Evidence for Nitrogen-bearing Species on Umbriel: Sourced from a Subsurface Ocean, Undifferentiated Crust, or Impactors?

Near-infrared spectra of Umbriel and the other classical Uranian moons exhibit 2.2 μm absorption bands that could result from ammonia (NH3) bearing species, possibly exposed in the geologically recent past. However, Umbriel has an ancient surface with minimal evidence for recent endogenic activity, raising the possibility that more refractory species are present, and/or that NH3 is retained over long timescales. We analyzed 33 spectra of Umbriel to investigate its 2.2 μm band, along with three other absorption features we identified near 2.14, 2.22, and 2.24 μm. We assessed the subobserver longitudinal distributions of these four bands, finding that they are present across Umbriel and may be spatially associated with geologic features such as craters and large basins. We compared the bands to 15 candidate constituents. We found that Umbriel’s 2.14 μm and 2.22 μm bands are most consistent with the spectral signature of organics, its 2.24 μm band is best matched by NH3 ice, and its 2.2 μm band is consistent with the signatures of NH3–H2O mixtures, aluminum-bearing phyllosilicates, and sodium-bearing carbonates. However, some of these candidate constituents do not match Umbriel’s spectral properties in other wavelength regions, highlighting the gaps in our understanding of the Uranian moons’ surface compositions. Umbriel’s 2.14 μm band may alternatively result from a 2 ν 3 overtone mode of CO2 ice. If present on Umbriel, these candidate constituents could have formed in contact with an internal ocean and were subsequently exposed during Umbriel’s early history. Alternatively, these constituents might have originated in an undifferentiated crust or were delivered by impactors.


Introduction
Carbonaceous and nitrogenous components represent some of the key chemical building blocks incorporated into icy bodies forming in the early solar system.Carbon-and nitrogenbearing species have been detected or inferred on many icy satellites in the giant planet systems (e.g., Kuiper 1944;Smith et al. 1989;Carlson et al. 1996;McCord et al. 1997McCord et al. , 1998;;Carlson 1999;Waite et al. 2006), the dwarf planets Pluto (e.g., Hart 1974;Cruikshank et al. 1976;Grundy et al. 2016) and Ceres (e.g., King et al. 1992;Rivkin et al. 2006;De Sanctis et al. 2015, 2017), and a bevy of trans-Neptunian objects in the Kuiper Belt and beyond (e.g., Barucci et al. 2005;Brown et al. 2005Brown et al. , 2007;;Licandro et al. 2006).N-and C-bearing constituents were likely incorporated into the planetesimals that formed the five largest and tidally locked classical satellites of Uranus (e.g., Lewis 1972;Prinn & Fegley 1989) and are the most commonly identified or suspected surface components on these moons after H 2 O ice (Bauer et al. 2002;Grundy et al. 2003).Along with a native origin for C-and N-bearing material, these constituents might also be delivered to planetary bodies through collisions with interstellar dust particles, heliocentric micrometeorites, intraplanetary dust grains, and even pebbles (e.g., Kerridge et al. 1987;Cronin et al. 1988;Bernstein et al. 1996;Maurette 1998;Maurette et al. 2000;Pendleton & Allamandola 2002;Dartois et al. 2013;Valletta & Helled 2022).
Near-infrared (NIR) reflectance spectra (∼0.7-2.5 μm) collected with ground-based telescopes have detected carbon dioxide (CO 2 ) ice on Ariel, Umbriel, Titania, and Oberon (Grundy et al. 2003(Grundy et al. , 2006;;Cartwright et al. 2015Cartwright et al. , 2022)).These data, along with spectra of Miranda, display subtle absorption bands between 2.12 and 2.27 μm, which have been tentatively attributed to the presence of ammonia (NH 3 ) and ammonium (NH 4 ) bearing constituents (Bauer et al. 2002;Cartwright et al. 2018Cartwright et al. , 2020c)).Similarly, 2.21 μm bands attributed to NH-bearing components have been detected on Pluto and its moon Charon in reflectance spectra collected with ground-based telescopes (Brown & Calvin 2000;Buie & Grundy 2000;Cook et al. 2007;Merlin et al. 2010;DeMeo et al. 2015), the Hubble Space Telescope (Dumas et al. 2001), and the Linear Etalon Imaging Spectral Array (LEISA; 1.25-2.5 μm) on the New Horizons spacecraft (e.g., Grundy et al. 2016;Cook et al. 2018;Dalle Ore et al. 2018, 2019; Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Cruikshank et al. 2019;Protopapa et al. 2020).On Pluto, the 2.21 μm band is primarily found in association with H 2 O icerich material that may have been exposed from its interior by endogenic activity (Cruikshank et al. 2019(Cruikshank et al. , 2021)).On Charon, the 2.21 μm band is widely distributed, with stronger concentrations in some fresh craters and their ejecta blankets (Protopapa et al. 2020).The spatial association with fresh craters suggests that the species contributing to Charon's 2.21 μm band are native components that are exposed by impact events and/or are delivered by impactors.
Reflectance spectra collected by ground-based telescopes (King et al. 1992;Rivkin et al. 2006) and the Visual and Infrared Imaging Spectrometer (VIR) on the Dawn spacecraft (e.g., Ammannito et al. 2016;De Sanctis et al. 2016, 2017;Carrozzo et al. 2018;Raponi et al. 2019) have also detected 2.2 μm bands and other spectral features on Ceres, attributed to NH 4 -bearing phyllosilicates, Na-bearing carbonates (CO 3 2- ), and other minerals.These constituents are concentrated in geologic features such as Ahuna Mons and Cerealia and Vinalia faculae in Occator crater, which could be mantled by salts that originated in Ceres' interior and were emplaced on its surface by past endogenic activity (e.g., Quick et al. 2019;Ruesch et al. 2019;De Sanctis et al. 2020a;Scully et al. 2020;Fagents et al. 2022).Additionally, ground-based observations of the Saturnian moon Enceladus detected subtle absorption features near 2.25 μm that were attributed to NH 3 (Emery et al. 2005;Verbiscer et al. 2006).
Determining whether NH 3 -bearing species, NH 4 salts and phyllosilicates, Na carbonates, or other components are present and contribute to the 2.2 μm band on Umbriel and the other Uranian moons is challenging because of the typical weakness of the detected features (1%-3% band depths), the faintness of these moons (Vmag 14.0-15.1),and the modest signal-to-noise ratio (S/N) of the available spectra covering this wavelength range (S/N 10-50).Furthermore, ground-based observations of the Uranian moons are disk integrated, and it is unknown whether the 2.2 μm band is spatially associated with specific geologic features on their surfaces, similar to Ceres, Pluto, and Charon.Nevertheless, disk-integrated spectra can still be used to measure longitudinal trends in the distribution of spectral features on the Uranian moons, providing important clues on the nature and origin of different species (e.g., Grundy et al. 2006;Cartwright et al. 2018;DeColibus et al. 2022).For example, CO 2 ice is primarily located on the trailing hemispheres of the inner moons Ariel and Umbriel, which is consistent with interactions between electrons and protons trapped in Uranus' magnetosphere and H 2 O ice and C-rich species on the surfaces of its moons, forming carbon oxides via radiolysis (e.g., Grundy et al. 2006;Cartwright et al. 2015Cartwright et al. , 2022)).In contrast, the 2.2 μm band on Ariel does not appear to display any notable longitudinal trends in its distribution, indicating that the species that cause this band are widespread and may originate in localized geologic landforms such as craters, chasmata, and possible cryovolcanic features (Cartwright et al. 2020c;Beddingfield & Cartwright 2021).
Laboratory experiments demonstrate that NH 3 hydrates and NH 3 ice are readily decomposed by irradiation and should be efficiently removed from the surfaces of the Uranian moons by magnetospheric charged particle bombardment, perhaps in as little as 0.01-1 Myr from the surface of the innermost classical moon Miranda (Moore et al. 2007).The predicted volatility of NH 3 has been cited as a possible tracer of recent geologic activity on Miranda and Ariel (Cartwright et al. 2020c;Beddingfield & Cartwright 2021;Beddingfield et al. 2022aBeddingfield et al. , 2022b)).However, other species that might contribute to the 2.2 μm band are more resistant to removal by irradiation and persist over longer timescales.For example, if NH 3 is present and exposed on the surfaces of the Uranian moons, then subsequent irradiation would generate NH 4 + ions (Moore et al. 2007).Some of these ions would likely interact with nearby H 2 O, CO 2 , and other molecules through a web of different reactions to form more refractory components, including NH 4 -bearing species such as NH 4 carbonate ((NH 4 ) 2 CO 3 ; e.g., Lister 1955;Ogura 1967).Other species, including simple hydrocarbon ices such as methane (CH 4 ) and ethylene (C 2 H 4 ) and nitriles such as hydrogen cyanide (HCN) can express absorption features between 2.12 and 2.27 μm (e.g., Cruikshank et al. 1991;Grundy et al. 2002;Hudson et al. 2014;Dartois 2021).Irradiation of organic species, in the presence of NH 3 and H 2 O, forms more complex and refractory molecules (e.g., Allamandola et al. 1988) that could continue to contribute to the 2.2 μm bands via a C ≡ N overtone mode that can be expressed by refractory organic residues called "tholins" (Cruikshank et al. 1991;Khare et al. 1993).
Although the 2.2 μm band appears to be concentrated in geologic features such as fresh craters on Charon and possible cryovolcanic constructs on Pluto, LEISA also detected prominent 2.2 μm bands on Pluto's small moons Nix and Hydra, which have ancient surfaces and minimal evidence for recent geologic resurfacing, suggesting that more refractory NH 4 -bearing compounds, and not NH 3 , could be present (Cook et al. 2018;Dalle Ore et al. 2018).Consequently, the association between 2.2 μm bands, NH 3 -bearing species, and recent geologic activity is not applicable in all cases, and more refractory species such as salts, organics, and phyllosilicates might contribute to 2.2 μm bands detected on icy bodies, especially those with ancient surfaces.
Similar to Miranda and Ariel, some ground-based spectra of Umbriel display 2.2 μm bands that were tentatively attributed to NH 3 -bearing species (Cartwright et al. 2018).Miranda and Ariel both exhibit heavily modified terrains (Schenk 1991;Pappalardo et al. 1997;Beddingfield et al. 2015;Beddingfield & Cartwright 2020, 2022) that could be geologically young (0.1 0.1 0.4 -+  Ga on Miranda and 1 0.5 0.8 -+  Ga on Ariel, Kirchoff et al. 2022) and exhibit evidence of high heat fluxes in the past (Peterson et al. 2015;Beddingfield et al. 2022aBeddingfield et al. , 2022b)), supporting the relationship between recent geologic activity, 2.2 μm bands, and the possible presence of NH 3 (Cartwright et al. 2020c).The larger outer moons Titania and Oberon may have long-lived internal oceans sustained by radiogenic heating (Bierson & Nimmo 2022) that hypothetically could have spurred the exposure or emplacement of ocean-derived material rich in NH 3 in the past.Although Umbriel may have a residual ocean (Castillo-Rogez et al. 2023), it has one of the most ancient icy satellite surfaces in the solar system (4.5 0.2 0.0 -+  Ga, Kirchoff et al. 2022) and displays little evidence for endogenic activity.The presence of the 2.2 μm bands in spectra of Umbriel, and other absorption features between 2.12 and 2.27 μm, raises important questions about the nature of the species that are contributing to its spectral properties, and whether NH 3 -and other N-bearing species are present.It is also uncertain whether these species formed through interactions with a subsurface ocean in Umbriel's early history, or if they were possibly sourced from undifferentiated crustal material.
Alternatively, perhaps these species were delivered by impactors, including dust grains from the retrograde irregular satellites that likely mantled the leading hemispheres of the classical Uranian moons with spectrally red material, possibly rich in tholins (Cartwright et al. 2018).
To investigate the nature and origin of the species contributing to Umbriel's spectral properties, we measured the band areas and depths of a 2.2 μm band, as well as bands centered near 2.14, 2.22, and 2.24 μm that have not been previously identified on Umbriel.We assessed the subobserver longitudinal distribution of these four absorption bands to gain insight into their origins.We compared the spectral signatures of the bands to an extensive suite of candidate constituents that exhibit absorption features between 2.12 and 2.27 μm, including a variety of NH 3 -bearing species, NH 4 salts, organics, the Na carbonate thermonatrite (Na 2 CO 3 •H 2 O), and the phyllosilicate kaolinite (Al 2 Si 4 O 5 (OH) 4 ).We consider the implications for the possible presence of these different constituents on Umbriel and discuss some relevant caveats for our comparisons between telescope, laboratory, and synthetic spectra (Section 4.2).

