# Water-Ice Dominated Spectra of Saturn’s Rings and Small Moons from JWST

###### Abstract

JWST measured the infrared spectra of Saturn’s rings and several of its small
moons (Epimetheus, Pandora, Telesto and Pallene) as part of Guaranteed Time
Observation program 1247. The NIRSpec instrument obtained near-infrared
spectra of the small moons between 0.6 and 5.3 microns, which are all
dominated by water-ice absorption bands. The shapes of the water-ice bands for
these moons suggests that their surfaces contain variable mixes of crystalline
and amorphous ice or variable amounts of contaminants and/or sub-micron ice
grains. The near-infrared spectrum of Saturn’s A ring has exceptionally high
signal-to-noise between 2.7 and 5 microns and is dominated by features due to
highly crystalline water ice. The ring spectrum also confirms that the rings
possess a 2-3% deep absorption at 4.13 microns due to deuterated water ice
previously seen by the Visual and Infrared Mapping Spectrometer onboard the
Cassini spacecraft. This spectrum also constrains the fundamental absorption
bands of carbon dioxide and carbon monoxide and may contain evidence for a
weak aliphatic hydrocarbon band. Meanwhile, the MIRI instrument obtained mid-
infrared spectra of the rings between 4.9 and 27.9 microns, where the observed
signal is a combination of reflected sunlight and thermal emission. This
region shows a strong reflectance peak centered around 9.3 microns that can be
attributed to crystalline water ice. Since both the near and mid-infrared
spectra are dominated by highly crystalline water ice, they should provide a
useful baseline for interpreting the spectra of other objects in the outer
solar system with more complex compositions.

JGR Planets

Department of Physics, University of Idaho, Moscow ID USA SETI Institute,
Mountain View, CA, USA School of Physics and Astronomy, University of
Leicester, Leicester, UK Royal Astronomical Society, UK Association of
Universities for Research in Astronomy, Washington, DC, USA Astrochemistry
Laboratory Code 691, NASA Goddard Space Flight Center, Greenbelt, MD, USA Carl
Sagan Institute, Cornell University, Ithaca NY USA Department of Astronomy,
University of California Berkeley, Berkeley CA USA Department of Earth and
Planetary Science, University of California Berkeley, Berkeley CA USA

M.M<EMAIL_ADDRESS>
Near-IR spectra of Saturn’s small moons and rings obtained by JWST’s NIRSpec
show water ice bands with different degrees of crystallinity.

Near-IR spectra of Saturn’s A ring confirm the existence of an O-D absorption
band and may contain a weak aliphatic hydrocarbon band.

Mid-IR spectra of Saturn’s rings obtained by JWST’s MIRI reveal a reflectance
peak at 9.3 microns due to highly crystalline water ice.

## Plain Language Summary

Saturn’s rings and small moons are all composed primarily of very pure water
ice, making them useful targets for characterizing the performance of the
various instruments onboard JWST. Observations of multiple small moons at
near-infrared wavelengths demonstrate the ability of JWST to detect faint
objects in the outer solar system, and reveal that the water ice on these
bodies is not always organized into large, pure crystals. Observations of
Saturn’s rings, by contrast, confirm that they are composed of very pure and
highly crystalline water ice. These data also provide new constraints on the
amounts of carbon-containing compounds that could be present in the rings.

##  1 Introduction

The infrared spectra of Saturn’s rings and small moons have been well
characterized thanks to extensive observations obtained by the Visual and
Infrared Mapping Spectrometer (VIMS) and Composite Infrared Spectrometer
(CIRS) onboard the Cassini spacecraft [RH. Brown . (2004), Flasar . (2004)].
Observations of these objects with the instruments onboard JWST therefore
provide opportunities not only to obtain new information about the rings and
small moons themselves, but also to evaluate the performance of JWST’s
instruments, particularly the Integral Field Unit (IFU) components of NIRSpec
and MIRI [Gardner . (2023), Jakobsen . (2022), Böker . (2022), Böker . (2023),
Wright . (2023)].

A wide range of spectroscopic observations demonstrate that Saturn’s main
rings are composed of very pure and highly crystalline water ice. While there
is evidence that the rings do contain variable amounts of non-icy materials
that influence both the overall brightness of the rings and the shape of their
spectra at visible and ultraviolet wavelengths [Clark . (2008), Clark .
(2019), Cuzzi . (2009), Cuzzi . (2018), Filacchione . (2012), Filacchione .
(2014), Hedman . (2013), Nicholson . (2008)], detailed modeling of infrared
and radio data indicate that most of the rings are over 99% water ice
[Ciarniello . (2019), Zhang . (2019)]. Indeed, while some observations by
Cassini-VIMS have indicated that there could be weak ($\sim 3\%$) hydrocarbon
features between 3.4 $\mu$m and 3.6 $\mu$m throughout the rings [Filacchione .
(2014)], extremely high signal-to-noise spectra of the B ring contained no
non-water-ice spectral features between 1 and 5 $\mu$m at the $\sim$1% level,
but did exhibit a 2-3% brightness dip around 4.13 $\mu$m that could be due to
deuterated water ice [Clark . (2019)]. Saturn’s rings therefore provide a
useful baseline for evaluating the quality and calibration of near-infrared
spectra obtained by NIRSpec.

Meanwhile, Cassini-CIRS observed the rings at wavelengths between 7.2 $\mu$m
and 1 mm, and data from its longer-wavelength channels have been used to
constrain the temperature of the ring material [Spilker . (2006), Altobelli .
(2008), Flandes . (2010), Filacchione . (2014), Spilker . (2018)]. These
temperature estimates can be compared with those extracted from the MIRI
observations in order to assess the current calibration pipeline. At the same
time, no one has yet used the CIRS data to publish a continuous brightness
spectrum of the rings between 7 and 20 $\mu$m (but see Morishima et al.,
2012). The JWST MIRI data can therefore provide the first detailed information
about the ring’s spectral properties over the wavelength range where the
signal transitions from being predominantly reflected sunlight to mainly
thermal emission from the rings themselves.

In addition, Cassini-VIMS observations of the small moons revealed that their
near-infrared spectra are also dominated by water ice features [Filacchione .
(2010), Filacchione . (2012), Buratti . (2019)]. Furthermore, these moons span
a wide range of sizes, with the average effective radii of the representative
small moons Epimetheus, Pandora, Telesto and Pallene being 58.6 km, 40 km,
12.3 km and 2.23 km, respectively [Thomas Helfenstein (2020)]. While all these
objects are point sources to JWST, the signals from these four moons span a
factor of nearly 700 because the observed flux scales with cross-sectional
area. The spectra derived from these moons can therefore provide a baseline
for evaluating the signal-to-noise for future observations of small bodies in
the outer solar system. In addition, NIRSpec does not have the same gaps in
wavelength coverage as VIMS around 1.6 $\mu$m and 3.0 $\mu$m [RH. Brown .
(2004)], so these spectra can provide new information about the crystallinity
of the ice on these objects.

For all of these reasons, as part of JWST Guaranteed Time Observation 1247,
near-infrared spectra of Saturn’s rings and a representative sample of its
small moons (Epimetheus, Pandora, Telesto and Pallene) were obtained with the
NIRSpec IFU, and mid-infrared spectra of Saturn’s rings were obtained with the
MIRI IFU. This paper provides an overview of these observations and summarizes
the early findings derived from them. Section 2 describes the NIRSpec
observations of Saturn’s rings and moons. This section begins with a general
description of the relevant observations, which is followed by descriptions of
how we extracted spectra of the small moons and rings from these data and
discussions of some interesting aspects of those spectra. Section 3 describes
the MIRI observations of Saturn’s rings. This section also begins by
describing the relevant observations and how they were processed to obtain
mid-infrared spectra of the B ring, followed by a brief discussion of the most
notable features of these spectra. Finally, Section 4 summarizes the key
findings of these preliminary analyses.

