<figure><img src="image_21.png" /><figcaption>**Figure 17.** Distributions of stratospheric species at 0.5 mbar: acetylene, ethane, ethylene, propane, $CO_2$, benzene, methacetylene (propyne), diacetylene (1,3-butadiyne), and methyl in Saturn's northern summer, compared to the predictions of the neutral photochemistry model for $L_8 = 150^\circ$ (Moses & Greathouse, 2005; Moses et al., 2007). The photochemical model is shown in red, and is referenced to the right-hand axis so that both the shape of the distribution and differences in absolute abundances can be compared. Error bars are shown in gray, but these do not include uncertainties due to scaling a latitudinally uniform a priori profile. If the shape of the profile changes significantly with latitude (as is expected for benzene and $C_4H_2$ from previous studies), this could influence the retrieved values. Vertical dotted lines show the latitudes of tropospheric eastward jets. Gases in panels (a–c) were derived as full vertical profiles, other gases are derived as scale factors for our a priori profiles. Gray points in panel (c) signify a lack of obvious detection of $C_2H_4$ emission by eye.</figcaption></figure>

uncertainties). The seasonal model overpredicts the estimated low-latitude abundance ($2.3 \pm 0.5$ ppb at 0.5 mbar compared to ~12 ppb from the model) by a factor of 4, but the data are more consistent with updated models for Saturn's stratosphere (1–2 ppb at 0.5 mbar from Figure 6 of Moses et al. (2015)).

Nevertheless, the polar maximum in $C_2H_4$ in Figure 17c is unexpected on the grounds of neutral photochemistry, and is suggestive of either subsidence of hydrocarbon-rich air from higher altitudes (leading to the maxima in ethane and acetylene), or due to an enhanced contribution from ion-neutral chemistry at the highest latitudes. Distinguishing these scenarios will require future modeling work for the chemistry of the NPSV.

### 5.4.3. $C_3$, $C_4$, and $C_6$ Hydrocarbons

Propane is detected as a perturbation to the stronger $C_2H_2$ lines near 13.37 µm (indicated in Figure 12), which results in the large uncertainties on the distribution in Figure 17d. Propane bands $\nu_{20}$ and $\nu_{21}$ are observed near 9.4 and 10.7 µm, respectively (see Figure S6 in Supporting Information S1), but primarily at high latitudes. The strongest detections are within the warm NPSV, with abundances near 200 ± 10 ppb, and a relatively uniform distribution with latitude, consistent with that found by Sylvestre et al. (2015). It is clear that $C_3H_8$ does not follow
<figure><img src="image_22.png" /><figcaption>**Figure 17.** Distributions of stratospheric species at 0.5 mbar: acetylene, ethane, ethylene, propane, $CO_2$, benzene, methacetylene (propyne), diacetylene (1,3-butadiyne), and methyl in Saturn's northern summer, compared to the predictions of the neutral photochemistry model for $L_8 = 150^\circ$ (Moses & Greathouse, 2005; Moses et al., 2007). The photochemical model is shown in red, and is referenced to the right-hand axis so that both the shape of the distribution and differences in absolute abundances can be compared. Error bars are shown in gray, but these do not include uncertainties due to scaling a latitudinally uniform a priori profile. If the shape of the profile changes significantly with latitude (as is expected for benzene and $C_4H_2$ from previous studies), this could influence the retrieved values. Vertical dotted lines show the latitudes of tropospheric eastward jets. Gases in panels (a–c) were derived as full vertical profiles, other gases are derived as scale factors for our a priori profiles. Gray points in panel (c) signify a lack of obvious detection of $C_2H_4$ emission by eye.</figcaption></figure>

uncertainties). The seasonal model overpredicts the estimated low-latitude abundance ($2.3 \pm 0.5$ ppb at 0.5 mbar compared to ~12 ppb from the model) by a factor of 4, but the data are more consistent with updated models for Saturn's stratosphere (1–2 ppb at 0.5 mbar from Figure 6 of Moses et al. (2015)).

Nevertheless, the polar maximum in $C_2H_4$ in Figure 17c is unexpected on the grounds of neutral photochemistry, and is suggestive of either subsidence of hydrocarbon-rich air from higher altitudes (leading to the maxima in ethane and acetylene), or due to an enhanced contribution from ion-neutral chemistry at the highest latitudes. Distinguishing these scenarios will require future modeling work for the chemistry of the NPSV.

### 5.4.3. $C_3$, $C_4$, and $C_6$ Hydrocarbons

Propane is detected as a perturbation to the stronger $C_2H_2$ lines near 13.37 µm (indicated in Figure 12), which results in the large uncertainties on the distribution in Figure 17d. Propane bands $\nu_{20}$ and $\nu_{21}$ are observed near 9.4 and 10.7 µm, respectively (see Figure S6 in Supporting Information S1), but primarily at high latitudes. The strongest detections are within the warm NPSV, with abundances near 200 $\pm$ 10 ppb, and a relatively uniform distribution with latitude, consistent with that found by Sylvestre et al. (2015). It is clear that $C_3H_8$ does not follow
the predictions of seasonal photochemistry, suggesting the influence of meridional circulation on the distribution of long-lived propane (e.g., similar to the argument for C₂H₆, Moses & Greathouse, 2005).

Unlike propane and ethane, the unsaturated hydrocarbons *methylacetylene* (C₃H₄) and *diacetylene* (C₄H₂) are relatively short-lived species that track the photochemical model predictions at latitudes equatorward of 45°N, with C₃H₄ in particular showing the expected equator-to-pole contrast in Figure 17g. C₄H₂ shows a local minimum near 20°–40°N that has evolved from the local maximum observed by Cassini in 2005–2006 (Guerlet et al., 2010), possibly related to the onset of low-latitude stratospheric upwelling during northern summer. Surprisingly, C₄H₂ then increases toward the NPSV where some of the highest abundances are obtained in Figure 17h. This is unlikely to be a circulation effect, as C₄H₂ has only a slightly shorter loss timescale than C₃H₄ (Guerlet et al., 2010), so we might expect to see the same structure in C₃H₄. Photolysis of C₂H₂ is the dominant production mechanism for C₄H₂ (Moses et al., 2005), so the polar C₄H₂ may simply be a result of the excess C₂H₂ within the NPSV at high altitudes in Figure 16, which is not captured by the photochemical models. Conversely, C₃H₄ is primarily formed from interconversion of other C₃ hydrocarbons (Moses et al., 2005), which do not display the same enrichment as C₂H₂. Furthermore, the model of Moses and Greathouse (2005) overestimates equatorial C₃H₄ and C₄H₂ by factors of ~5 and ~4, respectively, similar to that found by Guerlet et al. (2010).

*Methyl* (CH₃, first detected by ISO, Bézard et al., 1998) is also a short-lived species produced directly from methane photolysis (Moses et al., 2000). MIRI/MRS provides the first latitudinally resolved measurements of CH₃, showing the same equator-to-pole decline as C₂H₂ and C₃H₄, along with a local minimum near 30°N that may be related to the seasonal upwelling. The seasonal model underpredicts the equatorial abundances by ~2.5. Unlike most other hydrocarbon species, CH₃ appears to be most depleted within the NPSV, by a factor of ~5 compared to equatorial abundances. This may reflect the annual-average insolation, with less CH₄ photolysis at the highest latitudes, although we note that methyl-methyl recombination reactions are dominant producer of C₂H₆, so it may have been mostly converted into the enriched ethane of the NPSV. Alternatively, we note that the CH₃ abundance is very sensitive to the methane homopause pressure (Bézard et al., 1998), so the CH₃ depletion in the NPSV could potentially be caused by subsidence through the upper stratosphere and lower thermosphere that pushes the methane homopause to deeper pressures, providing a consistent picture with the enhanced C₂H₄, C₂H₆, and some other hydrocarbon abundances.

