Climate Instability on Planets with Large Day-Night Surface Temperature Contrasts
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Climate Instability on Planets with Large Day-Night Surface Temperature Contrasts
“Climate instability on tidally locked exoplanets”Kite, Gaidos & Manga, ApJ 743:41 (2011)
Edwin Kite (Caltech)
Eric Gaidos (Hawaii), Michael Manga (Berkeley), Itay Halevy (Weizmann)
Substellar magma pondsEdwin Kite (Caltech)
Discussions with: Eugene Chiang, Ray Pierrehumbert, Michael Manga.
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• Earth: – inference of a climate-stabilizing feedback
between greenhouse-gas control of surface temperature, and temperature-dependent weathering drawdown of greenhouse gases
• Exoplanets:– when can the weathering feedback be
destabilizing? – Enhanced substellar weathering instability
• Mars:– a nearby example of enhanced substellar
weathering instability?
• Conclusions and tests
Climate instability: Outline
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Long-term climate stability: Earth• Without a stabilizing mechanism, Earth’s observed long-term climate
stability is improbable.
• A good candidate stabilizing mechanism is temperature-dependent greenhouse gas
drawdown.– Walker et al., JGR, 1981
• There is suggestive, but circumstantial, evidence that the carbonate-silicate feedback does in fact moderate Earth’s climate. – Cohen et al., Geology, 2004; Zeebe & Caldeira, Nat. Geo., 2008;
Grotzinger and Kasting, J. Geol., 1993.
• If Earth’s climate-stabilizing feedback is unique, then habitable biospheres will be rare, young, or unobservable (buried/blanketed)
• The search for observable habitable environments beyond Earth depends on the generality of climate-moderating processes.
– Kasting et al., Icarus, 1993
Jet Rock,England
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“The closest habitable exoplanet orbits an M-dwarf”
JWST: no earlier than 2018TESS/ELEKTRA/PLATO + Warm Spitzer follow-up
Desert et al., ApJL, 2011; Bean et al. ApJ 2011
Planets in the M-dwarf Habitable Zone: Deep, frequent transits. M-dwarfs common.
Example: GJ 1214b (Charbonneau et al., Nature, 2009).1.5%-depth transit every 1.6 days. 40 ly distant; 6.6 Earth masses, 2.7 Earth radii
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Kite, Gaidos & Manga, ApJ 743:41 (2011)
Tidally locked exoplanet with a noncondensible, one-gas atmosphere:
WTG approximationPierrehumbert cookbook
What happens when atmospheric pressure is increased?
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… see also Mills, Abbott & Pierrehumbert poster
Pres
sure
in b
ars
Weathering rate varies strongly with distance from substellar point.
Kite, Gaidos & Manga, ApJ 743:41 (2011)
Dia
mon
ds: A
tmos
pher
ic te
mpe
ratu
res
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Berner & Kothavala, Am. J. Sci., 2001
Enhanced substellar weathering instability:
spee
d de
pend
s on
wea
ther
ing
kine
tics
and
resu
rfac
ing
rate
spee
d de
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s on
rate
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olca
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Stable equilibrium (examples)
Unstable equilibrium (examples)M= Mars insolationE = Earth insolationV = Venus insolation
Kite, Gaidos & Manga, ApJ 743:41 (2011)
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Is substellar dissolution feedback important for a steam atmosphere over a magma ocean?
Substellar dissolution feedback: faster than the weathering instability
Kite, Gaidos & Manga, ApJ 743:41 (2011)
CO2 in seawater
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A local test? The last 3 Ga on Mars
Resurfacing by wind and impacts is the limiting step for supply of weatherable materialUncertainty: Kinetics of carbonate formation under Marslike conditions?
NOW
-2 Ga
+2 Ga
TODAYsulfate eqb’m?(Halevy et al.Nature, 2007)
3±2 wt % carbonate in soil+dust, ~1 mbar CO2 per meter depth
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Conclusions and tests• Enhanced substellar weathering instability may destabilize
climate on some habitable-zone planets. The instability requires large ΔTs, but does not require 1:1 synchronous rotation.
• Substellar dissolution feedback is less likely to destabilize climate. It is only possible for restrictive conditions.
• Enhanced substellar weathering instability only works when most of the greenhouse forcing is associated with a weak greenhouse gas that also forms the majority of the atmosphere
- Does not work for Earth, but may work for Mars. - It would be incorrect to use our results to argue against prioritizing M-dwarfs for
transiting rocky planet searches.
• Test 1: Do GCMs reproduce the results from simple energy balance models?
• Test 2: If enhanced substellar weathering instability is widespread, we would expect to see a bimodal distribution of day-night temperature contrasts and thermal emission from habitable-zone rocky planets in synchronous rotation. Emission temperatures would be either close to isothermal, or close to radiative equilibrium.
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Bonus slides
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How many solar system climates are vulnerable to runaway weathering instability?
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Kite, Gaidos & Manga, ApJ 743:41 (2011)
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The magma planet opportunity
DetectabilityCharacterizataionNatural laboratoryFundamental planetary processesSolar system links
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Structure
Physics: Does magma circulation cause large changes in the phase curve? Chemistry: Are magma ponds sites of delayed differentiation?
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Progress
DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet
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Magma pond statics
DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet
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Magma pond circulation
DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet
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Magma pond as a gravity current
Magma pond as a gravity currentValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet
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Magma pond as a gravity current
Magma pond as a gravity currentValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet
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At and beyond the pond margin
DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet
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Potentially observable feedbacks
• Atmospheric blanket global mantle melting.
• Delayed differentiation volcanism, mantle melting.
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Processes and observables