# Saturn’s Atmosphere in Northern Summer Revealed by JWST/MIRI

###### Abstract

Saturn’s northern summertime hemisphere was mapped by JWST/MIRI (4.9-27.9
$\mu$m) in November 2022, tracing the seasonal evolution of temperatures,
aerosols, and chemical species in the five years since the end of the Cassini
mission. We provide algorithms to clean MRS spectral cubes, revealing Saturn’s
banded structure and discrete meteorological features. The transitional
spectral region between reflected sunlight and thermal emission (5.1-6.8
$\mu$m) is mapped 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 found in Jupiter’s equatorial zone. Saturn’s North Polar
Stratospheric Vortex has warmed since 2017, is entrained by westward winds at
$p<10$ mbar, and exhibits localised 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 $\mu$m compare favourably 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 new propane bands in
the 8-11 $\mu$m region.

JGR: Planets

School of Physics and Astronomy, University of Leicester, University Road,
Leicester, LE1 7RH, UK Association of Universities for Research in Astronomy,
Suite 1475, 1331 Pennsylvania Avenue NW, Washington DC 20004, USA Department
of Physics, University of Idaho, Moscow, Idaho 83844, USA Space Science
Institute, Boulder, CO, USA Laboratoire de Météorologie Dynamique/Institut
Pierre-Simon Laplace (LMD/IPSL), Sorbonne Université, CNRS, École
Polytechnique, Institut Polytechnique de Paris, École Normale Supérieure
(ENS), PSL Research University, Paris, France LESIA, Observatoire de Paris,
Université PSL, CNRS, Sorbonne Université, Univ. Paris-Diderot, Sorbonne
Paris-Cité, Meudon, France Astrochemistry Laboratory Code 691, NASA Goddard
Space Flight Center, 8800 Greenbelt Road, Greenbelt MD 20771, USA SETI
Institute, Mountain View, CA 94043, USA

Leigh N<EMAIL_ADDRESS>
Saturn’s northern summertime hemisphere was mapped by JWST/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 dataset 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 gases
due to summertime upwelling. MIRI provides access to spectral regions that
were not possible with the Cassini spacecraft, particularly in the 5-7 $\mu$m
region where reflected sunlight and thermal emission blend together. Our
measurements reveal a stacked system of aerosol layers modulating the infrared
brightness. Ammonia and phosphine are enriched at Saturn’s equator, suggesting
strong mixing from the deeper troposphere. The high sensitivity of MIRI
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, all of which provide insights into the circulation and
chemistry shaping Saturn’s seasonal atmosphere.

##  1 Introduction

Spectroscopic mid-infrared observations of the Saturn system by JWST [Gardner
. (2023)] were designed to build on the legacy of discoveries of the Cassini-
Huygens mission (2004-2017), exploiting the unprecedented spectral coverage
and sensitivity of the MIRI/MRS [<]Medium Resolution Spectrometer, 4.9-27.9
$\mu$m,¿[]23wright integral field units. As part of a Guaranteed-Time
programme for giant planet observations during JWST’s first cycle of
operations [L. Fletcher . (2021)], Saturn provided an ideal test of the
capabilities of this new facility. For example, Saturn’s large angular size
compared to the small fields-of-view of MIRI/MRS presented a challenge for
mapping extended, rotating, and moving sources. Saturn’s spectrum has a large
dynamic range, with some regions (e.g., near 6 $\mu$m) sufficiently dark as to
require long integrations, but others (e.g., near 25 $\mu$m) so bright that
they are close to the saturation limit of the sensitive MIRI/MRS detectors.
Observations of Saturn’s small satellites are challenged by scattered light
from Saturn’s atmosphere and rings. And given that Saturn’s forest of
molecular emission and absorption features were previously characterised in
detail by Cassini, the Saturn observations provided a sensitive check on the
calibration of JWST (e.g., wavelength and flux calibration, and the presence
of instrumental artefacts).

In this work, we provide a comprehensive first assessment of JWST mid-infrared
observations of Saturn in November 2022 as a baseline for a long-term seasonal
legacy for MIRI. The Cassini record of Saturn’s seasonal evolution came to an
end in 2017 [LN. Fletcher, Sromovsky . (2020)], shortly after Saturn passed
northern summer solstice in May 2017 (planetocentric solar longitude of
$L_{s}=90^{\circ}$). Five years later, the northern pole was receding from
view during Saturn’s mid-summer ($L_{s}=150^{\circ}$) as the planet approached
northern autumn equinox in June 2025 ($L_{s}=180^{\circ}$). Cassini-Huygens
did not have the opportunity to observe this particular northern-summer season
on Saturn, although it was captured one Saturnian year earlier by ground-based
imaging observations [<]e.g.,¿[]01stam, 08orton_qxo, 22blake and Hubble
observations [Westphal . (1992), Caldwell . (1993), Karkoschka Tomasko (1993)]
in the early 1990s. The mid-IR observations revealed the cooling of the
seasonal North Polar Stratospheric Vortex (NPSV), a region of elevated
tropospheric and stratospheric temperatures poleward of $75^{\circ}$N during
summer [Guerlet . (2015), LN. Fletcher, Orton . (2018)]. Observations in
1993-1995 also captured Saturn’s equatorial stratospheric oscillation during
its cool equatorial phase [Orton . (2008), Blake . (2022)], which should be
replicated in JWST/MIRI observations exactly one Saturnian year later if the
oscillation were semi-annual [<]i.e., a 15-year period,¿[]08fouchet,
08orton_qxo.

The MIRI/MRS instrument brings exceptional new capabilities for infrared
science that eluded even the Cassini-Huygens spacecraft. Saturn’s 5-$\mu$m
‘window,’ where deep thermal emission reveals the dynamics and morphology of
the cloud-forming region, was extensively mapped by Cassini/VIMS ($R\sim 300$
at 5 $\mu$m), but MRS provides an order of magnitude improvement in spectral
resolution ($R\sim 3500$) required to separate gaseous absorption features of
phosphine, ammonia, and water; plus it provides spatial mapping of the 5.1-6.9
$\mu$m range for the first time (reliable Cassini/CIRS spectroscopy started
near 7.0 $\mu$m with $R\sim 2800$). Although this region was previously
observed in the disc-averaged sense by ISO [<]the Short-Wave Spectrometer,
with $R\sim 1000-2000$,¿[]03encrenaz, the MRS data presented here are the
first to spatially map this transitional region between reflected sunlight and
thermal emission, revealing discrete features in the cloud deck, and providing
access to the spatial distribution of tropospheric water and aerosols, as well
as the stratospheric distributions of methane, ethane and other hydrocarbons
using previously inaccessible emission bands. Finally, MIRI/MRS provides a
dramatic improvement in signal-to-noise in Saturn’s 10-$\mu$m region, which
was plagued by noise in Cassini/CIRS observations (this was the overlap
between the two mid-infrared focal plane detectors near 9 $\mu$m).

The MIRI/MRS observations provide an exceptionally rich dataset that has
revealed how Saturn’s atmosphere evolved in the five years since the end of
the Cassini mission. Section 2 describes the processing steps applied to
MIRI/MRS data, including algorithms to address saturation and instrument
artefacts. Section 3 provides an overview of the spatial structure and
spectral features observable by MIRI/MRS, and compares the observations to
Cassini data in 2017 to reveal seasonal variability. Section 4 then describes
our spectral modelling approach, including both reflected sunlight and thermal
emission, which is used in Section 5 to assess Saturn’s northern-summer
temperatures and zonal wind shears; the spatial distribution of aerosols,
tropospheric condensables and disequilibrium species; and a broad range of
stratospheric species to trace stratospheric chemistry and circulation.

##  2 JWST MIRI Data Processing

###  2.1 MIRI/MRS Observations

Figure 1: Montage of JWST MIRI/MRS observations of Saturn. Panel (a) shows RGB
composites of the JWST observations (Saturn:
R=$10.3\text{\,}\mathrm{\SIUnitSymbolMicro m}$,
G=$10.1\text{\,}\mathrm{\SIUnitSymbolMicro m}$,
B=$11.6\text{\,}\mathrm{\SIUnitSymbolMicro m}$; rings:
R=$15.5\text{\,}\mathrm{\SIUnitSymbolMicro m}$,
G=$14.6\text{\,}\mathrm{\SIUnitSymbolMicro m}$,
B=$13.5\text{\,}\mathrm{\SIUnitSymbolMicro m}$) with an HST observation of
Saturn in the background [Simon . (2023)]. Panels (b-e) show spatial structure
on Saturn at a range of wavelengths as indicated by the grey shaded regions in
panel (f). (f) shows the average spectrum of Saturn with specific spectral
features labelled. Hubble image credit: NASA, ESA, and Amy Simon (NASA-GSFC);
Image Processing: Alyssa Pagan (STScI).

The MIRI/MRS instrument [Wells . (2015)] consists of four integral field units
(IFUs, channels 1-4) spanning the 4.9-27.9 $\mu$m range with spectral
resolutions from $R\sim 1330$ at 27.9 $\mu$m to $R\sim 3710$ at 4.9 $\mu$m
[Labiano . (2021)]. Each IFU has a different slice width (from 0.176-0.645”)
and pixel size (from 0.196-0.273”), and thus field of view, such that they
provide different spatial coverage and sampling on Saturn’s disc in Fig. 2.
Although all four IFUs observe simultaneously (channels 1 and 2 on a `SHORT`
detector, channels 3 and 4 on a `LONG` detector), a grating wheel with three
different settings (`A, B, C`) is needed for full coverage, leading to a short
delay between observations of adjacent portions of the spectra. The four IFUs
and three grating positions provide 12 individual sub-bands, each with its own
wavelength coverage and spectral resolution.

The Saturn system observations were part of the Solar System Guaranteed Time
Observations (GTO) awarded to H. Hammel, and collated as programme `1247`. The
MRS observations were the first science observations to be executed after a
brief hiatus of operations (2022-Aug-24 to 2022-Nov-12), when the MRS grating
wheel was found to be experiencing increased friction when moving between
short (A), medium (B) and long (C) wavelength settings. The Saturn
observations were redesigned to be executed in reverse-wavelength order (C, to
B, to A), with no detriment to the science, and were executed shortly before
Saturn left JWST’s field of regard in 2022 (i.e., moving below the
$85^{\circ}$ elongation angle from the Sun, as Saturn opposition was earlier
in the year on 2022-08-14).

MIRI/MRS provided a full latitude scan from Saturn’s equator to the north pole
using three separate mosaic tiles, with a final tile capturing the western
ring ansa, as shown in Fig. 1. Each tile covered the full 4.9-27.9 $\mu$m
spectrum, with saturation encountered in the brightest hydrocarbon emission
features. Saturn’s angular diameter was $16.9$” at the time of the
observations (9.8 AU from JWST, moving away from the observer at 29 km/s), and
the spatial coverage of each tile varies with wavelength, from $3.2\times
3.7$” at the shortest wavelength (4.9 $\mu$m) to $6.6\times 7.7$” at the
longest wavelength (27.9 $\mu$m).

The MIRI/MRS observations required five separate pointings on 2022-Nov-13 and
2022-Nov-14, as shown in Figure 2. The western ring ansa was observed first
(03:00-04:06UT), followed by an offset 90” north of Saturn (04:11-04:27UT) to
determine instrumental artefacts in the MRS observations. JWST then pointed to
Saturn’s northern hemisphere, targeting $45^{\circ}$N (05:40-06:44UT),
$15^{\circ}$N (06:50-07:55UT), but failed to re-acquire a guidestar to
complete the final pointing towards Saturn’s north pole. Given that Saturn was
about to depart from JWST’s field of regard, rapid instructions to re-execute
the failed MRS footprint were uploaded to the observatory, enabling the final
tile at $75^{\circ}$N on 2022-Nov-14 (21:58-23:05UT), around 36 hours after
the skipped observation was reported.

Figure 2: Spatial coverage of MIRI observations relative to Saturn’s disc (a)
and mapped to Saturn’s surface (b). Each dot represents the location of a
single spaxel with the colour indicating the MIRI channel. The background
observation was located 90” to the north of Saturn, so is not shown here
(Saturn’s disc has a diameter of $\sim 17$”).

With the exception of the $15^{\circ}$N observation, all MRS tiles used 5
groups (i.e., individual 2.8-second frames) with the `FASTR1` readout pattern,
8 integrations, and a 4-point extended-source dither pattern (no dithers were
used for the offset ‘background’ frame). The equatorial footprint used 4
groups and 10 integrations, to test the impact of using a smaller number of
groups on ability to radiometrically calibrate MRS (no problems were
identified).

