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 Like every talk about ability you have to go though the same and necessary questions.. Conversely on Earth, everywhere there is liquid water, there is life. 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) Mentionner No photosynthesis => life not very active, not easy t o detect (no dead fish) Class IV is for cold bodies but which are so rich in water ice that if there is an ocean trapped between the classical surface ice and a layer of ice which form at high pressure even at high temperature. That would be the case for the famous ocean planet and in the solar system. In that case you have a liquid water habitat without source of energy, and without … (à compléter) Today : I am going to talk about the classical habitable zone defined by the presence of liquid water at the surface.

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

Impact of temperature increase on water vapor distribution and escape H escape, water lost to space EUV radiation Altitude Photodissociation : H2O + hν → OH +H Temperature

Inner Edge of the Habitable zone Kasting et al. 1D radiative convective model; no clouds See also poster by Stracke et al. this week Water loss limit Runaway greenhouse limit H2O critical point of water reached at Ps=220 bar, 647K 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↑ Temperature ↓ Albedo↑ Ice and snow ↑ Climate model with current Earth atmosphere:  Global Glaciation beyond 101% à 110 % of distance Earth - Sun !

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 (CO2, CO2 + CO2 ice clouds, greenhouse gas cocktail…) can maintain a suitable climate Ts ↓ water cycle ↓ weathering ↓ Ts ↑ Greenhouse effect ↑ PCO2 ↑

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 Plate denser (e.g. cold) than asthenosphere, enough to drive subduction (Lithosphere) Mars : rapid interior cooling => thick lithosphere / weak convection. The origin of plate tectonics is poorly understood for the Earth, likely involving a complex interplay of rheological, compositional, melting and thermal effects, which makes it impossible to make reliable predictions for other planets. (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 ??

Earth-sized planet: R=1 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 McKenzie) 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 1D steady state radiative convective models. 3D 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 581d (see poster by Robin Wordsworth) Gliese 581D : a super Earth at 0.22 AU from M star Gl581, 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 CO2 ?  With a 1D model : mean Tsurf < 240K Franck Selsis et al. (Astronomy and Astrophysics, 2007)

A Global Climate Model for a terrestrial planet 1) 3D 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 581d (see poster by Robin Wordsworth)

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

Gliese 581d (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 1D results for a pure CO2 atmosphere, no clouds: → Global Annual mean temperatures : CO2 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 3D simulation of early Mars (faint sun, 2bars of CO2 Map of annual mean temperature (°C) CO2 ice cloud opacity Atmospheric mean temperature (K) CO2 ice clouds 0°C

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 TITAN VENUS TERRE Several GCMs (NASA Ames, Caltech, GFDL, LMD, AOPP, MPS, Japan, York U., Japan, etc…) Applications: Dynamics & assimilation CO2 cycle dust cycle water cycle Photochemistry thermosphere and ionosphere isotopes cycles paleoclimates etc… ~a few GCMs (LMD, Univ. Od Chicago, Caltech, Köln…) Coupled cycles: Aerosols Photochemistry Clouds ~2 true GCMs Coupling dynamic & radiative transfer (LMD, Kyushu/Tokyo university) Many GCM teams Applications: Weather forecast Assimilation and climatology Climate projections Paleooclimates chemistry Biosphere / hydrosphere cryosphere / oceans coupling Many other applications

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 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 3D 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 ?

New models are needed to understand runaway greenhouse effect Example: Boer et al. (Climate Dynamics, 2005) : «Climate sensitivity and climate change under strong forcing »  50 years with NCARR fully coupled Earth climate model with increased solar flux up to 50 % more. Complex behaviour controlled by cloud feedback.  Find a « runaway cloud feeback effect » more than a « runaway greenhouse effect » Mean Temperature Simulated Years