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


Sulfur gases substantially affect the photochemistry of planetary atmospheres in our Solar 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 prominent 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 (20–140 nm) for warm CO2-rich exoplanet atmospheres that possess H2S.

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Fig. 1: Simplified schematic of the PHAZER chamber for current work.
Fig. 2: AFM images of the particles on mica substrates.
Fig. 3: Size distribution of the haze particles formed in plasma and UV experiments, with H2S or no H2S.
Fig. 4: Haze particle production rate in the plasma and UV experiments.
Fig. 5: The mass spectra of the gas mixtures with H2S and without H2S for initial gas mixture, plasma on and UV on.
Fig. 6: Relative yields of the gas products in the plasma and UV experiments.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request. The data used to create Figs. 36 are available in the Source Data files.


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This work was supported by the NASA Astrophysics Research and Analysis Program NNX17AI87G. X.Y. is supported by a 51 Pegasi b Fellowship. S.E.M. is supported by NASA Earth and Space Science Fellowship 80NSSC18K1109.

Author information




C.H., S.M.H., N.K.L., M.S.M. and J.I.M. conceived the study. J.I.M. calculated the starting gas mixtures. C.H. carried out the experiments and MS measurements. C.H. and X.Y. performed the AFM measurements. C.H. conducted the data analysis and prepared the manuscript. All authors participated in discussions regarding interpretation of the results and edited the manuscript.

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Correspondence to Chao He.

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Source data

Source Data Fig. 3

Numerical data used to generate graphs in Fig. 3

Source Data Fig. 4

Numerical data used to generate graphs in Fig. 4

Source Data Fig. 5

Numerical data used to generate graphs in Fig. 5

Source Data Fig. 6

Numerical data used to generate graphs in Fig. 6

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He, C., Hörst, S.M., Lewis, N.K. et al. Sulfur-driven haze formation in warm CO2-rich exoplanet atmospheres. Nat Astron 4, 986–993 (2020).

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