###  2.2 MIRI/MRS Data Processing

The MIRI/MRS observations were reduced using the JWST pipeline version `1.9.4`
and calibration reference files `1046`111Available under calibration reference
context jwst_1046.pmap at https://jwst-
crds.stsci.edu/context_table/jwst_1046.pmap.. These were applied to the
stage-0 `UNCAL` raw data cubes downloaded from MAST. The final output of the
pipeline (stage-3), when run automatically for the archive, are spectral
images cubes for each MRS channel with individual dithers combined then
rotated and interpolated into a sky reference frame. However, the Saturn data
needed a significant amount of post-processing before being usable, such that
the pipeline was run locally, applying all three data reduction stages
separately to each dither position and tile. Stage 1 generates ‘slope images’
(count rates) from the raw `UNCAL` data; stage 2 applies wavelength
calibration and absolute flux calibration for each exposure; and stage 3
produces spectral image cubes from the input calibrated slope images - all
steps are described in the JWST data processing manual222https://jwst-
docs.stsci.edu/jwst-science-calibration-pipeline-overview/stages-of-jwst-data-
processing. The pipeline’s `ResidualFringeStep` was used to minimise the
effect of spectral fringing, although this remained a challenge at longer
wavelengths in Channels 3 and 4 [Wright . (2023)]. The default pipeline
rotates and interpolates the final cubes into a sky reference frame, but given
the significant artefacts from slice to slice (discussed below), we retained
the final stage-3 data in the coordinate system of the IFUs
(`cube_build.coord_system = ’ifualign’`) to allow later correction of flat
field effects. These corrections must be performed before any attempts to
combine individual MRS dithers, otherwise artefacts are blended together in
the final products. Finally, our pipeline included improved wavelength
calibration solutions333Known as FLT-5, now available as the ‘specwcs’ files
under calibration reference context jwst_1082.pmap at https://jwst-
crds.stsci.edu/context_table/jwst_1082.pmap. [Argyriou . (2023)], which were
generated by fitting Jupiter and Saturn spectral models (this work) to the MRS
data for each and every spaxel (the wavelength solution varied across the
IFU), and estimating the required wavelength shift for each spaxel to align
models and MRS data in the rest frame. These wavelength solutions were only
possible below 15 $\mu$m where strong and well-resolved spectral lines were
evident, but enabled a significant improvement in spectral fits over the
original pipeline.

As shown in Figure 3, significant flat field artefacts and saturation remained
in the pipeline output cubes in the IFU-aligned frame. Therefore, we developed
custom desaturation and flat field correction routines (discussed below) to
correct these effects and produce our final science cubes.

Figure 3: Example cube slices at different stages of our custom MIRI data
reduction process. The first column shows the output of the standard JWST
pipeline, which still contains significant flat field effects (a & d),
saturation (g), and partial saturation (dark pixels in g & j). The second
column shows the data after the desaturation step is applied, and the third
column shows the data after the flat field correction is applied.

###  2.3 Desaturation

Figure 4: Example spectra showing the desaturation routine. (a) shows the
spectra using different numbers of groups, where the 5 group data (in red) is
the ‘standard’ pipeline output, which shows both saturation and partial
saturation. The black dotted line in (a) shows the desaturated spectrum which
is constructed using a varying number of groups, as shown in (b), to minimise
saturation while maximising SNR. The majority of the spectral range uses the
full 5 group data, and only narrow regions (such as this one, in the $\nu_{9}$
ethane emission band) require desaturation.

In the brightest parts of Saturn’s spectrum, the MRS detector becomes
saturated, leading to a loss or corruption of data (e.g. Figure 3g). MIRI
observations are split into a series of ‘groups,’ which each record the
measured flux for a part of the whole exposure. The detector saturates when
the integrated flux reaches a certain threshold, meaning that even if the full
exposure is saturated, the first few groups in the exposure may still have
useful data that was recorded before the detector saturated. Therefore, data
reduced using different numbers of groups can be used to desaturate (i.e.
‘fill in’) saturated parts of the spectrum.

Our data processing routine modifies the `UNCAL` raw data cubes to create
versions containing the full range of groups (i.e., for the observations which
have 5 groups in total: 1, 2, 3, 4, and 5 group versions are created). These
different versions are all run through the standard JWST pipeline, creating 5
different versions of each science cube, each of which effectively has a
different integration time. These 5 cubes are then merged into a single
desaturated cube by dynamically selecting the highest number of groups
possible to maximise SNR while minimising saturation, as shown in Figure 4.

The desaturation routine operates by comparing the different versions of each
spectrum (the coloured lines in Figure 4a). The routine works iteratively,
starting with the largest number of groups $n$, then replacing bad regions of
the spectrum with the $n-1$ group spectrum. This is repeated until none of the
spectrum contains ‘bad’ data, or the 1-group spectrum is reached. The
following regions of the spectrum are treated as bad data and replaced:

  * •

Regions flagged as saturated by the JWST pipeline (marked as ‘full saturation’
in Figure 4a).

  * •

Regions that appear partially saturated, but have not been flagged as
saturated by the JWST pipeline. Regions are classed as partially saturated
where $n$ group data is $<90\%$ of the brightness of the $n-1$ group data.

  * •

Regions where the $n$ group data is $>120\%$ of the brightness of the $n-1$
group data are also flagged as outliers, likely caused by cosmic ray hits.

The desaturation routine is skipped for regions of the spectrum with a SNR
$<300$, as these would not be expected to experience saturation, and high
noise levels could lead to false positives when flagging bad regions of the
spectrum.

Initial testing of this routine found residual effects at the edge of spectral
regions flagged as bad data, so any regions of the spectrum flagged as bad
data are expanded by two spectral points. Additionally, the number of groups
used for neighbouring spectral points is only allowed to change by 1 (i.e. a
spectrum cannot immediately jump from 1 group at one wavelength to 5 groups at
the next wavelength). A similar filter is applied in image space, so at a
specific wavelength, neighbouring pixels can only vary by 1 group.

The parameters of the desaturation routine were selected by inspecting spectra
(e.g. Figure 4) and images (e.g. Figure 3) at a wide range of wavelengths,
including regions of the spectrum that do and do not experience saturation.
Almost all the spectral range uses the full 5 groups (or 4 groups for the
equatorial tile), with only the specific spectral and spatial regions ($<10\%$
of all spectral points) that experience saturation replaced with fewer groups.

###  2.4 Flat Field Correction

Figure 5: Example images at 5.07 $\mu$m (channel 1-SHORT) before (a-d) and
after (f-i) the synthetic flat field (e) is applied. The images are shown in
the IFU frame, where north is to the top left. The flat field effects in a-e
(such as the bright line across the centre of each image) are fixed in the IFU
frame, whereas the observed spatial structure on Saturn varies in position on
the detector with the different dithers, allowing flat field structure and
real spatial structure to be differentiated.

After desaturation, significant flat field effects remained visible in the
cubes, particularly at shorter wavelengths (e.g. Figure 3b,e). These effects
typically appeared as regular banding patterns aligned along the IFU slices,
stripes, and swirls that varied with wavelength, and in many cases completely
obscured any detail on Saturn. As shown in Figure 5, these patterns remained
fixed in location on the detector for different dithers (and tiles),
demonstrating that they are clearly an instrumental artefact.

These observed flat field patterns may be caused by small discrepancies
between the reference flat field images used in the pipeline and the ‘true’
flat field response of the detector. It is also possible that part of the
observed apparent flat field is caused by residual artefacts remaining after
the pipeline’s stray light correction step. For simplicity, we refer to the
entire observed pattern as the ‘flat field effect’, regardless of its origin.

To correct for these flat field effects, we used the Saturn observations
themselves to create a flat field for each channel and band. We assumed that
the flat field can be treated as a purely multiplicative effect, with a
corrected cube created by dividing the observed cube by the synthetic flat
cube. Note that the background observation, 90” north of Saturn, did not show
the same artefacts, suggesting that the flat field is sensitive to how the
target illuminates the detectors and IFU slices.

Our flat generation routine uses a set of four dithered observations to create
a synthetic flat field image for each wavelength. We match pairs of pixels
that observe (approximately) the same location on the surface of Saturn in
different dithers, and assume that any variation in brightness between these
pixels is caused by differences in the flat field for these pixels. The ratio
of brightness values of all of the pairs of pixels can then be used to
construct a flat field image at each wavelength:

  1. 1.

The input data files for each dither are ‘navigated’ to calculate the
latitude/longitude coordinates and illumination angles for each pixel. This
navigation uses the WCS metadata in the FITS headers (to convert from pixel to
celestial coordinates), which is derived from JWST’s pointing information. To
validate the navigation, we compared the navigated and observed positions of
Saturn’s limb and rings, and found that no additional manual adjustments were
needed.

  2. 2.

The four dither input images are filtered to set extreme values (outlier
pixels and pixels with emission angles $None$) to NaN to prevent any outliers
contaminating the flat. Wavelengths where the average SNR $<10$ are skipped as
the constructed flat would be too contaminated with noise.

  3. 3.

Corresponding pixels are identified by matching valid pixels that observe a
similar location on Saturn in any of the four input dithers, using the
navigated latitude/longitude coordinates for each pixel from step 1. An oval
footprint is used around each pixel (in latitude/longitude space), and any
pixels that fall within this footprint are treated as observing the same part
of Saturn. The height (North-South direction) of the oval $h$ is set at half
of the average difference in latitude between neighbouring pixels, and the
width (East-West direction) of the oval is set as $w=4h$. This elongated oval
footprint is used as Saturn has much more variation in the North-South
direction than the East-West direction, so it matches more similar pixels than
a simple circular footprint would.

  4. 4.

For each pair of corresponding pixels, $A$ and $B$, we calculate the ratio of
observed pixel fluxes $R_{AB}=O_{A}/O_{B}$. Assuming that the flat field is a
multiplicative effect, we can treat each pixel’s observed flux
$O_{i}=S_{i}F_{i}$ as the ‘true’ flux from Saturn $S_{i}$ multiplied by the
flat field for the given pixel $F_{i}$. As we define these corresponding
pixels to observe (approximately) the same region of Saturn’s surface, we
assume that the original flux from Saturn is equal for both pixels,
$S_{A}=S_{B}$, allowing the calculated ratio to be reduced to
$R_{AB}=(S_{A}F_{A})/(S_{B}F_{B})=F_{A}/F_{B}$, giving the ratio of the flat
field values for the two pixels.

  5. 5.

The set of calculated pixel ratios are then used to construct the flat field
image. A flat image is initialised with the central pixel value set to 1, and
all other values set to NaN, and we then iteratively construct the flat using
the calculated ratios to propagate values. At each iteration, every pixel
value in the flat is updated to $F^{\prime}_{A}=\mathrm{median}(R_{Ai}F_{i})$
where the $F_{i}$ are all the non-NaN corresponding pixels. After all the
updated values are calculated at each iteration, any pixels outside the range
$2/3<F_{i}<3/2$ are set to NaN to protect against outliers. After removing
outliers, the flat is then divided by the mean pixel value. The routine is run
for 50 iterations to allow the constructed flat to converge on a consistent
solution. Figure 6 shows an example flat field at different generation steps.

  6. 6.

This constructed flat image is slightly under-constrained, as all the pixels
can be multiplied by an arbitrary scaling factor and still provide a self-
consistent result. Therefore, we scale each flat image so that the mean value
of the pixels is unity, i.e. $\frac{1}{N}\sum^{N}_{i}{F_{i}}=1$. This ensures
that the application of the flat does not change a spectrum calculated by
averaging all the pixels in an entire cube.

Figure 6: Example flat field at 8.14 $\mu$m after the first 5 iterations of
the flat field generation routine, and after the final 50th iteration. The
flat is seeded with an initial value for the first iteration, which then uses
the corresponding pixel ratios (derived from the dithered observations) to
propagate the flat values and fill the image. White pixels have a value of
NaN, and the NaN values in the final flat field are regions of the detector
that do not contain any data.

Synthetic flat cubes were generated from the four dithers associated with the
$None$N and $None$N tiles, and then averaged to produce the final flat cubes
used to correct the data (the $None$N tile and ring observation included too
much background sky to be useful). The algorithm parameters were refined by
studying the flats generated from the different tiles and the quality of the
flat corrected data.

Special care was taken to ensure the flats did not contain any features of
Saturn’s atmosphere, and to ensure that the flats from different tiles
produced consistent results. Comparisons of sets of dithered images (e.g. Fig.
5) allows structure from the flat field (fixed in detector location, identical
between tiles) and real structure on Saturn (variable in detector location,
different in each tile) to be differentiated. Regions of the spectrum with and
without significant spatial structure were studied in detail, as well as the
entire spectral range using animations that compared sets of dithers at each
wavelength (see supplementary material). The $None$N and rings tiles also
provided useful checks, as these tiles were well corrected, even though they
were not used in generating the synthetic flats.

