Sulfate Aerosol Hazes and SO₂ 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 a 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.

Distinguishing between Wet and Dry Atmospheres of TRAPPIST-1 e and f

“…For the dry scenarios we do not consider any wet deposition. Loftus et al (2019) suggest that Figure 18. Number of transits required to detect CO with the cross-correlation technique between 2.3 and 2.45 μm with ELT (Southern sky) or TMT (Northern sky) in the atmosphere of hypothetical planets with the same properties as TRAPPIST-1e but around SPECULOOS targets.…”

Oxygen False Positives on Habitable Zone Planets Around Sun‐Like Stars

“…There are potentially other diagnostic spectral signatures of desertworlds such as spatial variation in atmospheric water vapor and photochemistry that could be tested using general circulation models and photochemical models. The presence of long‐lived sulfuric acid hazes (Loftus et al., 2019) has been proposed as putting an upper bound on surface water abundances, but the desertworlds considered here likely have larger surface water inventories than this threshold.…”

Unveiling Shrouded Oceans on Temperate sub-Neptunes via Transit Signatures of Solubility Equilibria versus Gas Thermochemistry

“…Therefore, the gas-phase thermochemical equilibrium would be achieved in the deep and hot part of a massive atmosphere, and in contrast, it would not be achieved in a small atmosphere overlying a liquid-water ocean. Instead, NH 3 and sulfur species would be sequestered by the ocean (Loftus et al 2019, and also see Section 3) and the abundance of CO 2 would be set by the ocean chemistry (Figure 2, with the cosmochemical and geological constraints detailed in the Appendix). This fundamental difference, coupled with atmospheric photochemistry, leads to distinctive gas abundances in the observable part (<∼0.1 bar) of the atmosphere.…”