and observability. Astronomy Astrophysics, 596:A112, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]

Radial velocity monitoring has found the signature of a Msini = 1.3M_⊕ planet located within the habitable zone (HZ) of Proxima Centauri. Despite a hotter past and an active host star, the planet Proxima b could have retained enough volatiles to sustain surface habitability. Here we use a 3D Global Climate Model (GCM) to simulate the atmosphere and water cycle of Proxima b for its two likely rotation modes (1:1 and 3:2 spin-orbit resonances), while varying the unconstrained surface water inventory and atmospheric greenhouse effect. Any low-obliquity, low-eccentricity planet within the HZ of its star should be in one of the climate regimes discussed here. We find that a broad range of atmospheric compositions allow surface liquid water. On a tidally locked planet with sufficient surface water inventory, liquid water is always present, at least in the substellar region. With a non-synchronous rotation, this requires a minimum greenhouse warming ( 10 mbar of CO₂ and 1 bar of N₂). If the planet is dryer, 0.5 bar or 1.5 bars of CO₂ (for asynchronous or synchronous rotation, respectively) suffice to prevent the trapping of any arbitrary, small water inventory into polar or nightside ice caps. We produce reflection and emission spectra and phase curves for the simulated climates. We find that atmospheric characterization will be possible via direct imaging with forthcoming large telescopes. The angular separation of 7λ/D at 1 μm (with the E-ELT) and a contrast of 10^-7 will enable high-resolution spectroscopy and the search for molecular signatures, including H₂O, O₂, and CO₂. The observation of thermal phase curves can be attempted with the James Webb Space Telescope, thanks to a contrast of 2 × 10^-5 at 10 μm. Proxima b will also be an exceptional target for future IR interferometers. Within a decade it will be possible to image Proxima b and possibly determine whether the surface of this exoplanet is habitable.

J. Yang, J. Leconte, E. T. Wolf, C. Goldblatt, N. Feldl, T. Merlis, Y. Wang, D. D. B. Koll, F. Ding, F. Forget, and D. S. Abbot. Differences in Water Vapor Radiative Transfer among 1D Models Can Significantly Affect the Inner Edge of the Habitable Zone. Astrophysical Journal, 826:222, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]

An accurate estimate of the inner edge of the habitable zone is critical for determining which exoplanets are potentially habitable and for designing future telescopes to observe them. Here, we explore differences in estimating the inner edge among seven one-dimensional radiative transfer models: two line-by-line codes (SMART and LBLRTM) as well as five band codes (CAM3, CAM4_Wolf, LMDG, SBDART, and AM2) that are currently being used in global climate models. We compare radiative fluxes and spectra in clear-sky conditions around G and M stars, with fixed moist adiabatic profiles for surface temperatures from 250 to 360 K. We find that divergences among the models arise mainly from large uncertainties in water vapor absorption in the window region (10 μm) and in the region between 0.2 and 1.5 μm. Differences in outgoing longwave radiation increase with surface temperature and reach 10-20 W m^-2 differences in shortwave reach up to 60 W m^-2, especially at the surface and in the troposphere, and are larger for an M-dwarf spectrum than a solar spectrum. Differences between the two line-by-line models are significant, although smaller than among the band models. Our results imply that the uncertainty in estimating the insolation threshold of the inner edge (the runaway greenhouse limit) due only to clear-sky radiative transfer is 10% of modern Earths solar constant (I.e., 34 W m^-2 in global mean) among band models and 3% between the two line-by-line models. These comparisons show that future work is needed that focuses on improving water vapor absorption coefficients in both shortwave and longwave, as well as on increasing the resolution of stellar spectra in broadband models.

