Sulfur dioxide in the mid-infrared transmission spectrum of WASP-39b.

The recent inference of sulfur dioxide (SO ₂ ) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations ^1-3 suggests that photochemistry is a key process in high-temperature exoplanet atmospheres ⁴ . This is because of the low (<1 ppb) abundance of SO ₂ under thermochemical equilibrium compared with that produced from the photochemistry of H ₂ O and H ₂ S (1-10 ppm) ^4-9 . However, the SO ₂ inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 μm and, therefore, the detection of other SO ₂ absorption bands at different wavelengths is needed to better constrain the SO ₂ abundance. Here we report the detection of SO ₂ spectral features at 7.7 and 8.5 μm in the 5-12-μm transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS) ¹⁰ . Our observations suggest an abundance of SO ₂ of 0.5-25 ppm (1σ range), consistent with previous findings ⁴ . As well as SO ₂ , we find broad water-vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 μm. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy-element content (metallicity) for WASP-39b of approximately 7.1-8.0 times solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.
(© 2024. The Author(s).)