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 ?