Transcript Day01_Kite

Climate Instability on Planets with Large
Day-Night Surface Temperature Contrasts
Edwin Kite (postdoc, Caltech)
Eric Gaidos (Hawaii), Michael Manga (Berkeley), Itay Halevy (Weizmann)
“Climate Instability on Tidally Locked Exoplanets”
Kite, Gaidos & Manga, ApJ 743:41 (2011)
Magma planets
Edwin Kite
Discussions: Michael Manga, Eugene Chiang, Ray Pierrehumbert.
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
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.
Ts, ln([CO2])
– Walker et al., JGR, 1981
Jet Rock,
Britain
• 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
“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?
Substellar
region
is key for
planet-integrated
weathering
Kite, Gaidos & Manga, ApJ 743:41 (2011)
Pressure in bars
Diamonds: Atmospheric temperatures
Weathering rate varies strongly
with distance from substellar point.
Also see work by Dorian Abbot’s group and
Robin Wordsworth
Enhanced substellar weathering instability:
Berner & Kothavala,
Am. J. Sci., 2001
speed depends
on rate of volcanism
speed depends on
weathering kinetics
and resurfacing rate
Planet-integrated weathering rate:
Stable equilibrium (examples)
Unstable equilibrium (examples)
Δtjump << Δtstar age
M= Mars insolation
E = Earth insolation
V = Venus insolation
Δtjump >> Observational
baseline
Climate instability phase diagram
An abundance of triggers exists:
Kite, Gaidos & Manga, ApJ 743:41 (2011)
Substellar dissolution feedback:
Consider slow increase in insolation of a planet with deep surface water ice.
CO2 in seawater
Kite, Gaidos & Manga, ApJ 743:41 (2011)
 faster than the weathering instability
 complements ice-albedo feedback
A local test? The last 3 Ga on Mars
3±2 wt % carbonate in soil+dust, ~1 mbar CO2 per meter depth
Assume large-scale carbonate formation requires liquid water:
TODAY
(Kahn, 1985)
+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.
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.
Magma planets: processes and observables
User “blackcat”, Planetside Forums
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.
Structure : Inside
Outside
the
magma
pond
the
magma
pond
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.
Progress
Recent review:
Léger et al., Icarus 2011
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
6 ppm
Inside the magma pond
Material properties
Dingwell et al., EPSL 2004
Insensitive to composition
across full Solar System
basalt range
Giordano & Dingwell, EPSL 2003
Giordano & Dingwell, EPSL 2003
Viscosity of liquid peridotite:
10-1 Pa s near solidus
Open question: Effect of melting on NIR spectral features, albedo?
Structure of a static magma pool
dH/dTsurf ~3.2 K/km (Earth gravity)
Sleep, Treatise Geophys., 2007
partial melt
fraction
z
x
magma sea
solid planet
Kite et al., ApJ 2009
U ~ 40 m/s, Ro~4 (!), Ek  0
 in geostrophic limit
 grossly unstable to
horizontal convection
“Earth-like” Disclaimer:
1. Lava slabs near pond margin sink (mixing source)
Circulation 2. Strongly variable viscosity (most important near margin)
Neglecting Coriolis forces:
 Convection must occur: no critical Ra#
 Almost all T variation confined to thin surface B.L.
 Strong plumelike downwelling near pond edge, passive/diffusive
upwelling elsewhere
2D Boussinesq numerical simulation,
validated by dye-tracer experiments:
EDGE OF POND
SUBSTELLAR POINT
TEMPERATURE: ORANGE=COLD
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.?
Can surface temperature be
homogenized by oceanic
heat transport alone?
Consider a planet with an entirely molten surface
and a radiatively unimportant atmosphere:
PERCOLATION/
FILTER PRESSING?
Cooling timescale for well-mixed B.L. on nightside:
( ρ cp ΔH 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?
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;
NIMS-derived model age
Rhoden and Kite, DPS/EPSC, 2011;
(blue = young, hot)
- 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 2003
Davies et al., GRL 2012.
Magma seas have “weather”:
The maximum length scale at which magma ponds can produce large-amplitude thermalemission 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?)
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 of nightside energy budget?
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
Bonus slides
How many solar system climates are vulnerable to
runaway weathering instability?
“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