A Volcanic Hydrogen Habitable Zone

Abstract

The classical habitable zone is the circular region around a star in which liquid water could exist on the surface of a rocky planet. The outer edge of the traditional N2-CO2-H2O habitable zone (HZ) extends out to nearly 1.7 AU in our Solar System, beyond which condensation and scattering by CO2 outstrips its greenhouse capacity. Here, we show that volcanic outgassing of atmospheric H2 on a planet near the outer edge can extend the habitable zone out to ~2.4 AU in our solar system. This wider volcanic hydrogen habitable zone (N2-CO2-H2O-H2) can be sustained as long as volcanic H2 output offsets its escape from the top of the atmosphere. We use a single-column radiative-convective climate model to compute the HZ limits of this volcanic hydrogen habitable zone for hydrogen concentrations between 1% and 50%, assuming diffusion-limited atmospheric escape. At a hydrogen concentration of 50%, the effective stellar flux required to support the outer edge decreases by ~35% to 60% for M to A stars. The corresponding orbital distances increase by ~30% to 60%. The inner edge of this HZ only moves out by ~0.1 to 4% relative to the classical HZ because H2 warming is reduced in dense H2O atmospheres. The atmospheric scale heights of such volcanic H2 atmospheres near the outer edge of the HZ also increase, facilitating remote detection of atmospheric signatures.