##  2 Near-Infrared observations of Saturn’s small moons and rings

###  2.1 General description of observations

JWST made NIRSpec IFU observations of the rings and small satellites using the
PRISM ($R\sim 100$, $\delta\lambda\sim$ 5 nm) mode to capture as much of the
0.6-5.3 $\mu$m spectrum as possible. These observations targeted Pandora
(2022-Nov-08, 21:55-22:11UT), Epimetheus (2022-Nov-08, 22:15-22:29UT), Telesto
(2022-Nov-10, 12:04-12:25UT) and Pallene (2023-Jun-20 01:14- 01:35 UT). Note
the Epimetheus observations were also a repeat of an earlier, skipped
observation. Given the expected brightness of Pandora and Epimetheus, the
observations of these moons were designed with 2 groups and 3 integrations in
the NRSRAPID readout mode (10.7-s frames), and a 2-point nod, yielding a total
exposure duration of 193.262 s. Telesto used 15 groups and 1 integration for
each dither, and Pallene used 44 groups and 1 integration with each dither.
Both these observations used the NRSIRS2RAPID readout mode (14.6-s frames).
This mode was intended to improve performance and sensitivity in the longer
exposures, and yielded total exposure times of 466.844 s for Telesto and
1313.0 s for Pallene. The Epimetheus, Pandora and Telesto observations were
processed using the JWST pipeline version 1.9.3 with CRDS context
jwst_1039.pmap, while the Pallene observations were processed using pipeline
version 1.10.2 with CDS context jwst_1094.pmap. All observations were
processed using the custom calibration pipeline described in King23.

Figure 1: NIRSpec image of Telesto in a coordinate system centered on and
aligned with Saturn. The inset in the upper left shows the context for the two
images shown in the two panels. Each image shows the average brightness of the
data derived from the two dithers over the indicated wavelength range. At 2-3
$\mu$m Telesto is clearly present, while at longer wavelengths various
background instrumental artifacts are more prominent. Note that the flux
density scale in these plots is per spatial pixel.

Figure 2: NIRSpec image of Pallene in a coordinate system centered on and
aligned with Saturn. The inset in the upper left shows the context for the two
images shown in the two panels. Each image shows the average brightness of the
data derived from the two dithers over the indicated wavelength range. At 2-3
$\mu$m Pallene is clearly present, while at longer wavelengths various
background instrumental artifacts are more prominent. Note that the flux
density scale in these plots is per spatial pixel.

The observations targeted at Telesto and Pallene shown in Figures 1 and 2 are
the most straightforward to interpret. Each observation consists of two
dithers pointed at slightly different locations with an overlap region that
should contain the moon. For the Telesto observations, a point source is
clearly visible in this overlap region at wavelengths shorter than 3 $\mu$m
(see Figure 1), and a fainter source is visible at nearly the same location in
the Pallene observations (see Figure 2). The signal from Pallene is much
weaker than the signal from Telesto simply because Pallene is significantly
smaller (see below). Neither Pallene nor Telesto is clearly visible at
wavelengths longer than 3 $\mu$m. This is consistent with both objects being
composed primarily of water ice, which is a strong absorber at these longer
wavelengths. In addition to the signal from the moons, these images also show
a variety of patterns that can be attributed to instrumental artifacts because
they have similar spatial structures in both dithers. These patterns are more
obvious in the Pallene images because the data had to be stretched further to
show the signal from the smaller moon.

Figure 3: NIRSpec images targeted at Epimetheus in a coordinate system
centered on and aligned with Saturn. The inset in the upper left shows the
context for the two images shown in the two panels. Each image shows the
average brightness of the data derived from the two dithers over the indicated
wavelength range. At 2-3 $\mu$m, signals from both Epimetheus and Pandora can
be identified, Saturn’s A ring is also present at the top left corner of the
images, which appears black in this panel because the pixels are overexposed.
Above 3 $\mu$m the signals from the moons are difficult to see, but the signal
from the rings are clearer because these channels are not overexposed. In
particular, the Encke Gap is clearly visible. Note that the flux density scale
in these plots is per spatial pixel.

Figure 4: NIRSpec images targeted at Pandora in a coordinate system centered
on and aligned with Saturn. The inset in the upper left shows the context for
the two images shown in the two panels. Each image shows the average
brightness of the data derived from the two dithers over the indicated
wavelength range. At 2-3 $\mu$m signals from Epimetheus can be identified, and
the signal from Pandora is barely visible due to contamination from the nearby
ring. Saturn’s A ring is also present at the top left corner of these images,
but appears black in this panel because the pixels are overexposed. Above 3
$\mu$m the signals from the moons are difficult to see, but the signal from
the rings is clearer because these channels are not overexposed. In
particular, the Encke Gap is clearly visible in this image. Note that the flux
density scale in these plots is per spatial pixel.

Figure 3 shows the images explicitly targeted at Epimetheus, which is clearly
visible in the 2-3 $\mu$m data. These images also clearly contain a portion of
Saturn’s rings, which are saturated at wavelengths shorter than 3 $\mu$m but
are clearly visible at longer wavelengths. In addition, a second bright spot
can be seen between Epimetheus and the rings at the predicted position of the
moon Pandora, so these data contain signals from both moons. Note that the
regions to the left of the field of view show a higher background brightness
level than the rest of the image due to stray light from the rings.

Finally, Figure 4 shows the images targeted at Pandora. Since these images
were obtained shortly before the images shown in Figure 3, the overall
geometry of the two observations are similar. However, since Pandora is closer
to the rings than Epimetheus, the rings take up a larger fraction of the field
of view. At the same time, the stray light levels outside the rings are
roughly an order of magnitude higher than they are in Figure 3. This makes the
moons more difficult to see in these images, with Epimetheus being just barely
visible.

###  2.2 Near-infrared spectra of the small moons

Figure 5: Images showing the regions considered for extracting the spectra of
the small moons (note the spatial dimensions of these cubes correspond to
right ascension and declination, and the typical pixel size is 700 km at
Saturn). In each image, the green box indicates the 9 pixels that were co-
added to obtain the spectra of the relevant moon, while a polynomial fit to
the region between the cyan and green boxes was used to estimate background
levels. Note the Pandora spectra were derived from the same observations as
the Epimetheus spectra, and only the first dither provided a useful spectrum
of Pandora.

Figure 6: Flux spectra of the small moons. This plot shows the large
differences in the absolute brightness of the various moons. Note that the
Epimetheus and Pandora spectra end between 1.35 and 1.5 $\mu$m because the
data at shorter wavelengths are contaminated by stray light from the nearby
rings. Strong water-ice absorptions can be seen at 1.5 $\mu$m, 2 $\mu$m and 3
$\mu$m in all of these spectra.

Figure 7: Near-Infrared reflectance spectra of the small moons. The spectra
for each moon shows the brightness coefficient of the moon’s surface computed
using Equation 2. Error bars show 1-$\sigma$ statistical uncertainties based
on the rms brightness variations in a region surrounding each moon. Note that
the Epimetheus and Pandora spectra end between 1.35 and 1.5 $\mu$m because the
data at shorter wavelengths are contaminated by stray light from the nearby
rings. For comparison, this figure also includes the reflectance spectra of
Methone derived from Cassini-VIMS observations [Hedman . (2020)]. The bottom
panel shows synthetic spectra for crystalline and amorphous water ice
regoliths computed using optical constants at 100 K from Mastrapa09 and the
formulas from Shkuratov99, assuming a typical regolith scattering length of 10
$\mu$m. Note the shapes and amplitudes of the 3.6 $\mu$m and 5.1$\mu$m peaks
are affected by errors in the Mastrapa09 optical constants that were
identified in Clark12.