Observations and Data Reduction
We reduced and analyzed a total of 15 new NIR reflectance spectra of Umbriel, one of which was collected in 2014 and the other 14 were collected between 2017 and 2021.Nine of these spectra were collected with the NIR SpeX spectrograph/imager at NASA's Infrared Telescope Facility (IRTF), operating in low resolution PRISM mode (4 spectra) and moderate resolution short cross-dispersed (SXD) mode (5 spectra; ∼0.7-2.5 μm; e.g., Rayner et al. 2003).The other 6 spectra were gathered with the TripleSpec spectrograph (0.95-2.46 μm) on the Astrophysical Research Consortium (ARC) 3.5 m telescope at the Apache Point Observatory (Wilson et al. 2004).We analyzed another 18 spectra collected with IRTF/SpeX between 2000 and 2017, and we refer the reader to prior studies for detailed accounting of their observation strategies and reduction techniques (Grundy et al. 2006;Cartwright et al. 2015Cartwright et al. , 2018)).Observation details for all 33 NIR spectra are summarized in Table 1.
Most of the spectra analyzed here were collected with the spectrograph slit oriented to match the parallactic angle, thereby minimizing atmospheric dispersion that can modify spectral slopes, in particular, at shorter wavelengths (1.2 μm).For a small number of observations, we rotated the spectrograph slit to avoid scattered light from Uranus, leading to a mismatch between the slit orientation and the parallactic angle.However, atmospheric dispersion is minor on Maunakea at wavelengths >2.1 μm (0 01 for an airmass ranging from 1.1 to 2), and changes to the spectral features of interest to this study, and the continuum between 2.12 and 2.27 μm, should be negligible.
IRTF/SpeX.On each night before and after observations of Umbriel, we observed nearby G-type standard stars (<4°s eparation), with typical airmass differences within ±0.1 of our science target.We repeated this star-Umbriel-star observing cadence multiple times on each night to increase the total integration time, making sure to observe standard stars at least once per hour when observing Umbriel at airmass <1.2 and two to three times per hour at higher airmass.The G-type stars we observed are summarized in Table 2. Science exposures were collected in two different positions (A and B) on SpeX's 15″ long slit, separated by 7 5. Exposures of Umbriel were 45-120 s long when operating SpeX in PRISM mode and 120-180 s long when using SXD mode.Standard star observations were limited to 4 or 5 s exposures in PRISM mode and 20-30 s in SXD mode.Sequential AB pairs of exposures for each target were then subtracted to provide firstorder removal of detector artifacts and sky emission.A-B pairs were normalized using flat-field frames, generated using SpeX's internal quartz lamp, to correct for sensitivity variations across the detector.Exposures of an internal argon lamp were recorded at the beginning and end of each night in order to provide wavelength calibration for the science exposures.
All collected science exposures were flatfielded and wavelength calibrated using the Spextool data reduction suite (Cushing et al. 2004).After calibration, Spextool extracted a 1D spectrum from each exposure.Using custom programs, we then divided the Umbriel spectra by standard star spectra, collected close in time and at comparable airmass, to remove the solar spectrum and provide additional removal of atmospheric contributions and instrument artifacts.We used subpixel shifting routines to minimize wavelength shifts between Umbriel and star spectra that can be introduced by instrument flexure, thereby providing modest improvements in the removal of telluric bands and solar features.Star-divided Umbriel spectra from each night were then coadded to boost the S/N, with uncertainties estimated using the standard error (σ/ n ) of each coadded pixel.
ARC 3.5 m/TripleSpec.G-type standard stars were observed before and after Umbriel, with at least one standard star observation occurring during each hour of clock time.Standard stars were typically within 4°of the Uranian system, and airmass disparities were relatively minor (±0.1).The stars we observed are summarized in Table 2.All targets were observed in A and B positions separated by 21″ along TripleSpec's 1 1 × 43″ slit.Exposures of Umbriel were 120-180 s in length and star exposures were 20-30 s.TripleSpec's long slit allowed for occasional observations of multiple Uranian moons (see the slit-viewing image of the Uranian system reported in Figure 2 of DeColibus et al. 2022).In these instances, observed moons were slightly off the standard A and B positions, but spectra were reduced and extracted using the same A-B techniques.Flat-field frames were generated using quartz lamps mounted on the telescope structure.Unilluminated exposures of the quartz lamps were used to remove thermal contributions from the telescope.Emission lines from atmospheric OH in the collected exposures were used to perform wavelength calibration.TriplespecTool, a modified version of the Spextool software package (Cushing et al. 2004), was used to calibrate the collected frames, extract a 1D spectrum from each exposure, divide Umbriel spectra by star spectra, and perform subpixel wavelength shifting.Star-divided Umbriel spectra were coadded using a robust weighted mean statistic.

Band Parameter Measurements
We measured the band areas and depths of four absorption features, centered near 2.14, 2.2, 2.22, and 2.24 μm.We made these measurements using a custom band parameter measurement program that we have used previously to assess NIR reflectance spectra of the Uranian moons and other icy bodies (e.g., Cartwright et al. 2020bCartwright et al. , 2020c)).The program reads in each spectrum and fits the continuum for each of the four bands between 2.1 and 2.3 μm.Because the central wavelength of the bands can shift slightly in different spectra, we visually confirm the wavelength range of each band before dividing them by their continua.The program calculated the average reflectance within ±0.003-0.005μm of a user-defined band center, propagating errors using standard techniques (e.g., Taylor 1997).This averaged band-center reflectance (b c ) was then subtracted from 1 to calculate the band depth.Continuumdivided band areas were calculated using the trapezoidal rule.Monte Carlo simulations were used to estimate band area errors, resampling the 1σ uncertainties for spectral channels included in each measured absorption band.Continuum modeling.The continuum for each spectrum was simulated with one-layer synthetic spectra generated using a Hapke-Mie radiative transfer program.The complex indices of refraction (i.e., optical constants) derived from laboratory spectra of crystalline H 2 O ice (80 K; Mastrapa et al. 2008), crystalline CO 2 ice (∼150 K; Hansen 1997), and amorphous carbon (Rouleau & Martin 1991) were used to generate these synthetic spectra (end-member synthetic spectra shown in Figure A1).These optical constants were passed to different routines that use Mie scattering theory (e.g., Bohren & Huffman 1983) to calculate the single scattering albedo ( ¯0 w ) for each constituent.Unlike the geometric optics routines of pure Hapke-based approaches, Mie theory is able to simulate scattering off submicron particles that are smaller than or comparable to the wavelength of incident light (e.g., Emery et al. 2006).These small particles could be important scattering components in the regoliths of the Uranian moons (e.g., Afanasiev et al. 2014;Cartwright et al. 2020a), potentially making Mie theory ideal for simulating their surface compositions.Mie scattering theory formally applies to isolated spherical particles, and therefore only approximates the structural complexities and particle packing regimes of planetary regoliths.Nonetheless, Mie theory is widely used to model the spectral properties of planetary surfaces, generally providing useful results.
The ¯0 w  values, along with phase function and opposition effect coefficients, were used to calculate the geometric albedo as a function of wavelength for each synthetic spectrum (e.g., Hapke 2012).All telescope spectra were scaled to the geometric albedo of Umbriel at 0.957 μm (0.26 ± 0.01; Figure 7 in Karkoschka 2001) prior to modeling their continua.Additional Hapke model inputs include the grain size and mixing ratio for each constituent and the mixing regime for each combination of modeled species (i.e., well-mixed or segregated constituents).To account for minor resonances that can occur when using Mie scattering theory, the spectral modeling program used a range of grain sizes for each constituent (±∼10% spread in diameters), which were then averaged together to represent the specified grain size.
Prior modeling results and quality assessment.This Hapke-Mie approach was used previously to model Ariel's continuum in order to measure its 2.2 μm band (Cartwright et al. 2020c).These prior modeling efforts indicate that multiple crystalline H 2 O ice grains sizes are required to adequately match the continuum, dominated by larger grains, with diameters ranging between 10 and 50 μm, and a small fraction (<1%) of submicron grains, with typical diameters ranging between 0.2 and 0.5 μm.To simulate the dark and spectrally neutral component that is well mixed with H 2 O ice on the Uranian moons (e.g., Clark & Roush 1984), we used large amounts of amorphous C (∼30%), with one grain diameter (5-10 μm).These models also included a modest amount of crystalline CO 2 ice (15%) with one grain diameter (5-15 μm).The inclusion of CO 2 increases the albedo of the synthetic spectrum without enhancing the 1.52 μm and 2.02 μm H 2 O ice bands, which are notably weaker on Umbriel than on the other Uranian moons (e.g., Cartwright et al. 2015Cartwright et al. , 2018)).All of the continuum models used particulate mixtures to simulate the likely well-mixed nature of Umbriel's dark regolith.
The quality of the fits between the synthetic spectra to Umbriel's continuum were assessed with reduced chi-square statistics: where a is the degrees of freedom, O i is the observed data, M i is the modeled data, and  values for all of the synthetic spectra we generated are <1, indicating that these models provide suitable fits to the data (Table A1).
Grand average spectra.After making measurements on the individual spectra and after assessing their subobserver longitudinal distribution, we generated four grand average spectra to compare to reflectance spectra of candidate constituents (Section 3.4).The grand average spectra were generated using the individual spectra that display >2σ band area and depth measurements for the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands.These four grand average spectra display a higher S/N (75-150) between 2.12 and 2.27 μm than the individual spectra (S/N 10-50).Note.
a Information obtained from the SIMBAD database, operated at the CDS, Strasbourg, France (Wenger et al. 2000).