*Benzene* (C₆H₆, first detected by Bézard et al. (2001a)) also differs substantially from neutral photochemical predictions, which would expect a distribution similar to that of C₂H₂. Instead, we see the C₆H₆ emission at all latitudes (e.g., between strong C₂H₂ lines in Figure 12), with enhancement by 1.5–2.0× within the NPSV compared to mid-latitudes. A similar latitudinal gradient was observed by Cassini/CIRS in the southern hemisphere between 2007 and 2012, with a slight enhancement within the SPSV (Guerlet et al., 2015). Nevertheless, the peak abundance at 0.5 mbar remains 50× smaller than the photochemical model prediction of Moses and Greathouse (2005), as previously found by Guerlet et al. (2015), and some 10× smaller than the coupled ion-neutral chemistry of Moses et al. (2023). As discussed by Koskinen et al. (2016) and Moses et al. (2023), benzene on Saturn is greatly enhanced by ion chemistry, and increased production due to auroral-induced ion chemistry may play a role at high latitudes, but finding a specific match between models and data remains a challenge.

It is interesting to note that, of all the hydrocarbon species observed by MIRI in 2022, none of them show the substantial chemical consequences predicted by neutral photochemistry (Moses et al., 2023) if the large influx of organic-rich ring material detected by Cassini during its Grand Finale in 2017 (Serigano et al., 2022) were vaporized/ablated upon entry into the equatorial stratosphere. Furthermore, no new nitriles were observed in 2022 (see below). We thus favor the hypothesis of Moses et al. (2023) that the ring material enters as small dust particles that do not ablate and affect the stratospheric composition.

### 5.4.4. Exogenic Species

Carbon dioxide (CO₂, first detected by ISO, Feuchtgruber et al., 1997) is detected at all latitudes by MIRI/MRS, with a relatively uniform abundance with latitude (0.60 ± 0.05 ppb at 0.5 mbar, Figure 17e) and hints of a slightly lower abundance (0.56 ± 0.03 ppb) within the NPSV. The distribution is approximately consistent with the few measurements available from Cassini (Abbas et al., 2013), but is inconsistent with photochemical models that assume a globally constant flux of incoming oxygen species and predict CO₂ abundances that are greatest near the equator and tail off toward high latitudes (Moses & Greathouse, 2005). There are no strong latitudinal gradients that might imply a spatially localized source (e.g., recent comets, Cavalié et al., 2010), or Enceladus plume and
the predictions of seasonal photochemistry, suggesting the influence of meridional circulation on the distribution of long-lived propane (e.g., similar to the argument for C₂H₆, Moses & Greathouse, 2005).

Unlike propane and ethane, the unsaturated hydrocarbons *methylacetylene* (C₃H₄) and *diacetylene* (C₄H₂) are relatively short-lived species that track the photochemical model predictions at latitudes equatorward of 45°N, with C₃H₄ in particular showing the expected equator-to-pole contrast in Figure 17g. C₄H₂ shows a local minimum near 20°–40°N that has evolved from the local maximum observed by Cassini in 2005–2006 (Guerlet et al., 2010), possibly related to the onset of low-latitude stratospheric upwelling during northern summer. Surprisingly, C₄H₂ then increases toward the NPSV where some of the highest abundances are obtained in Figure 17h. This is unlikely to be a circulation effect, as C₄H₂ has only a slightly shorter loss timescale than C₃H₄ (Guerlet et al., 2010), so we might expect to see the same structure in C₃H₄. Photolysis of C₂H₂ is the dominant production mechanism for C₄H₂ (Moses et al., 2005), so the polar C₄H₂ may simply be a result of the excess C₂H₂ within the NPSV at high altitudes in Figure 16, which is not captured by the photochemical models. Conversely, C₃H₄ is primarily formed from interconversion of other C₃ hydrocarbons (Moses et al., 2005), which do not display the same enrichment as C₂H₂. Furthermore, the model of Moses and Greathouse (2005) overestimates equatorial C₃H₄ and C₄H₂ by factors of ~5 and ~4, respectively, similar to that found by Guerlet et al. (2010).

*Methyl* (CH₃, first detected by ISO, Bézard et al., 1998) is also a short-lived species produced directly from methane photolysis (Moses et al., 2000). MIRI/MRS provides the first latitudinally resolved measurements of CH₃, showing the same equator-to-pole decline as C₂H₂ and C₃H₄, along with a local minimum near 30°N that may be related to the seasonal upwelling. The seasonal model underpredicts the equatorial abundances by ~2.5. Unlike most other hydrocarbon species, CH₃ appears to be most depleted within the NPSV, by a factor of ~5 compared to equatorial abundances. This may reflect the annual-average insolation, with less CH₄ photolysis at the highest latitudes, although we note that methyl-methyl recombination reactions are dominant producer of C₂H₆, so it may have been mostly converted into the enriched ethane of the NPSV. Alternatively, we note that the CH₃ abundance is very sensitive to the methane homopause pressure (Bézard et al., 1998), so the CH₃ depletion in the NPSV could potentially be caused by subsidence through the upper stratosphere and lower thermosphere that pushes the methane homopause to deeper pressures, providing a consistent picture with the enhanced C₂H₄, C₂H₆, and some other hydrocarbon abundances.

*Benzene* (C₆H₆, first detected by Bézard et al. (2001a)) also differs substantially from neutral photochemical predictions, which would expect a distribution similar to that of C₂H₂. Instead, we see the C₆H₆ emission at all latitudes (e.g., between strong C₂H₂ lines in Figure 12), with enhancement by 1.5–2.0× within the NPSV compared to mid-latitudes. A similar latitudinal gradient was observed by Cassini/CIRS in the southern hemisphere between 2007 and 2012, with a slight enhancement within the SPSV (Guerlet et al., 2015). Nevertheless, the peak abundance at 0.5 mbar remains 50× smaller than the photochemical model prediction of Moses and Greathouse (2005), as previously found by Guerlet et al. (2015), and some 10× smaller than the coupled ion-neutral chemistry of Moses et al. (2023). As discussed by Koskinen et al. (2016) and Moses et al. (2023), benzene on Saturn is greatly enhanced by ion chemistry, and increased production due to auroral-induced ion chemistry may play a role at high latitudes, but finding a specific match between models and data remains a challenge.

It is interesting to note that, of all the hydrocarbon species observed by MIRI in 2022, none of them show the substantial chemical consequences predicted by neutral photochemistry (Moses et al., 2023) if the large influx of organic-rich ring material detected by Cassini during its Grand Finale in 2017 (Serigano et al., 2022) were vaporized/ablated upon entry into the equatorial stratosphere. Furthermore, no new nitriles were observed in 2022 (see below). We thus favor the hypothesis of Moses et al. (2023) that the ring material enters as small dust particles that do not ablate and affect the stratospheric composition.

