Habitability

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

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

3. 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)

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

6. 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

7. 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 ?

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 !

9. 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 ↑

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

11. Habitable zone with no greenhouse effect ?

12. 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) (Lithosphere)

13. 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 ??

14. Earth-sized planet: R=1 R=1.07 R=1.1 O’Neil and Lenardic, 2007 Model

15. 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 ??

16. Venus : Ø 12100 km Earth : Ø 12750 km Why is there no plate tectonic on Venus ? • 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 ?

17. 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…

18. 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)

19. 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…

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

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

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

23. 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

24. 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

25. 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

26. Testing Universal equations-based Global climate models in the solar system : it works ! MARS TITAN TERRE VENUS • 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 • Many GCM teams • Applications: • Weather forecast • Assimilation and climatology • Climate projections • Paleooclimates • chemistry • Biosphere / hydrosphere cryosphere / oceans coupling • Many other applications ~2 true GCMs Coupling dynamic & radiative transfer (LMD, Kyushu/Tokyo university)

27. 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)

28. 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 ?