Figure 8: Close-ups of the 1.5 $\mu$m and 3 $\mu$m ice bands in the
reflectance spectra of Epimetheus and Telesto, compared with the same
predicted spectra for crystalline and amorphous ice regoliths shown in Figure
7. Note that Telesto shows a narrower peak near 3.1 $\mu$m and a stronger dip
around 1.65 $\mu$m than Epimetheus, indicating that the ice on Telesto’s
surface is more crystalline. Table 1: Parameters for the small-moon
observations

Parameter | Epimetheus 1 | Epimetheus 2 | Pandora 1 | Telesto1 | Telesto 2 | Pallene 1 | Pallene 2
---|---|---|---|---|---|---|---
Range to Sun (106 km) | 1473 | 1473 | 1473 | 1473 | 1473 | 1465 | 1465
Range to JWST (106 km) | 1460 | 1460 | 1460 | 1465 | 1465 | 1402 | 1402
Phase Angle (degrees) | 5.8 | 5.8 | 5.8 | 5.8 | 5.8 | 5.6 | 5.6
Sub-solar latitude (degrees) | 12.6 | 12.6 | 12.7 | 12.7 | 12.7 | 9.8 | 9.8
Sub-solar longitude (degrees) | 245 | 247 | 255 | 259 | 259 | 86 | 89
Sub-JWST latitude (degrees) | 15.2 | 15.2 | 15.2 | 15.2 | 15.2 | 7.3 | 7.3
Sub-JWST longitude (degrees) | 250 | 252 | 260 | 264 | 265 | 80 | 84
$a$ (km) | 64.8 | 64.8 | 51.5 | 16.6 | 16.6 | 2.88 | 2.88
$b$ (km) | 58.1 | 58.1 | 39.5 | 11.7 | 11.7 | 2.08 | 2.08
$c$ (km) | 53.5 | 53.5 | 31.5 | 9.6 | 9.6 | 1.84 | 1.84
Central x pixel∗ | 28 | 22 | 24 | 27 | 21 | 16 | 24
Central y pixel∗ | 35 | 12 | 43 | 35 | 12 | 14 | 35
Av. Brightness 1.36-1.40 $\mu$m | 0.876$\pm$0.011 | — | — | 1.209$\pm$0.016 | 1.125$\pm$0.026 | 0.850$\pm$0.050 | 0.860$\pm$0.070
Av. Brightness 1.48-1.52 $\mu$m | 0.600$\pm$0.017 | — | — | 0.735$\pm$0.064 | 0.543$\pm$0.101 | 0.521$\pm$0.066 | 0.490$\pm$0.129
Av. Brightness 1.77-1.81 $\mu$m | 0.741$\pm$0.009 | 0.776$\pm$0.007 | 1.180$\pm$0.112 | — | 0.948$\pm$0.011 | 0.765$\pm$ 0.028 | 0.850$\pm$0.241
Av. Brightness 1.84-1.88 $\mu$m | 0.714$\pm$0.018 | 0.571$\pm$0.219 | 1.244$\pm$0.088 | 0.972$\pm$0.021 | 0.914$\pm$0.023 | 0.659$\pm$0.058 | 0.682$\pm$0.101
Av. Brightness 2.03-2.07 $\mu$m | 0.336$\pm$0.017 | 0.327$\pm$0.003 | 0.386$\pm$0.013 | 0.397$\pm$0.048 | 0.376$\pm$0.008 | 0.318$\pm$0.022 | 0.344$\pm$0.062
Av Brightness 2.22-2.26 $\mu$m | 0.670$\pm$0.013 | 0.545$\pm$0.205 | 1.169$\pm$0.037 | 0.914$\pm$0.018 | 0.817$\pm$0.005 | 0.622$\pm$0.024 | 0.724$\pm$0.056
1.5 $\mu$m band depth | 0.28$\pm$0.02 | — | — | — | 0.49$\pm$0.09 | 0.37$\pm$0.08 | 0.43$\pm$0.16
2.0 $\mu$m band depth | 0.51$\pm$0.03 | 0.41$\pm$0.16 | 0.68$\pm$0.02 | 0.58$\pm$0.05 | 0.56$\pm$ 0.01 | 0.50$\pm$0.04 | 0.51$\pm$0.10

∗ All cubes have 47$\times$49 spatial pixels.

Disk-integrated spectra of the small moons were computed for each dither by
co-adding the signal from the nine pixels within the green squares shown in
Figure 5 (note the central pixels are listed in Table 1). after removing
estimates of the instrumental background signal levels. The background signal
level at each wavelength was derived by fitting a two-dimensional cubic
polynomial function to the region between the cyan and green squares shown in
Figure 5, excluding regions that fell outside the field of view. The rms
variations of the residuals from the fit in this region are also used to
estimate the statistical uncertainties of each moon’s brightness at each
wavelength.

The signal from the moon extends beyond the nine pixels used in this analysis.
We did consider larger regions in the flux calculation to capture a larger
fraction of the moon’s signal, but found that this did not change the signal
levels at short wavelengths by more than a few percent and also caused the
signal levels at wavelengths above 3.5 $\mu$m to strongly diverge from the
expected levels based on prior measurements [Filacchione . (2012)]. We
therefore elected to use a more limited number of pixels that provide more
stable information about the moons’ spectral shape and estimated the missing
flux using the NIRSpec observations of the solar analog star SNAP-2 (2MASS
J16194609+5534178) that were obtained as part of program 1128 [Gordon .
(2022)]. Specifically, we computed the ratio of the stellar signal within the
nine pixels centered on the star to the integrated brightness in all the
observed pixels for each wavelength channel (after removing the median
brightness in the entire image), and fit the resulting curve to a six-degree
polynomial over the entire wavelength range, excluding outliers. The resulting
function was
$R=0.5183+0.5545\lambda-0.4552\lambda^{2}+0.1943\lambda^{3}-0.04713\lambda^{4}+0.0059\lambda^{5}-0.0003\lambda^{6}$
(where the wavelength is measured in microns) and ranges between 0.8 and 0.6
across the observed wavelength range. For this initial investigation we simply
divided the observed flux from each moon by this ratio in order to approximate
the moon’s total flux.

It is also worth noting that the Pandora spectrum comes from one of the
dithers targeted at Epimetheus. This is because the signal from Pandora could
be better isolated from the ring backgrounds in those images due to the
overall background signal level being substantially lower (compare Figures 3
and 4). Even so, we found that we could not extract a reliable spectrum of
Pandora for dither 2 using the above procedure, so for the rest of this
analysis we will only consider the spectrum of Pandora derived from dither 1.

Figure 6 shows the derived flux spectra from each of the moons. The spectra of
Epimetheus and Pandora from dither 1 only extend down to 1.5 $\mu$m because
shorter wavelengths were saturated by stray light from the nearby rings, while
the spectrum of Epimetheus from dither 2 extends down to 1.35 $\mu$m before
saturation becomes an issue. The spectra of Telesto and especially Pallene
also become noticeably noisier at wavelengths shorter than 1.3 $\mu$m, perhaps
because of residual contamination from stray light from the rings.
Instrumental artifacts associated with errors in the background removal are
also responsible for signal levels from certain channels being obvious
outliers from the mean trend (most notably the narrow peak in the Telesto
dither 1 spectrum around 1.75 $\mu$m). Note that these outliers occur in
different channels for the two dithers, so they cannot be real spectral
features. These outliers also do not always correspond to wavelengths with
high statistical uncertainties, indicating that these features probably
involve localized systematic issues with those channels. More careful
processing could flag or remove these outliers, but for this preliminary
analysis we will simply note their existence and use them to document the
level of potential instrumental artifacts in these data.

Leaving aside the instrumental artifacts, the flux spectra for all the moons
show clear dips around 1.5 $\mu$m, 2 $\mu$m and 3 $\mu$m consistent with the
expected signatures of water ice. The different fluxes from the different
moons are also reasonably consistent with their different sizes [Thomas
Helfenstein (2020)], with Epimetheus (average radius 58.6 km) being somewhat
brighter than Pandora (average radius 40.0 km), and both of those moons being
over an order of magnitude brighter than Telesto (average radius 12.3 km), and
almost three orders of magnitude brighter than Pallene (average radius 2.23
km).

