paper

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

arXiv:2407.07965 · doi:10.1038/s41586-024-07040-9

Abstract

The recent inference of sulphur dioxide (SO $_{2}$ ) in the atmosphere of the hot ( $\sim$ 1100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations suggests that photochemistry is a key process in high temperature exoplanet atmospheres. This is due to the low ( $<$ 1 ppb) abundance of SO $_{2}$ under thermochemical equilibrium, compared to that produced from the photochemistry of H $_{2}$ O and H $_{2}$ S (1-10 ppm). However, the SO $_{2}$ inference was made from a single, small molecular feature in the transmission spectrum of WASP-39b at 4.05 $I^¼$ m, 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 $I^¼$ m in the 5-12 $I^¼$ m transmission spectrum of WASP-39b measured by the JWST Mid-Infrared Instrument (MIRI) Low Resolution Spectrometer (LRS). Our observations suggest an abundance of SO $_{2}$ of 0.5-25 ppm (1 $I ¨ $ range), consistent with previous findings. In addition to SO $_{2}$ , we find broad water vapour absorption features, as well as an unexplained decrease in the transit depth at wavelengths longer than 10 $I^¼$ m. Fitting the spectrum with a grid of atmospheric forward models, we derive an atmospheric heavy element content (metallicity) for WASP-39b of $\sim$ 7.1-8.0 $\times$ solar and demonstrate that photochemistry shapes the spectra of WASP-39b across a broad wavelength range.

Published in Nature