### 5.4.4. Exogenic Species

Carbon dioxide (CO₂, first detected by ISO, Feuchtgruber et al., 1997) is detected at all latitudes by MIRI/MRS, with a relatively uniform abundance with latitude (0.60 ± 0.05 ppb at 0.5 mbar, Figure 17e) and hints of a slightly lower abundance (0.56 ± 0.03 ppb) within the NPSV. The distribution is approximately consistent with the few measurements available from Cassini (Abbas et al., 2013), but is inconsistent with photochemical models that assume a globally constant flux of incoming oxygen species and predict CO₂ abundances that are greatest near the equator and tail off toward high latitudes (Moses & Greathouse, 2005). There are no strong latitudinal gradients that might imply a spatially localized source (e.g., recent comets, Cavalié et al., 2010), or Enceladus plume and
ring material entering at specific latitudes, but icy grain ablation remains too small a source to explain the measurements (Moses & Poppe, 2017). Indeed, the uniform CO₂ distribution derived here does not match the exogenic H₂O distribution derived from Herschel observations, which showed enhancements at low latitudes (Cavalié et al., 2019). More work is required to robustly compare the CO₂ and H₂O distributions to elucidate their sources.

Unless Saturn suffers a large-scale impact event, or the equatorial ring influx does provide excess nitrile production (Moses et al., 2023), HCN is not an expected photochemical product on Saturn, as the regions of photolytic destruction of CH₄ and NH₃ are separated by hundreds of kilometres in the vertical. Any detection of the 14.04 µm (ν₂) HCN line is challenging due to blending with lines of C₂H₂. We also note that the MRS wavelength calibration (Argyriou et al., 2023) and spectral resolution (Jones et al., 2023) creates fitting artifacts in channel 3B, preventing a rigorous study of upper limits in this preliminary study. Nevertheless, we find that the MRS data support HCN abundances no larger than 1 ppb at *p* < 1 mbar, an improvement over the previous upper limit of 22 ppb at *p* < 1 mbar in the sub-millimeter from Herschel/SPIRE (Fletcher et al., 2012). We do not see any evidence of the ν₅ band of HC₃N at 15.08 µm, again confirming an absence of nitriles related to ring influx (Moses et al., 2023).

## 6. Conclusions

This initial survey of JWST/MIRI observations of Saturn has revealed a wealth of new insights into the evolution of the seasonal atmosphere during northern summer (November 2022, L₅ = 150°); demonstrated MIRI/MRS capabilities to observe extended, bright, rotating, and moving planetary objects that are much larger than the fields-of-view; and provided a means to evaluate and mitigate challenges related to wavelength calibration, detector saturation, and instrumental artifacts for MIRI/MRS. Spatially resolved 4.9–27.9 µm maps of Saturn (three tiles spanning from the equator to the north pole) have been inverted to study the zonal-mean temperatures, windshears, aerosols, and gaseous composition from the cloud-forming region of the troposphere into the mid-stratosphere. This includes the first maps of the transitional region of Saturn's spectrum between 5.1 and 6.9 µm, where both thermal emission and scattered sunlight shape the spectrum, which were inaccessible to the VIMS and CIRS instruments on Cassini.

Although the JWST data reduction pipeline continues to evolve, we have presented algorithms for correcting artifacts such as wavelength calibration offsets, correction of partially saturated spectral regions, and the development of “flat-field” corrections by exploiting multiple dithers on the same target. As the MIRI/MRS spectral cubes required significant processing outside of the pipeline, we perform customized cleaning on each tile and dither independently, to prevent the blending of artifacts that, in the most extreme cases, can completely obscure the spatial structure of Saturn itself. With these artifacts removed, we summarize the initial survey of Saturn as follows:

1. **Saturn's banded structure:** Latitudinal gradients in temperatures and windshears show strong correlations with the locations of Saturn's mid-latitude eastward and westward jets (García-Melendo et al., 2011), with the contrasts between cool zones and warmer belts confirming the decay of the zonal winds with altitude via the thermal windshear equation. Conversely, gradients in reflectivity (and the derived aerosol structure from 4.9 to 7.3 µm) show similarities to albedo contrasts in observations acquired by Hubble in September 2022 (Simon et al., 2023), with a decrease in aerosols in distinct steps from equator to pole, with the highest cloud base and thickest tropospheric hazes within the broad reflective band equatorward of 15°N. A narrow equatorial band (<5°N) is dark at 5 µm, coinciding with the location where the narrow upper-tropospheric jet was previously detected (García-Melendo et al., 2011). The 5-µm brightness has evolved with time, such that a pole-encircling band from 62° to 78°N is now the brightest on the planet, coinciding with a dearth of aerosol opacity, whereas the polar domain (interior to the hexagon) is the darkest at 5 µm due to an absence of upper-tropospheric aerosols and an optically thick and compact cloud at higher pressures. We no longer see the 5-µm-bright, aerosol-depleted band near 35°–40°N that had dominated the appearance of the northern hemisphere after the 2010–2011 storm, consistent with a re-population of the band by Saturn's seasonal aerosols in the decade since the storm.

2. **NPSV:** The seasonal stratospheric vortex that developed poleward of 78°N during northern spring (Fletcher et al., 2018b) remained present in 2022, with warmer temperatures than those measured in 2017, now approaching the same temperatures as those observed in the SPSV during southern summer (2004–2005). Radiative models (e.g., Guerlet et al., 2014), combined with our Earth-based vantage point (Blake et al., 2022), suggest that the visibility of the warm NPSV will decline substantially in the next 1–2 years before autumn equinox. The sharp thermal gradient at the edge of the NPSV promotes negative windshear, and thus westward stratospheric winds entraining the NPSV for *p* < 10 mbar. MIRI/MRS did not detect evidence of the vertices of
ring material entering at specific latitudes, but icy grain ablation remains too small a source to explain the measurements (Moses & Poppe, 2017). Indeed, the uniform CO₂ distribution derived here does not match the exogenic H₂O distribution derived from Herschel observations, which showed enhancements at low latitudes (Cavalié et al., 2019). More work is required to robustly compare the CO₂ and H₂O distributions to elucidate their sources.

Unless Saturn suffers a large-scale impact event, or the equatorial ring influx does provide excess nitrile production (Moses et al., 2023), HCN is not an expected photochemical product on Saturn, as the regions of photolytic destruction of CH₄ and NH₃ are separated by hundreds of kilometres in the vertical. Any detection of the 14.04 µm (ν₂) HCN line is challenging due to blending with lines of C₂H₂. We also note that the MRS wavelength calibration (Argyriou et al., 2023) and spectral resolution (Jones et al., 2023) creates fitting artifacts in channel 3B, preventing a rigorous study of upper limits in this preliminary study. Nevertheless, we find that the MRS data support HCN abundances no larger than 1 ppb at *p* < 1 mbar, an improvement over the previous upper limit of 22 ppb at *p* < 1 mbar in the sub-millimeter from Herschel/SPIRE (Fletcher et al., 2012). We do not see any evidence of the ν₅ band of HC₃N at 15.08 µm, again confirming an absence of nitriles related to ring influx (Moses et al., 2023).