As shown in Figure 5, the synthetic flats are able to correct both small-scale
(1-2 pixel) and large scale ($>10$ pixel) variations in the sensitivity of the
detector, including in regions with significant spatial structure on Saturn.
The application of the flats helped to reveal spatial structure in the Saturn
observations that was often completely obscured by the sensitivity variations,
and prevented any spurious spatial variation being treated as real spatial
variation on Saturn’s surface.

###  2.5 Zonal Averages

Zonal averages were calculated from the observed data using the following
routine:

  1. 1.

All observed pixels, from all tiles and dithers, are binned into $None$
latitude bins.

  2. 2.

Within each bin, the median spectrum is calculated from all spectra in the
bin. The 1/3 of the spectra with the largest RMS relative to this median
spectrum are then discarded. This ensures the final zonal averages are
protected from the effect of outlier pixels. Median averaging is used here
(rather than mean) to ensure any extreme outlier pixels do not cause ‘good’
spectra to be discarded.

  3. 3.

The mean spectrum for each bin is calculated from the remaining 2/3 ‘good’
spectra. This mean spectrum is used as the zonal average for each latitude
bin, which is then used for spectral modelling in subsequent sections.

##  3 Dataset Overview

###  3.1 Saturn’s Spectrum

An average of MIRI/MRS observations of Saturn’s atmosphere is shown at the
bottom of Fig. 1 for 4.9-18.0 $\mu$m, omitting MRS data from the longest
channel (17.7-27.9 $\mu$m) due to ongoing challenges with fringe removal and
calibration. Below $\sim 7.3$ $\mu$m, the spectrum is shaped by a combination
of scattered reflected light from aerosols (notably within the deepest
absorption bands and near 6 $\mu$m) and thermal emission. The 5-$\mu$m window
is sculpted by PH3 lines below 5.2 $\mu$m ($\nu_{1}$ at 4.3 $\mu$m) and NH3
above 5.2 $\mu$m ($2\nu_{2}$ at 5.32 $\mu$m, $\nu_{4}$ at 6.15 $\mu$m), along
with narrow absorption bands of H2O (5.1-5.4 $\mu$m) and AsH3 (4.9-5.0
$\mu$m). Bright emission from the 5-$\mu$m window implies low aerosol opacity,
so cloud bands and small discrete features appear in silhouette in Fig. 1b-c
against the bright background glow from Saturn’s 4-6 bar region. Bright
reflection near 6 $\mu$m provides a means of constraining upper tropospheric
aerosols. Hydrocarbon emission from CH4 ($\nu_{2}$ at 6.5 $\mu$m) and C2H6
($\nu_{8}$ at 6.8 $\mu$m; $\nu_{6}$ at 7.3 $\mu$m) also contribute to this
4.9-7.3 $\mu$m range. This range is particularly noteworthy as it has only
been previously observed by ISO/SWS in the disc-average [Encrenaz (2003)], and
neither Cassini/VIMS ($R\sim 300$ at 5 $\mu$m) nor Cassini/CIRS ($R\sim 2800$
at 7.0 $\mu$m) could observe in the 5.1-6.9 $\mu$m range. Thus MIRI channel-1
provides access to tropospheric NH3 and H2O, along with the properties of
Saturnian aerosols, in this range for the first time, with spectral
resolutions of $R\sim 3100-3750$.

Aerosol contributions diminish at longer wavelengths in channels 2 and 3
(7.3-18.0 $\mu$m), which are dominated by the collision-induced absorption due
to H2 and He, with emission and absorption features superimposed. PH3
($\nu_{2}$ at 10.08 $\mu$m, $\nu_{4}$ at 8.94 $\mu$m) and NH3 ($\nu_{2}$ at
10.5 $\mu$m) provide absorption features that dominate the 8-12 $\mu$m range;
with strong emission features from methane (CH4 $\nu_{4}$ at 7.7 $\mu$m),
acetylene (C2H2 $\nu_{5}$ at 13.7 $\mu$m) and ethane (C2H6 $\nu_{9}$ at 12.2
$\mu$m); and weaker emission features from CO2 ($\nu_{2}$ at 14.9 $\mu$m),
diacetylene (C4H2 $\nu_{8}$ at 15.9 $\mu$m), methylacetylene (C3H4 $\nu_{9}$
at 15.8 $\mu$m), ethylene (C2H4 $\nu_{7}$ at 10.5 $\mu$m), propane (C3H8
$\nu_{26}$ at 13.4 $\mu$m) and benzene (C6H6 $\nu_{4}$ at 14.83 $\mu$m). The
H2 S(1) quadrupole at 17.03 $\mu$m and its associated dimer absorption can be
seen in Channel 3-Long, but the S(0) quadrupole at 28.2 $\mu$m is just outside
the MRS range.

The spectral database used in MRS modelling was initially based on that used
for Cassini retrievals [LN. Fletcher, Orton . (2018)], with updates to AsH3
[Coles . (2019)] and CH3 [Adam . (2019)] from the ExoMol database [Tennyson .
(2016)], and GeH4 from HITRAN [Gordon . (2022)]. Voigt broadening was used for
all bands - the sub-Lorentzian lineshape of 04bailly did not have an impact on
the quality of the spectral fits. The line database was used to calculate
$k$-distributions for each gas within each of the 12 MRS subbands, using the
wavelength grid from stage 3 of the standard pipeline, and the wavelength-
dependent resolving power of each channel determined from ground-based
measurements [Labiano . (2021)]. Note that in-flight commissioning updates to
the spectral resolution [Jones . (2023)] have not been incorporated into our
spectral models at this stage.

Collision-induced absorption of H2 and He was included based on their dimer
absorptions [LN. Fletcher, Gustafsson Orton (2018)]. During the course of the
spectral fitting, residuals between model and data were used to identify
missing bands of known species, and to search for any new species. Multiple
bands of propane are observed on Saturn for the first time - only $\nu_{26}$
at 13.4 $\mu$m had been included in our line database based in GEISA [Delahaye
. (2021)], and had been previously used to study the propane distribution
[Guerlet . (2009), LN. Fletcher, Orton . (2018)]. Residuals at high latitudes
revealed the presence of the $\nu_{7}$, $\nu_{20}$, $\nu_{21}$ and $\nu_{8}$
emission bands at 8.63, 9.49, 10.85 and 11.51 $\mu$m, respectively, for the
first time. These were introduced into our line database using the pseudo-
linelist of 13sung, and the improvement in spectral residuals are shown in
Supplemental Fig. 23.

The GeH4 $\nu_{2}$ band at 10.74 $\mu$m is too weak to be seen, lost amongst
numerous NH3 absorption features. The AsH3 $\nu_{4}$ band at 9.97 $\mu$m does
have a detectable signature on the edge of a PH3 absorption line, but in a
region of the spectrum that is affected by MRS fringing at the longward end of
channel 2-medium. The small spectral feature is reproduced by an abundance of
$\sim 0.4$ ppb, with a decline from equator to pole that cannot be explained
by fringing. Nevertheless, precise constraints must await more robust
defringing strategies. Finally, we see no evidence of emission from the HCN
$\nu_{2}$ at 14.05 $\mu$m, discussed in Section 5.4.

###  3.2 Spatial Structure

Figure 7: Composite images created by combining all three Saturn tiles to show
the equator-to-pole variation in brightness temperature at different
wavelengths. 5.05 $\mu$m senses aerosol opacity in the deep troposphere (3-6
bars), whereas 5.207 $\mu$m is in a strong NH3 absorption and senses a blend
of thermal emission and reflected sunlight from upper tropospheric aerosols.
7.67 $\mu$m senses stratospheric temperatures (0.1-5 mbar) via CH4 emission;
whereas 10.75 $\mu$m senses a blend of tropospheric temperature and ammonia
opacity near 400-600 mbars. 14.4 and 15.5 $\mu$m are primarily sensitive to
the H2-He continuum, sounding tropospheric temperatures in the 100-to-300-mbar
range. Note that each tile observed a different longitude range, but are shown
overlapping here for simplicity - this causes some artificial inconsistencies
in brightness at regions of overlap. Figure 8: Variation in Saturn’s zonal
average brightness temperature with wavelength and latitude. (a) shows the
difference from the mean brightness temperature spectrum (d), where red areas
are brighter than the average and blue areas are dimmer. (b) shows the
normalised zonal average brightness of HST observations of Saturn in September
2022 [Simon . (2023)] and (c) shows Saturn’s zonal wind profiles [García-
Melendo . (2011)]. Solid and dashed horizontal lines in (a-c) indicate the
peaks and troughs of the zonal wind profiles respectively. The background of
(c) shows part of the colour composite map of Saturn created from the HST
observations.

Fig. 7 shows selected wavelengths from the three MIRI/MRS 4.9-27.9 $\mu$m
cubes spanning from Saturn’s equator to the north pole. Given Saturn’s
relative longitudinal homogeneity, these can be taken as a good approximation
to a zonal mean, which is calculated as described in Section 2. Fig. 8 then
shows the difference between the zonally-averaged brightness and the mean
brightness temperature spectrum, highlighting strong gradients as a function
of latitude. These gradients are compared to the cloud-tracked zonal winds
from Cassini in both the continuum and methane bands [<]Fig. 8c,¿[]11garcia,
showing how thermal-infrared brightness is related to the peaks of the
eastward and westward jets. We also compare the MIRI/MRS maps (acquired in
November 2022) to visible-light reflectivity scans acquired by Hubble (HST) in
September 2022 [<]Fig. 8b¿[]23simon, to show how brightness temperature and
aerosol reflectivity are related. The zonally-averaged reflectivity in ten HST
WFC3/UVIS filters has been normalised for plotting purposes, to highlight the
similarities in the location of strong brightness gradients.

Together, the composite images of Fig. 7 and the zonal-mean brightness in Fig.
8 reveals a wealth of detail. The exquisite sensitivity of MRS, even compared
to previous spectroscopic maps from Cassini, reveal Saturn’s banded structure
in both reflected sunlight and thermal emission. The following three features
stand out in the meridional (latitudinal) direction:

  * •

Belt/Zone Structure: The strongest meridional gradients in tropospheric and
stratospheric brightness temperatures are co-located with the peaks of the
eastward and westward jets, as measured at Saturn’s cloud-tops [García-Melendo
. (2011)], supporting a geostrophic balance between the winds and temperature
gradients via the thermal windshear equation, and the decay of Saturn’s
tropospheric winds with altitude [Pirraglia . (1981), Conrath Pirraglia
(1983)]. At mid-latitudes where Ferrel-like meridional circulation cells are
expected to dominate [LN. Fletcher, Kaspi . (2020)], Saturn’s zones are
defined as cool, anticyclonic bands equatorward of eastward jets, whereas the
belts are warm, cyclonic bands poleward of eastward jets [Del Genio . (2009)].
MIRI continuum emission from the troposphere (e.g., Fig. 7e-f) reveals subtle
cool zones equatorward of eastward jets at $31.5^{\circ}$ (an inflection in
the broad equatorial jet), $47.8^{\circ}$, $61.5^{\circ}$, and
$78.0^{\circ}$N, in addition to the broad cool Equatorial Zone at
$<9.2^{\circ}$N where continuum-band cloud tracking reveals a maximum eastward
windspeed [García-Melendo . (2011)]. Stratospheric banding is more subtle, but
a bright equatorial band is observed in methane emission at 7.67 $\mu$m and in
the peak of the acetylene emission at 13.7 $\mu$m [<]corresponding to the
equatorial stratospheric oscillation,¿[]08orton_qxo,22blake.

  * •

North Polar Stratospheric Vortex (NPSV): The warm NPSV, defined by the strong
gradient in stratospheric brightness temperature near $78^{\circ}$N, is
visible throughout the MIRI/MRS dataset, particularly near 7-8 $\mu$m sensing
stratospheric CH4, and in regions of tropospheric continuum emission longward
of 14 $\mu$m. This should be contrasted with generally low polar brightness
temperatures in the 5-6.5 $\mu$m and 9-11 $\mu$m regions that probe higher
pressures. The NPSV formed during northern spring [LN. Fletcher, Orton .
(2018)] and is expected to have reached its maximum contrast with respect to
lower latitudes in 2021-22 [<]confirmed by ground-based thermal
imaging¿[]22blake, and to decline in visibility in the coming years - see
below for discussion of seasonal evolution since the end of the Cassini
mission in 2017. Embedded within the NPSV, the central north polar cyclone
discovered by Cassini [<]NPC,¿[]08fletcher_poles remains visible in the
MIRI/MRS maps as a peak in brightness temperature right at the pole (Fig.
7c-f).

