JWST NIRCam and NIRSpec Mapping of Bands, Waves, and Vortices in Saturn's Mid-Latitudes and Polar Domains

JWST’s infrared exploration of the Saturn system has been designed to extend the legacy of the Cassini-Huygens mission, using the exquisite sensitivity, spectral coverage, and spectral resolution of the integral field units (IFUs) to reveal new insights into the atmosphere, ionosphere, rings and moons.  Guaranteed-time programme GTO1247 used MIRI/MRS (4.9-28.5 µm) to explore Saturn’s seasonal circulation patterns during northern summer [1], and a combination of NIRSpec/IFU (1.8-5.3 µm) and MIRI/MRS to diagnose the icy composition of Saturn’s rings and small moons [2].  GTO1251 explored Titan’s atmosphere[3], GTO1250 revealed the extent of Enceladus’ watery plumes [4], while guest-observer programme GO3716 explored Saturn’s irregular satellites [5]. 

NIRCam Imaging (June 2023):  NIRCam observations of Saturn at 3.23 µm (GTO1247) reveal structures never-before-seen in Saturn’s upper atmosphere (Fig. 1). Observations in this strong methane absorption band, overlain by methane fluorescence, show diffuse patterns across the disc, with dark polar domains in the north and south in Fig. 2, a faint auroral oval in the north, and unexpected structure (patches of brightness and darkness) extending from the poles towards mid-latitudes suggesting high-altitude wave activity.  Conversely, images at 2.12 µm reveal bright reflection from polar hazes, with the northern hexagon (78^oN) and ribbon wave (48^oN) clearly visible, and small cyclones and anticyclones observed in both hemispheres.  Brightness gradients at 2.12 µm reveal the edges of Saturn’s bands (Fig. 2), coinciding with zonal jets measured by Cassini.  Intriguingly, zonally-averaged gradients at 3.23 µm also show peaks at the jet locations, suggesting that the winds provide zonal organisation to the 3.23 µm structure and that both stratospheric aerosols (sculpted by zonal winds) and CH₄ fluorescence (likely independent of the winds) are contributing to the unusual appearance at 3.23 µm.

Figure 1 NIRCam images of Saturn at 3.23 µm (left, with rings saturated to show atmospheric detail) and 2.12 µm (right), acquired in GTO1247 in June 2023.

 

Figure 2 Polar projections of NIRCam data at 3.23 µm (top row) and 2.12 µm (bottom row), for northern and southern hemispheres.  The auroral oval is evident at 3.23 µm (top left); and small vortices, the hexagon and ribbon can be seen at 2.12 µm (bottom left).  Zonally-averaged brightnesses (and their poleward gradients) are shown on the right, compared to the locations of Saturn’s prograde jet peaks (vertical dashed lines).

 

NIRSpec Spectroscopy (November 2024):  Missing from the JWST Saturn observations of Cycles 1 and 2 was NIRSpec/IFU spectroscopy, without which it would be challenging to distinguish ionospheric emissions from H₃⁺ or CH₄ fluorescence from reflection from stratospheric hazes.  In Cycle 3, GO 5308 [6] acquired a full NIRSpec/IFU map of Saturn’s northern hemisphere poleward of 45^oN, spanning 2.8-5.3 µm using the F290LP/G395H filter/grating.  Saturn was observed continuously between 04:00-13:58 UT on 29 November 2024 to observe all longitudes, resulting in 26 spectral cubes each constructed from four dithers.  Cubes were processed using a custom pipeline to reduce saturation and assign geometry [7], producing polar projections (Fig. 3) and zonally-averaged spectra (Fig. 4).

 

Results:  Combining the 2023 NIRCam observations with the 2024 NIRSpec observations, we observe the following:

• Extratropical and Polar Belt/Zone structure: Zonal jets organise both reflectivity and thermal emission, even in the CH₄ fluorescence band near 3.32 µm (sounding high altitude). Latitudinal banding at 5-µm is on a scale finer than the zonal jets, potentially due to different latitudinal distributions of at least two aerosol layers: an upper tropospheric haze p < 0.3 bars, and a deeper cloud layer at 1–2 bars.  Latitudinal gradients in aerosol opacity, phosphine, ammonia, and water, will be compared to the jet locations to search for evidence of vertical mixing on the scale of Saturn’s belts and zones. 
• Polar Domain and Hexagon: As previously reported by MIRI/MRS in 2022 [1], NIRSpec shows that the north pole remains dark at 5 µm and surrounded by a cloud-free band of bright emission, consistent with its appearance in Cassini/VIMS observations in 2016 [8].  This latitudinal contrast prevents us from seeing Saturn’s hexagon (78^oN) in thermal emission, but reflected sunlight near 2-3 µm clearly reveals the six vertices of the hexagon with a westward translation between 2023 and 2024.  These data are possibly our last view of the hexagon before it becomes hidden in autumn darkness after Saturn’s equinox (May 2025), and confirm that the hexagon has now existed for at least ~45 years (since discovery by Voyager).
• Discrete Atmospheric Features: NIRCam (Fig. 2) reveals the presence of a long-lived cyclone-anticyclone pair (one dark, one bright) near 60-65^oN that were also evident in Hubble observations in 2022-23.  The NIRSpec data at 3 µm (Fig. 3) reveal the same vortex pair near 60^oN, 135^oW in 2024.  In the southern hemisphere, NIRCam reveals reflective ovals near 44^oS and 55^oS that also appear to have persisted for several years (observed by Hubble as the south emerged from winter darkness).
• Large-Scale Upper Atmospheric Structure: Both NIRCam and NIRSpec reveal significant longitudinal structure in the 3.2-3.3 µm range sensitive to high-altitude aerosols and CH₄ fluorescence (Fig. 3-4). NIRSpec reveals at least four dark patches extending equatorward of the dark north polar domain, creating a wave-like appearance at mid-latitudes that has changed between 2023 and 2024.  NIRCam reveals hints of similar structure around the South Pole.  We will present spectral retrieval analyses to determine the likely origin (CH₄ fluorescence, stratospheric hazes) for these unexpected structures.

 

Figure 3 Polar projections of NIRSpec data in November 2024, showing brightness (top row) and residual from the zonal mean (bottom row) at 5.1 µm (showing deep thermal emission), 3.0 µm (showing aerosol reflection), and 3.32 µm (a blend of CH₄ fluorescence and aerosol reflection).

 

Figure 4 Zonally-averaged NIRSpec spectra at 55 and 75^oN, with key gaseous absorptions (and fluorescent emission) labelled.

 

References

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• Moore et al., Hunting for the source of Saturn's atmospherically driven aurora. 2024. 5308.
• King et al., 2023, DOI: 10.3847/2515-5172/ad045f.
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