The atmospheres of rocky exoplanets

Abstract

Context. Little is known about the interaction between atmospheres and crusts of exoplanets so far, but future space missions and ground-based instruments are expected to detect molecular features in the spectra of hot rocky exoplanets. Aims. We aim to understand the composition of the gas in an exoplanet atmosphere which is in equilibrium with a planetary crust. Methods. The molecular composition of the gas above a surface made of a mixture of solid and liquid materials was determined by assuming phase equilibrium for given pressure, temperature, and element abundances. We study total element abundances that represent different parts of the Earth’s crust (continental crust, bulk silicate Earth, mid oceanic ridge basalt), CI chondrites and abundances measured in polluted white dwarfs. Results. For temperatures between ~600 and ~3500 K, the near-crust atmospheres of all considered total element abundances are mainly composed of H2O, CO2, and SO2 and in some cases of O2 and H2. For temperatures ≲500 K, only N2-rich or CH4-rich atmospheres remain. For ≳3500 K, the atmospheric gas is mainly composed of atoms (O, Na, Mg, and Fe), metal oxides (SiO, NaO, MgO, CaO, AlO, and FeO), and some metal hydroxides (KOH and NaOH). The inclusion of phyllosilicates as potential condensed species is crucial for lower temperatures, as they can remove water from the gas phase below about 700 K and inhibit the presence of liquid water. Conclusions. Measurements of the atmospheric composition could, in principle, characterise the rock composition of exoplanet crusts. H2O, O2 and CH4 are natural products from the outgassing of different kinds of rocks that had time to equilibrate. These are discussed as biomarkers, but they do emerge naturally as a result of the thermodynamic interaction between the crust and atmosphere. Only the simultaneous detection of all three molecules might be a sufficient biosignature, as it is inconsistent with chemical equilibrium.

Magma Ocean Evolution of the TRAPPIST-1 Planets

“…Further work may be warranted to understand if and under which conditions a reducing mantle composition could occur, which would lead to outgassing of CO and H 2 instead of CO 2 and H 2 O as assumed here (Katyal et al, 2020;Ortenzi et al, 2020). In addition to its effect on the redox state of the outgassed atmosphere, the mantle, and in particular the crust, composition has an important influence on the stability of liquid water on the surface after the magma ocean solidified (Herbort et al, 2020).…”

Aerosols in Exoplanet Atmospheres

“…The hottest rocky exoplanets, with T eq ≥ 1,000 K, are likely to have either no atmosphere or thin atmospheres in equilibrium with a molten surface. These atmospheres would be made of metal oxides like SiO, atomic and molecular oxygen, and atomic magnesium, sodium, iron, and other refractory elements, with abundance ratios dependent on the surface composition (Herbort et al, 2020;Ito et al, 2015;Kite et al, 2016;Miguel et al, 2011). Under these conditions, clouds of oxidized minerals (silicates, corundum, perovskite) and alkali salts, much like those proposed for hot Jupiter atmospheres, are likely to form (Mahapatra et al, 2017;Schaefer & Fegley, 2009;Schaefer et al, 2012).…”

Coexistence of CH4, CO2, and H2O in exoplanet atmospheres

“…7). Figure 3 shows the results of some full equilibrium condensation models for 18 elements from Herbort et al (2020). In these models, the total (= condensed + gas phase) element abundances are taken from different materials found in the Earth crust, meteorites and polluted white dwarfs, see explanation of abbreviations in the figure caption.…”