## 6. Conclusions

This initial survey of JWST/MIRI observations of Saturn has revealed a wealth of new insights into the evolution of the seasonal atmosphere during northern summer (November 2022, L₅ = 150°); demonstrated MIRI/MRS capabilities to observe extended, bright, rotating, and moving planetary objects that are much larger than the fields-of-view; and provided a means to evaluate and mitigate challenges related to wavelength calibration, detector saturation, and instrumental artifacts for MIRI/MRS. Spatially resolved 4.9–27.9 µm maps of Saturn (three tiles spanning from the equator to the north pole) have been inverted to study the zonal-mean temperatures, windshears, aerosols, and gaseous composition from the cloud-forming region of the troposphere into the mid-stratosphere. This includes the first maps of the transitional region of Saturn's spectrum between 5.1 and 6.9 µm, where both thermal emission and scattered sunlight shape the spectrum, which were inaccessible to the VIMS and CIRS instruments on Cassini.

Although the JWST data reduction pipeline continues to evolve, we have presented algorithms for correcting artifacts such as wavelength calibration offsets, correction of partially saturated spectral regions, and the development of “flat-field” corrections by exploiting multiple dithers on the same target. As the MIRI/MRS spectral cubes required significant processing outside of the pipeline, we perform customized cleaning on each tile and dither independently, to prevent the blending of artifacts that, in the most extreme cases, can completely obscure the spatial structure of Saturn itself. With these artifacts removed, we summarize the initial survey of Saturn as follows:

1. **Saturn's banded structure:** Latitudinal gradients in temperatures and windshears show strong correlations with the locations of Saturn's mid-latitude eastward and westward jets (García-Melendo et al., 2011), with the contrasts between cool zones and warmer belts confirming the decay of the zonal winds with altitude via the thermal windshear equation. Conversely, gradients in reflectivity (and the derived aerosol structure from 4.9 to 7.3 µm) show similarities to albedo contrasts in observations acquired by Hubble in September 2022 (Simon et al., 2023), with a decrease in aerosols in distinct steps from equator to pole, with the highest cloud base and thickest tropospheric hazes within the broad reflective band equatorward of 15°N. A narrow equatorial band (<5°N) is dark at 5 µm, coinciding with the location where the narrow upper-tropospheric jet was previously detected (García-Melendo et al., 2011). The 5-µm brightness has evolved with time, such that a pole-encircling band from 62° to 78°N is now the brightest on the planet, coinciding with a dearth of aerosol opacity, whereas the polar domain (interior to the hexagon) is the darkest at 5 µm due to an absence of upper-tropospheric aerosols and an optically thick and compact cloud at higher pressures. We no longer see the 5-µm-bright, aerosol-depleted band near 35°–40°N that had dominated the appearance of the northern hemisphere after the 2010–2011 storm, consistent with a re-population of the band by Saturn's seasonal aerosols in the decade since the storm.

2. **NPSV:** The seasonal stratospheric vortex that developed poleward of 78°N during northern spring (Fletcher et al., 2018b) remained present in 2022, with warmer temperatures than those measured in 2017, now approaching the same temperatures as those observed in the SPSV during southern summer (2004–2005). Radiative models (e.g., Guerlet et al., 2014), combined with our Earth-based vantage point (Blake et al., 2022), suggest that the visibility of the warm NPSV will decline substantially in the next 1–2 years before autumn equinox. The sharp thermal gradient at the edge of the NPSV promotes negative windshear, and thus westward stratospheric winds entraining the NPSV for *p* < 10 mbar. MIRI/MRS did not detect evidence of the vertices of
Saturn's hexagon, meaning that we will need to wait until the 2040s for our next infrared views of the hexagon. However, the bright NPC was still visible, embedded within the NPSV. The NPSV was enriched in several stratospheric hydrocarbon species (ethane, acetylene, ethylene, benzene, diacetylene) due to a combination of polar subsidence over the summer pole, and potential ion-neutral chemistry at high latitudes.

3. *Cyclones and anticyclones*: Channel 1-short of MIRI/MRS offers the best opportunity to probe the fine-scale cloud banding, as well as discrete features. We observe contrasts associated with cyclones (bright and aerosol-free) and anticyclones (dark and cloudy) near 48°, 10°, and 62°N, but these could not be identified in contemporaneous amateur observations, nor Hubble observations ~7 weeks earlier. This strongly argues for near-simultaneous MIRI/MRS spectroscopy and NIRCAM (or Hubble) imaging in future observing programs, alongside long-term records of variability from ground-based facilities.

4. *Saturn's Equatorial Stratospheric Oscillation*: Windshears derived from retrieved temperature gradients reveal diagonally stacked shear zones that rise to higher altitudes with higher latitudes, connecting the stratosphere throughout the low-latitudes equatorward of 30°N. MIRI/MRS reveals warm and cool temperature anomalies at the equator (and off-equatorial anomalies at 13°N) that are consistent with the downward propagation of the oscillatory pattern over a decade of pressure in the 5 years since the 2017 Cassini observations. A warm equatorial anomaly centred near 0.7 mbar is responsible for the bright equatorial band of emission observed in CH₄ and C₂H₂ emission, but this is opposite to the dark band observed from the ground one Saturnian year earlier (1993–1995, Blake et al., 2022; Orton et al., 2008), raising doubts about the semi-annual nature (i.e., 15-year period) of the equatorial oscillation. Thermal wind calculations imply the presence of an equatorial westward jet near 1–5 mbar in 2022 superimposed onto a broader region of eastward flow, but future joint campaigns between JWST and ALMA are necessary to confirm the validity of the thermal winds derived here.

5. *Reversal of Saturn's interhemispheric stratospheric circulation*: MIRI/MRS observations in 2022 reveal cooler temperatures in the 10°–40°N domain compared to Cassini in 2017. This coincides with local minima in several hydrocarbon species in 2022 (notably C₃H₂, C₂H₆, C₄H₂, and CH₃), opposite to the local maxima detected by Cassini (Guerlet et al., 2009, 2010; Sylvestre et al., 2015). This adiabatic cooling and hydrocarbon-depleted air implies a transition from wintertime subsidence to summertime upwelling in the northern hemisphere, as part of the seasonal reversal of Saturn's interhemispheric stratospheric circulation (Bardet et al., 2022; Friedson & Moses, 2012).

6. *Stacked aerosol layers*: We present the first assessment of aerosol opacity in 5.1–6.8 µm range, requiring two aerosol layers to reproduce the thermal emission and reflected sunlight components. The *upper aerosol layer*, possibly a photochemically produced haze related to diphosphine (Ferris & Benson, 1980), is thickest, highest (near 200 mbar) and most extended at the equator, but deeper (300 mbar) and more compact at mid-latitudes, before becoming negligible poleward of 60°N. This layer is likely to be the same as that detected by Cassini/ISS (Roman et al., 2013), and coincides with a region of localized radiative heating detected by Cassini/CIRS (Fletcher et al., 2007). The *deeper aerosol layer*, possibly associated with condensates of NH₃ and other species, resides at 1–2 bars, being shallowest (1.2 bars) near 10°N and deepest (2.6 bars) and most compact in the polar domain, responsible for both the dark thermal emission at 5 µm and the absence of reflectivity at 5.2 µm. Aerosol fitting for MIRI/MRS is somewhat degenerate, and further constraints could be provided in future by NIRSpec observations of reflected sunlight.

