# A CO2 cycle on Ariel?
Radiolytic production and migration to low latitude cold traps

Richard J. Cartwrighta The Carl Sagan Center at the SETI Institute
189 Bernardo Ave., Suite 200
Mountain View, CA 94043, USA  Tom A. Nordheim  Jet Propulsion Laboratory,
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA 91109, USA  David Decolibus  New Mexico State University
PO Box 30001, MSC 4500
Las Cruces, NM 88001, USA  William M. Grundy  Lowell Observatory
1400 W Mars Hill Road
Flagstaff, AZ 86001, USA  Northern Arizona University
S San Francisco Street
Flagstaff, AZ 86011, USA  Bryan J. Holler  Space Telescope Science Institute
3700 San Martin Drive
Baltimore, MD 21218, USA  Chloe B. Beddingfield  The Carl Sagan Center at the
SETI Institute
189 Bernardo Ave., Suite 200
Mountain View, CA 94043, USA  NASA-Ames Research Center
Mail Stop 245-1
Building N245, Room 204
P.O. Box 1
Moffett Field, CA 94035, USA  Michael M. Sori  Purdue University
610 Purdue Hall
West Lafayette, IN 47907, USA  Michael P. Lucas  University of Tennessee
1621 Cumberland Avenue
Knoxville, TN 37996, USA  Catherine M. Elder  Jet Propulsion Laboratory,
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA 91109, USA  Leonardo H. Regoli  John Hopkins University Applied
Physics Laboratory
11100 Johns Hopkins Road
Laurel, MD 20723, USA  Dale P. Cruikshank  NASA-Ames Research Center
Mail Stop 245-1
Building N245, Room 204
P.O. Box 1
Moffett Field, CA 94035, USA  Joshua P. Emery  Northern Arizona University
S San Francisco Street
Flagstaff, AZ 86011, USA  Erin J. Leonard  Jet Propulsion Laboratory,
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA 91109, USA  Corey J. Cochrane  Jet Propulsion Laboratory,
California Institute of Technology
4800 Oak Grove Drive
Pasadena, CA 91109, USA

###### Abstract

CO2 ice is present on the trailing hemisphere of Ariel but is mostly absent
from its leading hemisphere. The leading/trailing hemispherical asymmetry in
the distribution of CO2 ice is consistent with radiolytic production of CO2,
formed by charged particle bombardment of H2O ice and carbonaceous material in
Ariel’s regolith. This longitudinal distribution of CO2 on Ariel was
previously characterized using 13 near-infrared reflectance spectra collected
at ‘low’ sub-observer latitudes between 30$\degree$S to 30$\degree$N. Here, we
investigated the distribution of CO2 ice on Ariel using 18 new spectra: two
collected over low sub-observer latitudes, five collected at ‘mid’ sub-
observer latitudes (31$\degree$ – 44$\degree$N), and eleven collected over
‘high’ sub-observer latitudes (45$\degree$ – 51$\degree$N). Analysis of these
data indicates that CO2 ice is primarily concentrated on Ariel’s trailing
hemisphere. However, CO2 ice band strengths are diminished in the spectra
collected over mid and high sub-observer latitudes. This sub-observer
latitudinal trend may result from radiolytic production of CO2 molecules at
high latitudes and subsequent migration of this constituent to low latitude
cold traps. We detected a subtle feature near 2.13 $\micron$ in two spectra
collected over high sub-observer latitudes, which might result from a
‘forbidden’ transition mode of CO2 ice that is substantially stronger in well
mixed substrates composed of CO2 and H2O ice, consistent with regolith-mixed
CO2 ice grains formed by radiolysis. Additionally, we detected a
2.35-$\micron$ feature in some low sub-observer latitude spectra, which might
result from CO formed as part of a CO2 radiolytic production cycle.

Uranian satellites (1750); Planetary surfaces (2113); Surface composition
(2115); Surface processes (2116); Surface ices (2117)

111aVisiting Astronomer at the Infrared Telescope Facility, which is operated
by the University of Hawaii under contract 80HQTR19D0030 with the National
Aeronautics and Space Administration.

##  1 Introduction

Ariel and the other ‘classical’ Uranian moons Miranda, Umbriel, Titania, and
Oberon are candidate ocean worlds (e.g., Hendrix et al., 2019; Cartwright et
al., 2021) with surfaces that have been modified by endogenic geologic
processes (e.g., Beddingfield et al., 2015; Beddingfield & Cartwright, 2020,
2021; Schenk & Moore, 2020) and interactions with the surrounding space
environment (e.g., Grundy et al., 2006; Cartwright et al., 2018; DeColibus et
al., 2020). Ground-based, near-infrared (NIR) observations have determined
that the surfaces of these moons are primarily composed of H2O ice mixed with
dark and spectrally neutral constituents (e.g., Cruikshank et al., 1977;
Cruikshank, 1980; Cruikshank & Brown, 1981; Soifer et al., 1981; Brown &
Cruikshank, 1983; Brown & Clark, 1984) that likely include a carbon-rich
component (Clark & Lucey, 1984). CO2 ice has also been detected on Ariel,
Umbriel, Titania, and Oberon via three prominent absorption features centered
near 1.966, 2.012, and 2.070 $\micron$ (Grundy et al., 2003, 2006; Cartwright
et al., 2015). These bands are stronger on the trailing hemispheres of these
moons (longitudes 181 – 360$\degree$) compared to their leading hemispheres
(longitudes 1 – 180$\degree$). Furthermore, these CO2 ice bands are strongest
on Ariel and get progressively weaker with increasing orbital radius, with the
weakest CO2 bands detected on the outermost classical moon Oberon (e.g.,
Cartwright et al., 2015).

Previous studies have suggested that the observed longitudinal and radial
trends in the distribution of CO2 ice are consistent with a radiolytic origin
(Grundy et al., 2006; Cartwright et al., 2015). In this scenario, charged
particles from Uranus’ magnetosphere bombard native H2O ice and carbonaceous
material exposed on the surfaces of the classical moons, driving radiolytic
production of CO2 molecules. Because Uranus rotates faster ($\sim$17.2 hours)
than the orbital periods of its classical moons ($\sim$1.4 – 13.5 days),
charged particles trapped in its magnetosphere should preferentially interact
with their trailing hemispheres as Uranus’ magnetic field lines sweep past
these moons. Voyager 2 observed depletions in magnetospheric charged particle
flux near the orbital location of some of the Uranian moons (including Ariel),
hinting at extensive interactions between Uranus’ magnetosphere and the
surfaces of its moons, possibly driving radiolytic chemistry (e.g., Paranicas
et al., 1996). Alternatively, CO2 ice could be a native constituent on the
classical moons that is sourced from their interiors by geologic processes
that expose and/or emplace material rich in CO2, perhaps similar to Enceladus
where plume activity has likely deposited large amounts of CO2 on its surface
(e.g., Waite et al., 2006; Combe et al., 2019).

The spectral signatures of the detected CO2 bands are remarkably consistent
with ‘pure’ CO2 ice measured in the laboratory (i.e., segregated from other
constituents in concentrated deposits with crystal structures dominated by CO2
molecules) (e.g., Hansen, 1997; Gerakines et al., 2005). At the estimated peak
surface temperatures of the Uranian moons (80 – 90 K, Hanel et al. 1986; Sori
et al. 2017), CO2 ice is thermodynamically unstable over geologic timescales
(Grundy et al., 2006; Sori et al., 2017). Because of Uranus’ large obliquity
($\sim$98$\degree$), the poles of the classical moons are bathed in continuous
sunlight during the Uranian system’s long summers. As a result, long-term cold
traps for CO2 ice are likely concentrated at low latitudes, where the
timescales of maximum solar heating are much shorter compared to high
latitudes (Grundy et al., 2006; Sori et al., 2017). Additionally, CO2
molecules should concentrate at similar longitudes to where they were formed
or exposed (Sori et al., 2017), likely explaining why CO2 ice remains
concentrated on the trailing hemisphere of Ariel and the other Uranian moons.
Thermodynamical models therefore predict that CO2 molecules generated or
exposed at polar latitudes should sublimate, migrate to equatorial latitudes,
and condense in cold traps, primarily on Ariel’s trailing hemisphere.

Previously analyzed NIR spectra were collected during late southern summer and
northern spring (subsolar latitudes 30$\degree$S – 25$\degree$N), when low
latitude regions that are likely rich in CO2 (30$\degree$S – 30$\degree$N)
represented $>$ 50$\%$ of these moons’ observed disks. Consequently, modeling
results that suggest CO2 could be generated/exposed at high latitudes and then
migrate to low latitude cold traps have not been tested using spectra
collected at higher sub-observer latitudes that better sample polar regions.
We present new results on the distribution of CO2 ice on Ariel, the Uranian
moon with the strongest CO2 ice bands, using NIR spectra collected over ‘mid’
sub-observer latitudes (31 – 44$\degree$N) and ‘high’ sub-observer latitudes
(45$\degree$ – 51$\degree$N) latitudes, thereby expanding on the latitudinal
coverage provided by previously collected spectra.

##  2 Data and Methods

###  2.1 Observations and Data Reduction

We analyzed 31 NIR reflectance spectra, ten of which are reported here for the
first time, with the other 21 spectra reported previously (observation details
summarized in Table 1) (Grundy et al., 2003, 2006; Cartwright et al., 2015,
2018, 2020b). Four of the new spectra, and all 21 previously reported spectra,
were collected using the NIR SpeX spectrograph/imager at NASA’s Infrared
Telescope Facility (IRTF), operating in moderate resolution short cross-
dispersed (SXD) mode ($\sim$0.8 – 2.5 $\micron$) (e.g., Rayner et al., 2003).
The other six new spectra were collected using the TripleSpec spectrograph
(0.95 – 2.46 $\micron$) on the Astrophysical Research Consortium (ARC) 3.5-m
telescope at the Apache Point Observatory (Wilson et al., 2004).

