Transcript Habitability - Pathways Towards Habitable Planets

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 :
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
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
(CO2, CO2 + CO2 ice
clouds, greenhouse
gas cocktail…) can
maintain a suitable
climate
Ts ↓
Ts ↑
water cycle ↓
Greenhouse effect ↑
weathering ↓
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
(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
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)
0°C
CO2 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
• CO2 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
• 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 ?