Quartz Clouds in the Dayside Atmosphere of the Quintessential Hot Jupiter HD 189733 b

Recent mid-infrared observations with JWST's Mid-Infrared Instrument Low Resolution Spectrometer (MIRI LRS) have resulted in the first direct detections of absorption features from silicate clouds in the transmission spectra of two transiting exoplanets, WASP-17 b and WASP-107 b. In this Letter, we measure the mid-infrared (5-12 mu m) dayside emission spectrum of the benchmark hot Jupiter HD 189733 b with MIRI LRS by combining data from two secondary-eclipse observations. We confirm the previous detection of H2O absorption at 6.5 mu m from Spitzer's Infrared Spectrograph (IRS) and additionally detect H2S as well as an absorption feature at 8.7 mu m in both secondary-eclipse observations. The excess absorption at 8.7 mu m can be explained by the presence of small (similar to 0.01 mu m) grains of SiO2[s] in the uppermost layers of HD 189733 b's dayside atmosphere. This is the first direct detection of silicate clouds in HD 189733 b's atmosphere, and the first detection of a distinct absorption feature from silicate clouds on the dayside of any hot Jupiter. We find that models including SiO2[s] are preferred by 6-7 sigma over clear models and those with other potential cloud species. The high-altitude location of these silicate particles is best explained by formation in the hottest regions of HD 189733 b's dayside atmosphere near the substellar point. We additionally find that HD 189733 b's emission spectrum longward of 9 mu m displays residual features not well captured by our current atmospheric models. When combined with other JWST observations of HD 189733 b's transmission and emission spectra at shorter wavelengths, these observations will provide us with the most detailed picture to date of the atmospheric composition and cloud properties of this benchmark hot Jupiter.