When possible, observations were made using slit orientations that matched or
were similar to the parallactic angle to minimize atmospheric dispersion,
which can introduce spectral slope changes to the continua of spectra over
visible and near-infrared wavelengths, in particular at wavelengths $\lesssim$
1.2 $\micron$. Some of the observations of Ariel were made using slit
orientations that deviated notably from the parallactic angle to minimize
scattered light from the bright disk of Uranus. Nevertheless, it is likely
that atmospheric dispersion over the wavelength range of the CO2 ice bands
between 1.5 to 1.7 $\micron$ did not exceed 0.05 and was typically $<$ 0.02
(relative to 2.2 $\micron$). Atmospheric dispersion over the wavelength range
of the CO2 ice bands between 1.9 to 2.1 $\micron$ did not exceed 0.02 and was
typically $<$ 0.01 (relative to 2.2 $\micron$). Consequently, the impact of
atmospheric dispersion on our measurements and modeling of the spectral
continuum proximal to these CO2 ice bands is likely negligible.

IRTF/SpeX observations were made by placing Ariel in two different positions
(‘A’ and ‘B’) separated by 7.5” on a 15” long slit. The resulting exposures
were separated into sequential ‘AB’ pairs. We then subtracted the B exposures
from the A exposures to provide a first order correction for sky emission. The
resulting A-B subtracted pairs were flatfielded to account for variations
across the detector. Flat frames were generated by illuminating SpeX’s
internal integrating sphere with a quartz lamp. Wavelength calibration was
performed using argon emission lines. We used the Spextool data reduction
suite (Cushing et al., 2004), along with custom programs, to calibrate and
extract all Ariel spectra. To remove the solar spectrum, perform additional
atmospheric correction, and remove instrument artifacts, all Ariel spectra
were divided by solar analog star spectra, which were observed on the same
night and close in time and airmass to the observations of Ariel (within $\pm
0.1$ airmass). We observed HD 12124 (G0) and HD 16275 (G5) in 2020. To
increase signal-to-noise (S/N), all star-divided frames from the same night
were co-added, and the resulting uncertainties were calculated using the
standard error ($\sigma$/$\sqrt{n}$) of each co-added pixel. The other 21 SpeX
spectra we analyzed were collected by different teams between 2000 and 2019
(Grundy et al., 2003, 2006; Cartwright et al., 2015, 2018, 2020b).

Spectra collected with ARC 3.5m/TripleSpec were obtained in a similar manner.
The A and B positions are separated by 21” along a 1.1x43” slit. The length of
the TripleSpec slit sometimes allowed for opportunities to place multiple
Uranian moons in the slit simultaneously. In these cases, the moons did not
fall exactly on the nominal A and B positions, but the spectra were reduced
and extracted using the same A - B procedure. Data reduction was performed
using TriplespecTool, a modified version of Spextool (Cushing et al., 2004).
Flat frames were obtained with quartz lamps mounted on the telescope
structure. We subtracted equivalent length, unilluminated exposures from the
lamp flats to remove intrinsic telescope thermal emission. Wavelength
calibration was performed using OH airglow emission lines in our science
frames. For our telluric correction, we observed the solar analog stars HD
16017 (G2V), BD+15 4915 (G2V), HD 19061 (G2V), and HD 224251 (G2V), in 2019
and 2020, and HD 16275 (G5) in 2021, usually within $\pm 0.1$ airmass of our
observations of Ariel.

Table 1: IRTF/SpeX and ARC 3.5-m/TripleSpec observations of Ariel. | Sub-
---
Observer
Long. ($\degree$)
| Sub-
---
Observer
Lat. ($\degree$)
UT Date |  | UT Time
---
(mid-expos)
| Integration
---
Time (min)
| Spectrograph
---
| Slit
---
Width
(”)
| Average
---
Resolving
Power ($\lambda$/$\Delta$$\lambda$)
References
6.8 | 44.8 | 1/18/20 | 4:20 | 76 | TripleSpec | 1.1 | 3500 | This work
15.3 | 27.8 | 9/15/14 | 11:35 | 88 | SpeX | 0.8 | 750 | Cartwright et al. (2018)
26.7 | 48.8 | 1/5/21 | 3:55 | 60 | TripleSpec | 1.1 | 3500 | This work
53.6 | -16.0 | 8/9/03 | 12:15 | 156 | SpeX | 0.3 | 2000 | Grundy et al. (2006)
79.8 | -19.4 | 7/17/02 | 13:25 | 108 | SpeX | 0.5 | 1200 | Grundy et al. (2003)
87.8 | 24.0 | 9/5/13 | 11:10 | 92 | SpeX | 0.8 | 750 | Cartwright et al. (2015)
93.5 | -18.1 | 10/4/03 | 5:45 | 108 | SpeX | 0.3 | 2000 | Grundy et al. (2006)
100.3 | 46.4 | 11/4/19 | 5:10 | 96 | TripleSpec | 1.1 | 3500 | This work
110.1 | 32.0 | 9/11/15 | 13:30 | 44 | SpeX | 0.8 | 750 | Cartwright et al. (2018)
132.2 | 28.5 | 8/24/14 | 14:05 | 40 | SpeX | 0.8 | 750 | Cartwright et al. (2018)
144.8 | 43.2 | 10/12/18 | 9:30 | 73 | SpeX | 0.5 | 1200 | Cartwright et al. (2020b)
152.9 | 51.2 | 10/17/20 | 9:20 | 60 | SpeX | 0.5 | 1200 | This work
159.9 | -11.1 | 7/15/04 | 12:00 | 112 | SpeX | 0.3 | 2000 | Grundy et al. (2006)
200.0 | -15.9 | 8/5/03 | 12:00 | 84 | SpeX | 0.3 | 2000 | Grundy et al. (2006)
205.2 | 50.5 | 11/4/20 | 9:30 | 84 | SpeX | 0.8 | 750 | This work
205.5 | 46.3 | 11/7/19 | 11:20 | 60 | SpeX | 0.5 | 1200 | Cartwright et al. (2020b)
219.8 | -17.2 | 9/7/03 | 9:35 | 90 | SpeX | 0.3 | 2000 | Grundy et al. (2006)
231.8 | 51.1 | 10/20/20 | 11:00 | 60 | SpeX | 0.8 | 750 | This work
233.8 | -23.1 | 7/5/01 | 14:10 | 50 | SpeX | 0.5 | 1200 | Grundy et al. (2003)
235.9 | 47.3 | 10/13/19 | 11:30 | 32 | TripleSpec | 1.1 | 3500 | This work
257.6 | -29.5 | 9/6/00 | 7:35 | 76 | SpeX | 0.8 | 750 | Cartwright et al. (2015)
260.2 | 47.0 | 10/21/19 | 5:05 | 36 | TripleSpec | 1.1 | 3500 | This work
263.7 | 46.8 | 10/26/19 | 6:40 | 32 | TripleSpec | 1.1 | 3500 | This work
268.3 | 39.0 | 10/15/17 | 8:00 | 40 | SpeX | 0.5 | 1200 | Cartwright et al. (2020b)
273.2 | 42.2 | 11/7/18 | 12:00 | 42 | SpeX | 0.5 | 1200 | Cartwright et al. (2020b)
278.3 | 24.8 | 8/7/13 | 13:20 | 44 | SpeX | 0.8 | 750 | Cartwright et al. (2015)
292.8 | 51.2 | 10/18/20 | 8:50 | 84 | SpeX | 0.5 | 1200 | This work
294.8 | 39.5 | 9/30/17 | 9:30 | 120 | SpeX | 0.5 | 1200 | Cartwright et al. (2020b)
294.8 | -19.3 | 7/16/02 | 13:10 | 140 | SpeX | 0.5 | 1200 | Grundy et al. (2003)
304.8 | -23.2 | 7/8/01 | 14:40 | 48 | SpeX | 0.5 | 1200 | Grundy et al. (2003)
316.6 | -18.2 | 10/8/03 | 7:55 | 132 | SpeX | 0.3 | 2000 | Grundy et al. (2006)

###  2.2 Band Parameter Analyses

We analyzed three combination and overtone CO2 ice absorption features
centered near 1.966, 2.012, and 2.070 $\micron$, which we hereafter refer to
as CO2 bands 1, 2, and 3, respectively. To assess the distribution of CO2 ice
on Ariel, we measured the areas of these three CO2 bands in each spectrum,
using a custom program (Cartwright et al., 2015). The program identified the
continua for each CO2 band (example shown in Figure A1), divided each CO2 band
by its continuum, and used the trapezoidal rule to measure the areas of the
resulting continuum-divided bands. To estimate band area errors, the program
used Monte Carlo simulations to resample the 1$\sigma$ uncertainties for the
spectral channels covered by each CO2 band (iterated 20,000 times). After
measuring all three CO2 bands in a spectrum, the program summed them into one
total band area and propagated errors.

###  2.3 Radiative transfer modeling

To complement our CO2 ice band area measurements, and provide additional
context on the spectral signature of CO2 ice on Ariel, we utilized a Hapke-Mie
spectral modeling program to generate synthetic spectra of the grand average
low, mid, and high sub-observer latitude spectra collected over Ariel’s
trailing hemisphere (shown in Figure 1). These synthetic spectra are one-layer
models that provide an estimate of the fractional area of Ariel’s trailing
hemisphere covered by CO2 ice, as well as providing additional context on the
spectral signature of this constituent. In order to focus our modeling efforts
on the spectral signature of CO2 ice, we generated synthetic spectra spanning
1.5 to 1.7 $\micron$ and 1.9 to 2.1 $\micron$.

