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

1. Edwin Kite (postdoc, Caltech) Eric Gaidos (Hawaii), Michael Manga (Berkeley), Itay Halevy (Weizmann) Climate Instability on Planets with Large Day-Night Surface Temperature Contrasts “Climate Instability on Tidally Locked Exoplanets” Kite, Gaidos & Manga, ApJ 743:41 (2011) Magma planets Edwin Kite Discussions: Michael Manga, Eugene Chiang, Ray Pierrehumbert.

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 (& substellar dissolution feedback) • Mars: • a nearby example of enhanced substellar weathering instability? • Conclusions and tests

3. Long-term climate stability: Earth Wt • 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 (hyperthermals; ice cores) – 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 Ts, ln([CO2]) Jet Rock, Britain

4. “M-dwarf opportunity” planets in the M-dwarf HZ will be tidally locked. Tidally locked exoplanet with a noncondensible, one-gas atmosphere: Pierrehumbert cookbook WTG approximation What happens when atmospheric pressure is increased?

5. Weathering rate varies strongly with distance from substellar point. Diamonds: Atmospheric temperatures Substellar region is key for planet-integrated weathering Pressure in bars Also see work by Dorian Abbot’s group and Robin Wordsworth Kite, Gaidos & Manga, ApJ 743:41 (2011)

6. Enhanced substellar weathering instability: Berner & Kothavala, Am. J. Sci., 2001 Planet-integrated weathering rate: Stable equilibrium (examples) Unstable equilibrium (examples) speed depends on rate of volcanism speed depends on weathering kinetics and resurfacing rate Δtjump << Δtstar age Δtjump >> Observational baseline M= Mars insolation E = Earth insolation V = Venus insolation

7. Climate instability phase diagram An abundance of triggers exists: Kite, Gaidos & Manga, ApJ 743:41 (2011)

8. Substellar dissolution feedback: Consider slow increase in insolation of a planet with deep surface water ice. CO2 in seawater • faster than the weathering instability • complements ice-albedo feedback Kite, Gaidos & Manga, ApJ 743:41 (2011)

9. A local test? The last 3 Ga on Mars Assume large-scale carbonate formation requires liquid water: TODAY (Kahn, 1985) 3±2 wt % carbonate in soil+dust, ~1 mbar CO2 per meter depth +2 Ga NOW -2 Ga Draws on Richardson & Mischna, JGR 2005 Resurfacing by wind and impacts is the limiting step for supply of weatherable material Uncertainty: Kinetics of carbonate formation under Marslike conditions? See also poster by Hollingsworth, Kahre et al.

10. Climate instability: 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 dampen the entirely justified excitement about targeting 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. Magma planets: processes and observables User “blackcat”, Planetside Forums

12. The magma planet opportunity Detectable: Much more likely to intersect stellar disk than HZ planet; given that a planet transits, 10x-1000x more transits than HZ equivalents. Intrinsically common (Howard et al., ArXiv 2011) Characterizable: Optical phase curve of Kepler-10b tentatively detected (Batalha et al., ApJ 2011). Much greater S/N for JWST. Natural laboratory: Composition probed by atmosphere, and magma pond size. Distillation by partial melting, sublimation and escape. Solar system links: e.g., Tidally sustained global subsurface magma ocean in Io today (Khuruna et al., Science 2011). Io also has a surface magma sea - Loki Patera. Surprisingly close dynamical similarity to Earth’s ocean. Fundamental processes: Runaway planetary differentiation (e.g. during formation) always produces magma oceans. These primordial magma oceans set the initial conditions for subsequent planetary evolution. Initial conditions matter: e.g. Earth’s LLVSPs and ULVZs are key to mantle circulation and mass extinctions (Jackson & Carlson, Nature 2011) and appear to be relics of Earth’s primordial magma ocean.

13. : Outside the magma pond : Inside the magma pond Structure Physics: Can magma circulation cause large changes in surface temperature (phase curve)? Chemistry: Are magma ponds sites of planetary-scale differentiation? Assumptions: Planet is in 1:1 spin-orbit synchronous rotation. Volatiles are absent – atmosphere consists of vaporized surface material.