These spectra can be more effectively compared to prior work by computing the
effective average reflectance spectra of the different moons. In principle,
these reflectances can be expressed in terms of the (phase-dependent) full-
disk average $I/F$ of the moon, which is given by:

$\langle I/F\rangle=\frac{\mathcal{F}_{\rm
obs}}{F_{\odot}(1AU)}\left(\frac{D_{\odot}}{1AU}\right)^{2}\frac{D_{\rm
obs}^{2}}{\pi R_{\rm eff}^{2}}$ (1)

where $\mathcal{F}_{\rm obs}$ is the measured flux from the moon,
$F_{\odot}(1AU)$ is the solar flux density (flux divided by $\pi$) at 1 AU,
$D_{\odot}$ is the distance from the moon to the Sun, $D_{\rm obs}$ is the
distance from the moon to the observer, and $R_{\rm eff}$ is the effective
average radius of the moon. However, while this is a reasonable quantity for
nearly spherical moons, for elongated moons like Pandora and Pallene this
quantity can vary significantly depending on how the moon is viewed due to
differences in the distribution of incidence and emission angles across the
visible surface [Helfenstein Veverka (1989), Muinonen Lumme (2015), Hedman .
(2020)]. Hence we will instead consider the average (phase-dependent)
brightness coefficient of each moon $B$, which is given by the following
formula:

$B=\frac{\mathcal{F}_{\rm
obs}}{F_{\odot}(1AU)}\left(\frac{D_{\odot}}{1AU}\right)^{2}\frac{D_{\rm
obs}^{2}}{a_{\rm pred}}$ (2)

where $a_{\rm pred}$ is the predicted effective cross-sectional area of the
moon computed using the python code in Hedman20. This code takes as input the
ellipsoidal shape parameters of the moon along with the sub-solar and sub-
observer latitudes and longitudes of the moon. For this particular calculation
we use shape parameters from Thomas20 and estimate the observation geometry
using tools checked against the PDS Ring-Moon Systems node website (all
parameters are provided in Table 1). We also assume the moon’s surface follows
a Lambertian scattering law to facilitate comparisons with prior calculations,
and we use a standard solar spectrum available from STScI
(https://archive.stsci.edu/hlsps/reference-atlases/cdbs/grid/solsys/) to
compute $F_{\odot}$ for each spectral channel. Due to the comparatively low
signal-to-noise of these satellite spectra, the solar spectrum is just
evaluated at the center of each wavelength band.

Figure 7 shows the derived reflectance spectra for the various moons.
Meanwhile, Table 1 provides the average brightness coefficients in several
different wavelength regions around the 1.5 $\mu$m and 2 $\mu$m bands, with
1-$\sigma$ error bars based on the observed scatter in the data points within
each wavelength. We also provide estimates of the band depths of the 1.5
$\mu$m and 2 $\mu$m bands. These band-depths are computed using the standard
formula $1-B_{b}/B_{c}$, where $B_{b}$ is the brightness in the middle of the
relevant band and $B_{c}$ is the average brightness on either side of the
band. For this particular study we use continuum regions consistent with prior
work by Filacchione12.

The overall shapes of all these satellite spectra are dominated by water-ice
absorption bands at 1.5 $\mu$m, 2 $\mu$m, 3 $\mu$m and 4.5 $\mu$m. The spectra
of Epimetheus, Pandora and Telesto also appear to be reasonably consistent
with previously published spectra of these moons derived from Cassini-VIMS
observations [Filacchione . (2012)]. For Telesto, our estimates of the 1.5
$\mu$m and 2.0$\mu$m band depths are 0.49$\pm$0.10 and 0.57$\pm$0.01, which
are consistent with the Filacchione12 values of 0.41$\pm$0.01 and
0.58$\pm$0.01. For Pandora we obtain a 2 $\mu$m band depth of 0.68$\pm$0.01,
which is consistent with the Filacchione12 value of 0.75$\pm$0.11. For
Epimetheus, our estimates of the 1.5$\mu$m and 2.0$\mu$m band depths are
0.28$\pm$0.02 and 0.51$\pm$0.03, while Filacchione12 find values of 0.35 and
0.58 (without error bars) for Epimetheus and 0.37$\pm$0.03 and 0.58$\pm$0.02
for Janus. The ice bands on Epimetheus measured by JWST therefore appear to be
slightly weaker than those observed by Cassini-VIMS. This could in part be
because the JWST data were obtained at a lower phase angles ($\sim$6∘) than
the VIMS observations ($>25^{\circ}$). Note that other moons demonstrate a
reduction in ice-band depth at phase angles below 10∘ [Filacchione . (2012)].

Figure 7 also shows the predicted spectra of regoliths composed of pure
crystalline or amorphous water ice at 100 K, computed using optical constants
from Mastrapa09 and the formulas from Shkuratov99 that translate these
constants into estimates of the surface albedo, assuming typical regolith
scattering lengths of 10 $\mu$m. Both model spectra show clear absorption
bands at 1.5 $\mu$m, 2 $\mu$m and 3 $\mu$m, but there are some notable
differences in the shapes of these bands. For the 1.5 $\mu$m band, crystalline
ice shows a distinct secondary band on its long-wavelength flank, while
amorphous ice shows a more subtle shoulder in the same region. The shape of
the 2 $\mu$m band also changes, with the minimum of this band shifting to
shorter wavelengths for amorphous ice. Finally, crystalline ice shows a
distinct peak at 3.1$\mu$m, while amorphous ice just shows a broad hump. These
features have been used to estimate the crystallinity of the icy surfaces of
Saturn’s larger moons [Dalle Ore . (2015)], but it is worth noting that
efforts to quantify the crystallinity of the ice can be complicated by
diffraction from small regolith grains [Clark . (2012)] and the potential
presence of non-ice contaminants [Ciarniello . (2021)].

Figure 8 provides close-ups of the 1.5 $\mu$m and 3 $\mu$m bands for the
higher signal-to-noise spectra of Telesto and Epimetheus, along with the
reference model spectra of crystalline and amorphous water ice. Both these
moons lack strong peaks at 3.1 $\mu$m and do not show a strong secondary
minima on the long-wavelength side of the 1.5 $\mu$m band. This implies that
the regoliths of both moons contain some amount of either amorphous or very
fine-grained ice. However, these data also show that Epimetheus and Telesto
have bands with different shapes. Most noticeably, Telesto’s 3-$\mu$m band
does contain a narrow peak near 3.1 $\mu$m, while Epimetheus’ band does not,
which suggests that Telesto has a larger fraction of crystalline ice than
Epimetheus. The detailed shape of the 1.5 $\mu$m band is also consistent with
Telesto’s regolith having a higher degree of crystallinity, with a stronger
1.65$\mu$m dip and a flatter band minimum between 1.5 and 1.55 $\mu$m. While
further work is needed to properly quantify the crystallinity of these moons,
these preliminary findings demonstrate that Saturn’s small moons can exhibit
crystalline signatures with a range of strengths.

The JWST data also provides the first near-infrared spectra of the small moon
Pallene. This moon’s spectrum is dominated by the same water-ice features as
the other moons, but the average reflectance of the moon’s surface is lower
than those of the other moons, which is consistent with previous photometric
analyses using data at visible wavelengths [Hedman . (2020)]. Furthermore, the
1.5 $\mu$m and 2 $\mu$m band depths for this moon are 0.37$\pm$0.08 and
$0.50\pm 0.04$, which are comparable to the band depths for the nearby and
comparably small moon Methone (that is, 0.45$\pm$0.04 and 0.60$\pm$0.06, see
Hedman et al. 2020). This demonstrates that JWST can provide sensible spectral
data for extremely small ice-rich objects in the outer solar system.

Furthermore, while the Pallene spectrum does not have sufficient signal-to-
noise to document the detailed shape of the 1.5 $\mu$m and 3 $\mu$m ice bands,
its 2 $\mu$m band has its minimum displaced towards shorter wavelengths than
seen in the other moons observed by JWST (see Figure 7). The wavelength of
Pallene’s 2-$\mu$m band minimum is also similar to that seen in the spectrum
of Methone previously observed by Cassini-VIMS [Hedman . (2020)] shown in
Figure 7. (Note that the Methone spectrum in Figure 7 has been re-normalized
from relative brightness to brightness coefficients. The brightness
coefficient of Methone is somewhat lower than for Pallene in part because the
Cassini-VIMS data were obtained at higher phase angles $\sim 58^{\circ}$.)
Since this band minimum location is also consistent with amorphous water ice,
this suggests that the ice on Pallene’s (and Methone’s) surface may have a
much higher amorphous fraction than the other small moons observed by JWST.
This finding is interesting because both Methone and Pallene experience 20-40
times higher flux of high-energy radiation than most of Saturn’s moons
(including Epimetheus and Telesto, cf. Hedman et al. 2020) and such radiation
can cause crystalline ice to become amorphous [Moore Hudson (1992), Famá .
(2010)]. Pallene’s and Methone’s spectra may therefore provide useful
information about how efficiently radiation can influence the crystallinity of
water ice in the Saturn system. However, it is important to note that the
limited signal-to-noise of these data and the relatively subtle changes in the
predicted shapes of the bands mean that more careful analysis will be needed
to properly quantify the significance and robustness of these variations in
the band minimum position, and translate these measurements into quantitative
estimates of the ice’s state (cf. Dalle Ore et al. 2015).

