Sulfur-driven haze formation in warm CO2-rich exoplanet atmospheres

Abstract

Sulfur gases significantly affect the photochemistry of planetary atmospheres in ourSolar System, and are expected to be important components in exoplanet atmospheres. However, sulfur photochemistry in the context of exoplanets is poorly understood due to a lack of chemical-kinetics information for sulfur species under relevant conditions.Here, we study the photochemical role of hydrogen sulfide (H2S) in warm CO2-rich exoplanet atmospheres (800 K) by carrying out laboratory simulations. We find that H2S plays a significant role in photochemistry, even when present in the atmosphere at relatively low concentrations (1.6%). It participates in both gas and solid phase chemistry, leading to the formation of other sulfur gas products (CH3SH/SO, C2H4S/OCS, SO2/S2, and CS2) and to an increase in solid haze particle production and compositional complexity. Our study shows that we may expect thicker haze with small particle sizes (20 to 140 nm) for warm CO2-rich exoplanet atmospheres that possess H2S.Observations 1-5 and laboratory simulations 6,7 have shown that clouds and/or hazes are likely ubiquitous in the atmospheres of exoplanets. These clouds and hazes play an important role in exoplanet atmospheres and affect the spectra of planets, therefore impacting our ability to observe their atmosphere and assess their habitability. Although a variety of atmospheric gases can condense at specific temperature and pressure conditions to form clouds, haze particles may be produced photochemically over a range of temperatures, pressures, and atmospheric compositions. 6−9 Among the various atmospheric components, sulfur gases significantly influence the photochemistry and haze formation in the atmospheres of Solar System bodies, such as Earth, Venus 10,11 , Jupiter 12,13 , and its moon, Io.

Aerosols in Exoplanet Atmospheres

“…Pavlov et al, 2001;Trainer et al, 2004;Wolf & Toon, 2010;G. Arney et al, 2016;He et al, 2018bHe et al, , 2020bVuitton et al, 2021), as well as sulfur (S 8 ) clouds (Hu et al, 2013). Many of these aerosols are highly reflective at optical wavelengths, facilitating future reflected light observations.…”

“…(2018) and He et al. (2018b, 2020a, 2020b) conducted a series of experiments targeting hazes in mini‐Neptunes and rocky planets with temperatures between 300 and 800 K incorporating gas mixtures dominated by H 2 , H 2 O, and CO 2 , with varying amounts of CH 4 , CO, NH 3 , N 2 , and H 2 S, as determined by equilibrium chemistry calculations. Both plasma discharge and UV energy sources were used.…”

“…Critically, exoplanet aerosol experiments have demonstrated that methane, long used in the exoplanet literature as an essential component of haze formation (e.g. Morley et al, 2015;Kawashima & Ikoma, 2018;Gao et al, 2020), is not always needed to produce substantial amounts of haze (Hörst et al, 2018;He et al, 2018b;Fleury et al, 2019;He et al, 2020bHe et al, , 2020a and that exoplanet hazes likely contain more than just hy- Figure 10. Laboratory hazes made from hydrogen-rich, water-rich, and carbon dioxide-rich atmospheres from 300 K to 600 K have a range of colors at visible wavelengths, some unlike those seen in solar system hazes (He et al, 2018a).…”

“…While methane may be an intermediary gas product in some of the experiments that use CO and CO 2 as the primarily carbon reservoir (e.g. Fleury et al, 2019), gas phase results show that it is not part of the chemical pathway in all cases, which instead seem more dependent on CO or CO 2 photolysis (He et al, 2019(He et al, , 2020b.…”

A major asymmetric ice trap in a planet-forming disk

“…Therefore, a quest has been ongoing to detect different molecules to try to account for the total volatile fraction of sulphur in >1 Myr protoplanetary disks especially as this is where gas-giant planets accrete the bulk of their atmospheres. Sulphur species may form hazes in the atmosphere (He et al 2020), and they are important for prebiotic chemistry (Ranjan et al 2018). CS is most commonly detected, followed by H 2 CS (Le Gal et al 2019;Codella et al 2021;Fedele & Favre 2020); yet although CS is the most abundant sulphur species in protoplanetary disks, it only accounts for <1% of the total sulphur budget.…”