Climate Instability on Planets with Large Day-Night Surface Temperature Contrasts

1. Edwin Kite (Caltech) Eric Gaidos (Hawaii), Michael Manga (Berkeley), Itay Halevy (Weizmann) Climate Instability on Planets with Large Day-Night Surface Temperature Contrasts Substellar magma ponds “Climate instability on tidally locked exoplanets” Kite, Gaidos & Manga, ApJ 743:41 (2011) Edwin Kite (Caltech) Discussions with: Eugene Chiang, Ray Pierrehumbert, Michael Manga.

2. Climate instability: Outline • 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

3. Long-term climate stability: Earth Jet Rock, England • 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

4. “The closest habitable exoplanet orbits an M-dwarf” 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 Desert et al., ApJL, 2011; Bean et al. ApJ 2011 JWST: no earlier than 2018 TESS/ELEKTRA/PLATO + Warm Spitzer follow-up

5. Tidally locked exoplanet with a noncondensible, one-gas atmosphere: Pierrehumbert cookbook WTG approximation What happens when atmospheric pressure is increased? Kite, Gaidos & Manga, ApJ 743:41 (2011)

6. Weathering rate varies strongly with distance from substellar point. Diamonds: Atmospheric temperatures Pressure in bars Kite, Gaidos & Manga, ApJ 743:41 (2011) … see also Mills, Abbott & Pierrehumbert poster

7. Enhanced substellar weathering instability: Berner & Kothavala, Am. J. Sci., 2001 speed depends on rate of volcanism speed depends on weathering kinetics and resurfacing rate Stable equilibrium (examples) Unstable equilibrium (examples) M= Mars insolation E = Earth insolation V = Venus insolation Kite, Gaidos & Manga, ApJ 743:41 (2011)

8. Substellar dissolution feedback: faster than the weathering instability CO2 in seawater Is substellar dissolution feedback important for a steam atmosphere over a magma ocean? Kite, Gaidos & Manga, ApJ 743:41 (2011)

9. A local test? The last 3 Ga on Mars sulfate eqb’m? (Halevy et al. Nature, 2007) TODAY 3±2 wt % carbonate in soil+dust, ~1 mbar CO2 per meter depth +2 Ga NOW -2 Ga Resurfacing by wind and impacts is the limiting step for supply of weatherable material Uncertainty: Kinetics of carbonate formation under Marslike conditions?

10. 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.

11. Bonus slides

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

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

14. The magma planet opportunity Detectability Characterizataion Natural laboratory Fundamental planetary processes Solar system links

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

16. Progress Detection Validation Internal modeling Atmospheric modeling Possible planet-sized rocky comet

17. Magma pond statics Detection Validation Internal modeling Atmospheric modeling Possible planet-sized rocky comet

18. Magma pond circulation Detection Validation Internal modeling Atmospheric modeling Possible planet-sized rocky comet

19. Magma pond as a gravity current Magma pond as a gravity current Validation Internal modeling Atmospheric modeling Possible planet-sized rocky comet

20. Magma pond as a gravity current Magma pond as a gravity current Validation Internal modeling Atmospheric modeling Possible planet-sized rocky comet

21. At and beyond the pond margin Detection Validation Internal modeling Atmospheric modeling Possible planet-sized rocky comet

22. Potentially observable feedbacks • Atmospheric blanket  global mantle melting. • Delayed differentiation  volcanism, mantle melting.

23. Processes and observables