###  2.3 Near Infrared Spectra of Saturn’s A Ring

The NIRSpec observations targeted at Pandora also provided exceptionally high
signal-to-noise spectra of one ansa of Saturn’s A ring at a phase angle of
5.8∘ and a ring opening angle of 15.2∘ with a typical pixel scale of 700 km.
We computed the average ring spectrum from the second dither of the Pandora
observations by taking the average of the $\sim$400 pixels with a signal
higher than $1.5\times 10^{-9}$ MJy between 3.5 and 3.6 microns. Again, the
flux measurements are converted to reflectance using the standard solar
spectrum (https://archive.stsci.edu/hlsps/reference-
atlases/cdbs/grid/solsys/), yielding the spectrum shown in Figure 9. Note that
in this case we average the solar flux over the nominal wavelength band for
each channel in order to better model the subtle signals from various solar
lines. We also compute the statistical uncertainty on this spectrum by
computing the standard deviation of the same data points after scaling to
remove overall brightness variations across the scene. Note that the rings
completely saturated the detector at all wavelengths below 2.71 $\mu$m, so the
available spectrum only covers longer wavelengths. The signal-to-noise of this
spectrum is much higher than that of any of the small moons because the A ring
fills many pixels, with the statistical uncertainties being below 0.2%
throughout most of the observed wavelength range.

Figure 9: Reflectance spectra of the A ring obtained from the second dither of
the Pandora observations. Note the spectrum only extends down to 2.71 $\mu$m
because the data at shorter wavelengths were saturated. The multiple peaks
around 3.1 $\mu$m are diagnostic of highly crystalline water ice (compare to
Figure 8). The bottom panel shows the statistical uncertainty in the spectra
based on the observed scatter in the brightness measurements among the
relevant pixels.

Figure 10: Normalized and de-trended near-infrared ring spectra. The top panel
shows the normalized ring reflectance spectrum computed using a standard solar
spectrum and the observed spectrum of a solar analog star SNAP-2. The
differences between these two spectra arise because SNAP-2 is a slightly
cooler star than the Sun. The lower panels show detrended versions of these
spectra using rolling quadratic and cubic fits to the observed spectra (see
text for details). There is a clear dip around 4.13 $\mu$m in all the
detrended spectra due to the fundamental O-D absorption band. There is no
evidence for a CO2 band around 4.26 $\mu$m or a CO band around 4.7 $\mu$m.
There may be a weak absorption band around 3.4 $\mu$m that could potentially
be due to aliphatic hydrocarbons.

The overall shape of the observed spectrum, with broad peaks at 3.6 $\mu$m and
5.1 $\mu$m, is consistent with very pure water ice [Mastrapa . (2009), Clark .
(2012), Clark . (2019)]. This spectrum also shows a clear peak at 3.1 $\mu$m
indicative of strongly crystalline water ice. This is consistent with Cassini-
VIMS observations that showed this feature was much stronger in the ring
specta than it was in most of the satellite spectra [Filacchione . (2012)].
These data are of exceptionally high quality and clearly show two smaller
peaks on either side of the 3.1 $\mu$m peak, which is consistent with the
measured optical constants of highly crystalline water ice [Mastrapa . (2009),
Clark . (2012)].

This spectrum covers a wavelength range where a number of more subtle spectral
features have been detected in various Cassini-VIMS observations of icy
objects in the Saturn system. More specifically, the fundamental O-D
absorption band has been identified at 4.13 $\mu$m in very high signal-to-
noise spectra of the B ring [Clark . (2019)]. While the published ring spectra
did not show any evidence for any absorption bands due to carbon-bearing
compounds deeper than 1-3% [Clark . (2008), Cuzzi . (2009), Filacchione .
(2014), Clark . (2019)], many of Saturn’s moons exhibit a carbon dioxide band
around 4.26 $\mu$m [Clark . (2008)], and Enceladus and Iapetus show aliphatic
hydrocarbon signatures between 3.4 and 3.6 $\mu$m [RH. Brown . (2006),
Cruikshank . (2014)]. The new JWST spectrum of the A ring is of comparable
quality to the highest signal-to-noise spectra obtained by Cassini-VIMS, and
so it is worth examining whether these data contain any evidence for carbon-
bearing compounds.

For this preliminary search for subtle spectral features, we convert the flux
from the rings into estimates of the rings’ reflectance using both the
standard solar spectrum provided by STScI and the spectrum of the solar analog
star SNAP-2 (2MASS J16194609+5534178) observed by JWST NIRSpec in the PRISM
configuration as part of program 1128 [Gordon . (2022)]. SNAP-2 is a G3V star
and so has a slightly lower effective temperature than the Sun. The
reflectance spectrum computed using the SNAP-2 data therefore has a different
overall slope than the one computed using the standard solar spectrum (see top
panel of Figure 10). Despite this, this spectrum is still useful for assessing
whether any weak spectral features could be due to residual solar lines.

In order to better visualize and isolate any weak features in these spectra,
we divided each reflectance spectrum at each wavelength channel by a rolling
quadratic or cubic function that best fit the data within $\pm 0.15$ $\mu$m of
the selected wavelength channel, excluding any data below 3.3 $\mu$m (where
the water-ice fresnel peaks complicate the fit) or between 4.1 and 4.2 $\mu$m
(which contains the O-D band). This process removes the smooth variations in
the spectral shape and normalizes the spectrum to a value of 1 at all
wavelengths outside of sharp features. These normalized and detrended spectra
are shown in the lower panels of Figure 10, along with similarly normalized
versions of the standard solar spectrum for reference. Note that the detrended
spectra computed using the cubic and quadratic fits have similar shapes,
although the cubic versions have slightly less dispersion around unity, as is
to be expected.

The most obvious feature in all the detrended spectra is the band centered at
4.13 $\mu$m, which corresponds to the O-D band identified by [Clark . (2019)]
in Cassini-VIMS spectra of the rings. The depth of this band in the JWST data
is between 2% and 3%, consistent with the prior analyses of the VIMS data
[Clark . (2019)], which indicated that the D/H ratio for Saturn’s rings is
close to the terrestrial value. The JWST observations therefore confirm the
existence and depth of this feature.

Outside of the O-D band, there are some differences between the detrended
reflectance spectra computed using the standard solar spectrum and those
computed using the SNAP-2 spectrum. Most notably, the spectra computed using
the standard solar spectrum exhibit a series of narrow spikes that correspond
to narrow dips in the predicted solar flux. Since these peaks are not seen in
the spectra computed using SNAP-2, they almost certainly represent residual
solar lines. Similarly, the spectra computed using the solar reference
spectrum show a 1.5% dip at 4.37 $\mu$m that is not seen in the SNAP-2
normalized spectra. This feature is therefore probably not a real spectral
feature and may instead reflect an issue with the calibration pipeline that is
still under active investigation (cf. Bockelee-Morvan et al. 2023). These
issues imply that we should exercise due caution when interpreting $\sim 1\%$
spectral features in these data sets. Despite this, such weak features in
these spectra can still provide useful information about potential non-water-
ice components of the ring that may merit further investigation.

None of the detrended spectra show evidence of a carbon dioxide band around
4.26 $\mu$m or a carbon monoxide band around 4.7 $\mu$m. We may therefore
conservatively conclude that both these bands must be weaker than 0.5% in
Saturn’s A ring. These findings are consistent with prior analyses of the
Cassini-VIMS observations [Cuzzi . (2009), Clark . (2019)], and so confirm
that oxygen-bearing carbon species are not currently detectable in Saturn’s
rings.

