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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? 1 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 2 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? 3 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 4 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) 6