7. *Tropospheric species*: Latitudinal variability of *phosphine* appears to be most confined to *p* < 1 bar, displaying an equatorial maximum (Fletcher, Orton, Teanby, & Irwin, 2009) and general decline in abundance toward the polar domain. Regions of higher aerosol opacity generally correspond to regions of elevated PH₃, suggesting this disequilibrium species is most abundant when aerosols shield the molecule from photolysis. *Ammonia* displays a strong equatorial enrichment <5° of the equator (450–650 ppm), similar to the NH₃-enriched column in Jupiter's equatorial zone (Achterberg et al., 2006; de Pater et al., 2016; Li et al., 2017) and suggesting similar dynamical processes at the equators of both gas giants. NH₃ also displays different latitudinal distributions above and below the condensed aerosols near 1.7 bars, suggestive of differing circulation patterns at different heights. Neither PH₃ nor NH₃ display consistent belt/zone contrasts at mid-latitudes, suggesting secondary circulation patterns associated with mid-latitude Ferrel cells may be weak. Tropospheric *water* is mapped for the first time in its condensation region (i.e., above the expected H₂O cloud), with estimates at 3.3 bars varying from 200 ppb at the equator to 10 ppb near 5°N, and a distinct step in the abundance near ~40°N from 145 at low latitudes to 80 ppb at higher latitudes.

8. *Stratospheric chemicals*: Short-lived hydrocarbons, such as *acetylene*, *diacetylene*, *methylacetylene*, *ethylene*, and *methyl*, tend to track the annual-average insolation (i.e., peaking at low latitudes, consistent with the
Saturn's hexagon, meaning that we will need to wait until the 2040s for our next infrared views of the hexagon. However, the bright NPC was still visible, embedded within the NPSV. The NPSV was enriched in several stratospheric hydrocarbon species (ethane, acetylene, ethylene, benzene, diacetylene) due to a combination of polar subsidence over the summer pole, and potential ion-neutral chemistry at high latitudes.

3. *Cyclones and anticyclones*: Channel 1-short of MIRI/MRS offers the best opportunity to probe the fine-scale cloud banding, as well as discrete features. We observe contrasts associated with cyclones (bright and aerosol-free) and anticyclones (dark and cloudy) near 48°, 10°, and 62°N, but these could not be identified in contemporaneous amateur observations, nor Hubble observations ~7 weeks earlier. This strongly argues for near-simultaneous MIRI/MRS spectroscopy and NIRCAM (or Hubble) imaging in future observing programs, alongside long-term records of variability from ground-based facilities.

4. *Saturn's Equatorial Stratospheric Oscillation*: Windshears derived from retrieved temperature gradients reveal diagonally stacked shear zones that rise to higher altitudes with higher latitudes, connecting the stratosphere throughout the low-latitudes equatorward of 30°N. MIRI/MRS reveals warm and cool temperature anomalies at the equator (and off-equatorial anomalies at 13°N) that are consistent with the downward propagation of the oscillatory pattern over a decade of pressure in the 5 years since the 2017 Cassini observations. A warm equatorial anomaly centred near 0.7 mbar is responsible for the bright equatorial band of emission observed in CH₄ and C₂H₂ emission, but this is opposite to the dark band observed from the ground one Saturnian year earlier (1993–1995, Blake et al., 2022; Orton et al., 2008), raising doubts about the semi-annual nature (i.e., 15-year period) of the equatorial oscillation. Thermal wind calculations imply the presence of an equatorial westward jet near 1–5 mbar in 2022 superimposed onto a broader region of eastward flow, but future joint campaigns between JWST and ALMA are necessary to confirm the validity of the thermal winds derived here.

5. *Reversal of Saturn's interhemispheric stratospheric circulation*: MIRI/MRS observations in 2022 reveal cooler temperatures in the 10°–40°N domain compared to Cassini in 2017. This coincides with local minima in several hydrocarbon species in 2022 (notably C₃H₂, C₂H₆, C₄H₂, and CH₃), opposite to the local maxima detected by Cassini (Guerlet et al., 2009, 2010; Sylvestre et al., 2015). This adiabatic cooling and hydrocarbon-depleted air implies a transition from wintertime subsidence to summertime upwelling in the northern hemisphere, as part of the seasonal reversal of Saturn's interhemispheric stratospheric circulation (Bardet et al., 2022; Friedson & Moses, 2012).

6. *Stacked aerosol layers*: We present the first assessment of aerosol opacity in 5.1–6.8 µm range, requiring two aerosol layers to reproduce the thermal emission and reflected sunlight components. The *upper aerosol layer*, possibly a photochemically produced haze related to diphosphine (Ferris & Benson, 1980), is thickest, highest (near 200 mbar) and most extended at the equator, but deeper (300 mbar) and more compact at mid-latitudes, before becoming negligible poleward of 60°N. This layer is likely to be the same as that detected by Cassini/ISS (Roman et al., 2013), and coincides with a region of localized radiative heating detected by Cassini/CIRS (Fletcher et al., 2007). The *deeper aerosol layer*, possibly associated with condensates of NH₃ and other species, resides at 1–2 bars, being shallowest (1.2 bars) near 10°N and deepest (2.6 bars) and most compact in the polar domain, responsible for both the dark thermal emission at 5 µm and the absence of reflectivity at 5.2 µm. Aerosol fitting for MIRI/MRS is somewhat degenerate, and further constraints could be provided in future by NIRSpec observations of reflected sunlight.

7. *Tropospheric species*: Latitudinal variability of *phosphine* appears to be most confined to *p* < 1 bar, displaying an equatorial maximum (Fletcher, Orton, Teanby, & Irwin, 2009) and general decline in abundance toward the polar domain. Regions of higher aerosol opacity generally correspond to regions of elevated PH₃, suggesting this disequilibrium species is most abundant when aerosols shield the molecule from photolysis. *Ammonia* displays a strong equatorial enrichment <5° of the equator (450–650 ppm), similar to the NH₃-enriched column in Jupiter's equatorial zone (Achterberg et al., 2006; de Pater et al., 2016; Li et al., 2017) and suggesting similar dynamical processes at the equators of both gas giants. NH₃ also displays different latitudinal distributions above and below the condensed aerosols near 1.7 bars, suggestive of differing circulation patterns at different heights. Neither PH₃ nor NH₃ display consistent belt/zone contrasts at mid-latitudes, suggesting secondary circulation patterns associated with mid-latitude Ferrel cells may be weak. Tropospheric *water* is mapped for the first time in its condensation region (i.e., above the expected H₂O cloud), with estimates at 3.3 bars varying from 200 ppb at the equator to 10 ppb near 5°N, and a distinct step in the abundance near ~40°N from 145 at low latitudes to 80 ppb at higher latitudes.

