Sulfur Hazes in Giant Exoplanet Atmospheres: Impacts on Reflected Light Spectra

TL;DR

Sulfur hazes in giant exoplanet atmospheres significantly alter their reflected light spectra by increasing albedo in certain wavelengths and masking molecular features, impacting direct imaging and atmospheric characterization.

Contribution

This study models the impact of sulfur hazes on exoplanet spectra, highlighting their potential to change observed colors and obscure molecular signatures in reflected light.

Findings

Sulfur hazes can increase albedo to ~0.7 between 0.5 and 1 μm.

Haze presence shifts planetary color from blue to orange.

Detection of sulfur hazes requires observations below 0.4 μm.

Abstract

Recent work has shown that sulfur hazes may arise in the atmospheres of some giant exoplanets due to the photolysis of H$_{2}$S. We investigate the impact such a haze would have on an exoplanet's geometric albedo spectrum and how it may affect the direct imaging results of WFIRST, a planned NASA space telescope. For temperate (250 K $<$ T$_{\rm eq}$ $<$ 700 K) Jupiter--mass planets, photochemical destruction of H$_{2}$S results in the production of $\sim$1 ppmv of \seight between 100 and 0.1 mbar, which, if cool enough, will condense to form a haze. Nominal haze masses are found to drastically alter a planet's geometric albedo spectrum: whereas a clear atmosphere is dark at wavelengths between 0.5 and 1 $\mu$m due to molecular absorption, the addition of a sulfur haze boosts the albedo there to $\sim$0.7 due to scattering. Strong absorption by the haze shortward of 0.4 $\mu$m results in albedos $<$0.1, in contrast to the high albedos produced by Rayleigh scattering in a clear atmosphere. As a result, the color of the planet shifts from blue to orange. The existence of a sulfur haze masks the molecular signatures of methane and water, thereby complicating the characterization of atmospheric composition. Detection of such a haze by WFIRST is possible, though discriminating between a sulfur haze and any other highly reflective, high altitude scatterer will require observations shortward of 0.4 $\mu$m, which is currently beyond WFIRST's design.