Mie theory, which is used to model scattering and absorption by randomly
spaced spherical particles, can simulate scattering off particles that are
smaller than or similar in size to the incident wavelength of light (e.g.,
Bohren & Huffman, 1983). Mie scattering theory is useful for simulating
planetary regoliths that include small grains like the Uranian moons’
regoliths, which could include a large number of sub-micron to micron sized
particles (e.g., Afanasiev et al., 2014; Cartwright et al., 2020a). In
contrast, pure Hapke approaches provide less robust models for planetary
regoliths that include grains comparable to, or smaller than, the wavelength
of incident light (e.g., Emery et al., 2006). Our hybrid Hapke-Mie technique
takes the complex indices of refraction (i.e., optical constants) for each
constituent and calculates the single scattering albedo ($\bar{\omega}$0)
using Mie theory. The $\bar{\omega}$0 values, along with the bidirectional
reflectance, opposition effect, and the phase function, are utilized to
calculate the geometric albedo of the synthetic spectrum (e.g., Hapke, 2012).
Small resonances can occur in scattering models that utilize Mie theory. To
account for these resonances, our spectral modeling software incorporates a
range of grain diameters (typically 10$\%$ spread in sizes) that are averaged
together to match the desired grain size of each constituent in the simulated
planetary regolith.

This spectral modeling program was used previously to generate best fit
synthetic spectra for the CO2 ice bands detected in Uranian moon reflectance
spectra collected at low sub-observer latitudes (Cartwright et al., 2015).
These synthetic spectra were generated using optical constants for crystalline
H2O ice (80 K) (Mastrapa et al., 2008), crystalline CO2 ice ($\sim$150 K)
(Hansen, 1997), and amorphous carbon (Rouleau & Martin, 1991), which simulates
the dark, spectrally-neutral absorber that is present on the Uranian moons
(e.g., Clark & Lucey, 1984). Using the same approach as these previous
modeling efforts, we generated particulate mixtures of H2O ice and amorphous
carbon grains to simulate a well mixed regolith dominated by ‘dirty’ H2O ice
where CO2 molecules could be formed via radiolysis. We then linearly combined
these dirty H2O ice models with CO2 ice to simulate concentrated deposits of
pure CO2 ice where migrating CO2 molecules might get cold trapped.

Prior spectral modeling efforts demonstrate that best fit synthetic spectra
for Ariel and the other Uranian moons include H2O ice grains with three
different grain diameters: a small fraction ($<$ 1$\%$) of sub-micron sized
grains (typically 0.2, 0.3, or 0.5 $\micron$ diameters) and larger fractions
of two different moderately sized H2O ice grains, typically with 10 and 50
$\micron$ grain diameters (Cartwright et al., 2015, 2018, 2020a). The
inclusion of multiple H2O ice grain sizes is required to simultaneously fit
the 1.52-$\micron$ and 2.02-$\micron$ H2O ice bands, in particular small
amounts of sub-micron H2O ice grains, which can significantly alter the
relative strengths of these two H2O ice bands in the resulting synthetic
spectrum. The synthetic spectra typically only require one amorphous C grain
diameter (ranging between 5 and 12.5 $\micron$) to simulate the low albedo
absorber that helps obscure shorter wavelength H2O bands and ‘flattens’ the
spectral continuum between 1.4 and 2.4 $\micron$. Of note, prior work has
demonstrated that amorphous pryoxene can also provide a suitable proxy for the
dark, spectrally neutral absorber on the surfaces of the Uranian moons
(Cartwright et al., 2018). Nevertheless, we included amorphous C in the
synthetic spectra present here to better simulate a regolith where CO2 ice is
generated by radiolysis of native H2O ice and carbonaceous species. Prior
spectral modeling work demonstrated that a blend of two different CO2 ice
grain diameters provides a good match to the band shape of the detected CO2
ice bands, with one smaller grain size (1 or 10 $\micron$ diameters) and
another larger grain size (50, 100, or 200 $\micron$ diameters). Additional
information on how grain size can modify synthetic spectra using this Hapke-
Mie modeling program was reported in Appendix B of Cartwright et al. (2015).
Because the CO2 ice bands between 1.5 and 1.7 $\micron$ require thick
substrates to detect and measure in the laboratory ($\sim$2 – 107 mm, Hansen
2005), and because CO2 ice on Ariel could be concentrated in large low
latitude cold traps, it is assumed to be optically thick (i.e., obscuring the
spectral signature of Ariel’s regolith beneath it).

The quality of the fits provided by the synthetic spectra was assessed using
reduced Chi Square statistics: $\chi_{R}^{2}$ = $\frac{1}{a}$ $\sum$ ($O_{i}$
\- $M_{i}$)2/$\sigma_{i}^{2}$, where a is the degrees of freedom, $O_{i}$ is
the observed data, $M_{i}$ is the modeled data, and $\sigma_{i}^{2}$ is the
variance (e.g., Bevington & Robinson, 1969). Typically, $\chi_{R}^{2}$
$\approx$ 1 indicates that a given model is a good match to a set of observed
data, whereas $\chi_{R}^{2}$ $\gg$ 1 indicates that a given model is likely a
poor match to a set of observed data. A $\chi_{R}^{2}$ $\ll$ 1 can also
indicate a good fit to the data and may result from overestimated measurement
errors. Additionally, fitting models to data with large uncertainties, as well
as uncertainty in the degrees of freedom (Andrae et al., 2010), can lower the
accuracy of $\chi_{R}^{2}$ statistics.

All of the synthetic spectra reported here provide non-unique solutions for
estimating the fractional coverage and grain size of the different
constituents on Ariel’s surface. Small adjustments in the grain sizes of
different constituents included in each synthetic spectrum can be accommodated
with complementary adjustments in the fractional coverage of different
constituents (i.e., there is a degeneracy between grain size and fractional
coverage). However, as demonstrated by prior work (Cartwright et al., 2015,
2018, 2020a), synthetic spectra that deviate substantially from the mixing
regimes, constituents, and grain sizes described above do not provide good
fits to the collected spectra of the Uranian moons.

##  3 Results and Analyses

###  3.1 Near-infrared spectra of Ariel

Ten new disk-integrated reflectance spectra of Ariel are shown in Figure A2.
Nine of these spectra display evidence for CO2 bands 1, 2, and 3, including
all seven spectra collected over Ariel’s trailing hemisphere (examples of CO2
bands 1, 2, and 3 are shown in Figure 1). We also observe the weak CO2 ice
combination and overtone features between 1.5 and 1.7 $\micron$, which were
first reported in Grundy et al. 2006, in some of the spectra collected over
Ariel’s trailing hemisphere. Additionally, we observe a subtle band near 2.13
$\micron$ in two of the new spectra, collected at similar sub-observer
latitudes (47.3$\degree$N and 51.1$\degree$N) and longitudes (231.8$\degree$
and 235.9$\degree$) (Figure 4). The detected 2.13-$\micron$ band may result
from a ‘forbidden’ overtone mode of CO2 ice, which is very weak in pure CO2
ice deposits and much stronger in substrates composed of CO2 intimately mixed
with H2O ice or methanol (Bernstein et al., 2005). We also observe a subtle
band centered near 2.35 $\micron$ in some of the spectra collected over
Ariel’s trailing hemisphere (Figure 5). This feature might result from CO
generated as part of a radiolytic production cycle of CO2 ice (e.g., Gerakines
et al., 2001; Zheng & Kaiser, 2007). We discuss the 2.13-$\micron$ and
2.35-$\micron$ absorption features in greater detail in Section 4.1.

Figure 1: Left: Diagram showing the mid-observation, sub-observer longitudes
and latitudes of reflectance spectra collected in the low (30$\degree$S –
30$\degree$N), mid (31$\degree$ – 44$\degree$N), and high (45$\degree$ –
51$\degree$N) sub-observer latitude zones over Ariel’s trailing hemisphere
(gold, green, and blue dots, respectively). Ariel’s Equator (0$\degree$) and
the center of its trailing hemisphere (270$\degree$ longitude) are indicated
(black lines). All collected spectra are disk-integrated and average over an
entire hemisphere. Right: ‘Grand average’ spectra and 1$\sigma$ uncertainties
for Ariel’s low, mid, and high sub-observer latitude zones (gold, green, and
blue, error bars respectively). The spectra were normalized to 1 using the
mean reflectance between 1.74 and 1.77 $\micron$ and then offset vertically
for clarity. The sub-observer latitude range for the individual spectra, and
the years they were collected in, are listed above each of the grand average
spectra (see Table 1 for observation details). The wavelength ranges of CO2
bands 1,2, and 3 are indicated (black dashed lines). The spectra were lightly
smoothed using a 2 to 4 pixel wide boxcar function.

All ten new spectra display the 1.52-$\micron$ and 2.02-$\micron$ H2O ice
bands and the 1.65-$\micron$ crystalline H2O ice feature detected previously
on Ariel (e.g., Grundy et al., 2006; Cartwright et al., 2018). We also observe
an absorption band centered near 2.2-$\micron$ in several of the new Ariel
spectra, which has been detected previously and attributed to NH3 and
NH4-bearing species (Cartwright et al., 2018, 2020b; Cook et al., 2018).
Analysis of these H2O ice bands and possible NH3 and NH4-bearing constituents
is beyond the scope of this paper and will be included in future work.

###  3.2 CO2 ice band parameter analyses

We measured the areas of CO2 bands 1, 2, and 3 in the ten new Ariel spectra,
as well as eight previously reported Ariel spectra (Cartwright et al., 2018,
2020b) that do not have published CO2 band area measurements. We report these
18 sets of CO2 band areas and their 1$\sigma$ uncertainties in Table 2, along
with CO2 ice band areas for another 13 Ariel spectra that were measured
previously (Grundy et al., 2003, 2006; Cartwright et al., 2015). All 31 of the
Ariel spectra were analyzed using the same CO2 ice band measurement procedure,
originally presented in Cartwright et al. (2015). To investigate possible
latitudinal trends in the distribution of CO2 ice, we calculated mean CO2 ice
band areas using spectra collect at low (30$\degree$S – 30$\degree$N), mid (31
– 44$\degree$N), and high (45$\degree$ – 51$\degree$N) sub-observer latitudes
(final column in Table 2).

