Linking the evolution of terrestrial interiors and an early outgassed atmosphere to astrophysical observations

Abstract

Context.A terrestrial planet is molten during formation and may remain molten due to intense insolation or tidal forces. Observations favour the detection and characterisation of hot planets, potentially with large outgassed atmospheres. Aims. We aim to determine the radius of hot Earth-like planets with large outgassing atmospheres. Our goal is to explore the differences between molten and solid silicate planets on the mass-radius relationship and transmission and emission spectra.Methods. An interior-atmosphere model was combined with static structure calculations to track the evolving radius of a hot rocky planet that outgasses CO 2 and H 2 O. We generated synthetic emission and transmission spectra for CO 2 and H 2 O dominated atmospheres. Results. Atmospheres dominated by CO 2 suppress the outgassing of H 2 O to a greater extent than previously realised since previous studies applied an erroneous relationship between volatile mass and partial pressure. We therefore predict that more H 2 O can be retained by the interior during the later stages of magma ocean crystallisation. Formation of a surface lid can tie the outgassing of H 2 O to the efficiency of heat transport through the lid, rather than the radiative timescale of the atmosphere. Contraction of the mantle, as it cools from molten to solid, reduces the radius by around 5%, which can partly be offset by the addition of a relatively light species (e.g. H 2 O versus CO 2 ) to the atmosphere. Conclusions. A molten silicate mantle can increase the radius of a terrestrial planet by around 5% compared to its solid counterpart, or equivalently account for a 13% decrease in bulk density. An outgassing atmosphere can perturb the total radius, according to its composition, notably the abundance of light versus heavy volatile species. Atmospheres of terrestrial planets around M-stars that are dominated by CO 2 or H 2 O can be distinguished by observing facilities with extended wavelength coverage (e.g. JWST).

Redox state of Earth’s magma ocean and its Venus-like early atmosphere

“…The degree to which cooling atmospheres remain closed to chemical exchange with the underlying mantle, as modeled above, depends on the thermal and dynamic evolution of the coupled magma ocean-atmosphere system. A rheological transition from a suspension to an interlocking matrix in partially molten systems occurs when the melt fraction decreases below ~0.4 ( 63 ), causing a decrease in convective vigor and therefore heat delivery to the base of the atmosphere ( 46 ). This threshold is crossed when mantle potential temperatures decline to ~1700 K ( 45 ), with atmospheric cooling occurring over short time scales of ~10 3 years thereafter ( 45 , 46 ).…”

“…A rheological transition from a suspension to an interlocking matrix in partially molten systems occurs when the melt fraction decreases below ~0.4 ( 63 ), causing a decrease in convective vigor and therefore heat delivery to the base of the atmosphere ( 46 ). This threshold is crossed when mantle potential temperatures decline to ~1700 K ( 45 ), with atmospheric cooling occurring over short time scales of ~10 3 years thereafter ( 45 , 46 ). At this point, the mantle and atmosphere are no longer in thermochemical equilibrium and can be considered as discrete thermodynamic entities.…”

“…The two most reduced samples had lost ~10% of their Fe by volatility, with a concomitant decrease in Si/Mg by ~4% relative. The average composition for the peridotite glass, by weight, was 46…”

Vertically Resolved Magma Ocean–Protoatmosphere Evolution: H2, H2O, CO2, CH4, CO, O2, and N2as Primary Absorbers

“…Recent studies exploring the super‐Earth to mini‐Neptune transition argue for a substantial influence of H 2 on the mass–radius relation due to its nonideal partitioning into the planetary interior (Chachan & Stevenson, 2018; Kite et al., 2019). Whether or not Earth‐sized planets in temperate orbits can acquire substantial H 2 atmospheres remains debated, but this could alter the planetary radius if the interior remains molten due to the difference in density between molten and solid rock (Bower et al., 2019). Probing this transition with upcoming space missions will inform whether young rocky planets experience long‐lived phases of melt–solid–atmosphere interaction as we predict for the H 2 case.…”

“…The partial pressure of a volatile then relates to the mass of the volatile in the atmosphere (Bower et al., 2019): with planetary radius at the interior–atmosphere interface R p , molar mass μ i of species i , mean atmospheric molar mass , and gravity g . We assume O 2 to be insoluble and thus its atmospheric mass is constant with time.…”