Subobserver Longitudinal Distribution of Spectral Features
Prior work indicates that the CO 2 ice detected on Ariel, Umbriel, Titania, and Oberon is mostly concentrated on their trailing hemispheres (181°-360°), supporting a radiolytic origin for this constituent (e.g., Grundy et al. 2006;Cartwright et al. 2015).In contrast, H 2 O ice bands are stronger and red material is more abundant on these moons' leading hemispheres (1°-180°), supporting regolith overturn by heliocentric micrometeorites and delivery of red dust from the irregular satellites (e.g., Cartwright et al. 2018).To determine whether the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands display similar subobserver longitudinal trends, we calculated the mean band area and depth measurements for the 13 spectra collected over Umbriel's leading hemisphere and 20 spectra collected over its trailing hemisphere and compared these values.As a another test, we fit the individual band area and depth measurements for each spectrum with a sinusoidal model (three coefficients: amplitude, phase shift, and vertical offset).To minimize the influence of noisy data, each measurement was weighted using its uncertainty.We then compared the sinusoidal fit to the mean measurement for each band using an F-test (e.g., Lomax & Hahs-Vaughn 2013), with the null hypothesis that there is no meaningful difference between the sinusoidal and mean models (2σ).If the sinusoidal model provides a better fit than the mean model (>2σ), then we reject the null hypothesis in these cases.

Laboratory and Synthetic Spectra of Candidate Constituents
To investigate the components that could be contributing to absorption bands between 2.12 and 2.27 μm, we compared the four grand average Umbriel spectra to reflectance spectra of 15 different species that have been identified or inferred on other icy bodies, including NH 3 , NH 4 salts, Na carbonates, short-chain organics, and Al phyllosilicates.NH 4 salts can exhibit small wavelength shifts in their spectral features as a function of temperature (Berg et al. 2016;Fastelli et al. 2022).To account for possible temperature-dependent wavelength shifts, we used multiple NH 4 salt spectra measured at relevant cryogenic temperatures (80-90 K) and room temperature.In total, we compared the grand average Umbriel spectra to 20 reflectance spectra of candidate constituents, 10 of which were measured in the laboratory, and another 10 that were included in synthetic spectra (Table 3).The synthetic spectra were generated with the previously described Hapke-Mie modeling program, using a particulate mixture of crystalline H 2 O ice, amorphous C, and crystalline CO 2 ice representing a base composition that simulates the spectral properties of Umbriel's regolith.We then added one other component that displays absorption features between 2.12 and 2.27 μm to these particulate mixtures (Table 4).

Near-infrared Spectra of Umbriel
We report 15 new disk-integrated spectra of Umbriel (Figures A2 and A3), which we collected between 2014 and 2021.Many of the new and previously reported Umbriel spectra display subtle absorption features between 2.12 and 2.27 μm, primarily centered near 2.14, 2.2, 2.22, and 2.24 μm.All of the new and previously reported spectra exhibit the 1.52 μm and 2.02 μm H 2 O ice absorption band complexes, as well as a feature centered near 1.65 μm that is consistent with crystalline H 2 O ice (e.g., Grundy & Schmitt 1998;Mastrapa et al. 2008).Some of the new spectra we collected over Umbriel's trailing hemisphere display narrow features between 1.9 and 2.1 μm that are consistent with crystalline CO 2 ice segregated from other constituents in concentrated deposits (Hansen 1997).Additionally, some of the new data show minor spectral reddening between 0.7 and 1.2 μm.Measurements and analyses of H 2 O ice (Grundy et al. 2006;Cartwright et al. 2015Cartwright et al. , 2018)), segregated CO 2 ice (Grundy et al. 2006;Cartwright et al. 2015), and spectrally red material (Bell & McCord 1991;Buratti & Mosher 1991;Cartwright et al. 2018) on Umbriel were reported in prior work and are beyond the scope of this study.

Band Parameter Measurements
Eight of the Umbriel spectra we analyzed display 2.14 μm bands with >2σ area and depth measurements (Table A2), nine spectra have 2.2 μm bands with area and depth measurements >2σ (Table A3), eight spectra exhibit 2.22 μm bands with >2σ Left: Nineteen Umbriel spectra (black, labeled a-s) and 1σ uncertainties (gray error bars) that display 2.14 μm (purple markers), 2.2 μm (blue markers), 2.22 μm (green markers), and/or 2.24 μm (red markers) absorption features with both band depths and areas >2σ (Tables A2-A5).The subobserver longitude for each spectrum is also listed (Table 1).All spectra were normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.To make spectral features easier to identify, the data have been lightly smoothed using boxcar functions ranging from 4 pixels wide (IRTF/SpeX PRISM mode spectra), 8 to 12 pixels wide (IRTF/ SpeX SXD mode spectra), and 16 pixels wide

Note.
a See Figure A1 for the end-member spectra of these constituents.
measurements (Table A4), and six spectra have 2.24 μm bands with >2σ measurements (Table A5) (Figure 1).Because the four spectral features we measured are subtle, with continuumdivided band depths ranging between 0% and 7%, robust assignment of their band centers can be difficult.Consequently, we only report the band centers and widths for spectra that have area and depth measurements >2σ for each of the four spectral features (Tables A2-A5).These band centers range between 2.133 and 2.148 μm for the 2.14 μm feature, 2.197-2.216μm for the 2.2 μm band, 2.218-2.231μm for the 2.22 μm feature, and 2.235-2.255μm for the 2.24 μm band.All band area and depth measurements, uncertainty estimates, and band center determinations were conducted using nonsmoothed reflectance spectra analyzed at their native spectral resolutions.(Figure 3, spectrum A, B, C, and D, respectively).Because of the low S/N of the individual Umbriel spectra, we base our analyses on spectra that exhibit only >2σ area and depth measurements for these four spectral features.Nonetheless, a small number of Umbriel spectra display >3σ area and depth measurements: 2.14 μm band, two spectra (Figure 1, spectra f and g); 2.2 μm band, three spectra (Figure 1, spectra c, f, and j); 2.22 μm band, one spectrum (Figure 1, spectrum j); and 2.24 μm band, one spectrum (Figure 1, spectrum o).The detection of each of these features with >3σ band measurements lends additional confidence to the results reported here.Other features that appear to be present in the spectra but are not analyzed in this study include narrow bands resulting from CO 2 ice (centered near 1.966, 2.012, and 2.070 μm) that were reported in prior work (Grundy et al. 2006;Cartwright et al. 2015).Additionally, spectral features at wavelength >2.27 μm are more uncertain because of increasing telluric contamination and decreasing S/N, and we refrain from analyzing these features.

Subobserver Longitudinal and Latitudinal Distribution of Spectral Features
Comparison between the mean band measurements for Umbriel's leading and trailing hemispheres indicates that there are no meaningful hemispherical trends in any of the four bands (Table 5).Similarly, the F-test statistics indicate that there are no meaningful differences between the mean and sinusoidal models in the individual area or depth measurements for the 2.14 μm, 2.22 μm, and 2.24 μm bands, and we do not reject the null hypothesis for these measurements (Table 6, Figure 2).The F-test results do indicate that there is a statistically relevant difference for the area and depth measurements for the 2.2 μm band, with stronger band measurements on Umbriel's leading side compared to its trailing side, and we reject the null hypothesis for this absorption band (Table 6).We consider the implications of the subobserver longitudinal distributions of these four bands in Section 4.1.
In the high-obliquity Uranian system (∼98°), the subobserver latitude ranges between 82°S to 82°N over the course of a Uranian year.Consequently, high-latitude regions (45°-90°S, 45°-90°N) begin to dominate the spectral properties of Umbriel's observed disk once the Uranian system approaches summer solstice.The data reported here were collected when the subobserver latitude ranged between 29.3°S (September 2000) to 54.9°N (2021 October).Defining high-latitude regions as subobserver latitudes 45° (Cartwright et al. 2022), we calculated the fraction of Umbriel's disk that was comprised by a The grand average spectrum for each band is shown in Figure 3. 2016 for disk-area calculation details).We found that highlatitude regions comprised between 13.6% and 41.0% of Umbriel's observed disk for subobserver latitudes 9.4°S (2005 September) to 54.9°N, respectively.The four spectral features between 2.12 and 2.27 μm appear in numerous spectra that were collected over this latitudinal range, including several spectra collected at subobserver latitudes 45°(spectra c, g, i, and j in Figure 1).Therefore, neither subobserver longitude nor latitude appears to control the distribution of the four bands.

