Sulfate Aerosol Hazes and SO 2 Gas as Constraints on Rocky Exoplanets’ Surface Liquid Water

Abstract

Despite surface liquid water’s importance to habitability, observationally diagnosing its presence or absence on exoplanets is still an open problem. Inspired within the solar system by the differing sulfur cycles on Venus and Earth, we investigate thick sulfate (H 2 SO 4 –H 2 O) aerosol haze and high trace mixing ratios of SO 2 gas as observable atmospheric features whose sustained existence is linked to the near absence of surface liquid water. We examine the fundamentals of the sulfur cycle on a rocky planet with an ocean and an atmosphere in which the dominant forms of sulfur are SO 2 gas and H 2 SO 4 –H 2 O aerosols (as on Earth and Venus). We build a simple but robust model of the wet, oxidized sulfur cycle to determine the critical amounts of sulfur in the atmosphere–ocean system required for detectable levels of SO 2 and a detectable haze layer. We demonstrate that for physically realistic ocean pH values (pH ≳ 6) and conservative assumptions on volcanic outgassing, chemistry, and aerosol microphysics, surface liquid water reservoirs with greater than 10 −3 Earth oceans are incompatible with a sustained observable H 2 SO 4 –H 2 O haze layer and sustained observable levels of SO 2 . Thus, we propose the observational detection of an H 2 SO 4 –H 2 O haze layer and of SO 2 gas as two new remote indicators that a planet does not host significant surface liquid water.