14. Recent review:Léger et al., Icarus 2011 Progress 6 ppm 2(+1) magma planets detected: CoRoT-7b, Kepler-10b, + disintegrating planet-sized rocky comet around KIC 12557548. Validation by Spitzer (Fressin et al., arXiv 2011) and RV. Possible optical phase curve (Rouan et al., ApJL 2011). Insolation gradient forces degree-1 mantle convection in simulation (e.g., van Summeren et al., ApJ 2011). Sophisticated models of 3D thermal-tidal runaways by Běhounková (ApJ 2011). Models do not treat melting. Atmospheric composition modelled by Schaeffer + Fegley + Lodders (e.g., Schaefer & Fegley, ApJ 2009) Na-only axisymmetric atmospheric simulation (Castan and Menou, ApJL 2011) – supersonic winds, pressures near surface-temperature equilibrium

15. Inside the magma pond Material properties Viscosity of liquid peridotite: 10-1 Pa s near solidus Dingwell et al., EPSL 2004 Insensitive to composition across full Solar System basalt range Giordano & Dingwell, EPSL 2003 Giordano & Dingwell, EPSL 2003 Open question: Effect of melting on NIR spectral features, albedo?

16. Structure of a static magma pool dH/dTsurf~3.2 K/km (Earth gravity) Sleep, Treatise Geophys., 2007 z partial melt fraction x magma sea solid planet subsolidus adiabat U ~ 40 m/s, Ro~4 (!), Ek 0 • in geostrophic limit • grossly unstable to horizontal convection Kite et al., ApJ 2009

17. Disclaimer: 1. Lava slabs near pond margin sink (mixing source) 2. Strongly variable viscosity (most important near margin) “Earth-like”Circulation • Convection must occur: no critical Ra# • Almost all T variation confined to thin surface B.L. • Strong plumelikedownwelling near pond edge, passive/diffusive • upwelling elsewhere Neglecting Coriolis forces: 2D Boussinesq numerical simulation, validated by dye-tracer experiments: TEMPERATURE: ORANGE=COLD EDGE OF POND SUBSTELLAR POINT STREAMFUNCTION STRATIFIED B.L. | SMALL-SCALE CONVECTION IN B.L. Hughes & Griffiths, Ann. Rev. Fluid Mech. 2008 Open questions: With Coriolis forces – substellar magma vortex, or gyres? How does strongly variable viscosity affect behavior of Ekman B.L.?

18. Can surface temperature behomogenized by oceanic heat transport alone? • Consider a planet with an entirely molten surface • and a radiatively unimportant atmosphere: • Cooling timescale for well-mixed B.L. on nightside: • ( ρ cpΔH ΔT ) / (εσT4) ~ 1 year for 10 km B.L.@1500K to cool 100K • Geostrophic velocity (true day-night mean velocity will be less – depends on eddy viscosity, Ekman B.L. properties): • usurf ~ (kageostrophic g ΔH ρα ΔT) / (fcor ρ L) ~ 1 m/s  distance: 3 x 107 m • ΔT ~ 100K may be enough to drive a magma flow that prevents further steepening of the temperature gradient (optimistic!) • Gradients in crystal fraction give much steeper density gradients / mixing (robust) • Wind-driven circulation? PERCOLATION/ FILTER PRESSING?

19. A nearby magma sea: Loki Patera, Io 200 km diameter, mostly covered by cool lava crust. Peak Io lava temps. ~1500K Galileo SSI image Matson et al., JGR 2006 Rathbun et al., GRL 2002; Rhoden and Kite, DPS/EPSC, 2011; - reanalysis indicates the 540-day (quasi)periodicity was real, and not an alias of the ~1.7 day tidal period due to observation geometry, but there is no evidence for periodicity in post-2003 data. Davies et al., GRL 2012. NIMS-derived model age (blue = young, hot) Davies et al., GRL 2003 • Magma seas have “weather”: • The maximum length scale at which magma ponds can produce large-amplitude thermal-emission variability is >=200km (decorrelation scale for activity is >=10^4 km^2). • Inference from Loki: Lava crust foundering may generate large-amplitude variability near the edge of the pond. (What are sources of variability above the liquidus?)

20. Outside the magma pond • Crustal flow timescale >> circulation time • Chemical fractionation by partial melting and sublimation: energy available from delayed differentiation ~ f m g H ~ 1034J (107 yr insolation) suppose 1% of insolation (1018 W) goes to vaporizing Fe2SiO4 (Lvap ~ 3.2 MJ/kg) and transporting Fe (~50% mass) to an Fe pool on the surface from which it sinks to the base of the mantle: gain is ~10x  Thermal emission can exceed insolation for some planets  A detectable component ofnightside energy budget?

21. The magma planet opportunity: Processes and observables GAS TAIL GRAIN (SPHERULE) TAIL ASH FROM ERUPTIONS PHASE CURVE EMISSION SPECTRUM PRECESSION ALBEDO WIND SPEEDS ELLIPTICITY? ASYNCHRONOUS ROTATION? ENSEMBLE OBS. (KEPLER) MAGNETIC EFFECTS? bold = already achieved

22. Bonus slides

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

24. “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 M-dwarf habitable zone  Tidally locked, assume 1:1 spin:orbit synchronous