Laboratory and Synthetic Spectra of Candidate Constituents
The telescope spectra that exhibit >2σ band measurements were incorporated into grand average Umbriel spectra (described in Section 2.2) for the 2.14 μm, 2.2 μm,2.22 μm, and 2.24 μm bands (Figure 3, spectrum A, B, C, and D, respectively).The laboratory and synthetic spectra (Table 3) that we compared to these grand average Umbriel spectra are shown in Figure 4 (numbered using the same sequence as shown in Tables 3 and 4).We also compared continuum-  A2-A5).The solid blue lines show the mean area and depth measurements for each band, and the dashed orange lines show sinusoidal fits to the data.Gray-toned zones represent duplicated subobserver longitudes.We compared the fits provided by the mean and sinusoidal models using an F-test (results reported in Table 6).Several data points on the 2.22 μm and 2.24 μm band measurement plots are notably larger than the mean band area or depth values, suggesting that the constituents contributing to these bands might be concentrated in specific geologic features or regions on Umbriel's surface.Spatially resolved spectra are required to further investigate this possibility.
divided versions of the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands to continuum-divided absorption features exhibited by representative candidate constituents (Figure 5).Relevant details pertaining to our comparison between telescope, laboratory, and synthetic spectra are discussed in the "Caveats" subsection of Section 4.2.
NH 3 -bearing species.Panel I of Figure 4 shows frozen NH 3 -H 2 O mixtures measured in the laboratory (spectra 1 and 2) and synthetic spectra that include amorphous NH 3 (spectrum 3) and crystalline NH 3 ice (spectra 4 and 5).None of these samples displays absorption features near 2.14 μm.The laboratory spectra of NH 3 -H 2 O mixtures exhibit features that can provide adequate matches to Umbriel's 2.2 μm band, but their band centers are shifted closer to 2.21 μm (Figure 5).If present on Umbriel, NH 3 -H 2 O mixtures could be well mixed with other components in its regolith, but the lack of optical constants for different types of NH 3 hydrates prevents us from exploring this possibility further with radiative transfer models.The synthetic spectrum that includes amorphous NH 3 exhibits a feature that is much broader than Umbriel's 2.22 μm band, but they share similar band centers (Figure 5).The synthetic spectra that include NH 3 ice display absorption features that provide good matches to the width and center of Umbriel's 2.24 μm band with somewhat different band shapes (Figure 5).
NH 4 carbonates.Panel II of Figure 4 shows laboratory spectra of NH 4 carbonate ((NH 4 ) 2 CO 3 ) measured at room temperature (spectra 6 and 7) and at 80 K (spectrum 8), and NH 4 bicarbonate (NH 4 HCO 3 ) measured at room temperature and 90 K (spectra 9 and 10, respectively).These five salt samples have band centers between 2.17 and 2.18 μm that do not match the centers of Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands.Furthermore, these NH 4 salt bands are considerably wider than the features we have identified on Umbriel (example shown in Figure 5).
NH 4 chlorides, Na carbonates, and Al phyllosilicates.Panel III of Figure 4 shows laboratory spectra of NH 4 chloride measured at 90 K and room temperature (spectra 11 and 12, respectively), the Na carbonate thermonatrite measured at 93 K (spectrum 13), and a synthetic spectrum that includes the Al phyllosilicate kaolinite measured at room temperature (spectrum 14).The NH 4 chloride samples have band centers that are similar to Umbriel's 2.2 μm band but shifted closer to 2.21 μm.Similar to NH 4 carbonate and NH 4 bicarbonate, the NH 4 chloride samples have substantially broader spectral features than the subtle bands we have identified on Umbriel (example shown in Figure 5).Thermonatrite provides a good match to the band center and shape of Umbriel's 2.2 μm feature (Figure 5).The synthetic spectrum that includes kaolinite also provides a good match to the center and shape of Umbriel's 2.2 μm feature (Figure 5).None of the species in Panel III show features that match Umbriel's 2.14 μm, 2.22 μm, or 2.24 μm bands.
Organics.Panel IV of Figure 4 displays synthetic spectra that include the amines methylamine (CH 3 NH 2 ) and ethylamine (C 2 H 5 NH 2 ), the nitrile propionitrile (C 2 H 5 CN), the hydrocarbon ices ethylene (C 2 H 4 ) and propyne (C 3 H 4 ), and methanol ice (CH 3 OH) (spectra 15, 16, 17, 18, 19, and 20, respectively).Methylamine and ethylamine both exhibit 2.14 μm absorption features with band centers and shapes similar to Umbriel's 2.14 μm band (Figure 5).Ethylene also displays a subtle absorption feature in this wavelength range, but it is shifted closer to 2.13 μm.None of these organics exhibit Figure 3. Grand average spectra (black) and 1σ uncertainties (light gray error bars) calculated using all individual spectra with band area and depth measurements >2σ for the 2.14 μm band (A, spectra b, c, f, g, i, l, m, and pin Figure 1), the 2.2 μm band (B, spectra b, c, e, f, h, i, j, l, and s in Figure 1), the 2.22 μm band (C, spectra a, d, g, i, j, m, n, and r in Figure 1), and the 2.24 μm band (D, spectra c, d, e, k, o, and q in Figure 1).All four grand average spectra are normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.The spectra have been smoothed using a 10-14 pixel wide boxcar function, whereas the 1σ uncertainties are shown at their native resolutions.The light purple zone highlights the wavelength range of the 2.14 μm band centers, the light blue zone shows the 2.2 μm band centers, the light green zone shows the 2.22 μm band centers, and the light red zone shows the range of 2.24 μm band centers (summarized in Figure 1 and the penultimate column of Tables A2, A3, A4, and A5, respectively).The dark gray line at the top of the plot is an atmospheric transmission model for Maunakea at zenith with 1.6 mm of precipitable water (50th percentile conditions), binned to simulate the spectral resolution of SpeX in SXD mode (R ∼ 750 at 2.2 μm, 0 8 slit).This atmospheric spectrum demonstrates that Earth's atmosphere is mostly transparent between 2.12 and 2.27 μm, with only a narrow CH 4 gas feature overlapping the 2.2 μm band.Subpixel shifting routines used during data reduction help minimize residual telluric features.Thus, telluric contamination of the 2.2 μm band should be negligible.
features that match Umbriel's 2.2 μm band, but ethylamine and propyne exhibit slight continuum slope changes in this wavelength range that might result from very subtle bands centered near 2.2 μm.Methylamine, ethylamine, propionitrile, and ethylene exhibit absorption features centered between 2.22 and 2.23 μm, consistent with Umbriel's 2.22 μm band.None of the organic species we investigated exhibit features that are consistent with Umbriel's 2.24 μm band.Additionally, methanol does not display any absorption features between 2.12 and 2.27 μm.R 2 c  Analyses.As an additional test, we compared the continuum-divided Umbriel spectra to continuum-divided synthetic spectra of candidate constituents using R 2 c  statistics (Table 7).We used the synthetic spectrum that includes methylamine (spectrum 15) as a representative of the different organic species investigated in this study, comparing it to Umbriel's 2.14 μm and 2.22 μm bands.We compared the synthetic spectra that include NH 3 ice (spectra 4 and 5) to Umbriel's 2.24 μm band, and we compared the synthetic spectrum including amorphous NH 3 (spectrum 3) to Umbriel's 2.22 μm band.We also compared kaolinite (spectrum 14) to Umbriel's 2.2 μm band.The results indicate that methylamine provides a good match to the shape and extent of Umbriel's 2.14 μm and 2.22 μm bands, kaolinite matches the 2.2 μm band, and NH 3 ice matches Umbriel's 2.24 μm ( 1   A2-A5) are highlighted by the purple, blue, green, and red zones, respectively.All spectra were normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.Some of the laboratory spectra were vertically scaled for easier presentation in panels II and III.The synthetic spectra were generated with the previously described Hapke-Mie modeling program (model components, grain sizes, and mixing ratios are summarized in Table 4).and the mean measurements for these bands, indicate that there is no statistically meaningful leading/trailing hemispherical variations in their subobserver longitudinal distribution.Although our F-Test analyses indicate that the 2.2 μm band is stronger on Umbriel's leading hemisphere (Table 6), the mean measurements for this band indicate that its leading/ trailing hemispherical variations are nonsignificant (Table 5).The mismatch between these results suggests that longitudinal variations in Umbriel's 2.2 μm band are subtle, and we favor a conservative approach and argue that the data do not constitute strong evidence that a leading/trailing asymmetry is present for any of the four bands we analyzed.
Nevertheless, prior work demonstrated that Ariel's 2.2 μm band is slightly stronger on its leading side compared to its trailing side (>1σ difference; Cartwright et al. 2020c).If this subtle hemispherical asymmetry in the 2.2 μm band on Ariel and Umbriel is real, then it could point to an association between the 2.2 μm band and H 2 O ice, which exhibits stronger spectral features on their leading sides (>2σ; Grundy et al. 2006;Cartwright et al. 2015Cartwright et al. , 2018)).The species contributing to the 2.2 μm band may have a crustal source that is exposed by higher impactor frequencies on the leading hemispheres of tidally locked icy moons (e.g., Zahnle et al. 2003), and/or they could be delivered in the impactors themselves.An association between the 2.2 μm band and crustal H 2 O ice indicates the presence of NH 3 -bearing components, which could originate in a subsurface ocean or in undifferentiated crustal materials.Alternatively, if NH 4 and CO 3 salts are contributing to Umbriel's 2.2 μm band, they could have formed in ancient internal oceans, reaching the surface via brine upwelling or impact excavation of salt pockets trapped in the crust (Section 4.3).
Another possibility is that the 2.2 μm band is spatially associated with red material, which is present across Umbriel's surface, but is somewhat more concentrated on its leading hemisphere compared to its trailing side (>1σ difference, Cartwright et al. 2018).In this scenario, the material contributing to the 2.2 μm band, and other spectral features between 2.12 and 2.27 μm, could be rich in organic residues that include both hydrocarbon and nitrile components.Future work that determines the distribution of the 2.2 μm on the outer moons Titania and Oberon, which have redder leading hemispheres compared to Umbriel, would provide an additional test of the possible association between red material, the 2.2 μm band, and complex organics.

