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Saturn’s Northern Summer Revealed by JWST

Presentation #401.02 in the session JWST Views of the Outer Planets and their Moons (Oral Presentation)

Published onOct 23, 2023
Saturn’s Northern Summer Revealed by JWST

The Saturn system, with its seasonal atmosphere, delicate rings, and myriad satellites, presented an ideal early target for JWST. Saturn’s extended disc, rapid rotation, and high infrared brightness provided a challenge for the small fields-of-view and high sensitivity of the mid-infrared MIRI integral field units. Saturn’s northern summertime hemisphere was mapped by JWST/MIRI (4.9-27.9 µm) in November 2022 using the Medium Resolution Spectrometer (MRS), as part of Guaranteed-Time programme 1247. This new dataset reveals the seasonal evolution of temperatures, aerosols, and chemical species in the five years since the end of the Cassini mission, showing 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 gaseous species due to summertime upwelling.

JWST reveals Saturn’s banded structure and discrete meteorological features in the troposphere. We are able to map the transitional spectral region between reflected sunlight and thermal emission (5.1-6.8 µm) for the first time, enabling retrievals of phosphine, ammonia, and water, alongside a stacked 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 observed in Jupiter’s equatorial zone. Reflectivity contrasts at 5-6 µm compare favourably with albedo contrasts observed by Hubble, and several discrete vortices are observed. The strongest latitudinal temperature gradients are co-located with the peaks of the zonal winds, implying that zonal winds decay with altitude above the clouds.

Saturn’s North Polar Stratospheric Vortex (NPSV) has warmed since 2017, is entrained by westward winds at p<10 mbar, and exhibits localised enhancements in several hydrocarbons. The detection of a warm equatorial stratospheric band in 2022 contradicts the expected 15-year repeatability of the equatorial oscillation. Saturn’s equatorial stratosphere is dominated by a stacked system of windshear zones, which 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. We present the first latitudinal distributions of stratospheric ethylene, benzene, methyl and carbon dioxide, and we report the first detection of new propane bands in the 8-11 µm region.

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