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

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 the Wide Field Infrared Survey Telescope (WFIRST), a planned NASA space telescope. For temperate (250 K<T eq <700 K) Jupiter-mass planets, photochemical destruction of H 2 S results in the production of ∼1 ppmv of S 8 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 μm, due to molecular absorption, the addition of a sulfur haze boosts the albedo there to ∼0.7, due to scattering. Strong absorption by the haze shortward of 0.4 μm 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 μm, which is currently beyond WFIRST's design.

Detectable Molecular Features above Hydrocarbon Haze via Transmission Spectroscopy with JWST: Case Studies of GJ 1214b-, GJ 436b-, HD 97658b-, and Kepler-51b-like Planets

“…Besides hydrocarbon haze, the existence of other aerosols can mute spectral features in transmission: condensation clouds such as KCl, ZnS, K 2 SO 4 , ZnO, and graphite clouds in the temperature range of interest in this study ( 1000 K) (Miller-Ricci Kempton et al 2012;Morley et al 2013;Mbarek & Kempton 2016;Ohno & Okuzumi 2018;Gao & Benneke 2018;Ormel & Min 2019) and photochemical haze made of sulfur species (Hu et al 2013;Zahnle et al 2016;Gao et al 2017).…”

A Comparison of Simulated JWST Observations Derived from Equilibrium and Non-equilibrium Chemistry Models of Giant Exoplanets

“…As for oxygen, its elemental abundance is harder to constrain as water is so much out of equilibrium on Jupiter (and the probe fell into a hot presumably dry spot and did not finish it's measurements). For exoplanet analysis, it has been a popular assumption to use solar metallicity Fortney et al 2006;Fortney 2007;Fortney et al 2008;Venot et al 2012;Moses et al 2011;Koskinen et al 2013;Zahnle & Marley 2014;Wakeford & Sing 2015;Heng 2016;Kataria et al 2016;Barstow et al 2017;Gao et al 2017), or a linear scaling of solar metallicity (Freedman et al 2014;Kataria et al 2015;Charnay et al 2015;Wakeford et al 2017), and hence why we chose to model it as one of our metallicity cases. The bulk composition of a planet's atmosphere plays a major role in the chemistry at work in the planet's atmosphere.…”

Exoplanet Clouds

“…Carbon-binding materials never dominate the cloud particle material in the summarised cases where C/O changes from oxygen-rich (C/O=0.54) to carbon-rich (C/O=1.1), except for the warm model for C/O=2.0. Sulfur haze has been suggested to emerge in cold gas giants (<700K, Gao et al 2017). An overabundance of sulfur was confirmed for Uranus (H2S ice particles detected, see de Pater 2018), and volcanic sulfur as trigger for snowball Earth (Macdonald & Wordsworth 2017).…”