James Webb Space Telescope Discovery

Saturn's Atmosphere in Northern Summer Revealed by JWST/MIRI

Abstract Saturn's northern summertime hemisphere was mapped by JWST/Mid-Infrared Instrument (4.9–27.9 µm) in November 2022, tracing the seasonal evolution of temperatures, aerosols, and chemical species in the 5 years since the end of the Cassini mission. The spectral region between reflected sunlight and thermal emission (5.1–6.8 µm) is mapped for the first time, enabling retrievals of phosphine, ammonia, and water, alongside a system of two aerosol layers (an upper tropospheric haze p < 0.3 bars, and a deeper cloud layer at 1–2 bars). Ammonia displays substantial equatorial enrichment, suggesting similar dynamical processes to those found in Jupiter's equatorial zone. Saturn's North Polar Stratospheric Vortex has warmed since 2017, entrained by westward winds at p < 10 mbar, and exhibits localized enhancements in several hydrocarbons. The strongest latitudinal temperature gradients are co-located with the peaks of the zonal winds, implying wind decay with altitude. Reflectivity contrasts at 5–6 µm compare favorably with albedo contrasts observed by Hubble, and several discrete vortices are observed. A warm equatorial stratospheric band in 2022 is not consistent with a 15-year repeatability for the equatorial oscillation. A stacked system of windshear zones dominates Saturn's equatorial stratosphere, and implies a westward equatorial jet near 1–5 mbar at this epoch. Lower stratospheric temperatures, and local minima in the distributions of several hydrocarbons, imply low-latitude upwelling and a reversal of Saturn's interhemispheric circulation since equinox. Latitudinal distributions of stratospheric ethylene, benzene, methyl, and carbon dioxide are presented for the first time, and we report the first detection of propane bands in the 8–11 µm region. Key Points Saturn's northern summertime hemisphere was mapped by JWST/Mid-Infrared Instrument (MIRI) to study seasonal evolution of temperatures, aerosols, and composition The data show evidence for changing temperatures and winds in the equatorial oscillation, polar vortices, and interhemispheric stratospheric circulation MIRI spectral coverage and sensitivity enables mapping of several gases for the first time, particularly in ranges inaccessible to Cassini Plain Language Summary The Saturn system, with its seasonally varying atmosphere, delicate rings, and myriad satellites, presented an ideal early target for JWST. Saturn's extended disc, rapid rotation, and infrared brightness provided a challenge for the small fields-of-view of the Mid-Infrared Instrument (MIRI), requiring a mosaic to map Saturn's northern summertime hemisphere. This exquisite data set reveals Saturn's banded structure, discrete vortices, the warm polar vortices, and the continued evolution of an oscillatory pattern of warm and cool anomalies over Saturn's equator. We show evidence that a stratospheric circulation pattern detected by Cassini during northern winter has now fully reversed in northern summer, with the low-latitude stratosphere being cool and depleted in aerosols due to summertime upwelling. MIRI provides access to spectral regions that were not possible with the Cassini spacecraft, particularly in the 5–7 µm region where reflected sunlight and thermal emission blend together. Ammonia and phosphine are enriched at Saturn's equator, suggesting strong mixing from the deeper troposphere. MIRI's high sensitivity enables the first identification of previously unseen emission propane bands, along with the first measurements of the distribution of several gaseous species: tropospheric water, and stratospheric ethylene, benzene, methyl, and carbon dioxide. Credit: JGR Planets Montage of JWST Mid-Infrared Instrument/Medium Resolution Spectrometer observations of Saturn. Panel (a) shows RGB composites of the JWST observations (Saturn: R = 10.3 µm, G = 10.1 µm, B = 11.6 µm and rings: R = 15.5 µm, G = 14.6 µm, B = 13.5 µm) with an HST observation of Saturn in the background (Simon et al., 2023). Panels (b–e) show spatial structure on Saturn at a range of wavelengths as indicated by the gray shaded regions in panel (f). (f) Shows the average spectrum of Saturn with specific spectral features labeled.
Credit: Space.com
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