Evolution and Spectral Response of a Steam Atmosphere for Early Earth with a Coupled Climate–Interior Model

The evolution of Earth’s early atmosphere and the emergence of habitable conditions on our planet are intricately coupled with the development and duration of the magma ocean (MO) phase during the early Hadean period (4–4.5 Ga). In this paper, we study the evolution of the steam atmosphere during the MO period. We obtain the outgoing longwave radiation (OLR) using the line-by-line radiative transfer code GARLIC. Our study suggests that an atmosphere consisting of pure H2O, built as a result of outgassing, extends the MO lifetime to several million years. The thermal emission as a function of the solidification timescale of an MO is shown. We study the effect of thermal dissociation of H2O at higher temperatures by applying atmospheric chemical equilibrium, which results in the formation of H2 and O2 during the early phase of the MO. A 1%–6% reduction in the OLR is seen. We also obtain the effective height of the atmosphere by calculating the transmission spectra for the whole duration of the MO. An atmosphere of depth 100 km is seen for pure water atmospheres. The effect of thermal dissociation on the effective height of the atmosphere is also shown. Due to the difference in the absorption behavior at different altitudes, the spectral features of H2 and O2 are seen at different altitudes of the atmosphere. Therefore, these species, along with H2O, have a significant contribution to the transmission spectra and could be useful for placing observational constraints on MO exoplanets.