Ranges of Atmospheric Mass and Composition of Super Earth Exoplanets

TL;DR

This paper models atmospheric formation during accretion of super Earth exoplanets, showing a wide range of possible atmospheric masses and compositions based on planetary and initial water contents.

Contribution

It introduces a model for atmospheric degassing during accretion, estimating atmospheric mass and composition ranges for super Earth exoplanets from 1 to 30 Earth masses.

Findings

Degassing can produce atmospheres from less than 1% to several percent of planetary mass.

Hydrogen-rich atmospheres can form through oxidation of metallic iron with water.

Planets with initial water content above 10% develop deep surface oceans after accretion.

Abstract

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 a percent of the planet's total mass up to ~6 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.