Evolution of Rocky Planets Laura Schaefer

Exoplanets in Our Backyard, Feb. 2020 Collaborators: 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- interaction • Deep volatile cycles Water in Earth’s

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

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

Nestola & Smyth (2015)

Bodnar et al. (2013)

Peslier et al. (2017) Water on Venus and Mars

McCubbin & Barnes (2019)

Lecuyer et al. (2000)

Peslier et al. 2019 Magma ?

• 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 Type I Planets Type II Planets have oceans. lose their water.

Hamano et al. (2013) Nature O2 uptake by magma ocean

Mantles composed mostly of Mg, Si, Fe, and O Mg2+ MgO 4+ Si + n O2- = SiO2 Fe2+ FeO 3+ Fe Fe2O3

FeO(melt) + 0.5 O(g) = FeO1.5 (melt) Atmospheric O 10 M 2 1 M buildup 

• most sensitive to • Orbit • Albedo • Planet mass

α = 0.7 100 bars CO2

Assumes no initial mantle Fe3+ and

perfect uptake of O2 by mantle during magma ocean stage.

Wordsworth et al. (2018) ApJ LHS 3844b – Atmosphere Detection?? Temperature Map

Observations with the Spitzer Space Telescope

The permanent dayside is 1200 degrees hotter than the nightside

| 10 Figures from Kreidberg et al. (2019) Nature LHS 3844b – Atmosphere Stability to Erosion

10% Can constrain maximum initial planet water abundance and minimum stellar 1% heating

Planet likely started with <2 wt% water 0.1% Earth has ~0.02 wt% water

0.01% Amount of initial water in the planet the in initial of water Amount 10-4 10-3 10-2 High energy radiation fraction Thin atmospheres aren’t stable: LHS 3844b is a bare rocky planet

Figures from Kreidberg et al. (2019) Nature | 11 Oxidation of Earth & Venus by atmosphere

Oxidation of the mantle Loss of Water

Venus

1.5 Earth

Earth

% FeO % Wt Venus

Radius of solidification (rs/Rp)

Based on Schaefer et al. (2016), Wordsworth et al. (2018) Oxidation of Earth & Venus by atmosphere

Loss of Water

Earth

Venus

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

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

Vazan et al. (2018) Large amounts of volatiles in “core”

Reaction with FeO (8wt%) Reaction with Fe metal (50 wt%)

Kite et al. (2020) ApJ, in revision H O + Fe (metal) = H + FeO H2 + FeO = Fe(metal) + H2O 2 2 Deep volatile cycles

• Volcanic outgassing • Recycling of volatiles into mantle • of oceanic plates  Plate tectonics • Plate delamination?  Stagnant lid recycling? • Plume/Drip magmatism? Deep

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 lid Kite (2009) al. et Kite

Plate tectonics doesn’t operate on the hot Hadean and Earth

Plate tectonics may have started between 3.2-2.2 Gyr (Brown et al. 2020) 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

Hirschmann (2018) 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?