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.

• 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

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

“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

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?

… 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

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

pend

s on

rate

of v

olca

nism

Stable equilibrium (examples)

Unstable equilibrium (examples)M= Mars insolationE = Earth insolationV = Venus insolation

Kite, Gaidos & Manga, ApJ 743:41 (2011)

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

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

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.

Bonus slides

How many solar system climates are vulnerable to runaway weathering instability?

Kite, Gaidos & Manga, ApJ 743:41 (2011)

The magma planet opportunity

DetectabilityCharacterizataionNatural laboratoryFundamental planetary processesSolar system links

Structure

Physics: Does magma circulation cause large changes in the phase curve? Chemistry: Are magma ponds sites of delayed differentiation?

Progress

DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet

Magma pond statics

DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet

Magma pond circulation

DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet

Magma pond as a gravity current

Magma pond as a gravity currentValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet

Magma pond as a gravity current

Magma pond as a gravity currentValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet

At and beyond the pond margin

DetectionValidationInternal modelingAtmospheric modelingPossible planet-sized rocky comet

Potentially observable feedbacks

• Atmospheric blanket global mantle melting.

• Delayed differentiation volcanism, mantle melting.

Processes and observables