Table 2: CO2 ice band areas.

| Sub-Observer
---
Long. ($\degree$)
| Sub-Observer
---
Lat. ($\degree$)
| CO2 Band 1
---
(10-4$\micron$)
| CO2 Band 2
---
(10-4$\micron$)
| CO2 Band 3
---
(10-4$\micron$)
|  †CO2 Total
---
Area (10-4$\micron$)
|  ††Mean CO2 Total
---
Areas (10-4$\micron$)
Leading Hemisphere |  |  |  |  |  |
6.8 | 44.8 | 3.59 $\pm$ 0.85 | 0.62 $\pm$ 1.21 | 1.25 $\pm$ 0.40 | 5.45 $\pm$ 1.53 | High Sub-Observer
15.3 | 27.8 | 2.76 $\pm$ 0.34 | 3.91 $\pm$ 0.38 | 0.11 $\pm$ 0.22 | 7.77 $\pm$ 0.55 | Latitudes ($\geq$ 45$\degree$):
26.7 | 48.8 | 3.58 $\pm$ 0.43 | 0.42 $\pm$ 0.60 | 1.05 $\pm$ 0.23 | 5.05 $\pm$ 0.78 | 3.68 $\pm$ 1.63
*53.6 | -16.0 | 1.36 $\pm$ 0.64 | 1.42 $\pm$ 1.19 | 0.23 $\pm$ 0.59 | 3.02 $\pm$ 1.47 |
*79.8 | -19.4 | 1.34 $\pm$ 0.44 | -1.23 $\pm$ 0.56 | 0.81 $\pm$ 0.26 | 0.92 $\pm$ 0.76 | Mid Sub-Observer
*87.8 | 24.0 | 1.26 $\pm$ 0.31 | 0.43 $\pm$ 0.44 | 0.49 $\pm$ 0.23 | 2.17 $\pm$ 0.59 | Latitudes (31 – 44$\degree$):
*93.5 | -18.1 | 0.87 $\pm$ 0.56 | 1.11 $\pm$ 0.72 | -0.24 $\pm$ 0.51 | 1.74 $\pm$ 1.05 | 3.12 $\pm$ 1.30
100.3 | 46.4 | 1.10 $\pm$ 0.39 | 1.76 $\pm$ 0.43 | 0.42 $\pm$ 0.17 | 3.27 $\pm$ 0.60 |
110.1 | 32.0 | 1.00 $\pm$ 0.40 | 2.81 $\pm$ 0.55 | 0.41 $\pm$ 0.26 | 4.22 $\pm$ 0.73 | Low Sub-Observer
132.2 | 28.5 | 1.45 $\pm$ 0.40 | 3.10 $\pm$ 0.61 | 0.32 $\pm$ 0.28 | 4.88 $\pm$ 0.78 | Latitudes (-30 – 30$\degree$):
144.8 | 43.2 | 0.42 $\pm$ 0.53 | 1.57 $\pm$ 0.66 | 0.04 $\pm$ 0.39 | 2.03 $\pm$ 0.93 | 3.83 $\pm$ 1.03
152.9 | 51.2 | -0.12 $\pm$ 0.77 | 0.39 $\pm$ 0.97 | 0.67 $\pm$ 0.47 | 0.94 $\pm$ 1.32 |
*159.9 | -11.1 | 2.30 $\pm$ 0.53 | 2.91 $\pm$ 0.78 | 1.08 $\pm$ 0.45 | 6.29 $\pm$ 1.04 |
Trailing Hemisphere |  |  |  |  |  |
*200.0 | -15.9 | 4.19 $\pm$ 0.67 | 5.94 $\pm$ 0.80 | 2.89 $\pm$ 0.45 | 13.02 $\pm$ 1.14 | High Sub-Observer
205.2 | 50.5 | 3.24 $\pm$ 0.29 | 3.10 $\pm$ 0.61 | 1.42 $\pm$ 0.16 | 7.76 $\pm$ 0.49 | Latitudes ($\geq$ 45$\degree$):
205.5 | 46.1 | 3.72 $\pm$ 0.29 | 4.22 $\pm$ 0.46 | 1.57 $\pm$ 0.18 | 9.51 $\pm$ 0.58 | 10.06$\pm$ 0.86
*219.8 | -17.2 | 5.77 $\pm$ 0.29 | 6.71 $\pm$ 0.68 | 2.52 $\pm$ 0.29 | 14.99 $\pm$ 0.80 |
231.8 | 51.1 | 3.54 $\pm$ 0.35 | 5.10 $\pm$ 0.39 | 2.21 $\pm$ 0.23 | 10.85 $\pm$ 0.57 | Mid Sub-Observer
*233.8 | -23.1 | 4.09 $\pm$ 0.78 | 6.31 $\pm$ 0.85 | 2.99 $\pm$ 0.45 | 13.40 $\pm$ 1.24 | Latitudes (31 – 44$\degree$):
235.9 | 47.3 | 6.25 $\pm$ 0.67 | 3.66 $\pm$ 0.80 | 2.51 $\pm$ 0.26 | 12.42 $\pm$ 1.04 | 12.52$\pm$ 1.24
*257.6 | -29.5 | 3.33 $\pm$ 0.50 | 6.47 $\pm$ 0.49 | 3.36 $\pm$ 0.27 | 13.16 $\pm$ 0.75 |
260.2 | 47.0 | 5.54 $\pm$ 0.60 | 4.17 $\pm$ 0.79 | 1.90 $\pm$ 0.30 | 11.61 $\pm$ 1.04 | Low Sub-Observer
263.7 | 46.8 | 4.15 $\pm$ 0.27 | 4.06 $\pm$ 0.33 | 2.16 $\pm$ 0.14 | 10.37 $\pm$ 0.45 | Latitudes (-30 – 30$\degree$):
268.3 | 39.0 | 3.63 $\pm$ 0.74 | 5.83 $\pm$ 0.80 | 3.07 $\pm$ 0.31 | 12.52 $\pm$ 1.14 | 14.15 $\pm$ 1.25
273.2 | 42.2 | 2.04 $\pm$ 1.37 | 5.49 $\pm$ 1.40 | 3.21 $\pm$ 0.66 | 10.74 $\pm$ 2.06 |
*278.3 | 24.8 | 6.06 $\pm$ 0.83 | 6.48 $\pm$ 1.02 | 2.83 $\pm$ 0.60 | 15.37 $\pm$ 1.45 |
292.8 | 51.2 | 2.68 $\pm$ 0.41 | 3.64 $\pm$ 0.57 | 1.85 $\pm$ 0.29 | 8.16 $\pm$ 0.76 |
294.8 | 39.5 | 4.49 $\pm$ 0.36 | 6.91 $\pm$ 0.56 | 2.90 $\pm$ 0.25 | 14.30 $\pm$ 0.71 |
*294.8 | -19.3 | 6.26 $\pm$ 0.85 | 5.56 $\pm$ 0.47 | 3.08 $\pm$ 0.25 | 14.90 $\pm$ 0.66 |
*304.8 | -23.2 | 5.12 $\pm$ 0.76 | 6.29 $\pm$ 0.96 | 3.35 $\pm$ 0.61 | 14.76 $\pm$ 1.36 |
*316.6 | -18.2 | 4.52 $\pm$ 0.58 | 6.72 $\pm$ 0.82 | 2.32 $\pm$ 0.42 | 13.56 $\pm$ 1.11 |

†Values shown graphically in Figure 2a.
††Values shown graphically in Figure 2b.
*Reported previously in (Grundy et al., 2003, 2006; Cartwright et al., 2015).

Figure 2: (a) CO2 ice total band areas and 1$\sigma$ uncertainties for Ariel’s
low (30$\degree$S – 30$\degree$N), mid (31$\degree$ – 44$\degree$N), and high
(45$\degree$ – 51$\degree$N) sub-observer latitude zones (gold, green, and
blue dots, respectively). Numerical values for these data points are shown in
the ‘CO2 Total Area’ column in Table 2. A sinusoidal fit to the low sub-
observer latitude data points (gold dashed line) highlights the clear
longitudinal trend in the distribution of CO2 ice on Ariel in this latitude
zone. Similarly, the sinusoidal fit to the mid and high sub-observer latitude
data points (black dashed line) highlights the reduction in CO2 ice band areas
on Ariel’s trailing hemisphere at sub-observer latitudes $\geq$ 30$\degree$N.
Repeat longitudes (gray-toned zones) are shown to highlight the periodicity in
the distribution of CO2 on Ariel. (b) Mean CO2 ice total band areas and
1$\sigma$ uncertainties (vertical error bars) for the low, mid, and high sub-
observer latitude zones (gold, green, and blue symbols, respectively) over
Ariel’s leading (diamonds) and trailing (circles) hemispheres. The horizontal
error bars represent the range of latitudes we use to define each sub-observer
latitude zone. Numerical values for these data points are shown in the ‘Mean
CO2 Total Areas’ column in Table 2. (c) Mean CO2 band 1, 2, and 3 areas and
1$\sigma$ uncertainties for the low, mid, and high sub-observer latitude zones
(gold, green, and blue dots, respectively) on Ariel’s trailing hemisphere.

###  3.3 Distribution of CO2 ice on Ariel

The 13 Ariel spectra analyzed previously demonstrate that CO2 bands are much
stronger in the spectra collected over Ariel’s trailing hemisphere compared to
its leading hemisphere (Grundy et al., 2006; Cartwright et al., 2015).
Utilizing the same measurement procedure presented in Cartwright et al. (2015)
for the analysis of those 13 previously analyzed Ariel spectra, we assessed
the distribution of CO2 ice in the 18 previously unanalyzed Ariel spectra
(Figure 2a). The measurements show similar trends in the longitudinal
distribution of CO2 ice in the spectra collected over low, mid, and high sub-
observer latitudes. However, the previously determined leading/trailing
hemispherical dichotomy in the distribution of CO2 ice appears to be weaker
for the spectra collected at mid and high sub-observer latitudes compared to
low sub-observer latitudes (Table 2, Figure 2a).

