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

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 $\mu$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 $\mu$m in the 5-12 $\mu$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$\sigma$ 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 $\mu$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

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