Habitability Franois Forget Institut PierreSimon Laplace LMD CNRS

Habitability François Forget, Institut Pierre-Simon Laplace LMD, CNRS, France

What’s needed for Life ? Liquid water & « food » • Indeed life without liquid water is – difficult to imagine – difficult to recognize and detect In this talk : life = liquid water …

4 kinds of « habitability » (Lammer et al. Astron Astrophys Rev 2009) • Class I: Planets with permanent surface liquid water: like Earth • Class II : Planet temporally able to sustain surface liquid water but which lose this ability (loss of atmosphere, loss of water, wrong greenhouse effect) : Early Mars, early Venus ? • Class III : Bodies with subsurface ocean which interact with silicate mantle (Europa) • Class IV : Bodies with subsurface ocean between two ice layers (Ganymede)

The habitable zone (Kasting et al. 1993) 100% vapour Liquid water Climate instability at the Inner edge Solar flux ↑ Greenhouse effect Temperature ↑ ↑ Evaporation ↑ 100% ice

Impact of temperature increase on water vapor distribution and escape H escape, water lost to space Altitude EUV radiation Photodissociation : H 2 O + hν → OH +H Temperature

Inner Edge of the Habitable zone Kasting et al. 1 D radiative convective model; no clouds See also poster by Stracke et al. this week Water loss limit Runaway greenhouse limit H 2 O critical point of water reached at Ps=220 bar, 647 K protection by clouds: Can reach 0. 5 UA assuming 100% cloud cover with albedo =0. 8 ?

The habitable zone (Kasting et al. 1993) 100% vapour Liquid water 100% ice Climate instability at the Outer edge Solar flux Climate model with current Earth atmosphere: Global Glaciation beyond 101% à 110 % of distance Earth - Sun ! ↑ Albedo Temperature ↑ ↓ Ice and snow ↑

HOWEVER : Earth remained habitable in spite of faint sun : • Greenhouse effect can play a role (if enough atmosphere) • Geophysical cycles like the « Carbonate-Silicate » cycle (Earth) can stabilize the climate May require : - Plate tectonic - Life ? ? Walker et al. (1981) Kasting et al. 1993: The outer edge of the habitable zone: where greenhouse effect (CO 2, CO 2 + CO 2 ice clouds, greenhouse gas cocktail…) can maintain a suitable climate Ts ↓ Ts ↑ water cycle ↓ Greenhouse effect ↑ weathering ↓ PCO 2 ↑

The classical habitable zone (Kasting et al. 1993, Forget and Pierrehumbert 1997)

Habitable zone with no greenhouse effect ?

Is plate tectonic likely on other terrestrial planets ? By default, planets could have a single « stagnant lid » lithosphere and no efficient surface recycling process. To enable plate tectonics one need : • Mantle Convective stress > lithospheric resistance lithospheric failure (Lithosphere) • Plate denser (e. g. cold) than asthenosphere, enough to drive subduction (Lithosphere)

Is plate tectonic likely on other terrestrial planets ? • On small planets (e. g. Mars) : rapid interior cooling : weak convection stress, thick lithosphere no long term plate tectonic • On large planets (e. g. super-Earth) : different views : – To first order : More vigorous convection stronger convective stress & thinner lithosphere (e. g. Valencia et al. 2007) – However, some models predict that the increase in mantle depth mitigate the convective stress (O’Neil and Lenardic, 2007): « supersized Earth are likely to be in an episodic or stagnant lid regime » – Moreover, In super-Earth, very high pressure increase the viscosity near the core-mantle boundary, creating a « low lid » reducing convection, primarily increasing the plate thickness and thus « reducing the ability of plate tectonics on super-Earth» (Stamenkovic, Noack, Breuer, EPSC, 2009, see also Tackley, P. J. ; van Heck, H. J. AGU 08). Earth size may be actually just right for plate tectonics ! So what about Venus ? ?

Earthsized planet: R=1. 07 R=1. 1 O’Neil and Lenardic, 2007 Model

Is plate tectonic likely on other terrestrial planets ? • On small planets (e. g. Mars) : rapid interior cooling : weak convection stress, thick lithosphere no long term plate tectonic • On large planets (e. g. super-Earth) : different views : – To first order : More vigorous convection stronger convective stress & thinner lithosphere (e. g. Valencia et al. 2007) – However, some models predict that the increase in mantle depth mitigate the convective stress (O’Neil and Lenardic, 2007): « supersized Earth are likely to be in an episodic or stagnant lid regime » – Moreover, In super-Earth, very high pressure increase the viscosity near the core-mantle boundary, creating a « low lid » reducing convection, primarily increasing the plate thickness and thus « reducing the ability of plate tectonics on super-Earth» (Stamenkovic, Noack, Breuer, EPSC, 2009, see also Tackley, P. J. ; van Heck, H. J. AGU 08). Earth size may be actually just right for plate tectonics ! So what about Venus ? ?