We then calculated the percentage of Ariel’s observed disk that was composed
of low latitude regions (30$\degree$S $-$ 30$\degree$N) for all of the spectra
collected over its trailing hemisphere (see Holler et al. 2016 for disk-area
calculation details). We found that for spectra where the disk of Ariel was
composed of $>$ 55$\%$ low latitude regions, the total band areas for detected
CO2 ice features ranges from 13.02 $\pm$ 1.14 $\cdot$ 10-4$\micron$ to 15.37
$\pm$ 1.45 $\cdot$ 10-4$\micron$. When Ariel’s disk was composed of 45 –
55$\%$ low latitude regions, the measured CO2 band areas decreased to 10.74
$\pm$ 2.06 $\cdot$ 10-4$\micron$ to 14.30 $\pm$ 0.71 $\cdot$ 10-4$\micron$.
When low latitudes regions composed $<$ 45$\%$ of Ariel’s observed disk, CO2
band areas decreased further to 7.76 $\pm$ 0.49 $\cdot$ 10-4$\micron$ to 12.42
$\pm$ 1.04 $\cdot$ 10-4$\micron$. Thus, CO2 ice band areas are gradually
decreasing as the sub-observer latitude moves further north and away from
Ariel’s low latitudes.

To further investigate latitudinal trends in the distribution of CO2 ice, we
compared the mean CO2 ice total band areas of each sub-observer latitude zone
(final column of Table 2, Figure 2b). This comparison shows that the low sub-
observer latitude zone has the largest mean CO2 total band area. The mid sub-
observer latitude zone has a smaller mean CO2 total band area, and the high
sub-observer latitude zone has the smallest mean CO2 total band area ($>$
1$\sigma$ difference between CO2 band areas for the low and high sub-observer
latitude zones). On Ariel’s leading hemisphere, these mean measurements show
negligible differences between the three sub-observer latitude zones ($<$
1$\sigma$ difference).

We also calculated the mean areas for each of the three CO2 bands (Figure 2c).
CO2 band 1 shows $<$ 1$\sigma$ difference between the different sub-observer
latitude zones, whereas CO2 bands 2 and 3 show $>$ 2$\sigma$ differences
between the high and low sub-observer latitude zones ($>$ 1$\sigma$ difference
between the high and mid sub-observer latitude zones). Of note, CO2 band 1 is
located on the long wavelength end of a strong telluric band resulting from
H2O vapor, possibly explaining the inconclusive results for this band compared
to CO2 bands 2 and 3, which are less contaminated by species in Earth’s
atmosphere. Thus, our measurements demonstrate that CO2 ice bands are stronger
in spectra collected at low and mid sub-observer latitudes compared to spectra
collected at high sub-observer latitudes over Ariel’s trailing hemisphere.
Spectra collected over Ariel’s leading hemisphere do not display discernible
differences in CO2 band areas at different sub-observer latitudes.

###  3.4 Radiative transfer modeling of CO2 ice bands

Figure 3: Left: Comparison between the grand average Ariel spectra for low,
mid, and high latitude zones (1$\sigma$ uncertainties indicated by gold,
green, and blue error bars, respectively) and synthetic spectra (numbered 1 –
7) that model the CO2 ice combination and overtone bands centered near 1.543,
1.578, and 1.609 $\micron$ (CO2 bands bracketed by black dashed lines). Right:
Comparison between the grand average Ariel spectra for low, mid, and high
latitude zones (orange, green, and blue error bars, respectively) and
synthetic spectra (numbered 8 – 10) that model the CO2 ice combination and
overtone bands centered near 1.966, 2.012, and 2.070 $\micron$ (CO2 bands
bracketed by black dashed lines). The low latitude synthetic spectrum (10) was
originally reported in Cartwright et al. (2015). The included constituents for
each synthetic spectrum are summarized in Tables 3 and 4. Spectra and models
in both plots are scaled to Ariel’s geometric albedo (0.564) at $\sim$0.958
$\micron$ (Karkoschka, 2001) and offset vertically for clarity. The grand
average spectra were lightly smoothed using a 2 to 4 pixel wide boxcar
function. Table 3: Model parameters for synthetic spectra: CO2 ice bands
between 1.5 and 1.7 $\micron$.

| Particulate Mixture Components | Areal Mixture Components
---|---|---
|  †Synthetic
---
Spectrum
| CO2 Ice
---
Band
| H2O ice:
---
Grain Size,
Fractional
Coverage
| H2O ice:
---
Grain Size,
Fractional
Coverage
| H2O ice:
---
Grain Size,
Fractional
Coverage
| Amorphous C:
---
Grain Size,
Fractional
Coverage
| CO2 ice:
---
Grain Size,
Fractional
Coverage
| CO2 ice:
---
Grain Size,
Fractional
Coverage
High Sub-Obs. Lat. |  |  |  |  |  |  |
1 | 1.58 $\micron$ | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 50 $\micron$, | 10 $\micron$,
|  | 26.8$\%$ | 57.0$\%$ | 0.3$\%$ | 0.9$\%$ | 7.5$\%$ | 7.5$\%$
2 | 1.61 $\micron$ | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 100 $\micron$, | 50 $\micron$,
|  | 26.5$\%$ | 56.3$\%$ | 0.3$\%$ | 0.9$\%$ | 8.8$\%$ | 7.2$\%$
Mid Sub-Obs. Lat. |  |  |  |  |  |  |
3 | 1.58 $\micron$ | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 50 $\micron$, | 10 $\micron$,
|  | 55.1$\%$ | 20.8$\%$ | 0.3$\%$ | 0.8$\%$ | 13.8$\%$ | 9.2$\%$
4 | 1.61 $\micron$ | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 200 $\micron$, | 50 $\micron$,
|  | 54.3$\%$ | 20.6$\%$ | 0.3$\%$ | 0.8$\%$ | 21.6$\%$ | 2.4$\%$
Low Sub-Obs. Lat. |  |  |  |  |  |  |
5 | 1.54 $\micron$ | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 50 $\micron$, | 10 $\micron$,
|  | 51.6$\%$ | 11.9$\%$ | 0.3$\%$ | 0.7$\%$ | 8.9$\%$ | 26.6$\%$
6 | 1.58 $\micron$ | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 200 $\micron$, | 50 $\micron$,
|  | 50.8$\%$ | 11.7$\%$ | 0.3$\%$ | 0.7$\%$ | 7.3$\%$ | 29.2$\%$
7 | 1.61 $\micron$ | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 100 $\micron$, | 10 $\micron$,
|  | 49.2$\%$ | 11.4$\%$ | 0.2$\%$ | 0.7$\%$ | 21.2$\%$ | 17.3$\%$

†Synthetic spectra shown graphically in Figure 3.

In Figure 3, we present ten spectral models that fit the CO2 ice bands
detected on Ariel’s trailing hemisphere between 1.5 and 1.7 $\micron$
(synthetic spectra 1 – 7) and 1.9 and 2.1 $\micron$ (synthetic spectra 8 –
10). The fractional coverage and grain sizes of all included components are
summarized in Table 3 (CO2 ice bands centered near 1.543, 1.578, and 1.609
$\micron$) and Table 4 (CO2 bands 1, 2, and 3). The previously reported
spectral models for CO2 bands 1, 2, and 3 in Ariel’s low sub-observer
latitude, grand average spectrum (Cartwright et al., 2015) are included here
(synthetic spectrum 10) along with new models of the mid and high sub-observer
latitude, grand average spectra (synthetic spectra 9 and 10, respectively).
The low sub-observer latitude synthetic spectrum includes 27$\%$ CO2 ice, the
mid sub-observer latitude model includes 23$\%$ CO2 ice, and the high sub-
observer latitude model includes 16$\%$ CO2 ice (model components are
summarized in Table 4).

The low sub-observer latitude, grand average spectrum shows the strongest
evidence for the 1.543-$\micron$, 1.578-$\micron$, and 1.609-$\micron$ CO2
bands, which we fit with three different models (synthetic spectra 5 – 7)
spanning 1.5 to 1.7 $\micron$. The amount of CO2 ice included in synthetic
spectra 5 – 7 ranges between 35.5$\%$ to 38.5$\%$, which is substantially
higher than the amount of CO2 included in synthetic spectrum 10 (27$\%$),
spanning 1.9 to 2.1 $\micron$. Furthermore, synthetic spectra 5 – 7 include
greater amounts of larger CO2 ice grains, including grains with diameters of
100 and 200 $\micron$, which are not included in any of the models spanning
1.9 to 2.1 $\micron$ (synthetic spectra 8 – 10).

Table 4: Model parameters for synthetic spectra: CO2 bands 1, 2, and 3.

| Particulate Mixture Components | Areal Mixture Components
---|---|---
|  †Synthetic
---
Spectrum
| H2O ice:
---
Grain Size,
Fractional
Coverage
| H2O ice:
---
Grain Size,
Fractional
Coverage
| H2O ice:
---
Grain Size,
Fractional
Coverage
| Amorphous C:
---
Grain Size,
Fractional
Coverage
| CO2 ice:
---
Grain Size,
Fractional
Coverage
| CO2 ice:
---
Grain Size,
Fractional
Coverage
High Sub-Obs. Lat. |  |  |  |  |  |
8 | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 50 $\micron$, | 1 $\micron$,
| 25.2$\%$ | 56.3$\%$ | 0.7$\%$ | 1.8$\%$ | 9.6$\%$ | 6.4$\%$
Mid Sub-Obs. Lat. |  |  |  |  |  |
9 | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 50 $\micron$, | 1 $\micron$,
| 54.6$\%$ | 21.1$\%$ | 0.6$\%$ | 1.7$\%$ | 7.7$\%$ | 14.3$\%$
Low Sub-Obs. Lat. |  |  |  |  |  |
*10 | 50 $\micron$, | 10 $\micron$, | 0.3 $\micron$, | 12.5 $\micron$, | 50 $\micron$, | 10 $\micron$,
| 36.5$\%$ | 34.3$\%$ | 0.6$\%$ | 1.6$\%$ | 22.5$\%$ | 4.5$\%$

†Synthetic spectra shown graphically in Figure 3.
*Previously reported in (Cartwright et al., 2015).

