Habitability

François Forget, Institut Pierre-Simon LaplaceLMD, CNRS, France

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 …

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)

100% vapour Liquid water 100% ice

The habitable zone(Kasting et al. 1993)

Solar flux↑ Temperature ↑

Greenhouse effect ↑ Evaporation ↑

Climate instability at the Inner edge

Alt

itu

de

Temperature

EUV radiation

Photodissociation :

H escape, water lost to space

Impact of temperature increase on water vapor distribution and escape

H2O + hν → OH +H

Inner Edge of the Habitable zone

Water loss limit

Runaway greenhouse limit

Kasting et al. 1D radiative convective model; no cloudsSee also poster by Stracke et al. this week

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 ?

100% vapour Liquid water 100% ice

The habitable zone(Kasting et al. 1993)

Solar flux↑ Temperature ↓

Albedo↑ Ice and snow ↑

Climate instability at the Outer edge

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 climateMay require :

- Plate tectonic- Life ??

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 ↑

Walker et al. (1981)

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)

(Lithosphere)

• 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 ??

Is plate tectonic likely on other terrestrial planets ?

O’Neil and Lenardic, 2007Model

Earth-sized planet:R=1

R=1.07

R=1.1

• 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 ??

Is plate tectonic likely on other terrestrial planets ?

Why is there no plate tectonic on Venus ?

Venus : Ø 12100 kmEarth : Ø 12750 km

• 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 ?

• High surface temperature prevent plate subduction ? Not likely (Van Thienen et al. 2004)

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

CO2 ice cloudsCO2 ice clouds

CO2 ice cloud opacity

GCM 3D simulation of early Mars(faint sun, 2bars of CO2

Atmospheric mean temperature (K)

0°C

Map of annual mean temperature (°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 ?

Fairbanks (AK) : -3ºC Barrow (AK) : -12ºC Antarctica Dry Valley :-15ºC – -30ºC

Examples of annual mean temperatures on the Earth:

VENUS

TERRE

MARS TITAN

Many GCM teamsApplications:• Weather forecast• Assimilation and climatology• Climate projections• Paleooclimates• chemistry• Biosphere / hydrosphere cryosphere / oceans coupling• Many other applications

~a few GCMs (LMD, Univ. Od Chicago, Caltech, Köln…)

Coupled cycles:• Aerosols• Photochemistry• Clouds

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…•

~2 true GCMs Coupling dynamic & radiative transfer(LMD, Kyushu/Tokyo university)

Testing Universal equations-based Global climate models in the solar system : it works !

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)

Toward a « universal climate model » :

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 ?