More interestingly, there are features between 3.3 $\mu$m and 4 $\mu$m in all
the detrended spectra. Two broad $\sim 0.5\%$ dips are found around 3.75
$\mu$m and 3.97$\mu$m. The origins of these features are not known at present.
A more sharp-edged band may also be present between 3.35 $\mu$m and 3.45
$\mu$m. This feature appears as a $\sim 1\%$ dip in both spectra detrended
using the quadratic fit, indicating that this feature is unlikely to be due to
residual solar lines. While this feature is shallower in the spectra detrended
using the cubic fit, its sharp edges are still visible even in these spectra,
along with the narrow peak around 3.4 $\mu$m. Previous analyses of VIMS
spectra found evidence of a weak potential absorption band around 3.42 $\mu$m
band with a depth of $\sim 3\%$ in the A ring [Filacchione . (2014)] that was
attributed to aliphatic hydrocarbons and could be consistent with this
feature. This structure also spans the same range of wavelengths as the
aliphatic hydrocarbon absorption bands seen in the spectra of Enceladus and
Iapetus [RH. Brown . (2006), Cruikshank . (2014)], and has some similarities
to the hydrocarbon bands recently observed by JWST on various trans-neptunian
objects [Emery . (2023), ME. Brown Fraser (2023)]. Detailed analysis and
modeling of this feature could therefore potentially provide new constraints
on the organic content of Saturn’s rings.

##  3 Mid-Infrared Observations of Saturn’s Rings

###  3.1 General description of observations

JWST observed the rings at mid-infrared wavelengths as part of a MIRI
observation sequence of both Saturn and its rings that is described in
Fletcher23. The MIRI observations targeted at Saturn’s rings were obtained on
2022-Nov-13 01:55:27 UT using the Medium-Resolution spectroscopy IFU mode,
yielding spectra between 4.9 $\mu$m and 27.9 $\mu$m at a time when the rings
were at a phase angle of around 5.8∘ and the ring opening angle was 15∘. These
observations used the FASTR1 readout pattern and employed 5 groups per
integration, 8 integrations per exposure and four dithers. This yielded a
total exposure time for each channel of 521.708 seconds. The data from these
observations are organized into four numbered channels covering overlapping
wavelength ranges (Channel 1 = 4.9-7.65 $\mu$m, Channel 2 = 7.51-11.7 $\mu$m,
Channel 3 = 11.55-17.98 $\mu$m, Channel 4 = 17.7-27.9 $\mu$m). Each of these
channels consists of three sub-bands designated Short, Medium and Long,
respectively, and have different spatial resolutions and fields of view (see
Figure 11). Note that for this particular study we will not consider data from
Channel 4 because the calibration of this channel is less reliable than the
other channels. Also, a single dither using the same observation parameters
(and so with a total exposure time of 130.427 seconds per sub-band) was
obtained at 2022-Nov-13 03:17:19UT in a region of the sky 90” north of Saturn
that provides a measure of the background signal levels in the instrument.
These cubes were all processed using pipeline version 1.11.4 with CDS context
jwst_1112.pmap. All observations were processed using the custom calibration
pipeline described in King23.

Figure 11: Mid-Infrared images of the rings derived from selected wavelength
ranges. Note that at short wavelengths the A and B rings are brighter than the
Cassini Division and C ring, while at long wavelengths the Cassini Division
and C ring are brighter than the A and B rings. This contrast reversal arises
because at short wavelengths the observed signal comes primarily from
reflected sunlight, while at longer wavelengths the signal comes primarily
from the rings’ thermal emission.

Figure 12: Maps of the ring’s flux and brightness temperature as functions of
ring radius and wavelength derived from the MIRI observations. The radial
extent of the three main rings are shown at the far right of each plot (the
Cassini Division is situated between the A and B rings). Note that the
brightness temperature is more useful on the right-hand side of the figure,
where thermal emission dominates over reflected sunlight. The brightness
temperature rapidly increases at short wavelengths because of the increasing
contribution of reflected sunlight. Narrow spectral features in the regions
indicated by the shaded bands around 8 $\mu$m, 12 $\mu$m and 14$\mu$m are due
to stray light from Saturn, but the bright line at 9.2 $\mu$m is due to the
presence of highly crystalline water ice throughout the ring.

Figure 11 shows images of the rings derived from a sample of the various
wavelength regions covered by the MIRI observations. All these images are
centered on the B ring, but also contain the Cassini Division and portions of
the C and A rings. At the shortest observed wavelengths (around 5 $\mu$m) the
A and B rings are both clearly brighter than the Cassini Division and C ring.
This is consistent with the appearance of the rings at visible and near-
infrared wavelengths [Cuzzi . (2009), Hedman . (2013), Filacchione . (2014),
Cuzzi . (2018), Ciarniello . (2019)], and occurs because the dominant signal
at these wavelengths is reflected sunlight. The Cassini Division and C ring
are less reflective than the A and B rings both because they have lower
optical depths and because the material in these regions is intrinsically
darker [Cuzzi . (2009), Hedman . (2013), Filacchione . (2014)]. However, at
longer wavelengths the situation reverses, with the Cassini Division and C
ring becoming brighter than the A and B rings around 10 $\mu$m. This happens
because at these wavelengths the dominant signal comes from the ring’s own
thermal emission, and the lower albedos of the particles in the Cassini
Division and C ring causes them to be warmer than the particles in the A and B
rings [Spilker . (2006), Altobelli . (2008), Flandes . (2010), Filacchione .
(2014), Spilker . (2018)].

There are also comparatively subtle wavelength-dependent brightness variations
within the B ring. Around 5 $\mu$m the inner B ring is slightly brighter than
the outer B ring. This is notable because at wavelengths shorter than 1 $\mu$m
the outer B ring is brighter at low phase angles [Hedman . (2013), Filacchione
. (2014)]. This contrast reversal probably arises because the outer B ring has
stronger water-ice absorption bands than the inner B ring [Hedman . (2013),
Filacchione . (2014)], which allows the contrast between the two regions to
reverse at wavelengths longer than 3 $\mu$m, where the water ice is strongly
absorbing (see for example Observations X an II in Fig 4. of Filacchione et
al. 2014). At even longer wavelengths a faint bright band can be seen near the
middle of the ring, which corresponds to regions of slightly elevated
temperatures that were previously observed by Cassini-CIRS [Spilker . (2006),
Spilker . (2018)].

It is also worth noting that none of the rings show obvious brightness
variations with longitude. This might at first be surprising because
observations at multiple wavelengths indicate that both the A and B ring are
composed of elongated canted structures called self-gravity wakes that
generate azimuthal variations in the ring’s opacity and brightness over a wide
range of wavelengths [Thompson . (1981), Lumme Irvine (1984), Franklin .
(1987), Dones . (1993), Dunn . (2004), Nicholson . (2005), French . (2007),
Ferrari . (2009), Jerousek . (2016)]. In practice, however, such brightness
variations will be subtle in these particular observations because the ring
opening angle is sufficiently low that the B ring will be almost completely
opaque at all observed azimuths. The A ring could exhibit substantial
brightness variations in this viewing geometry, but these are difficult to
discern in the MIRI data because substantial spans of A-ring longitudes are
only observable at longer wavelengths, where the A ring is difficult to
isolate from the brighter Cassini Division.

Another potential source of azimuthal brightness variations are changes in the
temperature of the ring material as it emerges from Saturn’s shadow and moves
around the planet. While such temperature variations have been observed by
Cassini-CIRS, it turns out these are quite small (1-2 K) over the range of
longitudes observed by JWST (which correspond to local times around 18 hours)
when the lit side of the rings are observed at low phase angles [Leyrat .
(2008), Spilker . (2018)]. This is again consistent with the lack of obvious
azimuthal brightness variations in the JWST data.