Table 5: $\chi_{R}^{2}$ Statistics for synthetic spectra.

|  | 1.543-$\micron$
---
CO2 Ice
Band
| 1.578-$\micron$
---
CO2 Ice
Band
| 1.609-$\micron$
---
CO2 Ice
Band
| Continuum
---
| 1.966-$\micron$
---
CO2 Ice
Band
| 2.012-$\micron$
---
CO2 Ice
Band
| 2.070-$\micron$
---
CO2 Ice
Band
| Continuum
---
Wavelength | 1.540 – | 1.574 – | 1.606 – | 1.5 – 1.7 | 1.962 – | 2.008 – | 2.068 – | 1.9 – 2.1
Range ($\micron$) | 1.545 | 1.581 | 1.613 |  | 1.969 | 2.015 | 2.072 |
Synthetic |  |  |  |  |  |  |  |
Spectra |  |  |  |  |  |  |  |
1 | - | 0.789 | 0.993 | 0.029 | - | - | - | -
2 | - | 0.849 | 1.004 | 0.029 | - | - | - | -
3 | - | 0.894 | 0.876 | 0.163 | - | - | - |
4 | - | 1.600 | 0.847 | 0.182 | - | - | - |
5 | 0.958 | 0.709 | 1.365 | 0.065 | - | - | - |
6 | 1.117 | 0.836 | 1.512 | 0.064 | - | - | - |
7 | 6.927 | 0.717 | 0.386 | 0.111 | - | - | - |
8 | - | - | - | - | 0.227 | 0.225 | 1.028 | 0.126
9 | - | - | - | - | 0.434 | 0.677 | 0.567 | 0.348
10 | - | - | - | - | 0.366 | 1.286 | 0.667 | 0.688

Bolded values highlight the primary CO2 ice band fit by each of the synthetic
spectra spanning 1.5 to 1.7 $\micron$.

In contrast, Ariel’s grand average mid and high sub-observer latitude, grand
average spectra display weaker 1.578-$\micron$ and 1.609-$\micron$ CO2 bands
compared to the low sub-observer latitude spectrum, and the weak
1.543-$\micron$ band is not visibly identifiable in either spectrum (Figure
3). The spectral models for Ariel’s mid sub-observer latitude spectrum that we
present here (synthetic spectra 3 and 4) include 23 and 24$\%$ CO2 to fit the
1.578-$\micron$ and 1.609-$\micron$ bands, respectively. These two synthetic
spectra include greater amounts of larger grains (10, 50, and 200 $\micron$
diameters) compared to synthetic spectrum 9 that spans 1.9 – 2.1 $\micron$ of
the mid sub-observer latitude, grand average spectrum (1 and 50 $\micron$
diameters). Similarly, the spectral models for Ariel’s high sub-observer
latitude, grand average spectrum (synthetic spectra 1 and 2) include 15 and
16$\%$ CO2 to fit the 1.578-$\micron$ and 1.609-$\micron$ bands, respectively.
Synthetic spectra 1 and 2 also include greater amounts of larger grains (10,
50, and 100 $\micron$ diameters) compared to synthetic spectrum 8 spanning 1.9
to 2.1 $\micron$ (1 and 50 $\micron$ diameters). Therefore, all of the
synthetic spectra spanning 1.5 to 1.7 $\micron$ include more CO2 ice, and
larger CO2 ice grains, than the corresponding synthetic spectra that span 1.9
to 2.1 $\micron$.

Along with visual assessment, we utilized $\chi_{R}^{2}$ tests to assess the
quality of the fits provided by the synthetic spectra. The $\chi_{R}^{2}$
statistics for the 1.543-$\micron$ CO2 ice band (synthetic spectra 5 – 7) and
the 1.578-$\micron$ and 1.609-$\micron$ CO2 bands (synthetic spectra 1 – 7)
demonstrate that the generated models are suitable fits to the observed
spectra (Table 5). Similarly, the $\chi_{R}^{2}$ values for CO2 ice bands 1,
2, and 3 (synthetic spectra 8 – 10) demonstrate that the generated models are
suitable fits to the observed spectra. The $\chi_{R}^{2}$ values for the
synthetic spectra spanning the 1.5 to 1.7 $\micron$ and 1.9 to 2.1 $\micron$
continua also appear to provide reasonable fits, albeit some of the
$\chi_{R}^{2}$ values for the synthetic spectra spanning 1.5 to 1.7 $\micron$
are quite low ($<$ 0.1) and may result from overestimated errors. Comparison
of the $\chi_{R}^{2}$ statistics for synthetic spectra 1 – 7 suggests that the
small differences in the grain size ranges and the amount of CO2 ice included
in each of these spectral models are not sufficient to cause significant
variations in their goodness-of-fit for each of the CO2 ice bands between 1.5
and 1.7 $\micron$ (Table 5). More sophisticated and computationally intensive
model assessment techniques may be necessary to better delineate between the
fits provided by synthetic spectra 1 – 7. Nevertheless, the small differences
between each of these synthetic spectra may reflect subtle spectral variations
resulting from differences in photon penetration depths as a function of
wavelength (see Section 4.2).

In summary, the synthetic spectra we generated for CO2 ice bands between 1.5
and 1.7 $\micron$ and 1.9 and 2.1 $\micron$ suggest that the fractional
coverage of CO2 ice is highest at low sub-observer latitudes, decreases at mid
sub-observer latitudes, and decreases further at high sub-observer latitudes.
These spectral modeling results are consistent with our CO2 band area
measurements, indicating that CO2 ice is likely concentrated at low latitudes
on Ariel.

##  4 Discussion

###  4.1 Nature and distribution of CO2 ice on Ariel

Thermodynamical models predict that CO2 molecules generated and/or exposed at
polar latitudes on the surfaces of the Uranian moons should readily sublimate
during summer when the pole is continuously exposed to direct sunlight (Grundy
et al., 2006; Sori et al., 2017). In the high-obliquity Uranian system,
equatorial regions experience less net sublimation than polar regions over the
course of a Uranian year. Consequently, CO2 should migrate over geologically
short timescales from these moons’ poles to their equators, thereby enriching
their low latitudes with CO2 ice. If CO2 molecules are distributed uniformly
across the surfaces of the Uranian moons, then CO2 would migrate to cold traps
at latitudes between 50$\degree$S and 50$\degree$N, with the removal of CO2
increasing steadily with latitude, peaking near their poles (see Fig. 9 in
Sori et al. 2017). Conversely, if CO2 is already depleted from latitudes $>$
50$\degree$ S or N, then the zone of CO2 accumulation may be smaller, spanning
only 30$\degree$S to 30$\degree$N, with peak CO2 removal concentrated near
45$\degree$S and 45$\degree$N. In both model scenarios, CO2 molecules get cold
trapped at similar longitudes to where they were initially generated and/or
exposed. Thus, the apparent concentration of CO2 ice on the trailing
hemispheres of Ariel and the other Uranian moons suggests that CO2 molecules
originate on their trailing sides (Sori et al., 2017).

Our band area measurements and synthetic spectra are consistent with a
reduction in the abundance of CO2 ice at higher latitudes on Ariel. However,
the disk-integrated nature of the data analyzed here limits our ability to
develop more detailed maps of the latitudinal distribution of CO2 ice. Our
analyses are therefore unable to discern whether the abundance of CO2 ice is
steadily decreasing with increasing sub-observer latitude, or whether CO2 ice
is mostly concentrated in one or perhaps a few large cold traps at low
latitudes and essentially absent at latitudes $\geq$ 30$\degree$N.

Other planetary bodies display large and bright volatile-rich regions, such as
the H2O and CO2 ice dominated Martian poles (e.g., Farmer et al., 1976;
Kieffer, 1979; Kieffer et al., 2000) and the N2 ice and CO ice dominated
composition of Sputnik Planita at low northern latitudes on Pluto (e.g.,
Bertrand & Forget, 2016; Hamilton et al., 2016; Earle et al., 2017).
Furthermore, the available photogeologic evidence (e.g., Smith et al., 1986)
and photometric measurements made with the Voyager 2 Imaging Science System
(e.g., Buratti & Mosher, 1991) indicate that a large and bright annulus of
material is mantling the floor of Wunda crater on Umbriel’s trailing
hemisphere, which has been interpreted to be a cold trap for CO2 ice (Sori et
al., 2017). Ariel, on the other hand, does not appear to display large and
bright regions on its trailing hemisphere that might serve as volatile sinks
for CO2 ice. Nevertheless, large CO2 ice cold traps could be present at low
northern latitudes, which were shrouded by winter darkness during the Voyager
2 flyby and were not imaged. Continued observations of Ariel as the Uranian
system migrates into northern summer in February 2030, reaching a maximum sub-
solar latitude of $\sim$82$\degree$N, could be used to help discern between
the two different modeling predictions for the latitudinal distribution of CO2
ice presented in Sori et al. (2017).

Radiolytic production of CO2 molecules? Charged particle bombardment of H2O
ice grains mixed with C-rich material in Ariel’s regolith could drive
radiolytic production of oxidized carbon-based species such as CO2.
Radiolytically generated CO2 molecules should not be thermodynamically stable
in Ariel’s dark regolith, in particular at high latitudes where the timescales
of maximum solar heating are considerably longer than at low latitudes (Grundy
et al., 2006; Sori et al., 2017). Consequently, CO2 molecules radiolytically
generated in Ariel’s polar regions should diffuse out of its regolith, hop
along its surface, and get cold trapped in its equatorial region over seasonal
timescales. Assuming the source(s) of CO2 molecules operate over long
timescales, then low latitude cold traps should gradually expand as they
accumulate more CO2, forming an optically thick layer of CO2 ice, with a
crystalline structure dominated by CO2 molecules (i.e., a pure CO2 ice
deposit). The detection of the weak CO2 ice band between 1.5 and 1.7
$\micron$, which are a factor of 60 to 200 weaker than CO2 ice bands 1, 2, and
3 (Hansen, 1997), supports the interpretation that thick deposits of CO2 ice
are present on Ariel. Once trapped in CO2 ice deposits, CO2 molecules should
be fairly stable and resistant to sublimation due to the deposit’s higher
albedo and thermal inertia compared to the surrounding regolith (Sori et al.,
2017).