8. *Stratospheric chemicals*: Short-lived hydrocarbons, such as *acetylene*, *diacetylene*, *methylacetylene*, *ethylene*, and *methyl*, tend to track the annual-average insolation (i.e., peaking at low latitudes, consistent with the
Acknowledgments

Fletcher, King, and Roman were supported by a European Research Council Consolidator Grant (under the European Union's Horizon 2020 research and innovation programme, Grant 723890) at the University of Leicester. Harkett was supported by an STFC studentship; Melin was supported by an STFC James Webb Fellowship (ST/W001527/1). Hammel and Milam acknowledge support from NASA JWST Interdisciplinary Scientist Grant 21-SMSS21-0013. We wish to express our gratitude to the JWST support team for their patience and perseverance as we designed these MRS observations—in particular Beth Perriello, Bryan Holler, Misty Cracraft, Tony Roman, and John Stansberry for their aid in setting up the observations in APT, and David Law for his tireless support as we developed codes to interpret MIRI/MRS data. We are grateful to Conor Nixon, Imke de Pater, Patrick Irwin, Pablo Rodriguez-Ovalle, and Thierry Fouchet for their helpful discussions during the development of this work. We thank amateur observers Chris Go, Trevor Barry, Anthony Wesley, and Tiziano Olivetti for their efforts to identify discrete features during the JWST MIRI observation epoch. We thank two reviewers, Glenn Orton and Bruno Bézard, for their thorough critiques of this article. This research used the ALICE High Performance Computing Facility at the University of Leicester. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program 1247 (PI: Fletcher). JWST observations were compared to data acquired from the NASA/ESA HST Space Telescope, associated with OPAL program (PI: Simon, GO13937), and archived by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. For the purpose of Open Access, the corresponding author has applied a CC-BY public copyright licence to any Author Accepted Manuscript (AAM) version arising from this submission.

predictions of seasonal photochemical models, Moses & Greathouse, 2005), whereas longer-lived species (*ethane*, *propane*, and possibly *benzene*) appear to be more influenced by long-term stratospheric circulation redistributing gases to higher latitudes. Acetylene has a steeper vertical gradient within the NPSV than elsewhere, and several species (*ethane*, *ethylene*, *benzene*, *diacetylene*) show strong enrichments within the NPSV. Methyl is mapped with latitude for the first time, showing a factor of ~5 decrease from equator to pole. Ethylene is detected in the NPSV for the first time. The benzene abundance remains an order of magnitude smaller than the predictions of photochemical models. *Carbon dioxide* is detected at all latitudes with a relatively uniform distribution, lacking strong latitudinal gradients that might imply a spatially localized source of exogenic oxygen species.

9. Absence of nitriles: None of the hydrocarbon distributions support the substantial chemical consequences predicted by neutral photochemistry (Moses et al., 2023) if the large influx of ring material detected by Cassini in 2017 (Serigano et al., 2022) were vaporized upon entry into the equatorial stratosphere. Neither HCN nor HC₃N, both predicted by nitrile chemistry related to ring influx, are detected by MIRI. This suggests that ring material influx is not strongly influencing Saturn's equatorial stratosphere.

We caution the reader that the MIRI/MRS calibration continues to evolve, and that refined cleaning techniques will enable future in depth studies - in particular, removal of instrument artifacts from the 17–28 µm domain will enable studies of temperature and para-H₂ at longer wavelengths; plus a more sensitive search for stratospheric emission lines throughout the MIRI spectrum. Future joint campaigns with other JWST instruments (NIRSpec and NIRCAM) would aid in breaking degeneracies in aerosol retrievals, and collaboration with ALMA would allow a direct comparison of stratospheric winds and thermal winds associated with equatorial and polar jets.

JWST observations of the Saturn system in November 2022 ($L_s = 150°$) have revealed the wealth of possibilities offered by IFU spectroscopy from space in the mid-infrared. But Saturn's atmosphere will continue to change with the onset of northern autumn (May 2025, $L_s = 180°$), and we hope that these Cycle-1 observations will mark the starting point for a long-term MIRI legacy program to track the seasonal evolution of Saturn's circulation and chemistry through to the next southern summer solstice (April 2032, $L_s = 270°$), completing the seasonal assessment begun by Cassini.

Data Availability Statement

Level-3 calibrated Saturn MIRI/MRS data from the standard pipeline are available directly from the MAST archive (MAST Archive, 2022). Hubble observations used for comparison were acquired by the Outer Planets Legacy Program (OPAL Archive, 2022).

The NEMESIS radiative transfer and retrieval code (Irwin et al., 2008) used in this study is open-access and is available for download (Irwin, 2022).

The JWST calibration pipeline is available via Bushouse et al. (2023), this work used version 1.9.4.

The custom pipeline and data processing code developed in this study is available at King et al. (2023).

The data products produced in this study, including synthetic flat fields, zonal average spectra and quick-look visualizations, are available from Fletcher (2023).

References

Abbas, M. M., LeClair, A., Woodard, E., Young, M., Stanbro, M., Flasar, F. M., et al. (2013). Distribution of CO₂ in Saturn's atmosphere from Cassini/CIRS infrared observations. *The Astrophysical Journal*, 776(2), 73. https://doi.org/10.1088/0004-637X/776/2/73

Achterberg, R. K., Conrath, B. J., & Gierasch, P. J. (2006). Cassini CIRS retrievals of ammonia in Jupiter's upper troposphere. *Icarus*, 182(1), 169–180. https://doi.org/10.1016/j.icarus.2005.12.020

Adam, A. Y., Yachmenev, A., Yurchenko, S. N., & Jensen, P. (2019). Variationally computed IR line list for the methyl radical CH₃. *Journal of Physical Chemistry A*, 123(22), 4755–4763. https://doi.org/10.1021/acs.jpca.9b02919

Argyriou, I., Glasse, A., Law, D. R., Labiano, A., Álvarez-Márquez, J., Patapis, P., et al. (2023). JWST MIRI flight performance: The Medium-Resolution Spectrometer. arXiv e-prints, arXiv:2303.13469. https://doi.org/10.48550/arXiv.2303.13469

Bailly, D., Birnbaum, G., Buechele, A., Flaud, P. M., & Hartmann, J. M. (2004). Absorption by pure NH₃ and NH₃-H₂ mixtures in the 5 µm window region. *Journal of Quantitative Spectroscopy and Radiative Transfer*, 83, 1–13. https://doi.org/10.1016/S0022-4073(02)00281-9

Baines, K. H., Drossart, P., Momary, T. W., Formisano, V., Griffith, C., Bellucci, G., et al. (2006). The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. *Earth, Moon, and Planets*, 96(3–4), 1–29. https://doi.org/10.1007/s11038-005-9058-2

Bardet, D., Spiga, A., & Guerlet, S. (2022). Joint evolution of equatorial oscillation and interhemispheric circulation in Saturn's stratosphere. *Nature Astronomy*, 6(7), 804–811. https://doi.org/10.1038/s41550-022-01670-7

Benmahi, B., Cavalié, T., Fouchet, T., Moreno, R., Lellouch, E., Bardet, D., et al. (2022). First absolute wind measurements in Saturn's stratosphere from ALMA observations. *Astronomy and Astrophysics*, 666, A117. https://doi.org/10.1051/0004-6361/20224200
Acknowledgments

