Evolution of Rocky PlanetsLaura Schaefer
Exoplanets in Our Backyard, Feb. 2020Collaborators:
Lindy Elkins-Tanton (ASU)
Bruce Fegley (WashU)
Edwin Kite (UChicago)
Kaveh Pahlevan (SETI)
Laura Kreidberg (CfA)
Robin Wordsworth (Harvard)
Dimitar Sasselov (CfA)
Outline
• Volatiles in rocky planet interiors
• Atmosphere-magma ocean interaction
• Deep volatile cycles
Karato (2015) TGC
Water in Earth’s Mantle
Reference H2O in mantle (OM)
Korenaga et al. 2017 0.56 – 1.3
Hirschmann (2018) 0.9 ± 0.2
Peslier et al. (2017) 1.1 - 7.4
Peslier et al. (2017)
Water in Earth’s Mantle
Reference H2O in mantle (OM)
Korenaga et al. 2017 0.56 – 1.3
Hirschmann (2018) 0.9 ± 0.2
Peslier et al. (2017) 1.1 - 7.4
Nestola & Smyth (2015)
Bodnar et al. (2013)
Water on Venus and Mars
Lecuyer et al. (2000)
McCubbin & Barnes (2019)
Peslier et al. 2019
Magma Oceans?
• Earth: inferred from giant impact scenario for Moon formation• Lunar MO most robust
• Venus: uncertain• Runaway greenhouse onset depends on uncertain stellar evolution• Core formation models (Jacobson et al. 2017) posit that Venus may not have
experienced a late giant impact
• Mars: rapid formation (~5-10 Myrs, Dauphas & Pourmand 2011) suggests at least a partial magma ocean• short-lived radionuclides and rapid accretion rate may be necessary (Saito &
Kuramoto 2018)
• Exoplanets: close-in planets, even M-dwarf habitable zone planets may experience extended runaway greenhouse driven magma oceans
Hamano et al. (2013) Nature
Type I Planets have oceans.
Type II Planets lose their water.
O2 uptake by magma ocean
Mg2+
Si4+
Fe2+
Fe3+
Mantles composed mostly of Mg, Si, Fe, and O
+ n O2-
MgOSiO2
FeOFe2O3
=
FeO(melt) + 0.5 O(g) = FeO1.5 (melt)
Atmospheric O2buildup
• most sensitive to• Orbit
• Albedo
• Planet mass
Wordsworth et al. (2018) ApJ
1 M
10 M
α = 0.7 100 bars CO2
Assumes no initial mantle Fe3+ and perfect uptake of O2 by mantle during magma ocean stage.
Temperature Map
LHS 3844b – Atmosphere Detection??
| 10
Observations with the Spitzer Space Telescope
The permanent dayside is 1200 degrees hotter than the nightside
Figures from Kreidberg et al. (2019) Nature
LHS 3844b – Atmosphere Stability to Erosion
| 11Figures from Kreidberg et al. (2019) Nature
Can constrainmaximum initial planet water abundance andminimum stellar heating
Planet likely started with <2 wt% water
Earth has ~0.02 wt% water
10%
1%
0.1%
0.01%Am
ou
nt
of
init
ial w
ater
in t
he
pla
net
10-4 10-3 10-2
High energy radiation fractionThin atmospheres aren’t stable: LHS 3844b is a bare rocky planet
Oxidation of Earth & Venus by atmosphere
Venus
Earth
Venus
Earth
Radius of solidification (rs/Rp)
Wt
% F
eO1
.5
Oxidation of the mantle Loss of Water
Based on Schaefer et al. (2016), Wordsworth et al. (2018)
Oxidation of Earth & Venus by atmosphere
Venus
Earth
Loss of Water
Lammer et al. (2018)
Water loss and oxidation will depend on stellar evolution (fast vs. slow rotator) and timing
Mars early magma ocean
Lammer et al. (2018)
Saito & Kuramoto (2018)
Most magma ocean models miss some heat sources (e.g. gravitational segregation), that may enhance melt production
Sub-Neptune “cores” are mostly molten
Vazan et al. (2018)
Evolution of atmosphere-mantle temperature for planets with 4.5 MEarth “cores” and variable masses of H2
atmospheres
Interiors of sub-Neptunes are mostly molten silicates
Large amounts of volatiles in “core”Reaction with Fe metal (50 wt%)Reaction with FeO (8wt%)
Kite et al. (2020) ApJ, in revision
H2 + FeO = Fe(metal) + H2O H2O + Fe (metal) = H2 + FeO
Deep volatile cycles
• Volcanic outgassing
• Recycling of volatiles into mantle• Subduction of oceanic plates
• Plate delamination?
• Plume/Drip magmatism?
Plate tectonics
Stagnant lid recycling?
Deep Water Cycle
Karato (2015) TGC
Water is transported into the mantle through subduction of hydrated minerals and sediments in a process called regassing or ingassing.
Water escapes from the mantle through volcanic eruptions at mid-ocean ridges in a process called degassing or outgassing.
Plate tectonics vs. Stagnant lidK
ite
et a
l. (2
00
9)
Plate tectonics doesn’t operate on the hot Hadean and Archean Earth
Plate tectonics may have started between 3.2-2.2 Gyr (Brown et al. 2020)
Hirschmann (2018)
Estimates of surface/mantle inventories suggest that most of Earth’s carbon is in the mantle, but most H2O and N is at the surface
Based on current outgassing rates, the inventories require significant ingassing of C, but early large surface inventories of H and N
Summary
• A large portion of planetary volatile components are locked in planetary interiors
• Initial solid mantle volatile abundances depend on solubilities, solid/melt partitioning, magma ocean lifetime and atmospheric escape
• Deep volatile cycles depend on style of tectonics (PT vs. stagnant lid)• Earth has not always had plate tectonics• Stagnant lid planets have slower return of materials to interior
• Exoplanets occupy a wider parameter space, so we have to ask, what are the limits in planet size/volatile content/etc that these models apply to?