Along with producing new CO2 molecules, interactions with energetic electrons
(e-) and protons (p+) should decompose CO2 molecules, forming CO (e.g.,
Gerakines et al., 2001; Zheng & Kaiser, 2007):

e- \+ CO2 $\rightarrow$ CO + O

p+ \+ CO2 $\rightarrow$ CO + O,

CO and O are likely transient and should back-react with surrounding species
to form new radiolytic products. As a result, CO and O fragments generated by
irradiation of pure CO2 ice deposits should primarily recombine into CO2,
possibly enhancing the persistence of CO2 ice deposits on the surface of Ariel
and the other Uranian moons. Supporting this scenario, some spectra collected
over Ariel’s trailing hemisphere exhibit a subtle band centered near 2.35
$\micron$ that could result from CO (e.g, Gerakines et al., 2001; Zheng &
Kaiser, 2007), in particular spectra collected over low sub-observer latitudes
where CO2 ice bands are strongest (Figure 4). This 2.35-$\micron$ band does
not appear to be present in spectra collected over Ariel’s leading hemisphere,
where CO2 ice is essentially absent. CO and O fragments are volatile at
Ariel’s estimated peak surface temperature (84 $\pm$ 1 K, Hanel et al. 1986)
and should be rapidly removed if exposed on Ariel’s surface. Fragments of CO2
molecules dissociated at depth within CO2 ice deposits, however, might be
retained long enough for some of the newly formed CO to back-react and re-form
CO2. Additionally, perhaps some CO and O fragments get trapped in void spaces
and defects within the crystalline structure of Ariel’s CO2 deposits instead
of back-reacting or escaping. In this scenario, O fragments would likely
combine into more stable O2 molecules before getting trapped. A possibly
analogous process is suspected to occur on the icy Galilean moons, which
display a subtle 0.577-$\micron$ absorption band attributed to trapped O2
formed by radiolysis of H2O ice on their surfaces, which is subsequently
trapped in defects and voids in the crystalline structure of H2O ice (e.g.,
Spencer et al., 1995; Spencer & Calvin, 2002).

Figure 4: Grand average low latitude spectra and 1$\sigma$ uncertainties
(gold) of Ariel’s trailing (top) and leading (bottom) hemisphere, highlighting
the position of a subtle 2.35 $\micron$ band (black dotted line) that is
observed on Ariel’s trailing hemisphere but is absent from its leading
hemisphere. Both spectra were normalized to 1 using the mean reflectance
between 1.74 and 1.77 $\micron$ and offset vertically for clarity. These data
were lightly smoothed using a 5-pixel wide boxcar function.

Regolith mixed CO2 ice on Ariel? The spectral signature of CO2 on the Uranian
moons is consistent with crystalline CO2 ice, segregated from other
constituents in concentrated deposits. However, if CO2 molecules are being
actively generated by charged particle bombardment of H2O ice grains and
C-rich material in Ariel’s regolith, then the spectral signature of these
radiolytic production sites might be consistent with CO2 well mixed with H2O
and other surface constituents. Laboratory experiments that investigated the
spectral signature of CO2 ‘intimately’ mixed with H2O ice and methanol ice
(CH3OH) detected a broad band centered near 2.134 $\micron$ (Bernstein et al.,
2005), which is a factor of $\sim$104 weaker in pure CO2 ice isolated in Ar
(Sandford et al., 1991) or N2 (Quirico & Schmitt, 1997) matrices and is absent
from pure H2O ice. This band could result from the 2$\nu$3 ‘forbidden’ first
overtone of the asymmetric stretching mode of CO2 ice, centered near 2.134
$\micron$ (Bernstein et al., 2005), and could be a useful indicator of the
presence of well mixed CO2 ice grains.

Two spectra collected at similar mid-observation, sub-observer longitudes and
latitudes on Ariel’s trailing hemisphere exhibit subtle absorption features
centered near 2.129 and 2.130 $\micron$ (Figure 5), hereafter referred to as
the ‘2.13-$\micron$’ feature. These two spectra were collected by different
teams using different telescopes (IRTF and ARC 3.5m), lending additional
confidence to the detection of this 2.13-$\micron$ feature. The 2.13-$\micron$
bands we detected span 2.123 to 2.137 $\micron$ (0.014 $\micron$ wide) along
the long wavelength shoulder of the 2.02-$\micron$ H2O ice band. The CO2
2$\nu$3 band presented in Fig. 3 of Bernstein et al. (2005) is centered
between $\sim$2.133 to 2.138 and is $\sim$0.016 to 0.041 $\micron$ wide, with
the band becoming narrower and shifting to shorter wavelengths as the amount
of CO2 ice relative to H2O ice is decreased. The closest match to the
2.13-$\micron$ band we detected on Ariel is a laboratory spectrum of a
H2O:CO2:CH3OH substrate (100:2.5:1 abundance ratio), displaying a band
centered near 2.133 $\micron$ that is $\sim$0.016 $\micron$ wide Bernstein et
al. (2005). Using this laboratory substrate as a guide, if CO2 is a well mixed
constituent in Ariel’s regolith, then it is possibly only a minor component
compared to dirty H2O ice.

If CO2 ice is well mixed with dirty H2O ice in Ariel’s regolith, then these
CO2 ice grains could be a tracer of a radiolytic production cycle. In this
scenario, CO2 molecules, and other oxidized carbon compounds, would be
produced by charged particle bombardment of Ariel’s regolith. The charged
particles would break apart H2O molecules, forming OH radicals that interact
with nearby C-rich grains to form CO2 molecules:

OH + C(s) $\rightarrow$ CO + H

OH + CO $\rightarrow$ CO2 \+ H,

where H efficiently diffuses out of Ariel’s regolith and is preferentially
removed via Jean’s escape. Laboratory experiments (e.g., Raut et al., 2012)
have demonstrated that OH is an efficient catalyst for the production of CO2
molecules in substrates composed of H2O ice and C-rich material that are
irradiated at cryogenic temperatures ($<$ 100 K) relevant to the Uranian
moons. Supporting the possible presence of OH on the Uranian moons, a 280-nm
absorption band, attributed to trapped OH, has been detected at high southern
latitudes on Ariel ($\sim$45$\degree$S) using the Faint Object Spectrograph on
the Hubble Space Telescope (HST) (Roush et al., 1998).

Other laboratory experiments have demonstrated that irradiation of
hydrogenated carbon grains (i.e., carbon grains encased in H2O ice) can also
lead to radiolytic generation of CO2 molecules (e.g., Mennella et al., 2004,
2006). Such an ‘intramixture’ of H2O ice with embedded carbon grains could
represent another useful analog for understanding the spectral signature of
radiolytic chemistry in Ariel’s regolith.

Figure 5: NIR spectra and 1$\sigma$ uncertainties (blue error bars) displaying
evidence for a subtle absorption band centered near 2.13 $\micron$ (black
arrows), located on the long wavelength shoulder of the 2.02-$\micron$ H2O ice
band. These two spectra were collected at mid-observation longitudes and
latitudes of (1) 231.8$\degree$ and 51.1$\degree$N (IRTF/SpeX) and (2)
235.9$\degree$ and 47.3$\degree$N (ARC 3.5m/TripleSpec). The 2.13-$\micron$
band spans 2.123 to 2.137 $\micron$ in these two spectra with continuum-
divided band depths of: (1) 1.8 $\pm$ 0.5$\%$ and (2) 1.7 $\pm$ 0.6$\%$. The
bottom spectrum (3) is an average of these two spectra, highlighting the
2.13-$\micron$ band. The positions of CO2 bands 1, 2, and 3 are delineated as
well (black dashed lines). All spectra are normalized to 1 using the mean
reflectance between 1.74 and 1.77 $\micron$ and offset vertically for clarity.
These data were lightly smoothed using a 7 pixel wide boxcar function.

In summary, a subtle 2.35-$\micron$ band we detected in near-infrared
reflectance spectra collected over Ariel’s trailing hemisphere hints at the
presence of CO, which if present, is probably a transient species. The
presence of a 2.13-$\micron$ band in two spectra collected at similar sub-
observer latitudes and longitudes suggests that CO2 molecules might be well
mixed with H2O ice in some high latitude locations. Such a mixture could be a
tracer of radiolytic production sites where CO2 molecules are being generated
by irradiation of H2O and C-rich material in Ariel’s regolith. Particulate
mixtures of CO2 ice, H2O ice, and C-rich material, or alternatively,
intramixtures where CO2 ice is formed at the boundary between C grains
embedded in H2O ice, could both represent suitable analogs for Ariel’s
regolith. Our results therefore support the presence of a sustained CO2 ‘life
cycle’ on Ariel, with radiolytic production of CO2 molecules and migration to
low latitude cold traps, as predicted by thermodynamical models (Grundy et
al., 2006; Sori et al., 2017).

It is also possible that CO2 ice on Ariel originates in its interior and has
been exposed by geologic processes, perhaps concentrated in deposits that are
also rich in carbonates and/or NH3-bearing species that could be contributing
to Ariel’s 2.2-$\micron$ band (Cartwright et al., 2020b). However, the
2.2-$\micron$ band has been identified across Ariel’s surface, possibly
consistent with concentration in local geologic landforms or regional
provinces, whereas CO2 ice is present primarily on Ariel’s trailing hemisphere
and is essentially absent from its leading hemisphere. It is therefore
difficult to explain the strong leading/trailing hemispherical asymmetries in
the distribution of CO2 ice on Ariel and the other Uranian moons if this
constituent primarily originates from geologic processes.