Given the lack of obvious azimuthal variations in the signal from the rings,
we transformed the calibrated MIRI cubes into maps of the ring’s brightness
versus radius and wavelength. To accomplish this, we first subtracted
instrumental backgrounds from each cube by subtracting the data from the
offset cube, and then computed the average brightness at each wavelength for a
series of 1000-km wide bins in the observed ring-plane radius. Figure 12 show
the resulting maps in two different formats. The top panel shows the measured
flux from the rings in the standard units of MJy/sr. In these units, the
signal from each radius in the ring declines with increasing wavelength
between 5 and 6.5 $\mu$m, which can be attributed to the decreasing
reflectance of the rings over these wavelengths. However, beyond about 8
$\mu$m the signal strongly increases with wavelength, which is due to the
steep increase in the ring’s thermal emission over this wavelength range.
Since the thermal emission flux spans several orders of magnitude, a clearer
picture of the radial variations in the ring’s spectra can be obtained by
converting the observed flux into a “Brightness Temperature”, which is
computed assuming the signal at each wavelength and location is entirely due
to thermal emission from a single-temperature source that fills the
appropriate region. At long wavelengths the ring’s brightness temperature at
each radius is approximately constant. However, at short wavelengths the
brightness temperature increases with decreasing wavelength due to increasing
importance of reflected sunlight.

The Cassini Division is clearly visible in both panels of Figure 12 as a
horizontal stripe around 120,000 km that transitions from dark to bright
around 8 $\mu$m. The C ring can also be seen below 90,000 km that transitions
from being dark to bright around the same wavelength. Note that the edge of
the A ring around 140,000 km is rather diffuse due to limited spatial
resolution of the instrument at long wavelengths. At the same time, there are
some features at particular wavelengths. There are multiple narrow bright
features around 8 $\mu$m, 12 $\mu$m and 14 $\mu$m that turn out to be due to
stray light from Saturn (see below). However, there is also a broader bright
band around 9.2 $\mu$m that does appear to come from the rings themselves.

Figure 13: Average ring spectra of the middle B ring between 100,000 km and
110,000 km. Shaded bands indicate regions most obviously contaminated by
emission lines in the stray light from Saturn. Note that the brightness in
various sub-channels were scaled to fixed factors to match across each overlap
region (see Table 2). These factors result in the longest wavelength channels
being multipled by a factor of roughly 1.4 relative the shortest wavelength
data. The bottom panel shows the scaled brightness spectra converted to
brightness temperature, which is not necessarily the physical temperature of
the rings and shows a steady decrease with increasing wavelengths even at
wavelengths where thermal emission dominates. The magenta lines in both plots
show an 86.3 K blackbody spectrum multiplied by the scaling factor
$\beta=0.663$ (see text). While this model reproduces the thermal component of
the spectrum well, the scaling factor is lower than expected. Table 2: Scaling
factors required for the B-ring spectra to match

Channel 1 Medium/Channel 1 Short 1.081 Channel 1 Long/Channel 1 Medium 1.057
Channel 2 Short/Channel 1 Long 1.055 Channel 2 Medium/Channel 2 Short 1.087
Channel 2 Long/Channel 2 Medium 1.074 Channel 3 Short/Channel 2 Long 0.961
Channel 3 Medium/Channel 3 Short 1.025 Channel 3 Long/Channel 3 Medium 1.001

###  3.2 Mid-infrared spectra of Saturn’s B ring

The images and maps indicate that the Cassini Division is barely resolved, and
the captured regions of the A and C rings do not have particularly uniform
brightness. Interpreting the spectral trends in these regions would therefore
require accounting for the wavelength-dependent point-spread function of
MIRI/MRS, as well as the azimuthal brightness asymmetries in the A ring
[Ferrari . (2009)], which is beyond the scope of this initial investigation.
Hence for this particular analysis we will consider only the average spectrum
of the region 100,000 and 110,000 km. This corresponds to the middle of the B
ring, where the observed ring brightness is relatively uniform in both radius
and longitude (note the variations in the brightness temperature across the B
ring seen in Figure 11 are less than 2 K). Figure 12 shows the average ring
flux and computed brightness temperature derived from the MIRI observations of
this region. Note that the spectrum constructed from the individual channel
sub-bands showed small offsets where they overlap with adjacent sub-bands.
Since the ring signal fills multiple pixels and is rather strong at most of
the sampled wavelengths, we interpret these offsets as due to differences in
the calibration between the different channels, and so assume that
multiplicative scaling factors are the appropriate way to bring these spectra
into alignment (note that we cannot directly compare the signals measured by
MIRI and NIRspec around 5 $\mu$m because the two instruments observed
different parts of the rings.) Table 2 shows the average brightness ratios
between adjacent bands in these overlap regions for the average B-ring
spectrum, which are all within 10% of unity. We use these factors to re-
normalize the various spectral channels relative to the shortest-wavelength
sub-channel, which meant that the longest wavelength sub-channel is multiplied
by a factor of 1.4. This is a relatively slight spectral slope over this
wavelength range, and is not a significant issue for this particular analysis,
which will focus primarily on the short-wavelength part of this spectrum. Also
note that this correction is applied before computing the brightness
temperature spectrum shown in the lower panel of Figure 13.

The average spectrum contains a number of interesting features. Again, the
many narrow lines near 8 $\mu$m, 12 $\mu$m and 14 $\mu$m are due to stray
light from Saturn, and correspond to strong emission lines from methane,
ethane and acetylene, respectively [Fletcher . (2023)]. These regions are
shaded in Figures 13 because this contamination from the planet prevents these
regions from providing reliable information about the composition of the
rings. However, below 10 $\mu$m we can see two distinct peaks in the flux at 5
$\mu$m and 9.3 $\mu$m, along with a dip around 6.2 $\mu$m. These features are
consistent with the expected reflectance spectra of crystalline water ice.
However, before we can examine these features in detail we first need to
consider the thermal component of these spectra.

At wavelengths longer than 10 $\mu$m, the flux increases smoothly over several
orders of magnitude due to the ring’s thermal emission. If this was a perfect
blackbody spectrum, then the derived brightness temperature at long
wavelengths would have a constant value corresponding to the temperature of
the ring material. However, in practice the brightness temperature above 10
$\mu$m shows an approximately linear decrease with increasing wavelength. This
trend cannot be attributed solely to the scaling factors applied to the
different sub-channels because this trend can be observed within individual
sub-channels. The most straightforward explanation for this sort of trend is
that the ring material either has a finite emissivity or does not completely
fill the field of view due to the gaps between the ring particles. Both of
these phenomena result in the observed thermal ring spectra being multiplied
by a scaling factor $\beta$ that is less than 1 [Spilker . (2006), Spilker .
(2018)]. In fact, if we fit the re-scaled data from sub-channel 3-Long (i.e.
15.41-17.98 $\mu$m) to a scaled blackbody spectrum, the best-fit model has a
temperature of 86.3 K and a $\beta$=0.663 (Note that $\beta$ would be 0.478 if
we did not re-scale the fluxes using the mean brightness ratios between
bands). This scaled blackbody spectrum is a good match to the observed spectra
in the regions free of stray-light from Saturn above 12 $\mu$m.

However, while the shape of the spectrum is consistent with a scaled
blackbody, the parameters derived from this fit are not entirely consistent
with previous observations of the B-ring’s thermal spectra obtained by the
Cassini CIRS instrument. The temperature of 86.3 K is reasonably compatible
with CIRS observations of the B ring taken at comparable phase and ring-
opening angles, which are generally between 85 K and 90 K [Spilker . (2006),
Altobelli . (2008), Flandes . (2010), Filacchione . (2014), Spilker . (2018)].
The issue is that the $\beta$ value is well below the expected value for the B
ring. The Cassini CIRS data indicates that the thermal scaling factor for the
B ring can be predicted quite well by the ring’s geometric filling factor
based on optical depth measurements [Spilker . (2018)]. For these particular
observations of the B ring the filling factor is above 0.95 for the region
between 100,000 and 110,000 km, which would imply $\beta>0.85$ [Spilker .
(2018)], which is 35% larger than the value needed to match the scaled MIRI
spectra (This discrepancy grows to 80% if we use $\beta=0.48$, which best fits
the unscaled channel 3-Long data). The reasons for this discrepancy are still
unclear, but could include wavelength-dependent contamination of the spectra
by either Saturn or blank sky due to MIRI’s extended point-spread-function,
both of which can potentially introduce spurious slopes and trends into the
spectrum. Since these aspects of MIRI’s performance are still under active
investigation, we will not make further attempts to interpret the detailed
shape of the thermal component of these spectra at this time. Instead, we will
focus on the spectral features observed at wavelengths below 10 $\mu$m, whose
shapes are relatively insensitive to uncertainties in the overall spectral
slope.