  * •

Deep Cloud Structure: As has been previously shown by ground-based imaging at
5.1-5.2 $\mu$m [Yanamandra-Fisher . (2001)] and Cassini/VIMS spectroscopy
[<]$<5.1$ $\mu$m,¿[]06baines, 11fletcher_vims, MIRI 5-$\mu$m maps in Fig. 7a
and the zonal mean in Fig. 8 reveal cloud banding on a much finer spatial
scale than the broad belts and zones defined by the temperature field. Indeed,
the distribution of aerosols revealed by MIRI spectroscopy between 4.9 and 7.3
$\mu$m is a good match for the albedo contrasts in Fig. 8c, but are not a good
proxy for Saturn’s belts and zones [LN. Fletcher, Kaspi . (2020)] \- aerosols
do not simply condense where it is cold, and sublimate where it is warm, with
a more complicated pattern emerging. Rather, the finescale cloud banding
observed at 5 $\mu$m is a closer match to that seen in visible light
[<]e.g.,¿[]05vasavada. Deep NH3 absorption features in the 5.1-5.3 $\mu$m
range, and in the 6-$\mu$m region, also reveal the contribution of reflected
sunlight from upper tropospheric aerosols (Fig. 7b) - without this reflection,
there would be almost no radiance from these deep bands. The equatorial region
is particularly bright in the 5.5-6.5 $\mu$m range in Fig. 8a due to the
presence of upper tropospheric aerosols observed in visible light in Fig. 8a.

The deep clouds revealed by MIRI/MRS are shown in more detail in Fig. 9, which
uses only the shortest MRS channel (1A). Animations in the Supporting Material
show that Saturn’s appearance changes dramatically with wavelength, from
inside a deep absorption band (sensing sunlight reflected from upper
tropospheric aerosols) to the intervening ‘continuum’ (sensing deeper cloud
opacity). The Equatorial Zone (0-9.2∘N) is generally dark (e.g., high aerosol
opacity), with a brighter band near 10∘N marking the boundary with the NEB. At
5.0 $\mu$m small low-contrast features are observed up to the prograde jet at
$48^{\circ}$N, but these cannot be seen at 5.2 $\mu$m where reflected sunlight
creates a more homogeneous appearance. Poleward of the $48^{\circ}$N jet, the
5-$\mu$m thermal emission increases substantially at the same time as the
reflectivity sharply declines. We no longer see the 5-$\mu$m-bright, aerosol-
depleted band near 35-40∘N that had dominated the appearance of the northern
hemisphere after the 2010-11 storm [Sromovsky . (2016)], consistent with a re-
population of the band by Saturn’s seasonal aerosols in the decade since the
storm, so that this band no longer stands out. This fading of the 5-$\mu$m
storm emission is consistent with the ground-based record [Bjoraker . (2020)].
Fine banding is observed up to $61.5^{\circ}$N, when the 5-$\mu$m emission
again rises substantially as 5.2 $\mu$m reflection falls, with bright emission
continuing to the latitude of the hexagon at $78.0^{\circ}$N. Interior to the
hexagon, the polar domain in Fig. 7a-b is 5-$\mu$m dark, and also dark in
reflected sunlight at 5.2 $\mu$m, consistent with its appearance in
Cassini/VIMS observations in 2016 [Sromovsky . (2021)].

Figure 9: Mapped spatial structure in channel 1-short for low- and mid-
latitudes. (d) and (h) show the zonal wind velocities [García-Melendo .
(2011)] and (i) shows an example brightness temperature spectrum. While all
panels sense a blend of thermal emission and reflected sunlight, panels (a)
and (e) primarily reveal deep thermal emission modulated by overlying aerosol
opacity, whereas panels (b)-(c) and (f)-(g) primarily reveal reflectivity
variations from aerosols in the upper troposphere. Solid and dashed horizontal
lines indicate the peaks and troughs of the zonal wind profiles, respectively.
The backgrounds of (a-c & e-g) show the colour composite map of Saturn created
from the HST observations.

Seasonally-generated aerosols are not homogeneous over the whole of Saturn’s
northern hemisphere, but are confined by the banded structure - the reflected
sunlight component in Fig. 9b and the 5-6 $\mu$m range in Fig. 8a decreases in
distinct steps from the equator to the pole, with notable boundaries between
the EZ and NEB ($None$N) and at $None$N; the thermal 5-$\mu$m emission
component shows a notable increase between $None$N and $None$N, consistent
with the lowest aerosol opacity there, and with the suggestion of cloud-
clearing near $65^{\circ}$N in 2019-20 [Bjoraker . (2020)]. A four-fold
increase in aerosol opacity of upper-tropospheric hazes interior to the
hexagon between 2013 and 2016 was observed by Cassini [Sromovsky . (2021)],
possibly accounting for the dark hexagon appearance to JWST in 2022. MIRI does
not observe the hexagon latitude as a bright 5-$\mu$m band, which had been
evident in VIMS observations in 2013 and 2016, suggesting continued increases
in aerosol opacity in the polar domain, spreading to lower latitudes, but not
yet reaching the $None$-$None$N range.

Right at the equator, equatorward of $5^{\circ}$N, both the 5-$\mu$m
brightness and the reflected sunlight drop to create an unusually dark band.
This can be seen in Fig. 8, where the dark band corresponds to subtle colour
contrasts in Hubble images from September 2022, and to a peak of reflectivity
in several of the individual HST filters, notably the strong CH4 band at 889
nm. Such a dark band was not evident in VIMS maps in 2006-11, although this
region was generally bland and diffuse at 5 $\mu$m, with several dark plume-
like discrete features bordering it at $None$-$None$N [LN. Fletcher . (2011)].
This dark band coincides with the rapid increase in the upper tropospheric
winds observed in the 890-nm CH4 band i Fig. 8c [García-Melendo . (2011)]. We
will return to this unique region, and the challenging spectral fits, in
Section 5.

####  3.2.1 Discrete Vortices and Hexagon

Although Saturn remains longitudinally homogeneous at most latitudes, we
subtracted zonal averages from the MRS images to search for evidence for
discrete features. Such longitudinal contrasts were only definitively observed
in the shortest MRS channels in Fig. 9, where significant structure is
observed. In particular, we see a bright cloud-free region near $None$N,
$None$W; at least two dark anticyclonic vortices near $None$N, $None$W and
$None$N, $None$W; a patch of high 5-$\mu$m brightness at the edge of the
equatorial zone near $None$N, $None$W; and spatial structure in the bright
band surrounding the dark polar domain near $None$N, $None$W. In an effort to
determine the history and longevity of these features, we compared the MIRI
maps (13-14 November 2022) to HST observations [<]21 September 2022,¿[]23simon
and observations by amateur astronomers [<]using the PVOL database throughout
November 2022,¿[]18hueso to search for the presence of these discrete features
in multiple datasets. Comparisons of the JWST and HST data are shown in the
Supplemental Material.

Unfortunately, there were no conclusive detections of any of the features
observed by MIRI in these supporting visible-light datasets. Given the $\sim
7$-week time gap between HST and JWST observations this is perhaps
unsurprising, despite attempts to account for zonal drifts during this
interval in the Supplemental Material. In particular, HST observations in
September revealed the continued presence of the anticyclonic vortex (AV) that
was generated by the 2010-11 storm [Sayanagi . (2013)] near $None$N, $None$W,
and the presence of a pair of vortices near $None$N, $None$W that could be
related to the coupled vortex system on the ‘double jet’ near $None$-$None$N
studied by 18gaztelurrutia. Whilst the shared latitudes are compelling, the
limited longitudinal coverage of the MRS maps, combined with the 7-week time
gap since Hubble, makes it unlikely that these are the same features. Indeed,
the long-lived AV at $None$N was expected to be near $None$W on 2022-Nov-13,
and was therefore missed by MIRI (Fig. 2). This strongly argues for
contemporaneous MIRI/MRS spectroscopy and NIRCAM (or HST) imaging in future
observing programmes.

Finally, when the MIRI observations had originally been designed (assuming a
2018 launch), we had hoped to detect the vertices of Saturn’s polar hexagon
during northern summer, both in tropospheric and stratospheric thermal
emission [LN. Fletcher, Orton . (2018)]. Unfortunately, the decreasing sub-
observer latitude in 2022 reduced the chances of success, and no convincing
evidence of hexagon vertices can be observed, despite considerable work to
clean up and combine the individual channel-1 dithers. We will likely need to
wait to the 2040s for our next infrared views of the hexagon itself.

###  3.3 Seasonal Change Since Cassini

Figure 10: Comparison of Cassini/CIRS observations in 2017 to MIRI/MRS
observations in 2022. MRS spectra were convolved to the same spectral
resolution as CIRS (15 cm-1), and three CIRS northern hemisphere maps were
zonally averaged onto a latitudinal grid. Brightness temperatures as a
function of wavelength are shown in (a)-(c), showing stratospheric CH4
emission on the left, ethane emission near 12 $\mu$m, and continuum H2-He
absorption on the right. In panel (d), the emission angles for MIRI (black)
and CIRS (red) for each latitude are approximately the same, so differences
are not due to limb brightening/darkening. CIRS measurements between $\sim
9$-10 $\mu$m are disregarded (white rectangle) as they had low signal-to-
noise.

Before modelling the MRS spectra, we compare the calibrated JWST data to other
observations in the mid-infrared. Although ground-based studies have been able
to monitor morphological changes to Saturn’s mid-IR emission since 2017 [Blake
. (2022)], these have typically been calibrated to match a low-latitude
average of Cassini/CIRS radiances due to the difficulties arising from
variable telluric contamination, making genuine assessments of global-scale
temperature changes rather challenging. Fig. 10 compares the MIRI 7-17 $\mu$m
brightness with those from Cassini/CIRS in 2017. Three CIRS northern
hemisphere maps (2017-Jan-19, 2017-Apr-17 and 2017-Aug-26), acquired from a
near-equatorial sub-spacecraft latitude at a spectral resolution of 15 cm-1,
were combined and zonally-averaged onto a $1^{\circ}$ latitude grid. The
MIRI/MRS zonal averages were convolved with the CIRS instrument lineshape to
achieve the same low spectral resolution, and the differences are shown in
Fig. 10. Note that the viewing geometry for each latitude was approximately
the same in 2022 and 2017, such that limb darkening/brightening should be
negligible.

The 7-8 $\mu$m CH4 emission in Fig. 10(c) shows the change in Saturn’s
equatorial stratospheric oscillation [Orton . (2008), Fouchet . (2008)], which
reveals a brighter equatorial band in 2022 compared to 2017. This warm band is
also visible at 7.65 $\mu$m in Fig. 7. This is consistent with the ground-
based record [Blake . (2022)] which showed continued warming at the equator
since the end of the Cassini mission. It is not, however, consistent with
expectations from one Saturnian year earlier, where the equatorial band was in
its cool phase in 1993-1995 during the same season. Thus the semi-annual
nature of Saturn’s equatorial oscillation remains in doubt [Sinclair . (2013),
Blake . (2022)].

The stratosphere from $10^{\circ}$ to $35^{\circ}$N appears to be cooler in
2022 than in 2017, consistent with the idea of upwelling and adiabatic cooling
in the summer hemisphere as part of an interhemispheric circulation from
summer to winter, reminiscent of the Earth’s Brewer-Dobson circulation
[Friedson Moses (2012), Bardet . (2022)]. During northern winter
($L_{s}=310^{\circ}$), enhancements of stratospheric hydrocarbons detected by
Cassini near 25∘N [Guerlet . (2010)] were consistent with stratospheric
subsidence as part of a meridional circulation from summer to winter [Friedson
Moses (2012)]. The cooler brightness temperatures measured by MIRI suggest
that this circulation has now reversed by $L_{s}=150^{\circ}$, having switched
direction near equinox in 2009 [Bardet . (2022)]. In the following sections,
we will attempt to verify this via measurements of trace hydrocarbon species
to determine vertical motions in the northern low-latitude stratosphere.

Poleward of $60^{\circ}$N, the MIRI/MRS observations reveal warmer
temperatures than the Cassini/CIRS observations, consistent with the continued
warming of the NPSV in Fig. 7 as northern summer progressed. This is also true
in the northern troposphere, sampled by H2-He emission longward of 14 $\mu$m,
which has warmed since 2017. Radiative models [Guerlet . (2014)], combined
with the viewing geometry from Earth [Blake . (2022)], predict the visibility
of the NPSV will drop considerably in the next 1-2 years as autumn approaches.
Cooler MIRI north-polar temperatures at $\sim 10$, 12.2, and 13.7 $\mu$m could
be a consequence of differences in spatial and spectral resolution
(particularly the spectral convolution in the Q-branches of ethane and
acetylene), rather than reflecting a real change between 2017 and 2022.

