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?