Download Venus Lecture Material

Last Friday, Oct. 3 Group exercise on
Mars’ Tectonics
Hypothesis: Mars had plate tectonics
(google: Mars, plate tectonics; Mars early plate tectonics)
Use the images provided, your reading and the
web (hints indicated) to argue in favor of ONE
of these two.
1) Mars had plate tectonics.
2) Mars did not have plate
tectonics.
TURN IN: 3 distinct bullets that define
your argument. Each bullet can have AT
MOST three sentences.
Hypothesis: Mars had plate tectonics
(google: Mars, plate tectonics; Mars early plate tectonics)
Tying things together:
Climate-Mantle coupling on Venus
(as well as Mars and Earth)
Couplings among:
-Atmosphere
-Hydrosphere
-Cryosphere
-Lithosphere
Can a runaway greenhouse effect kill
plate tectonics on Earth?
Is a greenhouse runaway one reason
why there is no plate tectonics on
Venus?
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EARTH
(GEOSAT
laser altimetry)
Earth Atmosphere and surface T
Venus Atmosphere and surface T
VENUS
(MAGELLAN radar
altimetry)
Climate
Concepts:
Climate
Concepts:
-Greenhouse forcing
- Volcanic outgassing
- Residence time of
climate forcings
- Weathering cycle
- Feedbacks
-Greenhouse forcing
- Volcanic outgassing
- Residence time of
climate forcings
- Weathering cycle
- Feedbacks
Atmosphere response to volcanism
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Venus surface temperature in more detail:
Volcanism, GH and the sulfur cycle on Venus
Bullock and Grinspoon (1998, 2001)
Observations and Inferences:
Venus in an episodic regime?
• Young surface age
IC:
- ~current temp
- atmo in eq. with surface
minerals CaCO3
(NOT true currently)
• 80% of the surface is covered in lava
• Coexistence of dynamically-supported BAT
coronae and the highlands --> A transient
thermal regime.
Climate Model:
Dynamic:
-1D radiative/convective
-Cloud chem/microphysics
SO2/H2O
clouds impossible:
Low Albedo, GH
Fluxes
IN:
SO2 and H20 gas flux =
12Gt/yr(t=0)*exp(-λt);
λ = 1/108 yrs
SO2/H2O clouds: high Albedo
OUT:
-SO2 to surface by
SO2 + CaCO3 = CaSO4 + CO
e-fold time ~ 20-50 Myr
-H to exosphere
e-fold time H2O ~ 160 Myr
SO2 loss=thin H2O
clouds, GH
Onset of plate tectonics on any planet:
Keys: Convective stresses and yield stresses
Robin, Jellinek, Thayalan, Lenardic, 2007
Some Modeling:
Venus as an adolescent planet…?
Stagnant Lid Convection: One plate planet
Plate tectonics possible
τyield
Lid
Cold Drip
Can flow into the
drip break the plate?
Driving Stress
(viscous drag)
increases with:
• Flow velocity
• Mantle viscosity
Restoring
Stress
Driving
Stress
Convective stress
τc < τyield
τconv. stress
Restoring Stress
(critical yield stress)
• Increases w/ lith thickness
• Decreases w/ water
• Increases w/Tsurf: Depth
of serpentinization less
One Plate Planet
Lith. Strength
Venus:
• Possible plate tectonics early.
• Episodic at present?
• To which regime will the Venus evolve with GH?
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The Greenhouse “runaway” on Venus:
Planets in transition…
- Extensive volcanic outgassing
- No sink for CO2
- High surface T (now ~ 482 deg C)
-How might a transitional regime respond to
forcing such as climate change on long time
scales (long enough for the mantle to “care”
or order 10-100 myr)?
-What is the effect on Venus of a large
increase in surface temperature over the last
500 myr or so due to greenhouse forcing?
-What do we learn about climate-mantle
coupling on Earth from this exercise?
The Greenhouse “runaway” on Venus:
Could it stabilize a stagnant lid regime?
A Mantle Thermostat (i.e. “boiling point”):
Variable viscosity + internal heating + convection
Progressive increase
Greenhouse Forcing;
Increase Tsurface
Maintain ΔT for 1
Lith. Response time
Q [watts]
Plate Tectonics /
Active Lid Mode
viscosity = A exp(-θT)
Teq (Tozer, 1972))
Internal Heating
(constant)
Convective
Cooling
Tsurf
ΔT GH
Tcmb
Internal Temperature [oC]
Stagnant Lid Mode
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Tozer’s Mantle Thermostat: enhanced further
-Variable viscosity and internal heating
+
-Convective regime
-Variable viscosity and internal heating
+
-Convective regime CHANGE via GH
viscosity = A exp(-θT)
Q [watts]
Active lid path
Teq (Tozer, 1972))
Stagnant lid path
Internal Heating
(constant)
Convective
Cooling
Tsurf
Tcmb
Internal Temperature
[oC]
NUMERICAL SIMULATIONS
Surface Velocity
Q [watts] , Convective Stress [Pa]
Tozer’s Mantle Thermostat: enhanced
Co
nve
cti
Active lid path
ve
St
res
s
Yield Stress
(constant)
Internal Heating
(constant)
Convective
Cooling
Active Lid
Stagnant Lid
?
Stagnant lid path
ΔT GH
Tsurf
Tcmb
Internal Temperature [oC]
How to kill plate tectonics on Earth
with climate change?
Mobile Lithosphere
Surface Temperature
Change Below Critical
Surface Temperature
Change above Critical
Earth
Stays Mobile
Goes Stagnant
Turn up the surface T by 50-100 degC….
5
What about Venus?
SO2/H2O
clouds impossible:
Low Albedo, GH
SO2/H2O clouds: high Albedo
SO2 loss=thin H2O
clouds, GH
Key: Changes occur over and persist
for long times (enough time for the
mantle to respond)
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