E. Bolmont, A.-S. Libert, J. Leconte, and F. Selsis. Habitability of planets on eccentric orbits: Limits of the mean flux approximation. Astronomy Astrophysics, 591:A106, 2016. [ bib | DOI | arXiv | PDF version | ADS link ]

Unlike the Earth, which has a small orbital eccentricity, some exoplanets discovered in the insolation habitable zone (HZ) have high orbital eccentricities (e.g., up to an eccentricity of ˜0.97 for HD 20782 b). This raises the question of whether these planets have surface conditions favorable to liquid water. In order to assess the habitability of an eccentric planet, the mean flux approximation is often used. It states that a planet on an eccentric orbit is called habitable if it receives on average a flux compatible with the presence of surface liquid water. However, because the planets experience important insolation variations over one orbit and even spend some time outside the HZ for high eccentricities, the question of their habitability might not be as straightforward. We performed a set of simulations using the global climate model LMDZ to explore the limits of the mean flux approximation when varying the luminosity of the host star and the eccentricity of the planet. We computed the climate of tidally locked ocean covered planets with orbital eccentricity from 0 to 0.9 receiving a mean flux equal to Earth's. These planets are found around stars of luminosity ranging from 1 L_&sun; to 10^-4L_&sun;. We use a definition of habitability based on the presence of surface liquid water, and find that most of the planets considered can sustain surface liquid water on the dayside with an ice cap on the nightside. However, for high eccentricity and high luminosity, planets cannot sustain surface liquid water during the whole orbital period. They completely freeze at apoastron and when approaching periastron an ocean appears around the substellar point. We conclude that the higher the eccentricity and the higher the luminosity of the star, the less reliable the mean flux approximation.

S. Guerlet, A. Spiga, M. Sylvestre, M. Indurain, T. Fouchet, J. Leconte, E. Millour, R. Wordsworth, M. Capderou, B. Bézard, and F. Forget. Global climate modeling of Saturns atmosphere. Part I: Evaluation of the radiative transfer model. Icarus, 238:110-124, 2014. [ bib | DOI | PDF version | ADS link ]

We have developed and optimized a seasonal, radiative-convective model of Saturns upper troposphere and stratosphere. It is used to investigate Saturns radiatively-forced thermal structure between 3 and 10^-6 bar, and is intended to be included in a Saturn global climate model (GCM), currently under development. The main elements of the radiative transfer model are detailed as well as the sensitivity to spectroscopic parameters, hydrocarbon abundances, aerosol properties, oblateness, and ring shadowing effects. The vertical temperature structure and meridional seasonal contrasts obtained by the model are then compared to Cassini/CIRS observations. Several significant model-observation mismatches reveal that Saturns atmosphere departs from radiative equilibrium. For instance, we find that the modeled temperature profile is close to isothermal above the 2-mbar level, while the temperature retrieved from ground-based or Cassini/CIRS data continues to increase with altitude. Also, no local temperature minimum associated to the ring shadowing is observed in the data, while the model predicts stratospheric temperatures 10 K to 20 K cooler than in the absence of rings at winter tropical latitudes. These anomalies are strong evidence that processes other that radiative heating and cooling control Saturns stratospheric thermal structure. Finally, the model is used to study the warm stratospheric anomaly triggered after the 2010 Great White Spot. Comparison with recent Cassini/CIRS observations suggests that the rapid cooling phase of this warm beacon in May-June 2011 can be explained by radiative processes alone. Observations on a longer timeline are needed to better characterize and understand its long-term evolution.

F. Forget and J. Leconte. Possible climates on terrestrial exoplanets. Philosophical Transactions of the Royal Society of London Series A, 372:20130084-20130084, 2014. [ bib | DOI | arXiv | PDF version | ADS link ]

What kind of environment may exist on terrestrial planets around other stars? In spite of the lack of direct observations, it may not be premature to speculate on exoplanetary climates, for instance to optimize future telescopic observations, or to assess the probability of habitable worlds. To first order, climate primarily depends on 1) The atmospheric composition and the volatile inventory; 2) The incident stellar flux; 3) The tidal evolution of the planetary spin, which can notably lock a planet with a permanent night side. The atmospheric composition and mass depends on complex processes which are difficult to model: origins of volatile, atmospheric escape, geochemistry, photochemistry. We discuss physical constraints which can help us to speculate on the possible type of atmosphere, depending on the planet size, its final distance for its star and the star type. Assuming that the atmosphere is known, the possible climates can be explored using Global Climate Models analogous to the ones developed to simulate the Earth as well as the other telluric atmospheres in the solar system. Our experience with Mars, Titan and Venus suggests that realistic climate simulators can be developed by combining components like a “dynamical core”, a radiative transfer solver, a parametrisation of subgrid-scale turbulence and convection, a thermal ground model, and a volatile phase change code. On this basis, we can aspire to build reliable climate predictors for exoplanets. However, whatever the accuracy of the models, predicting the actual climate regime on a specific planet will remain challenging because climate systems are affected by strong positive destabilizing feedbacks (such as runaway glaciations and runaway greenhouse effect). They can drive planets with very similar forcing and volatile inventory to completely different states.