As a further assessment of seasonal change, we compare zonal-mean scans of
ground-based VLT/VISIR observations of Saturn at 7.9, 12.3 and 17.6 $\mu$m
from 2016 to 2022 [Blake . (2022)] to the results from CIRS and MIRI (see
Supplemental Fig. 25). All three techniques capture the contrasts associated
with Saturn’s bands, with the 17.6-$\mu$m observations confirming the
tropospheric warming during northern summer; and 12.3 $\mu$m showing the warm
equatorial band and increased brightness of the NPSV. However, at 7.9 $\mu$m,
Supplemental Fig. 25c shows the problems associated with scaling ground-based
images to a low-latitude CIRS average, as it missed the stratospheric cooling
between $10^{\circ}$ and $50^{\circ}$N, and therefore overestimated the
brightness of the NPSV. So whilst the NPSV has warmed since 2017 (by 3-4 K in
brightness temperature at 7.9 $\mu$m), the magnitude is smaller than that
presented in ground-based studies [Blake . (2022)]. In summary, MIRI/MRS
observations in 2022 reveal changes to Saturn’s equatorial oscillation,
unexpected stratospheric cooling equatorward of $40^{\circ}$N, tropospheric
warming at most latitudes, and the continued warming of the NPSV.

##  4 Spectral Modelling

Although inspection of the cleaned MIRI/MRS cubes can demonstrate the spatial
contrasts in Saturn’s mid-IR emission associated with cloud banding and
discrete storms, further progress can be made by inverting the MIRI spectra to
determine Saturn’s temperatures, zonal winds, aerosols, and distributions of
gaseous species.

###  4.1 MRS Spectral Fitting

Zonally-averaged MRS spectra (Section 2.5) were fitted using the NEMESIS
optimal estimation retrieval algorithm [Irwin . (2008)], which has been
previously applied to Cassini/VIMS and Cassini/CIRS spectra of Saturn. Sources
of spectral linedata and the generation of $k$-distributions were described on
Section 3.1. Spectral uncertainties reported by the MRS pipeline (the `ERR`
backplane) were averaged for the pixels used for each latitude, but were found
to be unrealistically small - we retain the spectral shape of the uncertainty
envelope, but increase the error by factors of 10 to 40 (depending on MIRI
channel) to enable spectral fits with a goodness-of-fit of approximately one -
this is the equivalent of adding forward-modelling uncertainty during the
retrieval process [Irwin . (2008)].

Given the broad spectral coverage, and the high spectral resolution, we
adopted a multi-stage approach, first fitting undersampled broadband spectra
to constrain atmospheric temperatures and aerosols, before fitting narrowband
MRS spectra at their native sampling to study specific gaseous species. Each
MRS subband was fitted simultaneously, but used the correct geometry
(emission, incidence, and azimuthal angle) to calculate the atmospheric path,
as small differences arise from different pointings between tiles and dithers.
In addition, we divide the data at 7.3 $\mu$m, with longer wavelengths
considering only thermal emission and no scattering, but shorter wavelengths
considering multiple scattering of both reflected and thermal photons, as has
been typical of Cassini studies (see Supplemental Figure 24, which shows that
aerosol-free models and cloudy, scattering models converge in the 6.6-7.3
$\mu$m range, although this is somewhat dependent on the choice of refractive
indices described below). The sequential stages were as follows:

  1. 1.

Global Fit: Fitting the full 7.3-16.3 $\mu$m region, sampling every 4th point
in the spectrum to accelerate the retrieval process, to estimate the $T(p)$
profile as a function of latitude, along with initial assessments of gaseous
variability. Temperatures, continuous profiles of ethane and acetylene,
parameterised profiles of NH3 and PH3, and scaled abundances of C2H4, C4H2,
C3H4, C3H8, C6H6, CH3, and CO2 were retrieved during step 1 (see below for
discussion of priors). The gaseous abundances would be refined later, but this
provided a first zonal-mean $T(p)$ structure. Tropospheric temperatures and
gas abundances were determined by simultaneously fitting the H2-He continuum
observed beyond 15 $\mu$m and the absorption of NH3 and PH3 between 8-12
$\mu$m. Stratospheric temperatures are largely controlled by CH4 emission at
7.8 $\mu$m, but also by the simultaneous fitting of temperature and
composition from the ethane and acetylene emission. We omit wavelengths beyond
16.3 $\mu$m due to challenging fringing/ripples that dominate the long-
wavelength spectrum, and the lack of an in-flight MRS calibration for channel
4 at the time of writing. We omit 11.9-12.3 $\mu$m due to a known artefact in
this MRS subband, whereby light leaks through the MRS dichroic filter from 6.1
$\mu$m to 12.2 $\mu$m to create a source-dependent artefact in the data. This
artefact was identified due to difficulties in fitting Saturn’s C2H6 emission
band at 12.2 $\mu$m simultaneously with CH4 at 7.8 $\mu$m.

  2. 2.

Refined Temperatures: The initial $T(p)$ from step 1 was then used as a prior
for (i) a refined estimate of the stratospheric temperatures, using data from
channels 1C and 2A (7.3-8.4 $\mu$m) at their full spectral resolution; and
(ii) a refined estimate of tropospheric temperatures, ammonia, and phosphine
from 8.0-11.5 $\mu$m (channels 2A to 2C). Examples of the quality of the
spectral fits for latitudes at $None$N (representative of low latitudes) and
$None$N (representative of the bright polar emission) are shown in Fig. 11a
and Fig. 11b.

  3. 3.

Aerosol Fitting: Temperatures were then fixed for aerosol and gaseous
retrievals from the 4.9-7.3 $\mu$m range (channels 1A to 1C, sampling every
3rd spectral point). Unlike the longer wavelengths, this region requires
multiple scattering of reflected and thermal light to fit, and full details of
the aerosol model are provided below, with example spectral fits shown in Fig.
11c. The resulting aerosol profiles were then incorporated back into the
initial temperature inversions to check for any changes in the $T(p)$
structure, using only their absorption cross-sections (i.e., without
scattering). Changes to the resulting $T(p)$ were negligible, but would be
dependent on the refractive indices of the aerosols, which are not uniquely
constrained (see Section 4.3).

  4. 4.

Refined Composition: Finally, we adopted the $T(p)$ and aerosol cross-
sections, alongside the derived NH3 and PH3 distributions, as priors for
focused retrievals of specific spectral features such as the hydrocarbons,
HCN, CO2 and H2O. Examples for a range of gaseous species are shown in Fig.
12, with key features labelled.

Figure 11: Examples of the quality of spectral fits (solid lines) to MIRI/MRS
data (points with error bars) at different stages within our multi-stage
retrieval. Panel (a) is dominated by stratospheric CH4 emission, used to
determine the vertical temperature structure; panel (b) sounds tropospheric
species like PH3, NH3 and CH3D; panel (c) sounds reflection and absorption by
tropospheric aerosols, in addition to PH3, NH3, H2O, and emission from
stratospheric CH4 and C2H2. Spectra at $None$N have been offset from the
$None$N spectra by 20 K for clarity. Uncertainties are a scaled version of
those reported by the JWST pipeline, as described in Section 4. Figure 12:
Example fits (black line) to observations (points with error bars) in specific
regions of Saturn’s spectrum, including those regions sensing stratospheric
hydrocarbons (ethane in (a) with the central region omitted as described in
the text; acetylene and propane in (b); ethylene in (c); CO2 and benzene in
(d); methylacetylene, diacetylene, and methyl in (e)) and tropospheric
absorptions from ammonia, phosphine and (tentatively) water in (c) and (f).
Spectra for $None$N have been offset from $None$N for clarity by 10 K in (a)
and (c); 20 K in (b) and (f) - no offsets were used in (d) and (e).

###  4.2 Fitting Temperatures and Gases

Saturn’s prior $T(p)$ at $p>50$ mbar, as well as PH3 and NH3 profiles, are
based on a low-latitude ($\pm 30^{\circ}$ latitude) average from Cassini/CIRS
nadir observations [LN. Fletcher, Orton, Teanby Irwin (2009)]. The $T(p)$ is
extended deeper than 0.8 bar using a dry adiabatic lapse rate [LN. Fletcher .
(2011)], and higher ($p<50$ mbar) using a global average of Cassini/CIRS limb
observations [Guerlet . (2009)], resulting in a prior defined from 1 $\mu$bar
to 10 bars. We adopt the He/H2 ratio from Voyager [Conrath Gautier (2000)],
CH4 and its isotopologues from Cassini/CIRS [LN. Fletcher, Orton, Teanby,
Irwin Bjoraker (2009)]; C2H2, C2H6 and C3H8 from an average of CIRS limb
measurements [Guerlet . (2009)]; all other hydrocarbons (C2H4, C4H2, C3H4,
C6H6, CH3), and CO2 come from averages of the seasonal photochemical model of
05moses_sat, updated to a finer latitude grid with zero meridional mixing
[<]$K_{yy}=0$ m2/s,¿[]07moses. Prior deep abundances for CO [<]1.0
ppb,¿[]90noll, GeH4 [<]0.4 ppb,¿[]90noll, H2O [<]0.176 ppb,¿[]97degraauw and
AsH3 [<]2.2 ppb,¿[]11fletcher_vims come from previous investigations at 5
$\mu$m. HCN uses the stratospheric upper limit from Herschel [<]22 ppb at
$p<1$ mbar,¿[]12fletcher_spire.

With the exception of CO, AsH3, and GeH4, all species were allowed to vary
from their priors during the MIRI/MRS retrievals. Depending on the size and
strength of their spectral contributions in Fig. 13, gases were either (i)
retrieved as full, continuous profiles with height (C2H2, C2H6); (ii)
parameterised in terms of a deep mole fraction, transition pressure,
fractional scale height (compared to the atmospheric scale height) up to the
tropopause (NH3, H2O and PH3); or (iii) simply scaled versions of the prior
profiles (i.e., which implicitly assumes that the vertical profile is an
accurate representation of Saturn’s atmosphere). Contribution functions for
these spectral ranges are shown in Fig. 13, indicating the approximate
sensitivity levels to which the different MRS channels are sensitive.

Figure 13: Cloud-free contribution functions (Jacobians showing the rate of
change of spectral radiance with temperature) calculated for nadir viewing and
a typical mid-latitude composition on Saturn. These contribution functions
have been normalised at each wavelength, and darker shading indicates the
greatest contribution. No aerosols are included in this model. Key gaseous
features have been labelled, note that minor stratospheric species become
apparent with higher emission angles (not shown). The y-axis changes in
pressure range to emphasise dominant features.

###  4.3 Fitting Aerosols

Saturn’s aerosol distribution is best constrained via remote sensing at
visible and near-infrared wavelengths, but the opacity, location, and
wavelength-dependent scattering/absorption properties of aerosols contributes
significantly in the MIRI/MRS range, particularly at wavelengths below 7.3
$\mu$m. Studies in this range remain somewhat limited, and are dominated by
investigations of aerosol changes in discrete regions such as the polar domain
[Sromovsky . (2021), Sánchez-Lavega . (2020)] and the northern storm band
[Sromovsky . (2013), Sromovsky . (2016)]. Attempts to study the latitudinal
distribution of aerosols have used Hubble [Stam . (2001), Pérez-Hoyos .
(2016)], Cassini/ISS [Roman . (2013)], and Cassini/VIMS [LN. Fletcher .
(2011)], with the latter using only nightside 5-$\mu$m spectra to avoid the
complications of reflected sunlight, whereas JWST/MIRI spectra are a blend of
thermal emission and reflected sunlight. To our knowledge, there have been no
aerosol retrieval studies that utilise the 5.1-6.8 $\mu$m domain inaccessible
to Cassini.

Given the degeneracies inherent in fitting reflected sunlight spectra, as
evidenced by the range of different results in the literature [LN. Fletcher,
Sromovsky . (2020)], we initially adopted an Occam’s-razor approach to fitting
the 4.9-7.3 $\mu$m range. Fig. 13 shows that the 5-$\mu$m region senses high
pressures (3-7 bars) in the absence of aerosols, so we initially considered a
single aerosol population near $\sim 1.5$ bars (i.e., above the primary
contribution functions near 5.0-5.2 $\mu$m), rather than multiple different
compact cloud decks, and allowed the base pressure, top pressure, total
opacity and vertical extension to vary freely during the fitting process. We
also avoided imposing any particular spectral shape to the aerosol cross-
section, single-scattering albedo, and phase functions (modelled via two-term
Henyey Greenstein functions fitting the results of Mie scattering
calculations). Values for particle radius (from a standard gamma distribution
with a 5% variance) and the spectrally uniform real and imaginary refractive
indices were chosen after tests at several latitudes where these values were
allowed to vary freely. The radius had a small effect, with $r=1.0\pm 0.05$
$\mu$m selected as a best fit. The spectral fits were largely insensitive to
the real refractive index, with values representative of NH3 ice $n_{r}\sim
1.4$ [Martonchik . (1984)] and NH4SH solid $n_{r}\sim 2.3$ [Howett . (2007)]
fitting equally well, so a mean $n_{r}=1.8$ was selected. The most important
parameter was the imaginary refractive index $n_{i}$, which varies
considerably in the infrared depending on the assumed composition of the
aerosols. Fits were significantly improved with smaller values of $n_{i}$ in
the 5-$\mu$m region than would be typically expected for ‘pure’ NH3 and
NH$4$SH aerosols - the final selected value was $n_{i}=1\times 10^{-3}$ over
the whole range, resulting in weakly absorbing particles (single scattering
albedoes $>0.95$ in this spectral range).

