Hydrodynamic escape of mineral atmosphere from hot rocky exoplanet. I. Model description

Abstract

ABSTRACT Recent exoplanet statistics indicate that photo-evaporation has a great impact on the mass and bulk composition of close-in low-mass planets. While there are many studies addressing photo-evaporation of hydrogen- or water-rich atmospheres, no detailed investigation regarding rocky vapour atmospheres (or mineral atmospheres) has been conducted. Here, we develop a new 1D hydrodynamic model of the ultraviolet (UV)-irradiated mineral atmosphere composed of Na, Mg, O, Si, their ions and electrons, including molecular diffusion, thermal conduction, photo-/thermochemistry, X–ray and UV heating, and radiative line cooling (i.e. the effects of the optical thickness and non-local thermal equilibrium). The focus of this paper is on describing our methodology but presents some new findings. Our hydrodynamic simulations demonstrate that almost all of the incident X-ray and UV energy from the host star is converted into and lost by the radiative emission of the coolant gas species such as Na, Mg, Mg+, Si2+, Na3+, and Si3+. For an Earth-size planet orbiting 0.02 au around a young solar-type star, we find that the X-ray and UV heating efficiency is as small as 1 × 10−3, which corresponds to 0.3 M⊕ Gyr−1 of the mass-loss rate simply integrated over all the directions. Because of such efficient cooling, the photo-evaporation of the mineral atmosphere on hot rocky exoplanets with masses of 1 M⊕ is not massive enough to exert a great influence on the planetary mass and bulk composition. This suggests that close-in high-density exoplanets with sizes larger than the Earth radius survive in the high-UV environments.