Candidate Constituents on Umbriel
We investigated a wide variety of constituents that display absorption features between 2.12 and 2.27 μm that could contribute to Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands.We focused our analysis on NH 3 -and NH 4 -bearing components and organics, which have been detected or inferred in the Pluto system (e.g., Grundy et al. 2016;Cook et al. 2018Cook et al. , 2019Cook et al. , 2023)), and CO 3 -bearing components and phyllosilicates, detected on Ceres (e.g., De Sanctis et al. 2016Sanctis et al. , 2020a) ) and suspected on Callisto (Johnson et al. 2004).In the following sections, we consider these different candidate constituents in greater detail.
NH 3 -bearing species.Mixtures of NH 3 and H 2 O provide reasonable matches to Umbriel's 2.2 μm band, but their band centers are shifted to slightly longer wavelengths (∼2.21 μm;  3 and 4. All continuumdivided bands are offset vertically for clarity.The wavelength range in band centers displayed by the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (Tables A2-A5) are highlighted by the purple, blue, green, and red zones, respectively.The laboratory and synthetic spectra shown here represent the best matches to these four bands, based on comparison of their band centers: (A) 2.   2017).Crystalline NH 3 ice provides a reasonable match to Umbriel's 2.24 μm band in terms of its band center (Figure 5) and its wavelength extent and shape ( 7).Of the 15 different species we investigated, NH 3 ice is the most plausible candidate that could be contributing to Umbriel's 2.24 μm band.Prior work similarly suggested that a 2.24 μm band on Ariel could result from NH 3 ice (Cartwright et al. 2020c).
NH 3 -bearing compounds typically exhibit prominent absorption features near 2.0 μm (e.g., Hudson et al. 2022a) that have not been identified on the Uranian moons.Photon penetration depths into pure H 2 O ice and particulate mixtures of H 2 O ice and amorphous C are significantly lower at 2.0 μm (∼0.1 mm) than near 2.2 μm (∼1.2 mm; e.g., Cartwright et al. 2015Cartwright et al. , 2018)).Consequently, if NH 3 is primarily retained at depths 1 mm, its 2.2 μm band could be detected, whereas its 2 μm band might be obscured.NH 3 -bearing species also typically exhibit features near 1.54 and 1.64 μm (e.g., Roser et al. 2021; Figure A1), which are not present in the Umbriel spectra (Figure 6).However, these shorter-wavelength features are over an order of magnitude weaker than NH 3 absorption bands between 2.0 and 2.3 μm, and they are not exhibited by our synthetic spectra that include NH 3 ice (Figure 6).The absence of these shorter-wavelength NH 3 features in the Umbriel spectra is therefore unsurprising.
NH 3 ice and NH 3 -H 2 O mixtures should be thermodynamically stable at the estimated peak surface temperatures of Umbriel (80-90 K; Sori et al. 2017) and the other Uranian moons, hypothetically allowing them to persist over geologic timescales.However, NH 3 -bearing species could be decomposed by irradiation in 1 Myr (at least on Miranda; Moore et al. 2007), and it is uncertain whether they could persist on the ancient surface of Umbriel without recent endogenic activity to expose crustal NH 3 ice.As described previously, photon penetration depths into pure H 2 O ice and mixtures of H 2 O ice and amorphous C vary significantly as a function of wavelength, ranging between 0.1 and 1.6 mm at 2 and 2.24 μm, respectively.Ultraviolet photons and protons are mostly absorbed within the top 0.01 mm of planetary regoliths (e.g., Delitsky & Lane 1998), and NH 3 retained at greater depths could be shielded from these sources of irradiation.Energetic electrons (∼1 MeV), however, penetrate to centimeter-scale depths (e.g., Nordheim et al. 2017), and they should be able to interact with and decompose NH 3 molecules over the entire range of depths probed by NIR spectra.Nevertheless, deeper deposits might help explain how NH 3 ice could persist over geologic timescales.Additionally, irradiation of "pure" NH 3 ice (i.e., segregated from other constituents in concentrated deposits) should primarily result in fragmented NH 3 molecules back-reacting and recombining into NH 3 (Cruikshank et al. 2019).If NH 3 ice is present in concentrated deposits on Umbriel, then irradiation might not remove it as efficiently as predicted by laboratory experiments (Moore et al. 2007).New laboratory studies that investigate radiolytic modification of NH 3 ice under conditions that simulate the regoliths of Umbriel and the other Uranian moons are needed to further explore this possibility.
NH 4 -bearing species.Many NH 4 salts exhibit ν 1 + ν 4 and/ or ν 3 + ν 4 vibrational modes that result in a suite of absorption features between 2.12 and 2.27 μm that could match Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands.However, the band centers of the NH 4 carbonate (80 K and room temperature) and NH 4 bicarbonate (90 K and room temperature) samples we considered are near 2.18 μm and are considerably broader than the subtle features observed on Umbriel (Figures 4 and 5).Although NH 4 chloride has a band center that is close to Umbriel's 2.2 μm band (∼2.21 μm), it is much broader than the features observed in spectra of Umbriel (Figures 4 and 5).Furthermore, these NH 4 salts exhibit strong absorption bands and/or prominent blue spectral slopes between 1 and 2 μm (Fastelli et al. 2020), whereas Umbriel and the other classical Uranian moons exhibit neutral to slightly red surfaces over these wavelengths (Cartwright et al. 2018) (Figures 6 and A1).Miranda is a possible exception as it is neutral to slightly blue over VIS and NIR wavelengths (Karkoschka 2001), but prominent, non-H 2 O ice absorption features are also absent from spectra of Miranda (Bauer et al. 2002;Gourgeot et al. 2014;Cartwright et al. 2018;DeColibus et al. 2022).
Similar to NH 3 -bearing species, NH 4 salts exhibit features between 1.9 and 2.1 μm that may be present but obscured by H 2 O ice and CO 2 ice absorption bands.Other H 2 O ice-rich bodies such as Pluto's moons Charon, Nix, and Hydra exhibit bands near 2.2 μm, but do not display features near 2.0 μm, possibly because the 2.2 μm band in the Pluto system results from a different constituent, notably NH 4 chloride (Cook et al. 2018(Cook et al. , 2023;;Protopapa et al. 2020).To investigate this possibility further, we linearly combined scaled laboratory reflectance spectra of two NH 4 salts with synthetic spectra that simulate the spectral properties of Umbriel's regolith (Figure 7, Panels I and II).The resulting combined laboratory-synthetic spectra show minimal evidence for features between 1.9 and 2.1 μm and subtle bands between 2.12 and 2.27 μm, consistent with the spectra of Umbriel.However, these spectra also show prominent features between 1.2 and 1.7 μm that are absent from the Umbriel spectra (Figure 6) and the synthetic spectrum that simulates Umbriel's regolith (labeled SS in Figure 7).
The NH 4 combination modes expressed between 2.12 and 2.27 μm can shift in wavelength position, depending primarily on the ion bonded to NH 4 + (e.g., Berg et al. 2016;Fastelli et al. 2020Fastelli et al. , 2022)).Future work that investigates Umbriel's spectral properties should consider different anhydrous and hydrated NH 4 -bearing species to determine whether they can provide better matches to Umbriel's bands than the anhydrous NH 4 salts investigated here.Changes in grain size and temperature can also cause subtle wavelength shifts in the NH 4 vibrational modes (Fastelli et al. 2020(Fastelli et al. , 2022)), but our comparison to room temperature and cryogenic samples likely rules out temperature-related band shifts as a confounding variable in our analysis of NH 4 salts.Although subtle wavelength shifts resulting from grain size effects might occur, these alone are unlikely able to explain the large disparity between the width and shape of Umbriel's bands and the NH 4 salts we analyzed.Synthetic spectra that model particulate mixtures of NH 4 salts and other species could provide better matches to Umbriel's bands.However, the necessary optical constants are not available, and new laboratory studies that calculate and publish the complex indices of refraction for these NH 4 salts are needed.
Thermonatrite.The hydrated Na carbonate sample we investigated provides a good match to Umbriel's 2.2 μm band (Figures 4 and 5).Thermonatrite exhibits features between 1.9 and 2.1 μm (De Angelis et al. 2019) that could be obscured by H 2 O ice.Thermonatrite also displays a band centered near 1.595 μm (Figures 6 and A1) that matches a subtle feature in some of the grand average Umbriel spectra (spectra B and C in Figure 6).We linearly combined the laboratory spectrum of thermonatrite with a synthetic spectrum simulating Umbriel's regolith (Figure 7, Panel III).The combined laboratorysynthetic spectra show subtle 2.2 μm bands and slightly asymmetrical shapes to the 2.02 μm H 2 O ice band, but no other prominent bands, which is broadly consistent with the Umbriel spectra.Consequently, thermonatrite provides the best match to Umbriel's spectral properties of any of the salts we considered.New laboratory work that calculates optical constants for Na carbonates such as thermonatrite is needed to better compare the spectral properties of this constituent to the Umbriel spectra.Like NH 4 salts, Na carbonates are likely stable on Umbriel's surface over geologic timescales.
Kaolinite.Al phyllosilicates like kaolinite exhibit OH combination bands near 2.21 μm due to Al 2 stretching modes (Bishop et al. 2008) that can provide good matches to the center and shape of Umbriel's 2.2 μm band (Figure 5, Table 7).Kaolinite also show a prominent triplet band between 1.395 and 1.415 μm (Figure A1) due to Al 2 -OH stretching (Bishop Figure 6.Comparison between the four grand average spectra (black, A-D) and some representative laboratory and synthetic spectra shown in Figure 4 (gray).The number above each spectrum corresponds to the same spectrum number as shown in Figure 4 and Tables 5 and 6 We focused our analysis on kaolinite, but other hydrated silicates such as montmorillonite, illite, and attapulgite also display absorption bands near 2.2 μm that result from an Al-OH stretching mode (e.g., Clark et al. 1990;Bishop et al. 2008).Furthermore, ammoniated phyllosilicates, where the NH 4 + cation substitutes for alkali metals and/or is stored in the interlayer cations of phyllosilicates, have been detected on Ceres (e.g., De Sanctis et al. 2015;Ammannito et al. 2016;Ehlmann et al. 2018;Singh et al. 2021;Ammannito & Ehlmann 2022).Some of these NH 4 phyllosilicates, such as ammoniated montmorillonite, exhibit 2.2 μm bands (example shown in Figure 9.1 of Ammannito & Ehlmann 2022).Other absorption bands between 2.1 and 2.3 μm, resulting from Fe-OH, Mg-OH, and similar cation-OH stretching modes, are exhibited by other phyllosilicates that might also provide suitable matches to Umbriel's spectral properties.
Organics.The amines, nitrile, and hydrocarbons we investigated show absorption bands centered near 2.22 μm, resulting from a a N-H 2 bending mode (e.g., Workman & Weyer 2012) and a C-H combination mode (attributed to the ν 5 + ν 12 in C 2 H 4 ; e.g., Dartois 2021).Using methylamine as a proxy for these different organics, we found that the synthetic spectra that include organics provide good matches to the center, shape, and extent of Umbriel's 2.22 μm band, and the amines are also good matches for Umbriel's 2.14 μm band (Figure 5, Table 7).The amines we investigated exhibit a wide variety of additional absorption bands between 1.5 and 2.5 μm.Methylamine exhibits prominent features near 1.5 and 1.56 μm and between 1.68 and 1.74 μm (Figure 6), as well as near 2.28 and 2.31 μm (Figure 4).Ethylamine exhibits bands between 1.68 and 1.74 μm (Figure 6), and near 2.17, 2.24, and 2.33 μm (Figure 4).Propionitrile also exhibits a fairly strong band near 2.28 μm (Figure 4).Some of the Umbriel spectra hint at the presence of weak bands between 1.69 and 1.73 μm (Figure 6).Because these features are quite subtle, it is difficult to discern whether they exhibit similar structure to methylamine and ethylamine.The .Scaled laboratory spectra for three different salts (gray) linearly combined with a synthetic spectrum (SS, black) that simulates the composition of Umbriel's regolith, composed of a particulate mixture of crystalline H 2 O ice (0.3 and 20 μm grain sizes, 0.8 and 53.6%, respectively), amorphous C (8 μm grain size, 27.5%), and crystalline CO 2 ice (10 μm grain size, 18.1%).Spectra have been offset vertically for clarity.The range in band centers for Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (Tables A2-A5) is highlighted by the purple, blue, green, and red zones, respectively (shown in Figures 3 and 4).Additional putative features in the Umbriel spectra that may indicate the presence of NH 3 , salts, and organics are highlighted by pairs of dotted lines spanning 1.583-1.597,1.622-1.637,and 1.691-1.731μm (shown in Figure 6).The spectra shown in Panel I are NH 4 carbonate (80 K) scaled by a factor of 0.012 (1a) and combined with spectrum SS (1b); NH 4 carbonate (80 K) scaled by a factor of 0.007 (2a) and combined with spectrum SS (2b).The spectra shown in Panel II are NH 4 chloride (90 K) scaled by a factor of 0.004 (3a) and combined with spectrum SS (3b); NH 4 chloride (90 K) scaled by a factor of 0.002 (4a) and combined with spectrum SS (4b).The spectra shown in Panel III are thermonatrite (93 K) scaled by a factor of 0.016 (5a) and combined with spectrum SS (5b); thermonatrite (93 K) scaled by a factor of 0.008 (6a) and combined with spectrum SS (6b).Unscaled versions of these laboratory spectra are shown in Figures 4 and 6.
presence of subtle bands centered near 1.7 μm may broadly point to the presence of organic species that exhibit C-H stretching modes (e.g., Quirico & Schmitt 1997;Clark et al. 2009).However, these putative bands could be spurious and result from incomplete removal of OH airglow emission, which is much greater between 1.55 and 1.75 μm compared to 2.12-2.27μm (Roth et al. 2016).
The Umbriel spectra also indicate the presence of subtle features beyond 2.27 μm, but telluric contamination increases sharply beyond 2.3 μm, making identification of solid-state absorption features more challenging.Consequently, distinguishing between subtle features resulting from surface or atmospheric species is more difficult at these longer wavelengths, and we leave this task for future work.Similar to amines and nitriles, the hydrocarbons we investigated display a bevy of features between 2 and 2.5 μm, with the most prominent expressed near 2.13 and 2.33 μm in ethylene and 2.18, 2.28, and 2.31 μm in propyne (Figure 4).Although the optical constants we used for crystalline ethylene were measured at 60 K, this constituent can exhibit similar spectral properties over a range of temperatures that overlap Umbriel's estimated peak surface temperatures (ethylene clathrates, 6-160 K; Dartois 2021).Because the organic species we analyzed exhibit many of the same C-N and C-H combination and overtone modes, it seems likely that other nitriles and hydrocarbons would match the subtle absorption bands we identified between 2.12 and 2.27 μm.Future studies that compare Umbriel's spectral properties to different organic species would complement the work reported here.
Mixtures of H 2 O ice and CO 2 ice.Mixtures of CO 2 ice and H 2 O ice in Umbriel's regolith could contribute to its 2.14 μm band.Laboratory experiments demonstrate that intimate mixtures of these two constituents, along with methanol ice, can generate a broad absorption feature centered between 2.133 and 2.138 μm, attributed to a theoretically forbidden overtone (2ν 3 ) of the asymmetric stretching mode (ν 3 ) of CO 2 , which is absent from pure H 2 O ice or pure CO 2 ice (Bernstein et al. 2005).Umbriel's 2.14 μm band is centered between 2.133 and 2.148 μm (Table A2), with a mean band center near 2.139 μm that overlaps the range of band centers reported in Bernstein et al. (2005).The closest match to the mean band center of Umbriel's 2.14 μm band is provided by a sample of intimately mixed H 2 O ice and CO 2 ice (5:1 abundance ratio, shown in Figure 3 of Bernstein et al. 2005).A similar feature detected on Ariel is centered closer to 2.13 μm, with a band center best matching an intimate mixture of H 2 O ice, CO 2 ice, and methanol ice (100:2.5:1abundance ratio; Cartwright et al. 2022).Based on the compositional analysis presented here, we find no evidence for methanol on Umbriel (Figure 4), suggesting that different mixtures of species could be contributing to the spectral properties of Ariel and Umbriel (Section 4.4).
Synopsis.A bevy of different constituents could be present on Umbriel and contribute to one or more absorption bands between 2.12 and 2.27 μm.Our analysis shows that different N-bearing species provide good matches to the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands.These results suggest that nitrogen-rich material is likely present on Umbriel's surface in some form, possibly including crystalline NH 3 ice, which is the only candidate constituent we investigated that provides a suitable match to the 2.24 μm band.The possible presence of N-bearing constituents on Umbriel is consistent with prior identification of NH 3 -bearing species on Ariel and Miranda (Bauer et al. 2002;Cartwright et al. 2018Cartwright et al. , 2020c)).Some of our results also support the presence of species lacking N, including hydrated silicates and Na carbonates (2.2 μm band) and hydrocarbons (2.22 μm band).
Caveats.The constituents we investigated that provide good matches to Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands tend to provide less ideal matches to the Umbriel spectra in the 1.4-1.8μm wavelength range (Figure 6).Differences in photon penetration depths into Umbriel's regolith, between 1.4 and 1.8 μm (0.2-0.5 mm depths) and 2.12-2.27μm (1.0-1.6 mm depths; Cartwright et al. 2018), may contribute to the dissimilarities between the candidate constituents and the Umbriel spectra in these two wavelength regions.Furthermore, the averaging procedure we used, in order to investigate Umbriel's spectral properties using higher S/N spectra, may have obscured or blended absorption bands, or enhanced spurious features from the inclusion of noisy spectra in the grand averages, thereby generating mismatches between the telescope spectra and laboratory and synthetic spectra.However, we see no obvious evidence of obscured, blended, or spurious absorption bands when comparing the four grand average spectra to the 33 individual spectra.Larger groundbased telescopes and the James Webb Space Telescope could collect higher S/N reflectance spectra compared to the SpeX and TripleSpec data reported here, thereby providing additional tests of our results and compositional interpretations.
Other organic species exhibiting C-H, C-N, and N-H combination and overtone modes, additional anhydrous and hydrated salt species, and other phyllosilicates exhibiting Al-OH modes, or ammoniated phyllosilicates could match Umbriel's spectral properties.Variations in sample preparation, experimental design, measurement techniques, improvements in instrument sensitivity, and other confounding variables could alter the spectral signatures of the samples we investigated.The comparisons we made between telescope spectra and laboratory spectra of candidate constituents are therefore extensive, but not exhaustive.Reevaluation of candidate constituents with spectra collected by different laboratory teams and/or under different conditions should be considered in future work.Additionally, the lack of optical constants for NH 3 hydrates, NH 4 salts, and Na carbonates limits more precise comparisons between these species and the Umbriel data.