Fletcher, King, and Roman were supported by a European Research Council Consolidator Grant (under the European Union's Horizon 2020 research and innovation programme, Grant 723890) at the University of Leicester. Harkett was supported by an STFC studentship; Melin was supported by an STFC James Webb Fellowship (ST/W001527/1). Hammel and Milam acknowledge support from NASA JWST Interdisciplinary Scientist Grant 21-SMSS21-0013. We wish to express our gratitude to the JWST support team for their patience and perseverance as we designed these MRS observations—in particular Beth Perriello, Bryan Holler, Misty Cracraft, Tony Roman, and John Stansberry for their aid in setting up the observations in APT, and David Law for his tireless support as we developed codes to interpret MIRI/MRS data. We are grateful to Conor Nixon, Imke de Pater, Patrick Irwin, Pablo Rodriguez-Ovalle, and Thierry Fouchet for their helpful discussions during the development of this work. We thank amateur observers Chris Go, Trevor Barry, Anthony Wesley, and Tiziano Olivetti for their efforts to identify discrete features during the JWST MIRI observation epoch. We thank two reviewers, Glenn Orton and Bruno Bézard, for their thorough critiques of this article. This research used the ALICE High Performance Computing Facility at the University of Leicester. This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program 1247 (PI: Fletcher). JWST observations were compared to data acquired from the NASA/ESA HST Space Telescope, associated with OPAL program (PI: Simon, GO13937), and archived by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. For the purpose of Open Access, the corresponding author has applied a CC-BY public copyright licence to any Author Accepted Manuscript (AAM) version arising from this submission.

predictions of seasonal photochemical models, Moses & Greathouse, 2005), whereas longer-lived species (*ethane*, *propane*, and possibly *benzene*) appear to be more influenced by long-term stratospheric circulation redistributing gases to higher latitudes. Acetylene has a steeper vertical gradient within the NPSV than elsewhere, and several species (*ethane*, *ethylene*, *benzene*, *diacetylene*) show strong enrichments within the NPSV. Methyl is mapped with latitude for the first time, showing a factor of ~5 decrease from equator to pole. Ethylene is detected in the NPSV for the first time. The benzene abundance remains an order of magnitude smaller than the predictions of photochemical models. *Carbon dioxide* is detected at all latitudes with a relatively uniform distribution, lacking strong latitudinal gradients that might imply a spatially localized source of exogenic oxygen species.

9. Absence of nitriles: None of the hydrocarbon distributions support the substantial chemical consequences predicted by neutral photochemistry (Moses et al., 2023) if the large influx of ring material detected by Cassini in 2017 (Serigano et al., 2022) were vaporized upon entry into the equatorial stratosphere. Neither HCN nor HC₃N, both predicted by nitrile chemistry related to ring influx, are detected by MIRI. This suggests that ring material influx is not strongly influencing Saturn's equatorial stratosphere.

We caution the reader that the MIRI/MRS calibration continues to evolve, and that refined cleaning techniques will enable future in depth studies - in particular, removal of instrument artifacts from the 17–28 µm domain will enable studies of temperature and para-H₂ at longer wavelengths; plus a more sensitive search for stratospheric emission lines throughout the MIRI spectrum. Future joint campaigns with other JWST instruments (NIRSpec and NIRCAM) would aid in breaking degeneracies in aerosol retrievals, and collaboration with ALMA would allow a direct comparison of stratospheric winds and thermal winds associated with equatorial and polar jets.

JWST observations of the Saturn system in November 2022 ($L_s = 150°$) have revealed the wealth of possibilities offered by IFU spectroscopy from space in the mid-infrared. But Saturn's atmosphere will continue to change with the onset of northern autumn (May 2025, $L_s = 180°$), and we hope that these Cycle-1 observations will mark the starting point for a long-term MIRI legacy program to track the seasonal evolution of Saturn's circulation and chemistry through to the next southern summer solstice (April 2032, $L_s = 270°$), completing the seasonal assessment begun by Cassini.

Data Availability Statement

Level-3 calibrated Saturn MIRI/MRS data from the standard pipeline are available directly from the MAST archive (MAST Archive, 2022). Hubble observations used for comparison were acquired by the Outer Planets Legacy Program (OPAL Archive, 2022).

The NEMESIS radiative transfer and retrieval code (Irwin et al., 2008) used in this study is open-access and is available for download (Irwin, 2022).

The JWST calibration pipeline is available via Bushouse et al. (2023), this work used version 1.9.4.

The custom pipeline and data processing code developed in this study is available at King et al. (2023).

The data products produced in this study, including synthetic flat fields, zonal average spectra and quick-look visualizations, are available from Fletcher (2023).

References

Abbas, M. M., LeClair, A., Woodard, E., Young, M., Stanbro, M., Flasar, F. M., et al. (2013). Distribution of CO₂ in Saturn's atmosphere from Cassini/CIRS infrared observations. *The Astrophysical Journal*, 776(2), 73. https://doi.org/10.1088/0004-637X/776/2/73

Achterberg, R. K., Conrath, B. J., & Gierasch, P. J. (2006). Cassini CIRS retrievals of ammonia in Jupiter's upper troposphere. *Icarus*, 182(1), 169–180. https://doi.org/10.1016/j.icarus.2005.12.020

Adam, A. Y., Yachmenev, A., Yurchenko, S. N., & Jensen, P. (2019). Variationally computed IR line list for the methyl radical CH₃. *Journal of Physical Chemistry A*, 123(22), 4755–4763. https://doi.org/10.1021/acs.jpca.9b02919

Argyriou, I., Glasse, A., Law, D. R., Labiano, A., Álvarez-Márquez, J., Patapis, P., et al. (2023). JWST MIRI flight performance: The Medium-Resolution Spectrometer. arXiv e-prints, arXiv:2303.13469. https://doi.org/10.48550/arXiv.2303.13469

Bailly, D., Birnbaum, G., Buechele, A., Flaud, P. M., & Hartmann, J. M. (2004). Absorption by pure NH₃ and NH₃-H₂ mixtures in the 5 µm window region. *Journal of Quantitative Spectroscopy and Radiative Transfer*, 83, 1–13. https://doi.org/10.1016/S0022-4073(02)00281-9

Baines, K. H., Drossart, P., Momary, T. W., Formisano, V., Griffith, C., Bellucci, G., et al. (2006). The atmospheres of Saturn and Titan in the near-infrared: First results of Cassini/VIMS. *Earth, Moon, and Planets*, 96(3–4), 1–29. https://doi.org/10.1007/s11038-005-9058-2

Bardet, D., Spiga, A., & Guerlet, S. (2022). Joint evolution of equatorial oscillation and interhemispheric circulation in Saturn's stratosphere. *Nature Astronomy*, 6(7), 804–811. https://doi.org/10.1038/s41550-022-01670-7

Benmahi, B., Cavalié, T., Fouchet, T., Moreno, R., Lellouch, E., Bardet, D., et al. (2022). First absolute wind measurements in Saturn's stratosphere from ALMA observations. *Astronomy and Astrophysics*, 666, A117. https://doi.org/10.1051/0004-6361/20224200
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Irwin, P. G. J. (2022). NEMESIS retrieval code [Software]. https://doi.org/10.5281/zenodo.5816724

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Yanamandra-Fisher, P. A., Orton, G. S., Fisher, B. M., & Sánchez-Lavega, A. (2001). NOTE: Saturn's 5.2-µm cold spots: Unexpected cloud variability. *Icarus*, *150*(1), 189–193. https://doi.org/10.1006/icar.2000.6580
Noll, K. S., & Larson, H. P. (1990). The spectrum of saturn from 1990–2230 cm⁻¹: Abundances of AsH₃, CH₃D, CO, GeH₄, and PH₃. *Icarus*, *89*(1), 168–189. https://doi.org/10.1016/0019-1035(91)90096-c

OPAL Archive. (2022). Outer planets legacy program archive [Data]. https://doi.org/10.17909/T9G593

