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

Abstract

The recent inference of sulfur dioxide (SO2) in the atmosphere of the hot (approximately 1,100 K), Saturn-mass exoplanet WASP-39b from near-infrared JWST observations1–3 suggests that photochemistry is a key process in high-temperature exoplanet atmospheres4. This is because of the low (<1 ppb) abundance of SO2 under thermochemical equilibrium compared with that produced from the photochemistry of H2O and H2S (1–10 ppm)4–9. However, the SO2 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 SO2 absorption bands at different wavelengths is needed to better constrain the SO2 abundance. Here we report the detection of SO2 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 SO2 of 0.5–25 ppm (1σ range), consistent with previous findings4. As well as SO2, 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.

Automated Chemical Reaction Network Generation and Its Application to Exoplanet Atmospheres

“…Figure 7 shows the comparison between the averaged theoretical transmission spectra generated by EPACRIS and JWST observations of WASP-39 b (Alderson et al 2023;Feinstein et al 2023;Powell et al 2024). The EPACRISpredicted transmission spectra are broadly consistent with the near-and mid-infrared observations.…”

“…However, they overpredict the transit depth in the 7-8 μm wavelength range, which corresponds to the dominant SO 2 absorption feature when compared to the MIRI data reported by Powell et al (2024). Consequently, using the same photochemical networks resulted in the lowest χ 2 value of 2.51 when assuming 7.5× solar metallicity, while the χ 2 for 10× solar metallicity was 2.91 (Powell et al 2024). In contrast, the predicted transmission spectra using the photochemical network from the current study showed an even lower χ 2 of 2.10.…”

“…The predicted transmission spectra generated by the previous photochemical networks, assuming 10× solar metallicity, as discussed in Tsai et al (2023), align well with the NIRSpec/G395H spectra (Feinstein et al 2023). However, they overpredict the transit depth in the 7-8 μm wavelength range, which corresponds to the dominant SO 2 absorption feature when compared to the MIRI data reported by Powell et al (2024). Consequently, using the same photochemical networks resulted in the lowest χ 2 value of 2.51 when assuming 7.5× solar metallicity, while the χ 2 for 10× solar metallicity was 2.91 (Powell et al 2024).…”

“…These choices facilitate a direct comparison between the EPACRISsimulated WASP-39 b and WASP-80 b atmospheres and published results. After the model has converged and reached the steady state, we compute the synthetic transmission spectra of WASP-39 b and WASP-80 b based on the molecular mixing ratio profiles (Figures 4 and 8), using the transmission spectra generation module of EPACRIS (Hu et al 2013), and compare the resulting transmission spectra with JWST observations (Bell et al 2023;Rustamkulov et al 2023;Powell et al 2024).…”

“…This integration facilitates the fast and reliable construction of tailored reaction networks for specific exoplanet atmospheres. This paper details our methodology and demonstrates its effectiveness using the well-characterized atmosphere of WASP-39 b and the atmosphere of WASP-80 b as case studies for two different types of H 2 -dominated hot or warm Jupiters, compared with recent JWST observations and photochemical modeling studies (Ahrer et al 2023;Alderson et al 2023;Bell et al 2023;Feinstein et al 2023;JWST Early Release Science Team et al 2023;Rustamkulov et al 2023;Tsai et al 2023;Powell et al 2024).…”