Figure 14: A closer look at the mid-infrared reflectance spectra of Saturn’s
rings. The top panel shows the average Saturn spectrum from the SATURN-15N
observation, converted to equivalent $I/F$ for comparison with the rings. Note
particularly the series of strong emission lines around 8 $\mu$m and beyond 9
$\mu$m. The middle panel shows three different versions of the average ring
spectrum between 100,000 and 110,000 km. The cyan curve shows the brightness
data after the scaling to remove offsets between subchannels. The blue curve
shows the same curve after removing the best-fit average thermal signal with a
temperature of 86.3 K and a $\beta=0.663$. The violet curve further has stray
light from Saturn removed by subtracting the Saturn spectrum scaled by 0.0033.
Note that this removed the lines around 8 $\mu$m, but introduces spectral
features beyond 9 $\mu$m that might be spurious. The bottom panel shows the
predicted spectra of crystalline and amorphous water ice regoliths with
effective scattering lengths of 10 $\mu$m and temperatures of 100 K in this
same wavelength range.

Figure 14 shows the average reflectance spectra of the rings between 100,000
and 110,000 km before and after removing the 86.3 K blackbody scaled by a
factor of 0.663. The conversion from flux density to reflectance again uses
the standard solar spectrum available from STScI
(https://archive.stsci.edu/hlsps/reference-atlases/cdbs/grid/solsys/). Note
that removing the thermal signal only strongly affects the signal at
wavelengths longer than 8 $\mu$m. In addition, it is reasonable to assume
$\beta$ is independent of wavelength because the generally low reflectance of
the rings at these wavelengths means that the emissivity should be close to
unity throughout this wavelength range [Morishima . (2012)]. Finally, we can
note that around 5 $\mu$m, the peak reflectance is around 0.03, which is
consistent with the near-infrared spectrum shown in Figure 9, especially if we
realize that the B ring is slightly brighter than the A ring at near-infrared
wavelengths when viewed at low phase angles [Filacchione . (2012), Hedman .
(2013), Filacchione . (2014)].

Stray light from Saturn is clearly present in these spectra around 8 $\mu$m
(corresponding to stratospheric methane emission), but contamination from the
planet also may be having more subtle effects on other parts of this spectrum.
For comparison, Figure 14 also shows a Saturn spectrum derived from the
average brightness in the cubes targeted at low Saturn latitude (designated
SATURN-15N), converted to reflectance to facilitate comparisons with the ring
spectrum. While the lines around 8 $\mu$m are the obvious features in the
Saturn spectrum, there are also features in the Saturn spectrum that could be
influencing the ring spectrum around 5 $\mu$m and 10 $\mu$m. In order to
estimate how much stray-light from Saturn could be influencing the ring
spectrum, we estimate the likely strength of the Saturn signal in the ring
spectrum by plotting the ring’s brightness versus Saturn’s brightness in the
wavelength range between 7.25 and 8.15 $\mu$m and fitting these data to a
linear trend. This fit indicated that the lines in the ring spectrum are about
0.0033 the intensity of the lines in the Saturn spectrum. We therefore
subtracted a version of the Saturn spectrum scaled by this factor from the
ring spectrum to produce the violet curve in Figure 14. This correction
changes the shape of the 5 $\mu$m peak slightly, and also introduces narrow
features into the ring spectrum between 8.5 $\mu$m and 10 $\mu$m. The latter
implies that assuming a fixed scaling factor is overcorrecting the ring
spectrum beyond 8.5 $\mu$m. This is likely due to a combination of variations
in Saturn’s spectrum across its disk [Fletcher . (2023)] and wavelength-
dependent variations in the MIRI point-spread function. The stray-light
signals from Saturn therefore vary with wavelength in rather complex ways that
will need to be addressed in future work. Fortunately the overall shape of the
peak at 9.3 $\mu$m is insensitive to details of this subtraction.

Comparing all these spectra with the predicted spectra of amorphous and
crystalline water ice computed using the Mastrapa09 optical constants and the
Shkuratov99 scattering formulas, we can see that the overall shape of the
observed spectra matches those expected for crystalline ice extremely well.
Most dramatically, the peak in the observed spectrum at 9.3 $\mu$m is only
found in the crystalline ice spectrum, and is absent in amorphous ice,
providing independent confirmation that the ice in Saturn’s rings is strongly
crystalline. The shape of the 5 $\mu$m peak (after correcting for Saturn
shine) is also closer to that of the crystalline model than the amorphous
model. Differences between the crystalline ice model and the observed ring
spectrum in this range can mostly be attributed to issues with removing the
thermal signal and the straylight from Saturn, particularly at longer
wavelengths. Since these spectra match crystalline water ice so well, they can
serve as a useful baseline for interpreting mid-infrared spectra of other
bodies in the outer solar system with more complex compositions.

##  4 Summary of Key Findings

The key findings of this initial analysis of the JWST spectra of Saturn’s
rings and small moons are as follows:

  * •

NIRSpec could clearly detect and measure spectral features of Saturnian moons
with radii between 60 km and 2 km (or angular radii between 8.5 and 0.3
milliarcseconds).

  * •

The near-infrared spectra of Epimetheus, Pandora and Telesto are dominated by
water ice features, consistent with prior observations, and the shapes of the
1.5 $\mu$m and 2.0 $\mu$m bands, as well as the lack of clear 3.1 $\mu$m
peaks, suggest that either the surfaces of these moons are composed of a
mixture of crystalline and amorphous water ice, or the surface includes
significant amounts of sub-micron grains and/or contaminants. Overall, Telesto
appears to have a more crystalline spectrum than Epimetheus does.

  * •

The near-infrared spectrum of Pallene is also dominated by water ice features,
and its water ice bands are similar to those previously observed on the nearby
and comparably small moon Methone. The shape of Pallene’s 2 $\mu$m band
suggests that its surface ice could be more amorphous than the other small
moons observed by JWST, which may happen because Pallene’s surface is exposed
to relatively high doses of high-energy radiation [Hedman . (2020)].

  * •

The near-infrared spectrum of Saturn’s A ring is dominated by features
consistent with highly crystalline water ice, including a prominent set of
Fresnel peaks around 3.1 $\mu$m.

  * •

The JWST ring spectrum confirms the existence and depth of the 4.13 $\mu$m
band due to O-D absorption, and therefore supports previous work suggesting
that the rings have a D/H ratio close to terrestrial values [Clark . (2019)].

  * •

The JWST near-infrared ring spectrum does not show any clear spectral features
at 4.26 $\mu$m due to carbon dioxide or at 4.7 $\mu$m due to carbon monoxide.
However, there may be a weak feature around 3.4 $\mu$m that could be due to
aliphatic hydrocarbons.

  * •

The mid-infrared ring observations show a clear contrast reversal, with the A
and B rings being brighter than the Cassini Division and C ring at shorter
wavelengths due to their higher reflectance, while the Cassini Division and C
ring are brighter at longer wavelengths because of their higher temperatures.

  * •

The rings have a reflectance peak at 9.3 $\mu$m due to highly crystalline
water ice, and the overall shape of the B-ring spectrum between 5 and 10
$\mu$m is consistent with very pure, highly crystalline water ice. More work
is needed to understand the stray light from Saturn and overall radiometric
calibration of MIRI.

## Open Research Section

Level-3 calibrated data from the standard pipeline are available directly from
the MAST archive https://mast.stsci.edu. The JWST calibration pipeline is
described in Bushouse23, and the custom pipeline and initial data processing
code is described in King23. The Jupyter notebooks used to produce the spectra
shown in this study, along with relevant input files and csv files of the flux
spectra for the rings and small moons, are available at
https://github.com/JWSTGiantPlanets/SaturnRingsMoons and via Hedman24.

## Acknowledgments

Fletcher, King, and Roman were supported by a European Research Council
Consolidator Grant (under the European Union’s Horizon 2020 research and
innovation programme, Grant 723890) at the University of Leicester. Harkett
was supported by an STFC studentship; 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 observations—in particular Beth Perriello,
Bryan Holler, Stephen Birkman, 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. 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
JWST. 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).
We also wish the thank Mauro Ciarniello and Gianrico Filacchione for their
helpful comments on an earlier version of this manuscript.

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