Implications for Umbriel's Surface Chemistry and Geochemistry
The possible presence of N-bearing species on Umbriel and the other Uranian moons is consistent with cosmochemical models that predict that NH 3 -rich planetesimals were incorporated into these moons as they formed (e.g., Lewis 1972;Prinn & Fegley 1989).Along with a native origin, the presence of N-bearing species could be consistent with infall of meteoritic material of a cometary nature, which is supported by the detection of N-bearing species on comets, including molecular nitrogen in comet 67P/Churyumov-Gerasimenko by the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis (ROSINA) mass spectrometer (Rubin et al. 2015).Nitrogen has been detected on planets, icy satellites, and small bodies across the solar system, yet confirmation of N-bearing species has been challenging due to the faintness of the Uranian moons and the prevalence of stronger absorbers such as H 2 O ice, dark material, and CO 2 ice.Nevertheless, the presence of nitrogen on the Uranian moons would have important implications for their internal states, thermal evolutions, and activity, at present or in the past (Beddingfield et al. 2022a(Beddingfield et al. , 2022b;;Castillo-Rogez et al. 2023).
Radiolysis of N-bearing ices.The possible presence of crystalline NH 3 ice and crystalline methylamine (or other organic ices) on the ancient surface of Umbriel raises important questions regarding the origin of these species, and whether they can persist over geologic timescales once exposed or emplaced.Laboratory experiments demonstrate that irradiation of substrates composed of mixed H 2 O ice, CO 2 ice, and NH 3 ice generates complex nitriles and hydrocarbons (e.g., Allamandola et al. 1988), providing a possible chemical pathway for the generation of refractory organic residues like tholins that could continue to contribute to Umbriel's spectral properties between 2.12 and 2.27 μm.The flux of charged particles at Umbriel, however, is not well constrained, and the ∼59°offset between the orbital plane of the classical moons and Uranus' magnetosphere (Ness et al. 1986) confounds our understanding of the irradiation environment at Umbriel and the other moons (e.g., Kollmann et al. 2020).Nonetheless, interactions between Uranus' magnetosphere and its moons could spur radiolytic chemical reactions that might form complex organics and other species out of exposed surface constituents.Consequently, Umbriel's absorption bands between 2.12 and 2.27 μm may persist over geologic timescales as original inventories of exposed N-bearing ices are modified into more refractory organics that exhibit similar spectral features.
Internal sources of salts and hydrated silicates.Surface deposits rich in NH 4 salts, Na carbonates, and other salts could result from emplacement of briny material formed in Umbriel's interior.Na carbonates form in specific environments that involve interactions between rock and a large volume of water (Castillo-Rogez et al. 2022), as might occur at the water-rock interface in ocean world interiors.For example, Na carbonates have been found in abundance on Ceres, likely due to extensive rock processing in an oceanic environment that includes NH 3 (Castillo-Rogez et al. 2018).Similarly, the extensive plains on Charon may have formed via a large NH 3 -rich cryoflow sourced from its internal ocean (e.g., Beyer et al. 2019), which may have been enriched in CH 4 as well (Menten et al. 2022).Analogous environments are predicted for the Uranian moons' internal oceans (Castillo-Rogez et al. 2023) that could have spurred emplacement of ocean-derived salts and other species in their early histories.Hydrated silicates could be native to these moons, formed by aqueous alteration of silicate-rich material in contact with liquid H 2 O (e.g., McSween Jr. 1979).If these moons have (or had) internal oceans, then phyllosilicates could form as a consequence of aqueous alteration during differentiation, leading to the separation of a rockdominated core.
Brines may have been delivered to Umbriel's surface by diapirism or other endogenic mechanisms in its early history.Similarly, salts and other minerals could have been exposed following the disruption (by impacts or freezing stress) of a thin crust freezing over an early ocean (Neveu et al. 2015).Fine rock particles in suspension in the oceans could reach the surfaces of the moons via mud-ice volcanism, similar to surface deposits rich in carbonates and other carbonaceous materials on Ceres that are suspected to have formed in its interior (Quick et al. 2019;Ruesch et al. 2019;De Sanctis et al. 2020a;Scully et al. 2020;Fagents et al. 2022).On Umbriel, early endogenic activity might be associated with the large "bright" and "dark" polygonal basins identified in contrast-stretched images of Umbriel collected by the Imaging Science System (ISS) on Voyager 2 (Helfenstein et al. 1989).These polygonal basins were interpreted to be relics of an early stage of tectonic disruption, which could have exposed material rich in salts and other minerals retained in Umbriel's crust.
The presence of bright, salt-rich brines in Occator crater and elsewhere on Ceres, a relict ocean world (De Sanctis et al. 2020b), points to the viability of internally processed material reaching the ancient surfaces of icy bodies little modified by endogenic activity (Stein et al. 2019).Similar to Occator on Ceres, Wunda crater (white arrow in Figure 1) and three other large craters on Umbriel have floors that are significantly brighter than the surrounding terrains (Figure 1 in Helfenstein et al. 1989).These bright crater floors could be mantled by post-impact cryovolcanic deposits (Smith et al. 1986), perhaps rich in salts and other high-albedo components originating in Umbriel's interior.Our comparison between telescope and laboratory spectra supports the presence of Na carbonates (Figures 5 and 7), indicating that salts might be present and contributing to bright spots on Umbriel (albeit our results do not favor the presence of NH 4 salts).Alternatively, these bright deposits could be cold traps for CO 2 ice (Sori et al. 2017).Along with material originating in Umbriel's deep interior, another possible scenario is that the Uranian moons' upper crusts have remained undifferentiated and the observed material is representative of the planetesimals that accreted into the moons (Castillo-Rogez et al. 2023).
Impactor-delivered material.The spectral features we analyzed could also result from material delivered to Umbriel's surface in bolides, heliocentric micrometeorites, and irregular satellite dust grains.Impactors rich in anhydrous silicate minerals could collide with the Uranian moons, and the energy from the impact could warm and melt native H 2 O ice, spurring hydration of impactor-delivered silicates and other minerals (e.g., Suttle et al. 2021;Yasui et al. 2021), with no requirement for aqueous alteration in subsurface oceans.However, Na carbonates are not found in carbonaceous chondrites, presumably due to the low waterto-rock ratio characterizing the environments in the parent bodies of these meteorites (Castillo-Rogez et al. 2018).Consequently, if present, Na carbonates on Umbriel would support formation of these salts in a subsurface ocean instead of exogenic delivery and post-impact processing.
Spectrophotometric data for Caliban, the second largest Uranian irregular satellite, show a reduction in flux near 0.7 μm (Vilas et al. 2006), which hints at the presence of a broad band, generally attributed to a Fe 2+ → Fe 3+ charged transfer transition in phyllosilicates detected on some asteroids (Vilas & Gaffey 1989).More recent observations have detected a broad band centered near 0.7 μm in reflectance spectra of the largest irregular satellite Sycorax (Sharkey & Reddy 2022).Although dust from the irregular satellites could be the primary source of red material on the classical moons (Buratti & Mosher 1991;Tamayo et al. 2013;Cartwright et al. 2018), similar broad bands centered near 0.7 μm have not been detected on Uranus' large moons.The work presented here favors the presence of Al-bearing hydrated silicates such as kaolinite and not Fe-bearing phyllosilicates.Nevertheless, Febearing minerals might also be present within the dark material that mantles Umbriel and the other Uranian moons.For example, magnetite (Fe 2+ Fe 3+ 2 O 4 ) has been tentatively identified on Neptune's moon Nereid, which has a low-albedo surface (p 0 = 0.25-0.27,Kiss et al. 2016) and weak H 2 O ice bands, similar to Umbriel (Sharkey et al. 2021).
The dark material on Umbriel could also be rich in organics that were delivered in exogenic material and contribute to its spectral properties between 2.12 and 2.27 μm.The Saturnian moon Iapetus has a dark leading hemisphere that is coated by dust grains originating on Phoebe and Saturn's other irregular satellites (e.g., Soter 1974;Burns et al. 1979;Verbiscer et al. 2009;Tamayo et al. 2011Tamayo et al. , 2014)).Analysis of data collected by the Visual and Infrared Mapping Spectrometer (VIMS) on the Cassini spacecraft demonstrated that a broad feature centered near 3.29 μm is present in pixels covering Iapetus' dark leading hemisphere, consistent with a C-H stretching mode in polycyclic aromatic hydrocarbon (PAH) molecules (e.g., Clark et al. 2005Clark et al. , 2012;;Dalle Ore et al. 2012;Cruikshank et al. 2014) that are possibly delivered in dust grains from Phoebe.Determining whether organic-related spectral features between 3.2 and 3.5 μm are present on Umbriel is a key goal of upcoming observations by the James Webb Space Telescope (Cartwright et al. 2021a).