Orton, G., Yanamandra-Fisher, P., Fisher, B., Friedson, A., Parrish, P., Nelson, J., et al. (2008). Semi-annual oscillations in Saturn's low-latitude stratospheric temperatures. *Nature*, *453*(7192), 196–198. https://doi.org/10.1038/nature06897

Pérez-Hoyos, S., Sanz-Requena, J. F., Sánchez-Lavega, A., Irwin, P. G. J., & Smith, A. (2016). Saturn's tropospheric particles phase function and spatial distribution from Cassini ISS 2010-11 observations. *Icarus*, *277*, 1–18. https://doi.org/10.1016/j.icarus.2016.04.022

Pirraglia, J. A., Conrath, B. J., Allison, M. D., & Gierasch, P. J. (1981). Thermal structure and dynamics of saturn and Jupiter. *Nature*, *292*(5825), 677–679. https://doi.org/10.1038/292677a0

Read, P. L., Conrath, B. J., Fletcher, L. N., Gierasch, P. J., Simon-Miller, A. A., & Zuchowski, L. C. (2009). Mapping potential vorticity dynamics on Saturn: Zonal mean circulation from Cassini and Voyager data. *Planetary and Space Science*, *57*(14–15), 1682–1698. https://doi.org/10.1016/j.pss.2009.03.004

Roman, M. T., Banfield, D., & Gierasch, P. J. (2013). Saturn's cloud structure inferred from Cassini ISS. *Icarus*, *225*(1), 93–110. https://doi.org/10.1016/j.icarus.2013.03.015

Sánchez-Lavega, A., García-Muñoz, A., del Río-Gaztelurrutia, T., Pérez-Hoyos, S., Sanz-Requena, J. F., Hueso, R., et al. (2020). Multilayer hazes over Saturn's hexagon from Cassini ISS limb images. *Nature Communications*, *11*(1), 2281. https://doi.org/10.1038/s41467-020-16110-1

Sayanagi, K. M., Dyudina, U. A., Ewald, S. P., Fischer, G., Ingersoll, A. P., Kurth, W. S., et al. (2013). Dynamics of Saturn's great storm of 2010–2011 from Cassini ISS and RPWS. *Icarus*, *223*(1), 460–478. https://doi.org/10.1016/j.icarus.2012.12.013

Serigano, J., Hörst, S. M., He, C., Gautier, T., Yelle, R. V., Koskinen, T. T., et al. (2022). Compositional measurements of Saturn's upper atmosphere and rings from Cassini INMS: An extended analysis of measurements from Cassini's Grand Finale orbits. *Journal of Geophysical Research: Planets*, *127*(6), e07238. https://doi.org/10.1029/2022JE007238

Simon, A. A., Hedman, M. M., Nicholson, P. D., Tiscareno, M. S., Showalter, M. R., McDonald, T., & Callos, S. (2023). Hubble detects the start of a new saturn ring spoke season. *Geophysical Research Letters*, *50*(3), e022GL101904. https://doi.org/10.1029/2022GL101904

Sinclair, J. A., Irwin, P. G. J., Fletcher, L. N., Moses, J. I., Greathouse, T. K., Friedson, A. J., et al. (2013). Seasonal variations of temperature, acetylene and ethane in Saturn's atmosphere from 2005 to 2010, as observed by Cassini-CIRS. *Icarus*, *225*(1), 257–271. https://doi.org/10.1016/j.icarus.2013.03.011

Sromovsky, L. A., Baines, K. H., & Fry, P. M. (2013). Saturn's Great Storm of 2010–2011: Evidence for ammonia and water ices from analysis of VIMS spectra. *Icarus*, *226*(1), 402–418. https://doi.org/10.1016/j.icarus.2013.05.043

Sromovsky, L. A., Baines, K. H., & Fry, P. M. (2021). Evolution of Saturn's north polar color and cloud structure between 2012 and 2017 inferred from Cassini VIMS and ISS observations. *Icarus*, *362*, 114409. https://doi.org/10.1016/j.icarus.2021.114409

Sromovsky, L. A., Baines, K. H., Fry, P. M., & Momary, T. W. (2016). Cloud clearing in the wake of Saturn's Great Storm of 2010–2011 and suggested new constraints on Saturn's He/H₂ ratio. *Icarus*, *276*, 141–162. https://doi.org/10.1016/j.icarus.2016.04.031

Stam, D. M., Banfield, D., Gierasch, P. J., Nicholson, P. D., & Matthews, K. (2001). Near-IR spectrophotometry of Saturnian aerosols-meridional and vertical distribution. *Icarus*, *152*(2), 407–422. https://doi.org/10.1006/icar.2001.6641

Sung, K., Toon, G. C., Mantz, A. W., & Smith, M. A. H. (2013). FT-IR measurements of cold C₃H₈ cross sections at 7–15 µm for Titan atmosphere. *Icarus*, *226*(2), 1499–1513. https://doi.org/10.1016/j.icarus.2013.07.028

Sylvestre, M., Guerlet, S., Fouchet, T., Spiga, A., Flasar, F. M., Hesman, B., & Bjoraker, G. L. (2015). Seasonal changes in Saturn's stratosphere inferred from Cassini/CIRS limb observations. *Icarus*, *258*, 224–238. https://doi.org/10.1016/j.icarus.2015.05.025

Tennyson, J., Yurchenko, S. N., Al-Refaie, A. F., Barton, E. J., Chubb, K. L., Coles, P. A., et al. (2016). The ExoMol database: Molecular line lists for exoplanet and other hot atmospheres. *Journal of Molecular Spectroscopy*, *327*, 73–94. https://doi.org/10.1016/j.jms.2016.05.002

Vasavada, A. R., & Showman, A. P. (2005). Jovian atmospheric dynamics: An update after Galileo and Cassini. *Reports on Progress in Physics*, *68*(8), 1935–1996. https://doi.org/10.1088/0034-4885/68/8/R06

Wells, M., Pel, J.-W., Glasse, A., Wright, G. S., Aitink-Kroes, G., Azzollini, R., et al. (2015). The mid-infrared instrument for the James Webb space telescope, VI: The medium resolution spectrometer. *Publications of the Astronomical Society of the Pacific*, *127*(953), 646–664. https://doi.org/10.1086/682281

Westphal, J. A., Baum, W. A., Ingersoll, A. P., Barnet, C. D., de Jong, E. M., Danielson, G. E., & Caldwell, J. (1992). Hubble Space Telescope observations of the 1990 equatorial disturbance on Saturn - Images, albedos, and limb darkening. *Icarus*, *100*(2), 485–498. https://doi.org/10.1016/0019-1035(92)90112-K

Wright, G. S., Rieke, G. H., Glasse, A., Ressler, M., Marín, M. G., Aguilar, J., et al. (2023). The mid-infrared instrument for JWST and its in-flight performance. *Publications of the Astronomical Society of the Pacific*, *135*(1046), 048003. https://doi.org/10.1088/1538-3873/acbe66

Yanamandra-Fisher, P. A., Orton, G. S., Fisher, B. M., & Sánchez-Lavega, A. (2001). NOTE: Saturn's 5.2-µm cold spots: Unexpected cloud variability. *Icarus*, *150*(1), 189–193. https://doi.org/10.1006/icar.2000.6580