J. Leconte, F. Forget, B. Charnay, R. Wordsworth, and A. Pottier. Increased insolation threshold for runaway greenhouse processes on Earth-like planets. Nature, 504:268-271, 2013. [ bib | DOI | arXiv | PDF version | ADS link ]

The increase in solar luminosity over geological timescales should warm the Earth's climate, increasing water evaporation, which will in turn enhance the atmospheric greenhouse effect. Above a certain critical insolation, this destabilizing greenhouse feedback can `run away' until the oceans have completely evaporated. Through increases in stratospheric humidity, warming may also cause evaporative loss of the oceans to space before the runaway greenhouse state occurs. The critical insolation thresholds for these processes, however, remain uncertain because they have so far been evaluated using one-dimensional models that cannot account for the dynamical and cloud feedback effects that are key stabilizing features of the Earth's climate. Here we use a three-dimensional global climate model to show that the insolation threshold for the runaway greenhouse state to occur is about 375 W m^-2, which is significantly higher than previously thought. Our model is specifically developed to quantify the climate response of Earth-like planets to increased insolation in hot and extremely moist atmospheres. In contrast with previous studies, we find that clouds have a destabilizing feedback effect on the long-term warming. However, subsident, unsaturated regions created by the Hadley circulation have a stabilizing effect that is strong enough to shift the runaway greenhouse limit to higher values of insolation than are inferred from one-dimensional models. Furthermore, because of wavelength-dependent radiative effects, the stratosphere remains sufficiently cold and dry to hamper the escape of atmospheric water, even at large fluxes. This has strong implications for the possibility of liquid water existing on Venus early in its history, and extends the size of the habitable zone around other stars.

B. Charnay, F. Forget, R. Wordsworth, J. Leconte, E. Millour, F. Codron, and A. Spiga. Exploring the faint young Sun problem and the possible climates of the Archean Earth with a 3-D GCM. Journal of Geophysical Research (Atmospheres), 118:10, 2013. [ bib | DOI | arXiv | PDF version | ADS link ]

Different solutions have been proposed to solve the “faint young Sun problem,” defined by the fact that the Earth was not fully frozen during the Archean despite the fainter Sun. Most previous studies were performed with simple 1-D radiative convective models and did not account well for the clouds and ice-albedo feedback or the atmospheric and oceanic transport of energy. We apply a global climate model (GCM) to test the different solutions to the faint young Sun problem. We explore the effect of greenhouse gases (CO₂ and CH₄), atmospheric pressure, cloud droplet size, land distribution, and Earth's rotation rate. We show that neglecting organic haze, 100 mbar of CO₂ with 2 mbar of CH₄ at 3.8 Ga and 10 mbar of CO₂ with 2 mbar of CH₄ at 2.5 Ga allow a temperate climate (mean surface temperature between 10degC and 20degC). Such amounts of greenhouse gases remain consistent with the geological data. Removing continents produces a warming lower than +4degC. The effect of rotation rate is even more limited. Larger droplets (radii of 17 μm versus 12 μm) and a doubling of the atmospheric pressure produce a similar warming of around +7degC. In our model, ice-free water belts can be maintained up to 25degN/S with less than 1 mbar of CO₂ and no methane. An interesting cloud feedback appears above cold oceans, stopping the glaciation. Such a resistance against full glaciation tends to strongly mitigate the faint young Sun problem.

J. Leconte, F. Forget, B. Charnay, R. Wordsworth, F. Selsis, E. Millour, and A. Spiga. 3D climate modeling of close-in land planets: Circulation patterns, climate moist bistability, and habitability. Astronomy Astrophysics, 554:A69, 2013. [ bib | DOI | arXiv | PDF version | ADS link ]

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.