Ranges of Atmospheric Mass and Composition of Super-Earth Exoplanets

By L. Elkins‐Tanton, Sara Seager. 2008

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Terrestrial-like exoplanets may obtain atmospheres from three primary sources: capture of nebular gases, degassing during accretion, and degassing from subsequent tectonic activity. Here we model degassing during accretion to estimate the range of atmospheric mass and composition on exoplanets ranging from 1 to 30 Earth masses. We use bulk compositions drawn from primitive and differentiated meteorite compositions. Degassing alone can create a wide range of masses of planetary atmospheres, ranging from less than 1% of the planet’s total mass up to ~6 percent by mass (mass%) of hydrogen, ~20 mass% of water, and/or ~5 mass% of carbon compounds. Hydrogen-rich atmospheres can be outgassed as a result of oxidizing metallic iron with water, and excess water and carbon can produce atmospheres through simple degassing. As a byproduct of our atmospheric outgassing models we find that modest initial water contents (10 mass% of the planet and above) create planets with deep surface liquid water oceans soon after accretion is complete.

Subfields: Astronomy and Astrophysics, Geophysics

View Paper: https://www.semanticscholar.org/paper/246bdc286700cf72efe32aef2073215f6527d73f

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GEODYNAMICS AND RATE OF VOLCANISM ON MASSIVE EARTH-LIKE PLANETS (2008, background): “Elkins-Tanton & Seager (2008a) estimate the mass and composition of super-Earth atmospheres degassed during accretion.”
Exoplanet Atmospheres: Physical Processes (2010, background): “How much depends on the composition of planetesimals from which the planet formed (Elkins-Tanton & Seager 2008; Schaefer and Fegley 2010).”
CHEMISTRY OF SILICATE ATMOSPHERES OF EVAPORATING SUPER-EARTHS (2009, background): “So far, atmospheric models of super-Earths have focused on traditional compositions containing H, C, and N (e.g., Miller-Ricci et al. 2009, Elkins-Tanton & Seager, 2008, Ehrenreich et al. 2006).”
The Evaporation Valley in the Kepler Planets (2017, background): “…least a few percent of H/He envelopes (“water-worlds” with water/steam envelopes are ruled out), this amount of gas lies above the most optimistic estimate for outgassing (Elkins-Tanton 2008; Elkins-Tanton & Seager 2008) and suggests that the envelopes were accreted from the protoplanetary discs.”
A FRAMEWORK FOR QUANTIFYING THE DEGENERACIES OF EXOPLANET INTERIOR COMPOSITIONS (2009, background): “Super-Earths that have outgassed an extended hydrogen envelope would lack helium (Elkins-Tanton & Seager 2008b).”
Linked magma ocean solidification and atmospheric growth for Earth and Mars (2008, methodology): “Under the assumption in these models, to reach saturation in an Earth-sized planet, the magma ocean must begin with at least 10 wt.% of water (Elkins-Tanton and Seager, 2008).”
THREE POSSIBLE ORIGINS FOR THE GAS LAYER ON GJ 1214b (2009, background): “…is 2 orders of magnitude below the maximal amount of hydrogen that might be outgassed by a rocky planet (6% of the planet mass), which corresponds to the extreme where the planet formed from planetesimals similar to EH chondrites and fully oxidized during formation (Elkins-Tanton & Seager 2008).”
PHOTOCHEMISTRY IN TERRESTRIAL EXOPLANET ATMOSPHERES. I. PHOTOCHEMISTRY MODEL AND BENCHMARK CASES (2012, background): “Super Earth atmospheres may originate from the capture of nebular gases, degassing during accretion, and/or degassing from subsequent tectonic activity (Elkins-Tanton & Seager 2008; Schaefer & Fegley 2010).”
How to Characterize Habitable Worlds and Signs of Life (2017, background): “…more massive than Earth, a process like plate tectonics might be needed to cool a planet’s surface sufficiently to maintain such high heat flux with high interior pressures limiting the development of a magnetic field (Elkins-Tanton & Seager 2008, Barnes et al. 2009, Stamenkovic et al. 2012).”
DENSITY AND ECCENTRICITY OF KEPLER PLANETS (2012, background): “Alternatively, as suggested by Rogers et al. (2011) and Elkins-Tanton & Seager (2008), outgassing and the subsequent break-down of water could produce a hydrogen envelope ≤ 1% in mass.”
UNDERSTANDING THE MASS–RADIUS RELATION FOR SUB-NEPTUNES: RADIUS AS A PROXY FOR COMPOSITION (2013, background): “Elkins-Tanton & Seager (2008) showed that low-mass planets can outgass up to∼5% of their mass after formation in H2.”
FORMATION AND STRUCTURE OF LOW-DENSITY EXO-NEPTUNES (2011, background): “Out of the Solar System chondrites, this composition should be near optimum for outgassing of H2 due to the high proportion of unoxidized iron (in metal or sulfide form) (Elkins-Tanton & Seager 2008b).”