We attempted to fit the 4.9-7.3 $\mu$m range (step 3, above) using this
single-cloud layer and the choices of optical properties described above.
Retrievals with multiple scattering of reflected and thermal photons are
numerically intensive, requiring numerical evaluation of Jacobians at each
step of the inversion, so MIRI/MRS spectra from channels 1A, 1B and 1C were
undersampled by a factor of 4 to ensure a good fit across the whole 4.9-7.3
$\mu$m range. The latitudinally-resolved $T(p)$ derived from steps 1 and 2 of
our retrieval scheme were required to correctly reproduce emission from the
CH4 $\nu_{2}$ band at 6.5 $\mu$m and the C2H6 emission band at 6.8 $\mu$m,
seen in Fig. 11c, particularly at high-latitudes where emission from the warm
NPSV dominates. We varied the vertical location and extent of the cloud
simultaneously with parametric profiles of NH3, PH3 and H2O.

This single-cloud model was remarkably successful in fitting most of the
4.9-7.3 $\mu$m range, allowing us to then search for discrepancies which might
hint at more complicated cloud structure, such as the multi-layer clouds of
21sromovsky, or the potential wavelength-dependent absorptions of
photochemical hazes such as those of 15guerlet. Poor fits in the dark
equatorial band and polar latitudes led us to experiment with a second cloud
layer in the upper troposphere sitting at higher altitudes than the original
layer, providing further degrees of freedom but informed by the observation of
hazes in Saturn’s upper troposphere [Roman . (2013)]. A real refractive index
of $n_{r}=1.74$ was selected, similar to the value for diphosphine at 195 K
(P2H4) that had been adopted in Cassini/VIMS studies of upper-tropospheric
hazes [Sromovsky . (2021)], based on the expected photochemical production
from PH3. The resulting fits were weakly sensitive to the aerosol radius
($1.0\pm 0.05$ $\mu$m was selected) and imaginary refractive index
($n_{i}=5\times 10^{-3}$ was selected). This double-cloud scheme can be
thought of as representing the upper tropospheric haze and the top-most
condensate cloud, with their base pressures, opacities, and vertical
extensions all freely varying during the 4.9-7.3 $\mu$m fitting, and producing
the high-quality fits shown in Fig. 11(c). The results will be discussed in
Section 5.

Finally, although Cassini/CIRS observed aromatic and aliphatic hydrocarbon
aerosols in the polar stratosphere ($p<8$ mbar) in limb observations [Guerlet
. (2015)], there is limited need to include them in the nadir MIRI/MRS
spectral fitting. The CIRS results suggested a peak in opacity near $6.9\pm
0.3$ $\mu$m, and there is a subtle but compelling residual in the spectral
fits in the same location (see Supplemental Fig. 22), which will be the topic
of future investigations.

##  5 Saturn’s Temperatures, Aerosols and Composition in 2022

The results of the multi-stage retrievals of zonally-averaged temperatures,
aerosols, and gaseous species are described in the following subsections.

###  5.1 Temperatures and Winds

Saturn’s zonal-mean temperatures during northern summer are shown in Fig. 14,
as derived from step two (i.e., refined spectral fitting at full spectral
resolution after a ‘global fit’ to the 7.3-16.4 $\mu$m spectrum). The
temperature inversion confirms many of the conclusions available from the
brightness temperature maps alone. The troposphere is characterised by a cool
EZ; temperature gradients between mid-latitude belts and zones that correlate
with the peaks of the cloud-tracked zonal winds; and a warm polar domain. The
cool EZ is coincident with the highest aerosol opacity (Section 5.2), and it
is therefore possible that our aerosol-free assumption for wavelengths beyond
7.3 $\mu$m is inadequate, despite tests in Section 4 suggesting that the
derived aerosols had minimal absorption at these wavelengths. Further
refinement of the aerosol refractive indices, incorporating wavelengths longer
than 7.3 $\mu$m, would be needed to fully resolve this potential degeneracy
between aerosols and temperatures.

The stratosphere exhibits warming within the polar domain, reaching maximum
temperatures of $154\pm 1$ K within the NPSV (poleward of $None$N) and $160\pm
2$ K within the NPC (poleward of $None$N). These peak temperatures are only
slightly cooler than those observed within the southern stratosphere in
2004-05 - the SPSV and SPC - by Cassini/CIRS [LN. Fletcher, Orton . (2018)], a
seasonal asymmetry possibly due to Saturn’s orbital eccentricity. Radiative
models demonstrate that the bulk of this warming is driven by seasonal
radiative heating [Guerlet . (2014), Hue . (2016), Blake . (2022)], but the
sharp boundaries are related to dynamics (e.g., stratospheric winds encircling
the NPSV and NPC). Temperature inversions indicate a moderate stratospheric
cooling at pressures below 300 $\mu$bar, with temperatures approaching a
$140\pm 3$ K quasi-isothermal structure up to the base of the thermosphere.
While retrieved low-pressure temperatures are somewhat influenced by the
choice of prior, this isothermal structure is consistent with radiative
modelling [<]e.g.,¿[]14guerlet.

The most prominent feature of the MIRI temperature field is the vertical
structure of Saturn’s Equatorial Stratospheric Oscillation (we refer to this
as the SESO, as the semi-annual nature of the oscillation is questionable).
During the November-2022 phase, a prominent warm anomaly ($153\pm 0.8$ K) is
observed between the equator and $\sim$$None$N centred near 0.7 mbar. This is
some 12-14 K warmer than temperatures at 0.1 mbar, and is responsible for the
warm band of bright stratospheric emission observed in Fig. 7 and Fig. 8. This
is accompanied by off-equatorial temperature maxima near $None$N, a strong
warm anomaly near 0.05 mbar, and a weaker cool anomaly near 1 mbar. Similar
vertical patterns were observed $\sim 16-17$ years earlier by Cassini/CIRS
limb spectroscopy [Fouchet . (2008)] in 2005-06, and are revealed here due to
the high spectral resolution of MIRI/MRS.

Latitudinal temperature gradients $dT/dy$ (where $y$ is the north-south
distance in km) are converted to vertical windshears $du/dz$ (where $z$ is
altitude) via the thermal wind equation [<]e.g.¿[]04holton, omitting the
equatorial region where the Coriolis parameter tends to zero. These are shown
in Fig. 14b, and show intriguing shear structure equatorward of $None$N
(broadly the domain occupied by Saturn’s equatorial jet). The positive
equatorial shearzone in near 1 mbar in 2022 is likely to be the one that was
seen near 0.1-0.3 mbar in 2017 by Cassini [LN. Fletcher . (2017)], having
descended over a decade in pressure in five years. Positive and negative shear
zones associated with the SESO and its off-equatorial structures appear to
move diagonally to higher pressures with decreasing latitude. For example, the
negative shear zone in the upper troposphere appears continuously connected to
the negative shear zone at 1 mbar and $None$N. The same connection is seen for
the negative shear zone at 10 mbar ($None$N) and 0.05 mbar ($None$N). Such a
system of stacked shear zones connecting the equator and off-equatorial
features was nicely captured by the model of 22bardet.

Windshears derived from nadir infrared data provide a good picture of the
shearzones as a function of altitude and latitude, but using them to integrate
winds with altitude and across regions of low information content (e.g., the
tropopause and lower stratosphere) can generate significant uncertainties
[<]e.g.,¿[]08fouchet, 16fletcher. Nevertheless, we display the estimated
thermal winds in Fig. 14c, assuming that the continuum-band winds measured by
11garcia are placed at 500 mbar. Treating these thermal winds with appropriate
caution, we infer a localised equatorial westward jet (exceeding -200 m/s)
near 1-5 mbar and equatorward of $None$N, embedded within a larger region of
eastward flow that spans the tropics equatorward of $None$N. The westward jet
is below an eastward equatorial jet ($\sim 200$ m/s) near 0.1-0.5 mbar, with
the peak-to-peak contrast of $\sim 400$ m/s between eastward and westward
maxima being comparable to that derived from Cassini limb observations
[Fouchet . (2008)]. Direct observations of Saturn’s stratospheric winds by
ALMA [Benmahi . (2022)] four years earlier (2018) observed a $\sim 300$ m/s
eastward jet between $None$S and $None$N but with a coarse vertical
resolution, covering 0.01-20 mbar. Although this is qualitatively consistent
with the broad eastward flow inferred from MIRI in Fig. 14c, the coarse
resolution of the ALMA data may average over any oscillatory wind patterns
over a decade of pressure. Alternatively, differences between ALMA and MIRI
might simply reflect the downward propagation of these stacked zonal jets over
four years, and future joint campaigns between ALMA and JWST would be welcome
to confirm this, alongside ground-based spectroscopic monitoring of the
equatorial CH4 emission at high spatial and spectral resolution.

Beyond the equator, the correlation between $dT/dy$ and the cloud-tracked
zonal winds causes the decay of the cloud-top winds with height (e.g., the
westward jet near $None$N becomes eastward at $p<80$ mbar), as previously
observed by Cassini [Read . (2009)]. Finally, the strong $dT/dy$ at the edge
of the NPSV implies negative windshear and the inference of westward flow
around the edge of the vortex for $p<10$ mbar [LN. Fletcher, Orton . (2018)],
at a latitude consistent with the westward winds directly observed by ALMA
near $None$N planetographic [Benmahi . (2022)]. This westward wind is zonally
symmetric and in balance with the seasonal temperature gradients from
radiative heating within the NPSV.

Figure 14: Temperatures, windshear, and thermal winds as a function of
latitude derived from MIRI/MRS spectroscopy. The peaks of cloud-tracked
eastward winds are shown as vertical dashed lines for context. In panels b and
c, regions of negative windshear/westward winds are shown as blue with dotted
contours; regions of positive windshear/eastward winds are shown as red with
solid contours. Temperature contours are every 4 K; windshear and wind
contours are logarithmic to show structure, but windshears are labelled at
$\pm 0.1$, $\pm 0.5$ and $\pm 1.0$ m/s/km; winds are labelled at 10, 50, 100,
150, and 200 m/s. Absolute values for derived thermal winds are subject to
significant uncertainties related to the integration of the thermal wind
equation [LN. Fletcher . (2016)], so should be used as a guideline to trends
only.

###  5.2 Aerosols

The zonal-mean distribution of Saturn’s aerosols are shown in Fig. 15(a),
derived from fits to the 4.9-7.3 $\mu$m region using the double-cloud scheme
and multiple scattering of reflected and thermal photons as outlined in
Section 4.3. Although significant degeneracy exists in the choices of aerosol
refractive indices and size distributions, the need for two clouds was evident
from the difficulty fitting equatorial latitudes (where clouds are most
reflective). The base pressures, vertical extensions, top-most pressures, and
total opacity were then varied as free parameters for the two aerosol
populations, resulting in the stacked cloud decks observed here.

The upper cloud, which may comprise a photochemically-produced haze
potentially associated with diphosphine [Ferris Benson (1980)], is optically
thickest, highest (a base near 200 mbar), and most extended at the equator,
reaching into the lower stratosphere near 60-70 mbar. This upper haze is found
deeper (near 300 mbar) and more compact from $None$-$None$N, and then declines
considerably at higher latitudes beyond $None$N. The latitudinal dependence
and altitude of this haze layer is well-matched to the small inflection in the
$T(p)$ profile observed by Cassini/CIRS and known as the ‘temperature knee’
[LN. Fletcher, Sromovsky . (2020)], suggesting that seasonal heating of this
aerosol population is responsible for the change in the curvature of the
vertical temperature profile. The absence of this aerosol at high latitudes
may be partially responsible for the bright 5-$\mu$m emission between
$None$-$None$N in Figs. 7, 8 and 9. The latitudinal distribution of this upper
haze also matches that derived from Palomar visible-light observations
acquired in 1995 [Stam . (2001)], and Cassini/ISS determinations of aerosols
during 2004-2007 [Roman . (2013)], which identified haze-top pressures ranging
from 40 to 150 mbar, with the thickest and highest at the equator, becoming
deeper and thinner at mid-latitudes.