Comparing the Spectral Properties of Umbriel and Ariel
Umbriel and Ariel exhibit starkly different surface geologies (e.g., Croft & Soderblom 1991;Schenk & Moore 2020), raising important questions about whether their 2.2 μm bands result from the same species and share common origins.We compared the grand average spectra of Umbriel to a grand average spectrum of Ariel (Figure 8).The 2.2 μm bands on these two moons have comparable band depths (∼1%-3%) and span similar wavelength ranges, but the central wavelength of Ariel's 2.2 μm band (∼2.21 μm) is shifted to slightly longer wavelengths than Umbriel's 2.2 μm band (blue zone in Figure 8).NH 3 -bearing species and the other candidate constituents we considered for Umbriel's 2.2 μm band could be plausible for Ariel's 2.2 μm band as well.
Unlike Umbriel, the grand average spectrum of Ariel does not display 2.14 μm, 2.22 μm, or 2.24 μm bands.Some individual spectra of Ariel do display 2.24 μm bands, which were tentatively attributed to NH 3 ice (Cartwright et al. 2020c).Umbriel's 2.14 μm band could result from a forbidden transition mode of CO 2 ice, which has been suggested for a 2.13 μm band that was detected in two spectra of Ariel (Cartwright et al. 2022).The offset central wavelengths of these two bands, however, suggest that the constituents contributing to them may be slightly different, and/or they formed under different conditions, possibly because of the much higher concentration of dark material and weaker H 2 O and CO 2 ice bands on Umbriel.The 2.22 μm band has yet to be detected on Ariel, suggesting that it could result from organics associated with the dark material mantling Umbriel.In this scenario, the 2.22 μm band might also be present on Titania and Oberon, which both have considerably more dark material on their surfaces than Ariel (e.g., Cartwright et al. 2018).

Conclusions and Future Work
We measured the band strengths and subobserver longitudinal distributions of four subtle absorption features centered near 2.14, 2.2, 2.22, and 2.24 μm (Figures 1 and 2, Tables A2-A5).We compared these four bands to the spectral signatures of 15 candidate constituents (Figures 4 and 5).Our  , E).The grand average Ariel spectrum was generate using the five IRTF/SpeX spectra that display >3σ band area and depth measurements for Ariel's 2.2 μm band (reported in Cartwright et al. 2020c).All five spectra are normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.Each spectrum has been smoothed using a 10-14 pixel wide boxcar function, whereas the 1σ uncertainties are shown at their native resolutions.The range in band centers for Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (Tables A2-A5) is highlighted by the purple, blue, green, and red zones, respectively.
comparison shows that NH 3 -H 2 O mixtures and NH 3 ice provide suitable matches to the 2.2 and 2.24 μm bands, respectively, similar to a prior analysis of Ariel (Cartwright et al. 2020c).A variety of nitrogenous organics provides good matches to Umbriel's 2.14 μm and 2.22 μm bands.Thus, our results and analyses best support the presence of N-bearing species on Umbriel, contributing to its spectral properties through a variety of subtle bands.
Other species that lack nitrogen can match some of the spectral features we investigated.The 2.14 μm band could result from a 2ν 3 overtone of CO 2 ice.Al phyllosilicates such as kaolinite and Na carbonates such as thermonatrite can provide reasonable matches to Umbriel's 2.2 μm band (Figures 5 and 7, Table 7).Hydrocarbons such as ethylene can match Umbriel's 2.22 μm band.In contrast, our analysis demonstrates that amorphous NH 3 , methanol, and NH 4 salts do not provide good matches to Umbriel's spectral properties (Figures 4-7).We also compared Umbriel's spectra properties at shorter wavelengths (1.55-1.75μm) to some of the candidate constituents, including NH 4 chloride and NH 4 carbonate, finding no matches between these NH 4 salts and the Umbriel spectra (Figure 6).Some of the Umbriel spectra show subtle bands centered near 1.7 μm, hinting at the presence of organics (Figure 6).However, it is uncertain whether these features are real or result from incomplete removal of OH airglow emission.
A fuller assessment of the possible contributions from NH 4 salts and Na carbonates would greatly benefit from the calculation of optical constants for these constituents, which could be used to model their spectral contributions in particulate mixtures with other species using radiative transfer models.Continuing NIR observations of Umbriel will improve the overall S/N of the data set presented here.Analysis of weak 2.2 μm features detected on Titania and Oberon would improve our understanding of the species contributing to these absorption bands across the Uranian system.Planned observations of Umbriel, Ariel, Titania, and Oberon by the NIRSpec spectrograph on the James Webb Space Telescope (Jakobsen et al. 2022) will provide key information on whether NH 3 , organics, and other species are present on these moons (∼3-5 μm; Cartwright et al. 2021a).Finally, spectral observations made by a NIR mapping spectrometer on a Uranus orbiter would provide unparalleled access to the surface composition of Umbriel and the other Uranian moons (Beddingfield et al. 2020;Cartwright et al. 2021b;Leonard et al. 2021;Cohen et al. 2022; National Academies of Sciences, Engineering, and Medicine 2022).
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.This work is partially based on observations obtained with the ARC 3.5 m telescope at Apache Point Observatory, which is owned and operated by the Astrophysical Research Consortium.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.This research made use of the SIMBAD database, operated at CDS, Strasbourg, France.Here we report our assessment of the fit between the synthetic spectra we generated and the continuum of each of the 33 NIR spectra of Umbriel, using R 2 c statistics (Table A1).The results are organized using the same subobserver longitude information as shown in Table 1.A.2. Methods: End-member Synthetic Spectra Here we show end-member synthetic spectra for the constituents included in the particulate mixtures we analyzed in this study (Figure A1).    .Seven reflectance spectra of Umbriel.All 15 spectra (A2, A3) were normalized to unity between 1.74 and 1.77 μm.These spectra were smoothed using the boxcar function described in the caption of Figure 1 (black), along with the 1σ uncertainties shown at their native resolutions (gray).The mid-observation subobserver longitude is listed in the top left corner of each plot.Relevant observing information for each spectrum is summarized in Table 1, organized by subobserver longitude.The greater point-to-point variation and lower S/N between 1.08 and 1.14 μm, 1.31 and 1.46 μm, 1.78 and 1.96 μm, and 2.35 μm results from increased telluric absorption in these wavelength regions.The sensitivities of SpeX (SXD mode) and TripleSpec are lower at wavelengths <1.4 μm and >2.3 μm, further reducing the S/N beyond these wavelengths.Additional structure at wavelengths <1.3 μm in some of the spectra results from scattered-light contributions from Uranus.

i 2 s
 is the variance (e.g.,Bevington & Robinson 1969).R 2 c  values close to 1 are generally indicative of a good match between a model and observed data.can be indicative of good matches between observed and modeled data, but can also result from overestimating observed data uncertainties.Observed data with large errors can lower the accuracy of R 2 c  statistics.The R 2 c