###  4.2 Compositional stratification of Ariel’s regolith?

Analysis of synthetic spectra generated by prior spectral modeling efforts
suggests that Ariel’s surface could be compositionally stratified, with a thin
layer of small H2O ice grains capping concentrated deposits of CO2 ice grains
underneath (Cartwright et al., 2015, 2018, 2020a). These spectral modeling
conclusions were based on the premise that near-infrared photons at different
wavelengths travel different distances into planetary surfaces, depending on
the composition of the grains within the regolith (e.g., Clark & Roush, 1984).
Comparison of a planetary body’s spectral properties in different wavelength
regions might therefore yield insight into the vertical structure of its
regolith. One technique that can be used to gain such insight is the mean
optical path length (MOPL), which provides an estimate for the average travel
distance of photons into a particulate-dominated regolith before they are
absorbed, accounting for inter-grain boundary scattering (for calculation
details, see Clark & Roush 1984).

Utilizing prior estimates of the MOPL for the Uranian moons’ regoliths
(Cartwright et al., 2018), we find that photons penetrate to depths of
$\sim$0.1 mm over the wavelength range of CO2 bands 1, 2, and 3 (1.9 – 2.1
$\micron$). In contrast, photons penetrate to depths of $\sim$0.2 mm over the
wavelength range of the 1.543-$\micron$ and 1.578-$\micron$ CO2 bands and
$\sim$0.4 mm over the wavelength range of the 1.609-$\micron$ CO2 band. The
CO2 ice grains included in synthetic spectra 1 – 7, which fit the CO2 bands
between 1.5 to 1.7 $\micron$, are larger than the CO2 grains included in
synthetic spectra 8 – 10, which fit CO2 bands 1, 2, and 3 between 1.9 and 2.1
$\micron$. The larger CO2 grains included in synthetic spectra 1 – 7 could be
simulating the factor of 2 to 4 longer path lengths that photons achieve
through Ariel’s regolith between 1.5 and 1.7 $\micron$. Relatively stronger
absorption by H2O ice between 1.9 and 2.1 $\micron$ reduces photon path
lengths, thereby possibly explaining the smaller CO2 grains included in the
synthetic spectra that fit CO2 bands 1, 2, and 3. Thus, our spectral modeling
results are consistent with prior studies that suggest the Uranian moons’
regoliths are compositionally stratified, with larger grains of CO2 ice
retained at greater depths (Cartwright et al., 2015, 2018, 2020a).

The spectral modeling results presented here might instead hint at a different
regolith structure, where CO2 ice is formed beneath a thin H2O ice cap, at the
boundary between C grains embedded into a dominantly H2O ice matrix (i.e.,
intramixture). Albeit, visible polarimetric observations of the Uranian moons
are consistent with crumbly regoliths that include sub-micron sized grains
(Afanasiev et al., 2014), supporting the interpretation that particulate
mixtures are good analogs for Ariel’s regolith. Determining whether
particulate mixtures or intramixtures are better analogs for the structure of
Ariel’s regolith is beyond the scope of this work and should be considered by
future studies. As described in Section 3.4, minor variations in grain size
and fractional coverage in each of synthetic spectra 1 – 7 do not cause
significant changes in their goodness-of-fit to the CO2 bands between 1.5 and
1.7 $\micron$ (Table 5). Consequently, these synthetic spectra may not be
entirely sensitive to the factor of $\sim$2 change in photon penetration depth
between the 1.543-$\micron$ and 1.578-$\micron$ bands and the 1.609-$\micron$
band described above.

##  5 Conclusions and Future Work

We measured the distribution and spectral signature of CO2 ice as a function
of sub-observer latitude and longitude to investigate whether a radiolytic
production cycle, and associated transport of CO2 molecules to low latitude
cold traps, is occurring on Ariel. We measured the integrated areas of three
CO2 ice bands between 1.9 and 2.1 $\micron$ in 31 reflectance spectra
collected over a wide range of sub-observer longitudes and latitudes
(30$\degree$S – 51$\degree$N). Our results and analyses indicate that CO2 ice
band areas are decreasing with increasing sub-observer latitude on Ariel’s
trailing hemisphere but do not vary on its leading hemisphere. These results
suggest that the amount of CO2 ice on Ariel’s surface is higher at low
latitudes, as predicted by thermodynamical models. Furthermore, two spectra
collected at high northern sub-observer latitudes (47$\degree$N and
51$\degree$N) display subtle 2.13-$\micron$ bands, hinting at the presence of
CO2 ice grains well mixed with H2O ice and carbonaceous material, possibly
representing a spectral tracer of radiolytic production sites for CO2
molecules in Ariel’s regolith. Similarly, a subtle 2.35-$\micron$ band
detected in low sub-observer latitude spectra of Ariel hints at the presence
of CO, which could be a transient product generated by irradiation and
fragmentation of CO2 molecules in low latitude cold traps. The work presented
here is therefore consistent with radiolytic production of CO2 molecules and
subsequent migration and cold trapping of this constituent in concentrated
deposits of CO2 ice at low latitudes on Ariel.

Continued observations of Ariel over the next decade as the Uranian system
approaches northern summer solstice, when the sub-solar latitude will reach
82$\degree$N, will provide key measurements of the changing distribution and
spectral signature of CO2 ice. These future measurements could be compared to
the results presented here to further test whether CO2 ice is a radiolytic
product that migrates to low latitude cold traps. Furthermore, follow up
observations are needed to confirm the presence of the 2.13-$\micron$ band
reported here, and determine whether this feature is present in other
locations. Similar to Ariel, CO2 ice has been detected in reflectance spectra
collected over low sub-observer latitudes on the trailing hemispheres of
Umbriel, Titania, and Oberon (Grundy et al., 2003, 2006; Cartwright et al.,
2015), but the nature of CO2 ice at high sub-observer latitudes on these moons
has yet to be investigated. If CO2 ice originates as part of a radiolytic
production cycle on Umbriel, Titania, and Oberon, then CO2 ice band areas
should decrease in spectra collected at higher sub-observer latitudes on these
moons, similar to the observed trends on Ariel. Furthermore, determining
whether subtle 2.13-$\micron$ bands are present in spectra collected over high
sub-observer latitudes of these three moons could provide insight into whether
this feature is a tracer of radiolytic production sites in the Uranian
satellites’ regoliths. Follow up near-UV observations are also needed to
confirm whether a 280-nm band attributed to trapped OH, a key catalyst for the
radiolytic production of CO2 molecules, is present at high northern latitudes
on Ariel and the other Uranian moons.

To fully characterize the life cycle of CO2 ice on the classical Uranian
satellites, a spacecraft equipped with a visible wavelength camera and near-
infrared mapping spectrometer, making multiple close flybys of these moons, is
needed to reveal their previously unimaged northern hemispheres, measure the
localized distribution and spectral signature of CO2 ice, and characterize any
possible associations between CO2 ice and geologic landforms like craters and
chasmata that might serve as cold traps (e.g., Cartwright et al., 2021;
Leonard et al., 2021). Similarly, a Uranus orbiter equipped with a plasma
spectrometer and an energetic particle detector could measure Uranus’
magnetosphere proximal to Ariel and the other moons, thereby providing
critical context on moon-magnetosphere interactions that could be driving CO2
production and volatile cycling on these moons (e.g., Kollmann et al., 2020).
New measurements made by a thermal camera onboard a Uranus orbiter are also
needed to measure local and regional variations in the surface temperatures of
these moons, which would provide important information on the location and
longevity of CO2 ice cold traps and the source regions where CO2 molecules are
being generated by radiolysis or exposed by geologic processes.

##  6 Acknowledgments

This project was funded by the JPL Research and Technology Development Fund.
Portions of this work were carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under contract to the National Aeronautics
and Space Administration. Many of the observations reported here were made
from the summit of Maunakea, and we thank the people of Hawaii for the
opportunity to observe from this special mountain. Feedback and edits provided
by two anonymous reviewers were incorporated into this paper.

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## Appendix

###  A1 Methods: Band parameter measurements

In this appendix, we show the wavelength ranges of CO2 ice bands 1, 2, and 3
and their associated continua.

Figure A1: A graphical demonstration of the wavelength ranges of CO2 ice bands
1, 2, and 3, and their associated continua, using the grand average low
latitude spectrum of Ariel (1$\sigma$ uncertainties shown as gold error bars).
The data points used to calculate the areas of CO2 bands 1, 2, and 3 (spanning
1.962 – 1.969 $\micron$, 2.008 – 2.015 $\micron$, and 2.068 – 2.072 $\micron$,
respectively) are shown as red data points. The data points utilized for the
continua of these three CO2 bands are shown as white data points that are
connected by black lines.

###  A2 Results: IRTF/SpeX and ARC 3.5 m/TripleSpec spectra

Here we show ten near-infrared reflectance spectra of the Uranian satellite
Ariel at their native spectral resolutions.

Figure A2: Four new IRTF/SpeX spectra (longitudes 153$\degree$, 205$\degree$,
232$\degree$, and 293$\degree$) and six new ARC 3.5m/TripleSpec spectra
(longitudes 07$\degree$, 27$\degree$, 100$\degree$, 236$\degree$,
260$\degree$, and 264$\degree$) of Ariel and their 1$\sigma$ uncertainties,
collected in 2019 and 2020. The mid-observation, sub-observer longitude for
each spectrum is included in the bottom left-hand corner of each plot (see
Table 1 for observation details). All spectra have been normalized to 1
between 2.24 to 2.25 $\micron$. The sensitivities of SpeX (SXD mode) and
TripleSpec are lower at wavelengths $<$ 1.4 $\micron$ and $>$ 2.3 $\micron$,
resulting in lower S/N compared to wavelengths between 1.4 to 2.3 $\micron$.
Scattered light from Uranus contributes additional structure to some of these
spectra at wavelengths $<$ 1.3 $\micron$.