The deeper cloud, which may be associated with condensates of NH3
[<]potentially coated in other material, e.g.,¿[]21sromovsky, resides between
1-2 bars, but with an extension to lower pressures that maybe merges with the
upper haze. The base pressure of this cloud varies with latitude, reaching the
lowest pressures (1.2 bars) near $None$N, which was the site of the highest
reflectivity in Fig. 9, and the deepest pressure (2.6 bars) within the polar
domain. Indeed, the inversions poleward of $None$N suggest that this deep
cloud resides at $p>2$ bars and is vertically compact, responsible for both
the dark thermal emission at 5 $\mu$m and the absence of reflectivity at 5.2
$\mu$m - the dark north pole is therefore caused by this deep aerosol layer.
The compact nature of this cloud deck appears to be consistent with Cassini
observations [LN. Fletcher . (2011), Sromovsky . (2021)], although we caution
that the vertical extension is a rather poorly constrained parameter in these
retrievals. The mean pressure of the cloud base is again consistent with that
found by Cassini/ISS [<]$1.75\pm 0.4$ bars,¿[]13roman, even though optical
measurements only reveal this deep cloud when there are gaps in the overlying
haze.

This simple two-cloud scheme does not provide constraints on two other
proposed cloud layers: neither a deep cloud [<]2.7-4.5 bars, potentially due
to NH4SH¿[]21sromovsky; nor the stratospheric hazes [<]near 50
mbar,¿[]13roman, 15guerlet, 21sromovsky. The deep cloud does not seem to be
required to reproduce the MIRI data, but this may be due to lack of constraint
from reflected sunlight at shorter wavelengths (e.g., from NIRSpec). The very
low opacities ($0.08\pm 0.05$ at 619 nm) and small particle sizes ($<0.3$
$\mu$m) of the stratospheric aerosols inferred by 13roman make it very
unlikely that they would contribute significant opacity at longer mid-IR
wavelengths. Nevertheless, very subtle residuals in our fits to north polar
latitudes near $6.8\pm 0.2$ $\mu$m could be related to the stratospheric
aerosols (see Supplemental Fig. 22c), and will be the topic of future studies,
given their photochemical nature and potential importance in the balance of
radiative heating and cooling in the polar domain [Guerlet . (2015)].

###  5.3 Tropospheric Gases

Saturn’s tropospheric gases - namely NH3, PH3, and H2O - are accessible in the
4.9-6.0 $\mu$m region (primarily channel 1A) and the 8-11 $\mu$m region
(spanning channels 2A to 2C). The latter sounds higher altitudes in the upper
troposphere, whereas the former provides access to the deeper cloud-forming
layers.

####  5.3.1 Phosphine

Phosphine (PH3) is retrieved parametrically from both spectral regions (i.e.,
varying the deep mole fraction and scale height for a fixed transition
pressure of 1 bar), and the results from 5 $\mu$m are shown in Fig. 15b. For
$p>1$ bar, the deep abundance varies between 4.0-5.0 ppm over most of the
northern hemisphere, consistent with results from Cassini/VIMS [LN. Fletcher .
(2011), Sromovsky . (2021)], but with no notable contrasts at the equator, and
a higher abundance $6.5\pm 1.0$ ppm poleward of $None$N. Much of the
latitudinal structure is therefore found in the upper troposphere, driven by
the fractional scale height of the gas. For $p<1$ bar, PH3 is enriched
equatorward of $None$N with evidence for a slight depletion right at the
equator. Further enriched bands are found at $None$N, $None$N and between
$None$-$None$N, with a general decline in abundance towards the north pole.
The equatorial peak and presence of bands of elevated PH3 were also observed
by Cassini/CIRS [LN. Fletcher, Orton, Teanby Irwin (2009)], but the precise
locations differ - in particular, there is no good correspondence between mid-
latitude PH3 bands and zones of cooler temperatures, as we might expect if
only dynamics (i.e., upwelling) were controlling the mid-latitude
distribution. Conversely, there is a good correspondence between higher cloud
bases and the elevated PH3, reinforcing links between PH3 and aerosols
shielding the gas from UV photolysis. This correspondence breaks down in the
5-$\mu$m-bright band near $None$-$None$N, where we have thin clouds but
enriched PH3, suggesting a more complex balance between aerosol shielding and
vertical mixing. Where clouds are deepest and most compact poleward of
$None$N, PH3 could be depleted either by polar subsidence [LN. Fletcher .
(2008)] or by diminished aerosol shielding.

Inversions from the 10-$\mu$m region show a similar morphological structure
but primarily sense $p<1$ bar (see Fig. 13). However, the mid-latitude peaks
are at different locations - $None$N and $None$N, and the deep abundances vary
between 7-10 ppm, a factor of $\sim 2$ higher than those derived from the
5-$\mu$m region. This is a known discrepancy between PH3 abundances derived
from the two regions and occurs on both Saturn [LN. Fletcher . (2011)] and
Jupiter [Giles . (2015)], and reconciliation will require joint fitting of
both spectral domains with multiply-scattering aerosols.

####  5.3.2 Ammonia

The latitudinal distribution of NH3 was also retrieved parametrically (with a
transition pressure at 1.75 bars), with the 5-$\mu$m results shown in Fig.
15c. As previously observed by Cassini/RADAR [Laraia . (2013)] and VIMS [LN.
Fletcher . (2011)], MIRI reveals a strong equatorial enhancement within $None$
of the equator, with deep abundances below $p>1.75$ bars of 450-650 ppm,
compared to a minimum of $\sim 150$ ppm at $None$N. This equatorial
enhancement on Saturn shares similarities with that observed on Jupiter [de
Pater . (2016), Li . (2017)], and suggests shared dynamics driving the unique
compositions of their cold Equatorial Zones, compared to strong NH3 depletion
at other latitudes. Despite the strong equatorial maximum at depth, the
abundance falls steeply with height equatorward of $None$N, such that NH3 at
the 200-mbar level displays a local equatorial minimum, perhaps due to the
enhanced condensation through cold equatorial temperatures to form the thicker
clouds observed here.

Further local maxima in the deep abundance occur at $None$N and $None$N, which
are similar in size (but at different latitudes) compared to peaks observed by
Cassini/VIMS in 2006 [LN. Fletcher . (2011)], suggesting temporal variability
in the abundance of NH3 in the mid-latitude bands. The region near $None$N is
notable as displaying the shallowest gradient in NH3 for $p<1$ bar, coinciding
with the coldest tropospheric band in Fig. 14a, but is actually a local
minimum ($\sim 100$ ppm) in the deep NH3 abundance for $p>1.75$ bars.
Similarly, there is a general increase in deep abundance towards the polar
domain ($\sim 600$ ppm for $p>1.75$ bars), but also a shallower upper
tropospheric gradient implying a local polar minimum at 200 mbar. These MIRI
results imply that ammonia displays different latitudinal distributions above
and below the condensed aerosols near 1.75 bars, suggestive of differing
circulation patterns at different heights [LN. Fletcher, Kaspi . (2020)]. To
reinforce this, NH3 was also retrieved from the 10-$\mu$m region, sensing
higher altitudes of the upper troposphere, and resulted in a relatively
meridionally uniform distribution similar to that derived by Cassini/CIRS
[Hurley . (2012)]. The strongest NH3 contrasts are therefore only seen in the
cloud-forming region sensed near 5 $\mu$m, with only small latitudinal
gradients in the stably-stratified upper troposphere sensed near 10 $\mu$m.

####  5.3.3 Water

Perhaps the most tantalising prospect offered by MIRI/MRS is the possibility
of mapping Saturn’s tropospheric water in the 5.1-5.5 $\mu$m region, which
remained out of reach to Cassini/VIMS because of the lack of spectral coverage
beyond 5.1 $\mu$m. 97degraauw detected Saturn’s tropospheric water with disc-
averaged observations from ISO/SWS, fitting it with 0.2 ppm at $p>3$ bars and
generally sub-saturated conditions. H2O was retrieved parametrically, with
initial testing at mid-latitudes suggesting the transition pressure should lie
at $p>4$ bars (5 bars was chosen), a weak sensitivity to the chosen deep
abundance (10 ppm was selected), but a stronger sensitivity to the fractional
scale height, so only the latter parameter was allowed to vary.

Tropospheric water lines are difficult to observe between the forest of PH3
features in Fig. 11f, and are most visible as small notches either side of the
narrow NH3 feature near 5.2 $\mu$m. These H2O features are present at all
latitudes, but are close to the level of the spectral residuals seen elsewhere
in the channel-1 data (see Supplemental Fig. 22c). Nevertheless, the MIRI/MRS
data provide tentative evidence of latitudinal variability of H2O in Fig. 15d.
At 3.3 bars, the water abundance varies from a maximum of $\sim 200$ ppb right
at the equator [<]consistent with ISO estimates from¿[]97degraauw, to $\sim
10$ ppb between $None$-$None$N, to $\sim 145$ ppb from $None$-$None$N, then
declines to $\sim 80$ ppb from $None$-$None$N. Poleward of $None$N, the
changes in abundance with latitude mirror those in the aerosol distribution,
with a tendency for enhanced H2O where the aerosols have a higher optical
depth, although we caution that this could reflect a model-dependent
degeneracy given the weakness of the H2O features. The low values near $None$N
are very poorly constrained due to the thicker aerosol opacity, but are
possibly due to enhanced condensation in the cool EZ. The shallower gradient
in the polar domain are possibly due to the warmer upper-tropospheric
temperatures there and the absence of upper tropospheric aerosols, but the
same is not seen for NH3. Conversely, the increase in H2O right at the equator
mimics the equatorial column of NH3, suggesting a volatile-rich domain at
Saturn’s equator.

We note that the challenge of calibrating MRS channel 4 (i.e., beyond 18
$\mu$m) limits our sensitivity to the distribution of stratospheric H2O, which
will be the topic of future investigations.

Figure 15: Aerosols, phosphine, ammonia and water derived from the 4.9-7.3
$\mu$m region. Aerosols in panel (a) are plotted in opacity/km at a reference
wavelength of 5 $\mu$m, calculated following the scheme in Appendix C of
22irwin. A logarithmic colour bar is used to show structure within the aerosol
cross-section. For the gases in panels (b)-(d), the contours are also
logarithmic to allow for the rapid decline of the abundance with altitude. PH3
and NH3 are provided in ppm, H2O is given in ppb. Vertical dotted lines show
the latitudes of tropospheric eastward jets.

###  5.4 Stratospheric Chemistry

The spatial distribution of chemical species provides a means to trace
Saturn’s stratospheric circulation, to understand the photochemical lifetimes
of different products, the exogenic supply of oxygenated species, and the
potential influence of ionisation on the chemistry of the polar domains. As
described in Section 3.1, MIRI/MRS provides access to a host of stratospheric
chemicals with a higher spectral resolution and sensitivity than Cassini/CIRS.
However, fringing artefacts that still plague wavelengths beyond 10 $\mu$m
make precise quantitative measurements challenging, particular for minor
species. In the following sections, we present an initial assay of Saturn’s
stratospheric composition based on MIRI/MRS data.

####  5.4.1 Acetylene and Ethane

Latitudinal cross-sections of ethane (C2H6) and acetylene (C2H2) are shown in
Fig. 16, and at the 0.5-mbar level in Fig. 17a,b, based on spectral fits shown
in Fig. 12. Cassini observations revealed that both species are time-variable,
responding to Saturn’s seasonally-evolving circulation and solar flux. The
equator-to-pole gradient of C2H2 changes with height, with the upper
stratosphere showing polar enrichment (poleward of $None$N and $p<0.1$ mbar),
but the lower stratosphere showing polar depletion ($p>0.1$ mbar) due to the
stronger gradient of C2H2 near the poles compared to at other latitudes. C2H6
also shows strong polar enrichment (poleward of $None$N and $p<0.1$ mbar,
within the NPSV). Both species show enhancements at the equator, as previously
observed by Cassini [Guerlet . (2009), Sinclair . (2013), Sylvestre . (2015)]
and predicted by photochemical models based on the annual-average insolation
[Moses Greathouse (2005), Hue . (2015)], but this too is time variable, with
evidence that the equatorial C2H6 peak has strengthened with time whereas C2H2
has remained reasonably constant [LN. Fletcher, Sromovsky . (2020)].