Figure 1 .
Figure1.Left: Nineteen Umbriel spectra (black, labeled a-s) and 1σ uncertainties (gray error bars) that display 2.14 μm (purple markers), 2.2 μm (blue markers), 2.22 μm (green markers), and/or 2.24 μm (red markers) absorption features with both band depths and areas >2σ (TablesA2-A5).The subobserver longitude for each spectrum is also listed (Table1).All spectra were normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.To make spectral features easier to identify, the data have been lightly smoothed using boxcar functions ranging from 4 pixels wide (IRTF/SpeX PRISM mode spectra), 8 to 12 pixels wide (IRTF/ SpeX SXD mode spectra), and 16 pixels wide (3.5 m/TripleSpec spectra).Right: Modified simple cylindrical map projection of the Voyager 2/Imaging Science System image mosaic of Umbriel (NASA/JPL/Caltech/USGS).The subobserver longitude and latitude for all 33 spectra are indicated with filled circles that represent the center of the target disk.These observations were disk integrated and averaged over an entire hemisphere.The 19 spectra on the left are represented by colored circles (labeled a-s), with colors and band centers defined below the map.Spectra that display two bands are represented by two-color semicircles, spectra that display three bands are represented by three-color wedges, and spectra without band depth and area measurements >2σ are shown as black circles.The white arrow points to the bright annulus of material mantling the floor of Wunda crater.The candidate constituents listed below the map are shown in Figure4and discussed in further detail in Sections 3.4 and 4.2.
Figure1.Left: Nineteen Umbriel spectra (black, labeled a-s) and 1σ uncertainties (gray error bars) that display 2.14 μm (purple markers), 2.2 μm (blue markers), 2.22 μm (green markers), and/or 2.24 μm (red markers) absorption features with both band depths and areas >2σ (TablesA2-A5).The subobserver longitude for each spectrum is also listed (Table1).All spectra were normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.To make spectral features easier to identify, the data have been lightly smoothed using boxcar functions ranging from 4 pixels wide (IRTF/SpeX PRISM mode spectra), 8 to 12 pixels wide (IRTF/ SpeX SXD mode spectra), and 16 pixels wide (3.5 m/TripleSpec spectra).Right: Modified simple cylindrical map projection of the Voyager 2/Imaging Science System image mosaic of Umbriel (NASA/JPL/Caltech/USGS).The subobserver longitude and latitude for all 33 spectra are indicated with filled circles that represent the center of the target disk.These observations were disk integrated and averaged over an entire hemisphere.The 19 spectra on the left are represented by colored circles (labeled a-s), with colors and band centers defined below the map.Spectra that display two bands are represented by two-color semicircles, spectra that display three bands are represented by three-color wedges, and spectra without band depth and area measurements >2σ are shown as black circles.The white arrow points to the bright annulus of material mantling the floor of Wunda crater.The candidate constituents listed below the map are shown in Figure4and discussed in further detail in Sections 3.4 and 4.2.

Figure 2 .
Figure2.Band area (left column) and band depth (right column) measurements and 1σ uncertainties for Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands, displayed as a function of subobserver longitude (measurements reported in TablesA2-A5).The solid blue lines show the mean area and depth measurements for each band, and the dashed orange lines show sinusoidal fits to the data.Gray-toned zones represent duplicated subobserver longitudes.We compared the fits provided by the mean and sinusoidal models using an F-test (results reported in Table6).Several data points on the 2.22 μm and 2.24 μm band measurement plots are notably larger than the mean band area or depth values, suggesting that the constituents contributing to these bands might be concentrated in specific geologic features or regions on Umbriel's surface.Spatially resolved spectra are required to further investigate this possibility.

Figure 5 .
Figure 5.Comparison between continuum-divided absorption features in the four grand average spectra (black, A-D) and representative laboratory and synthetic spectra (gray).The number to the right of each spectrum corresponds to the same number shown in Figure 4 and Tables3 and 4. All continuumdivided bands are offset vertically for clarity.The wavelength range in band centers displayed by the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (TablesA2-A5) are highlighted by the purple, blue, green, and red zones, respectively.The laboratory and synthetic spectra shown here represent the best matches to these four bands, based on comparison of their band centers: (A) 2.14 μm band compared to methylamine (15); (B) 2.2 μm band compared to NH 4 chloride (11 * ), NH 4 carbonate (7 * ), flash-frozen NH 3 -H 2 O solution (2), mixed NH 3hydrates (1), kaolinite (14), and thermonatrite (13); (C) 2.22 μm band compared to amorphous NH 3 (3) and methylamine (15); and (D) 2.24 μm band compared to NH 3 ice (4 and 5).Continuum-divided bands 7 * and 11 * only represent the central portion of the broad absorption features present in these spectra (Figure 4).
Figure 5.Comparison between continuum-divided absorption features in the four grand average spectra (black, A-D) and representative laboratory and synthetic spectra (gray).The number to the right of each spectrum corresponds to the same number shown in Figure 4 and Tables3 and 4. All continuumdivided bands are offset vertically for clarity.The wavelength range in band centers displayed by the 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (TablesA2-A5) are highlighted by the purple, blue, green, and red zones, respectively.The laboratory and synthetic spectra shown here represent the best matches to these four bands, based on comparison of their band centers: (A) 2.14 μm band compared to methylamine (15); (B) 2.2 μm band compared to NH 4 chloride (11 * ), NH 4 carbonate (7 * ), flash-frozen NH 3 -H 2 O solution (2), mixed NH 3hydrates (1), kaolinite (14), and thermonatrite (13); (C) 2.22 μm band compared to amorphous NH 3 (3) and methylamine (15); and (D) 2.24 μm band compared to NH 3 ice (4 and 5).Continuum-divided bands 7 * and 11 * only represent the central portion of the broad absorption features present in these spectra (Figure 4).
Figure6.Comparison between the four grand average spectra (black, A-D) and some representative laboratory and synthetic spectra shown in Figure4(gray).The number above each spectrum corresponds to the same spectrum number as shown in Figure4and Tables5 and 6.All spectra were normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.Wavelength regions that show subtle features in the Umbriel spectra are highlighted by pairs of dotted lines spanning 1.583-1.597,1.622-1.637,and 1.691-1.731μm.The laboratory spectra shown in Panel I are NH 4 chloride (11), NH 4 bicarbonate (10), and thermonatrite (13).The synthetic spectra shown in Panel II include kaolinite (14), amorphous NH 3 (3), NH 3 ice (4), methylamine (15), and ethylamine (16).

Figure 7
Figure7.Scaled laboratory spectra for three different salts (gray) linearly combined with a synthetic spectrum (SS, black) that simulates the composition of Umbriel's regolith, composed of a particulate mixture of crystalline H 2 O ice (0.3 and 20 μm grain sizes, 0.8 and 53.6%, respectively), amorphous C (8 μm grain size, 27.5%), and crystalline CO 2 ice (10 μm grain size, 18.1%).Spectra have been offset vertically for clarity.The range in band centers for Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (TablesA2-A5) is highlighted by the purple, blue, green, and red zones, respectively (shown in Figures3 and 4).Additional putative features in the Umbriel spectra that may indicate the presence of NH 3 , salts, and organics are highlighted by pairs of dotted lines spanning 1.583-1.597,1.622-1.637,and 1.691-1.731μm (shown in Figure6).The spectra shown in Panel I are NH 4 carbonate (80 K) scaled by a factor of 0.012 (1a) and combined with spectrum SS (1b); NH 4 carbonate (80 K) scaled by a factor of 0.007 (2a) and combined with spectrum SS (2b).The spectra shown in Panel II are NH 4 chloride (90 K) scaled by a factor of 0.004 (3a) and combined with spectrum SS (3b); NH 4 chloride (90 K) scaled by a factor of 0.002 (4a) and combined with spectrum SS (4b).The spectra shown in Panel III are thermonatrite (93 K) scaled by a factor of 0.016 (5a) and combined with spectrum SS (5b); thermonatrite (93 K) scaled by a factor of 0.008 (6a) and combined with spectrum SS (6b).Unscaled versions of these laboratory spectra are shown in Figures4 and 6.

Figure 8 .
Figure 8. Umbriel grand average spectra (black, A-D) and 1σ uncertainties (light gray error bars) compared to a grand average spectrum and 1σ uncertainties (light gray error bars) of Ariel (black, E).The grand average Ariel spectrum was generate using the five IRTF/SpeX spectra that display >3σ band area and depth measurements for Ariel's 2.2 μm band (reported inCartwright et al. 2020c).All five spectra are normalized to unity between 1.74 and 1.77 μm and offset vertically for clarity.Each spectrum has been smoothed using a 10-14 pixel wide boxcar function, whereas the 1σ uncertainties are shown at their native resolutions.The range in band centers for Umbriel's 2.14 μm, 2.2 μm, 2.22 μm, and 2.24 μm bands (TablesA2-A5) is highlighted by the purple, blue, green, and red zones, respectively.

c
Comparison between Umbriel Spectra and Continuum Models

A
.3.Results: IRTF/SpeX and ARC 3.5 m/TripleSpec Spectra Here we show a total of 15 NIR reflectance spectra of Umbriel, separated into Figure A2 and Figure A3.

Figure A3
Figure A3.Seven reflectance spectra of Umbriel.All 15 spectra (A2, A3) were normalized to unity between 1.74 and 1.77 μm.These spectra were smoothed using the boxcar function described in the caption of Figure1(black), along with the 1σ uncertainties shown at their native resolutions (gray).The mid-observation subobserver longitude is listed in the top left corner of each plot.Relevant observing information for each spectrum is summarized in Table1, organized by subobserver longitude.The greater point-to-point variation and lower S/N between 1.08 and 1.14 μm, 1.31 and 1.46 μm, 1.78 and 1.96 μm, and 2.35 μm results from increased telluric absorption in these wavelength regions.The sensitivities of SpeX (SXD mode) and TripleSpec are lower at wavelengths <1.4 μm and >2.3 μm, further reducing the S/N beyond these wavelengths.Additional structure at wavelengths <1.3 μm in some of the spectra results from scattered-light contributions from Uranus.

Table 2
Solar Analogs and Telluric Standard Stars

Table 3
Laboratory and Synthetic Spectra of Candidate Constituents a Downloaded from the Solid Spectroscopy Hosting Architecture of Databases and Expertise (SSHADE; Schmitt et al. 2018).b Optical constants downloaded from the Cosmic Ice Laboratory, located at the NASA Goddard Space Flight Center (https://science.gsfc.nasa.gov/691/cosmicice/constants.html)).

Table 4
Component Grain Sizes and Mixing Ratios for Synthetic Spectra that Include Candidate Constituents

Table 7
Table 7).Furthermore, amorphous NH 3 transitions to a polycrystalline state at temperatures near 65 K (Dawes et al. 2007) and might not be stable at Umbriel's estimated peak surface temperatures (80-90 K; Sori et al.