Why is there no plate tectonic on Venus ? Venus : Ø 12100 km Earth : Ø 12750 km • High surface temperature prevent plate subduction ? Not likely (Van Thienen et al. 2004) • More likely : Venus mantle drier than Earth (e. g. Nimmo and Mc. Kenzie) Higher viscosity mantle Thicker lithosphere Does tectonic requires a « wet » mantle ? Speculation : if the presence of water in the Earth mantle results from the moon forming impact, is such an impact necessary for plate tectonic ?

From Global scale habitability to local/seasonal habitability • Study on habitability have mostly been performed with simple 1 D steady state radiative convective models. • 3 D time-marching models can help better understand : – Cloud distribution and impact (key to inner and outer edge of the habitable zone). – Transport of energy by the atmosphere and possible oceans – Local (latitude, topography) effects – Seasonal and diurnal effects…

One example: Gliese 581 d (see poster by Robin Wordsworth) • • Gliese 581 D : a super Earth at 0. 22 AU from M star Gl 581, at the edge of the habitable zone. Excentric orbit (e=0. 38) + low rotation rate (tidal locking, resonnance 2/1 ou 5/2) What can be the climate on such a planet with, say 2 bars of CO 2 ? With a 1 D model : mean Tsurf < 240 K Franck Selsis et al. (Astronomy and Astrophysics, 2007)

A Global Climate Model for a terrestrial planet 1) 3 D Hydrodynamical code to compute large scale atmospheric motions and transport 2) At every grid point : Physical parameterizations to force the dynamic to compute the details of the local climate • • • Radiative heating & cooling of the atmosphere Surface thermal balance Subgrid scale atmospheric motions Turbulence in the boundary layer Convection Relief drag Gravity wave drag • Specific process : ice condensation, cloud microphysics, etc…

Tidal locked Gliese 581 d (see poster by Robin Wordsworth)

Gliese 581 d (resonnance 2/1) (see poster by Robin Wordsworth)

Gliese 581 d (resonnance 2/1) (see poster by Robin Wordsworth) Annual mean Surface temperature (K)

Another example at the edge of the habitable zone: Early Mars • Early Mars was episodically habitable in spite of faint sun. – Typical 1 D results for a pure CO 2 atmosphere, no clouds: – → Global Annual mean temperatures : – CO 2 pressure Temperature 0. 006 bar -72ºC 0. 1 bar -61ºC 0. 5 bar -50ºC 2. 0 bar -41ºC Remnant of a River delta on Mars

GCM 3 D simulation of early Mars (faint sun, 2 bars of CO 2 Map of annual mean temperature (°C) CO 2 ice cloud opacity Atmospheric mean temperature (K) 0°C CO 2 ice clouds

The meaning of local surface temperature and liquid water : (assuming pressure >> triple point of water) • Local Annual mean temperature > 0°C Deep ocean, lakes, rivers are possible • Summer Diurnal mean temperature > 0°C Rivers, lakes are possible and flow in summer, but you get permafrost in the subsurface. • Maximum temperature > 0°C (e. g. summer afternoon temperature): Limited melting of glacier. Possible formation of ice covered lake though latent heat transport ? Examples of annual mean temperatures on the Earth: Fairbanks (AK) : -3ºC Barrow (AK) : -12ºC Antarctica Dry Valley : -15ºC – -30ºC

Testing Universal equations-based Global climate models in the solar system : it works ! MARS VENUS ~2 true GCMs Coupling dynamic & radiative transfer (LMD, Kyushu/Tokyo university) TERRE Many GCM teams Applications: • Weather forecast • Assimilation and climatology • Climate projections • Paleooclimates • chemistry • Biosphere / hydrosphere cryosphere / oceans coupling • Many other applications Several GCMs (NASA Ames, Caltech, GFDL, LMD, AOPP, MPS, Japan, York U. , Japan, etc…) Applications: • Dynamics & assimilation • CO 2 cycle • dust cycle • water cycle • Photochemistry • thermosphere and ionosphere • isotopes cycles • paleoclimates • etc… TITAN ~a few GCMs (LMD, Univ. Od Chicago, Caltech, Köln…) Coupled cycles: • Aerosols • Photochemistry • Clouds

Toward a « universal climate model » : A model designed to predict climate on a given planet around a given star with a given atmosphere… • The key of the project : a semi automatic «chain of production » of radiative transfer code suitable for GCMs, for any mixture of gases and aerosols. • Robust dynamical core • Boundary Layer model, • convection parametrization, • simplified oceans, • etc… Contact in our team: Robin Wordsworth, Ehouarn Millour, F. Forget (LMD) F. Selsis (Obs. Bordeaux)

Conclusions: Extrasolar planet habitability. We have no observable yet , but many scientific questions to adress • Habitability depends on plate tectonic (and sometime magnetic field) more modelling of planet internal dynamic work required • 3 D climate modelling should allow « realistic » prediction of climate conditions with a minimum of assumptions. The major difficulty : how can we generalize our experience in geophysics based on a planet which « works » so well ?