The different latitudinal trends in the lower stratosphere (C2H2 declining
towards high latitudes, C2H6 increasing) have been observed previously, with
short-lived C2H2 more closely following photochemical predictions of
05moses_sat (Fig. 17a), whereas long-lived C2H6 is more sensitive to
stratospheric circulation. Intriguingly, local maxima between $None$-$None$N
observed in both species during northern winter by Cassini
[<]2005-2012,¿[]09guerlet, 15sylvestre have now been replaced by local minima
between $None$ and $None$N during northern summer observed by JWST. This
supports the idea that wintertime subsidence has been replaced by summertime
upwelling in this latitude range (i.e., upwelling of hydrocarbon-depleted air
from the lower stratosphere), associated with the seasonal reversal of the
inter-hemispheric Hadley cell [Friedson Moses (2012), Bardet . (2022)]. This
upwelling may provide an explanation for why MIRI observed colder
stratospheric temperatures in 2022 (Section 5.1) compared to Cassini in 2017.

Figure 16: Distributions of acetylene and ethane, derived as continuous
profiles to capture changes in the vertical gradients of these species.
Contours are logarithmic, and labelled in ppmv. Vertical dotted lines show the
latitudes of tropospheric eastward jets. A cross-section at 0.5 mbar is shown
in Fig. 17. Figure 17: Distributions of stratospheric species at 0.5 mbar:
acetylene, ethane, ethylene, propane, CO2, benzene, methyacetylene (propyne),
diacetylene (1,3-butadiyne), and methyl in Saturn’s northern summer, compared
to the predictions of the neutral photochemistry model for $L_{s}=150^{\circ}$
[Moses . (2007), Moses Greathouse (2005)]. 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 grey, 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 C4H2 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. Grey points in panel (c) signify a lack of obvious
detection of C2H4 emission by eye.

####  5.4.2 Ethylene

Ethylene was previously only detected within Saturn’s storm-perturbed
stratosphere [Hesman . (2012), Moses . (2015)], due to the elevated
temperatures and a photochemical increase in the C2H4 abundance in 2011. Other
than the storm and a reported ground-based detection [Bézard, Moses . (2001)],
C2H4 had proven elusive until the high sensitivity of MIRI/MRS, which reveals
10.5-$\mu$m C2H4 emission within the NPSV for the first time (Fig. 17c,
0.5-mbar abundances of $4.0\pm 0.5$ ppb poleward of $None$N). This abundance
is a factor of 2-3$\times$ lower than that expected (but not seen) at the
equator due to neutral photochemistry [Moses Greathouse (2005)]. Note that the
emission features are only readily detectable in spectra poleward of $None$N -
at lower latitudes, our model provides the highest C2H4 abundance that would
be consistent with the non-detection of emission (within uncertainties). The
seasonal model overpredicts the estimated low-latitude abundance ($2.3\pm 0.5$
ppb at 0.5 mbar compared to $\sim 12$ 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 Fig. 6 of¿[]15moses.

Nevertheless, the polar maximum in C2H4 in Fig. 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
modelling work for the chemistry of the NPSV.

####  5.4.3 $C_{3}$, C4 and C6 Hydrocarbons

Propane is detected as a perturbation to the stronger C2H2 lines near 13.37
$\mu$m (indicated in Fig. 12), which results in the large uncertainties on the
distribution in Fig. 17d. Propane bands $\nu_{20}$ and $\nu_{21}$ are observed
near 9.4 and 10.7 $\mu$m, respectively (see Supplemental Fig. 23), 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 15sylvestre. It is
clear that C3H8 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,¿[]05moses_sat.

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 $None$N, with C3H4
in particular showing the expected equator-to-pole contrast in Fig. 17g. C4H2
shows a local minimum near $None$-$None$N that has evolved from the local
maximum observed by Cassini in 2005-06 [Guerlet . (2010)], possibly related to
the onset of low-latitude stratospheric upwelling during northern summer.
Surprisingly, C4H2 then increases towards the NPSV where some of the highest
abundances are obtained in Fig. 17h. This is unlikely to be a circulation
effect, as C4H2 has only a slightly shorter loss timescale than C3H4 [Guerlet
. (2010)], so we might expect to see the same structure in C3H4. Photolysis of
C2H2 is the dominant production mechanism for C4H2 [Moses . (2005)], so the
polar C4H2 may simply be a result of the excess C2H2 within the NPSV at high
altitudes in Fig. 16, which is not captured by the photochemical models.
Conversely, C3H4 is primarily formed from interconversion of other C3
hydrocarbons [Moses . (2005)], which do not display the same enrichment as
C2H2. Furthermore, the model of 05moses_sat overestimates equatorial C3H4 and
C4H2 by factors of $\sim 5$ and $\sim 4$, respectively, similar to that found
by 10guerlet.

Methyl [<]CH3, first detected by ISO,¿[]98bezard is also a short-lived species
produced directly from methane photolysis [Moses . (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
$None$N that may be related to the seasonal upwelling. The seasonal model
underpredicts the equatorial abundances by $\sim 2.5$. Unlike most other
hydrocarbon species, CH3 appears to be most depleted within the NPSV, by a
factor of $\sim 5$ 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 . (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¿[]01bezard 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 Fig. 12), with enhancement by 1.5-2.0$\times$
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 . (2015)]. Nevertheless,
the peak abundance at 0.5 mbar remains $50\times$ smaller than the
photochemical model prediction of 05moses_sat, as previously found by
15guerlet, and some $10\times$ smaller than the coupled ion-neutral chemistry
of 23moses. As discussed by 16koskinen and 23moses, 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 . (2023)] if the large influx of
organic-rich ring material detected by Cassini during its Grand Finale in 2017
[Serigano . (2022)] were vapourised/ablated upon entry into the equatorial
stratosphere. Furthermore, no new nitriles were observed in 2022 (see below).
We thus favour the hypothesis of 23moses 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,¿[]97feuchtgruber is detected at
all latitudes by MIRI/MRS, with a relatively uniform abundance with latitude
($0.60\pm 0.05$ ppb at 0.5 mbar, Fig. 17e) and hints of a slightly lower
abundance ($0.56\pm 0.03$ ppb) within the NPSV. The distribution is
approximately consistent with the few measurements available from Cassini
[Abbas . (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-localised source [<]e.g., recent comets,¿[]10cavalie, 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 . (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 . (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 $\mu$m ($\nu_{2}$) HCN line is
challenging due to blending with lines of C2H2. We also note that the MRS
wavelength calibration [Argyriou . (2023)] and spectral resolution [Jones .
(2023)] creates fitting artefacts 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-millimetre
from Herschel/SPIRE [LN. Fletcher . (2012)]. We do not see any evidence of the
$\nu_{5}$ band of HC3N at 15.08 $\mu$m, again confirming an absence of
nitriles related to ring influx [Moses . (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, $L_{s}=150^{\circ}$); 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 artefacts for MIRI/MRS. Spatially-resolved
4.9-27.9 $\mu$m 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 5.1-6.9 $\mu$m, 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 artefacts 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 customised cleaning on each tile and
dither independently, to prevent the blending of artefacts that, in the most
extreme cases, can completely obscure the spatial structure of Saturn itself.
With these artefacts removed, we summarise the initial survey of Saturn as
follows:

  1. 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 . (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 4.9-7.3
$\mu$m) show similarities to albedo contrasts in observations acquired by
Hubble in September 2022 [Simon . (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 $None$N. A
narrow equatorial band ($<$$None$N) is dark at 5 $\mu$m, coinciding with the
location where the narrow upper-tropospheric jet was previously detected
[García-Melendo . (2011)]. The 5-$\mu$m brightness has evolved with time, such
that a pole-encircling band from $None$-$None$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 $\mu$m due to an absence of
upper-tropospheric aerosols and an optically thick and compact cloud at higher
pressures. We no longer see the 5-$\mu$m-bright, aerosol-depleted band near
35-40∘N that had dominated the appearance of the northern hemisphere after the
2010-11 storm, consistent with a re-population of the band by Saturn’s
seasonal aerosols in the decade since the storm.

  2. 2.

North Polar Stratospheric Vortex (NPSV): The seasonal stratospheric vortex
that developed poleward of $None$N during northern spring [LN. Fletcher, Orton
. (2018)] 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 (2004-05). Radiative models
[<]e.g.,¿[]14guerlet, combined with our Earth-based vantage point [Blake .
(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 North Polar Cyclone (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. 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 $None$N, $None$N and $None$N,
but these could not be identified in contemporaneous amateur observations, nor
Hubble observations $\sim 7$ weeks earlier. This strongly argues for near-
simultaneous MIRI/MRS spectroscopy and NIRCAM (or Hubble) imaging in future
observing programmes, alongside long-term records of variability from ground-
based facilities.

  4. 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 $None$N. MIRI/MRS reveals warm and
cool temperature anomalies at the equator (and off-equatorial anomalies at
$None$N) that are consistent with the downward propagation of the oscillatory
pattern over a decade of pressure in the five 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,¿[]08orton_qxo, 22blake, 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. 5.

Reversal of Saturn’s interhemispheric stratospheric circulation: MIRI/MRS
observations in 2022 reveal cooler temperatures in the $None$-$None$N domain
compared to Cassini in 2017. This coincides with local minima in several
hydrocarbon species in 2022 (notably C2H2, C2H6, C4H2 and CH3), opposite to
the local maxima detected by Cassini [Guerlet . (2009), Guerlet . (2010),
Sylvestre . (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 [Friedson Moses (2012), Bardet .
(2022)].

  6. 6.

Stacked aerosol layers: We present the first assessment of aerosol opacity in
5.1-6.8 $\mu$m 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 $None$N. This layer is likely to be the same as that
detected by Cassini/ISS [Roman . (2013)], and coincides with a region of
localised radiative heating detected by Cassini/CIRS [LN. Fletcher . (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 $None$N
and deepest (2.6 bars) and most compact in the polar domain, responsible for
both the dark thermal emission at 5 $\mu$m and the absence of reflectivity at
5.2 $\mu$m. Aerosol fitting for MIRI/MRS is somewhat degenerate, and further
constraints could be provided in future by NIRSpec observations of reflected
sunlight.

  7. 7.

Tropospheric species: Latitudinal variability of phosphine appears to be most
confined to $p<1$ bar, displaying an equatorial maximum [LN. Fletcher, Orton,
Teanby Irwin (2009)] and general decline in abundance towards 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 $<5^{\circ}$ of the equator (450-650 ppm), similar to
the NH3-enriched column in Jupiter’s equatorial zone [Achterberg . (2006), Li
. (2017), de Pater . (2016)] 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 $None$N, and a distinct step in the
abundance near $\sim$$None$N from 145 ppb at low latitudes to 80 ppb at higher
latitudes.

  8. 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
predictions of seasonal photochemical models,¿[]05moses_sat, 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 $\sim 5$ 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-localised source of exogenic oxygen species.

  9. 9.

Absence of nitriles: None of the hydrocarbon distributions support the
substantial chemical consequences predicted by neutral photochemistry [Moses .
(2023)] if the large influx of ring material detected by Cassini in 2017
[Serigano . (2022)] were vapourised 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 artefacts from the 17-28 $\mu$m 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 ($L_{s}=150^{\circ}$)
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, $L_{s}=180^{\circ}$), and we hope
that these Cycle-1 observations will mark the starting point for a long-term
MIRI legacy programme to track the seasonal evolution of Saturn’s circulation
and chemistry through to the next southern summer solstice (April 2032,
$L_{s}=270^{\circ}$), completing the seasonal assessment begun by Cassini.

## Open Research Section

Level-3 calibrated Saturn MIRI/MRS data from the standard pipeline are
available directly from the MAST archive [MAST Archive (2022)] via
http://dx.doi.org/10.17909/wjpz-7383. Hubble observations used for comparison
were acquired by the Outer Planets Legacy Program [OPAL Archive (2022)] via
https://archive.stsci.edu/prepds/opal/, also available here:
http://dx.doi.org/10.17909/T9G593.

The NEMESIS radiative transfer and retrieval code [Irwin . (2008)] used in
this study is open-access and is available for download [Irwin (2022)] from
GitHub or Zenodo (https://doi.org/10.5281/zenodo.5816724).

The JWST calibration pipeline [Bushouse . (2023)] is available at
https://github.com/spacetelescope/jwst, this work used version `1.9.4`.

The custom pipeline and data processing code [King . (2023)] developed in this
study is available at https://github.com/JWSTGiantPlanets/pipelines/
(https://doi.org/10.5281/zenodo.7891560).

The data products produced in this study [LN. Fletcher (2023)], including
synthetic flat fields, zonal average spectra and quick-look visualisations,
are available at https://github.com/JWSTGiantPlanets/saturn-atmosphere-miri
(https://doi.org/10.5281/zenodo.7891588).

###### Acknowledgements.

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 agreement No 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-SMDSS21-0013. 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.

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