Outgassing

Physics

Geology

Atmosphere (unit)

Astronomy

Terrestrial planet

Atmospheric escape

Silicate

Astrophysics

Planet

Mantle (geology)

Geophysics

Astrobiology

Thermodynamics

Astronomy and Astrophysics

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).

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