Outgassing on stagnant-lid super-Earths

¹ Institute of Computational Sciences, University of Zurich, Winterthurerstrasse 109, 8057, Zurich, Switzerland
e-mail: cdorn@physik.uzh.ch
² Department of Reference Systems and Geodynamics, Royal Observatory of Belgium, Avenue Circulaire 3, 1180 Brussels, Belgium
³ Institute of Geological Sciences, Free University Berlin, Malteserstr. 74-100, 12249 Berlin, Germany
⁴ Institute of Geophysics, Department of Earth Sciences, ETH Zurich, Sonneggstrasse 5, 8092 Zurich, Switzerland

Received: 5 July 2017
Accepted: 25 February 2018

Abstract

Aims. We explore volcanic CO₂-outgassing on purely rocky, stagnant-lid exoplanets of different interior structures, compositions, thermal states, and age. We focus on planets in the mass range of 1–8 M_⊕ (Earth masses). We derive scaling laws to quantify first- and second-order influences of these parameters on volcanic outgassing after 4.5 Gyr of evolution.

Methods. Given commonly observed astrophysical data of super-Earths, we identify a range of possible interior structures and compositions by employing Bayesian inference modeling. The astrophysical data comprise mass, radius, and bulk compositional constraints; ratios of refractory element abundances are assumed to be similar to stellar ratios. The identified interiors are subsequently used as input for two-dimensional (2D) convection models to study partial melting, depletion, and outgassing rates of CO₂.

Results. In total, we model depletion and outgassing for an extensive set of more than 2300 different super-Earth cases. We find that there is a mass range for which outgassing is most efficient (~2–3 M_⊕, depending on thermal state) and an upper mass where outgassing becomes very inefficient (~5–7 M_⊕, depending on thermal state). At small masses (below 2–3 M_⊕) outgassing positively correlates with planet mass, since it is controlled by mantle volume. At higher masses (above 2–3 M_⊕), outgassing decreases with planet mass, which is due to the increasing pressure gradient that limits melting to shallower depths. In summary, depletion and outgassing are mainly influenced by planet mass and thermal state. Interior structure and composition only moderately affect outgassing rates. The majority of outgassing occurs before 4.5 Gyr, especially for planets below 3 M_⊕.

Conclusions. We conclude that for stagnant-lid planets, (1) compositional and structural properties have secondary influence on outgassing compared to planet mass and thermal state, and (2) confirm that there is a mass range for which outgassing is most efficient and an upper mass limit, above which no significant outgassing can occur. Our predicted trend of CO₂-atmospheric masses can be observationally tested for exoplanets. These findings and our provided scaling laws are an important step in order to provide interpretative means for upcoming missions such as JWST and E-ELT, that aim at characterizing exoplanet atmospheres.