Mantle redox state drives outgassing chemistry and atmospheric composition of rocky planets

Abstract

Volcanic degassing of planetary interiors has important implications for their corresponding atmospheres. The oxidation state of rocky interiors affects the volatile partitioning during mantle melting and subsequent volatile speciation near the surface. Here we show that the mantle redox state is central to the chemical composition of atmospheres while factors such as planetary mass, thermal state, and age mainly affect the degassing rate. We further demonstrate that mantle oxygen fugacity has an effect on atmospheric thickness and that volcanic degassing is most efficient for planets between 2 and 4 Earth masses. We show that outgassing of reduced systems is dominated by strongly reduced gases such as $$\text {H}_{2}$$H2, with only smaller fractions of moderately reduced/oxidised gases ($$\text {CO}$$CO, $$\text {H}_{2}\text {O}$$H2O). Overall, a reducing scenario leads to a lower atmospheric pressure at the surface and to a larger atmospheric thickness compared to an oxidised system. Atmosphere predictions based on interior redox scenarios can be compared to observations of atmospheres of rocky exoplanets, potentially broadening our knowledge on the diversity of exoplanetary redox states.

Characterisation of the hydrospheres of TRAPPIST-1 planets

“…The minimum limit for the thicknesses is zero, which corresponds to the case of a dry composition. Ortenzi et al (2020) estimated that for a planet of 1-1.5 M ⊕ the maximum atmospheric thickness due to the outgassing of an oxidised mantle is 200 km, which is compatible with the ranges we obtain for the atmospheric thicknesses. Scenario 2 shows the same trends for the atmospheric parameters but with lower atmospheric mass and surface pressure.…”

“…Planet d appears to be compatible with a planet with a CO 2 -dominated atmosphere and CMF between 0.2 and 0.3, which is a very likely CMF range for TRAPPIST-1 planets based on our analysis. Surface pressures lower than 300 bar would yield lower atmospheric thicknesses, and so it would be necessary to consider a lower CMF to explain the observed density of planet d. CO 2 in the case of planet d can be provided by volcanic outgassing (Ortenzi et al 2020), as its internal heat flux produced by tidal heating is in the range 0.04-2 W m −2 , which favours plate tectonics (Papaloizou et al 2018). Secondary CO 2 -dominated atmospheres could have traces of O 2 , N 2 , and water vapour.…”

Magma Ocean Evolution of the TRAPPIST-1 Planets

“…However, even for early Earth this assumption has come under scrutiny (Kasting et al, 1993b;Gaillard and Scaillet, 2014;Ehlmann et al, 2016;Fegley et al, 2016;Schaefer et al, 2016;Armstrong et al, 2019;Del Genio et al, 2020). 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).…”

Oxygen False Positives on Habitable Zone Planets Around Sun‐Like Stars

“…Scenarios 1 (high CO 2 :H 2 O perpetual runaway greenhouse) and 2 (waterworlds) are viable under a more reducing (iron‐wüstite buffer) initial mantle (supporting information Section ). This counterintuitive result occurs because, even though degassed volatiles are likely to be more reducing, total volatile concentrations in the melt phase are typically lower due to graphite saturation (Grott et al., 2011; Ortenzi et al., 2020). Moreover, crustal sinks are precluded by high overburden pressure, regardless of the redox state of the crustal material.…”