The science case for spacecraft exploration of the Uranian satellites: Candidate ocean worlds in an ice giant system

The 27 satellites of Uranus are enigmatic, with dark surfaces coated by material that could be rich in organics. Voyager 2 imaged the southern hemispheres of Uranus' five largest 'classical' moons Miranda, Ariel, Umbriel, Titania, and Oberon, as well as the largest ring moon Puck, but their northern hemispheres were largely unobservable at the time of the flyby and were not imaged. Additionally, no spatially resolved datasets exist for the other 21 known moons, and their surface properties are essentially unknown. Because Voyager 2 was not equipped with a near-infrared mapping spectrometer, our knowledge of the Uranian moons' surface compositions, and the processes that modify them, is limited to disk-integrated datasets collected by ground- and space-based telescopes. Nevertheless, images collected by the Imaging Science System on Voyager 2 and reflectance spectra collected by telescope facilities indicate that the five classical moons are candidate ocean worlds that might currently have, or had, liquid subsurface layers beneath their icy surfaces. To determine whether these moons are ocean worlds, and investigate Uranus' ring moons and irregular satellites, close-up observations and measurements made by instruments onboard a Uranus orbiter are needed.


INTRODUCTION AND RATIONALE FOR A SPACECRAFT MISSION TO URANUS
The surfaces of Uranus' large and tidally-locked 'classical' moons Miranda, Ariel, Umbriel, Titania, and Oberon exhibit photogeologic evidence for endogenic activity. The ubiquitous evidence hinting at geologic communication between the interiors and surfaces of these moons, in particular on Miranda and Ariel, makes them candidate ocean worlds that may have, or once had, subsurface liquid H 2 O layers beneath their icy exteriors (Hendrix et al. 2019;Beddingfield & Cartwright 2020;Schenk & Moore 2020). In 1986, the Voyager 2 spacecraft flew by the Uranian system and collected tantalizing snapshots of these classical moons, measured Uranus' offset and tilted magnetic field, and discovered ten new ring moons orbiting within Uranus' ring system (e.g., Smith et al. 1986). Since this brief flyby, exploration of Uranus and its satellites has remained in the purview of ground-and space-based telescopes. At nearinfrared (NIR) wavelengths, these telescope observations have revealed that the surfaces of the five classical moons are dominated by H 2 O ice mixed with dark material that could be rich in organics and silicate minerals (e.g., Cruikshank et al. 1977;Cruikshank 1980;Cruikshank & Brown 1981;Soifer et al. 1981;Brown & Cruikshank 1983;Brown & Clark 1984). NIR observations have also detected carbon dioxide (CO 2 ) ice on the classical moons (Grundy et al. 2003(Grundy et al. , 2006Cartwright et al. 2015), and nitrogen-bearing constituents like ammonia (NH 3 ) and ammonium (NH 4 + ) could be present in surface deposits as well (Bauer et al. 2002;Cartwright et al. 2018;Cook et al. 2018;Cartwright et al. 2020c;DeColibus et al. 2020). The surfaces of the classical moons could therefore be rich in C, H, O, and N-bearing species, representing some of the key chemical requirements for life as we know it.
Although data collected by Voyager 2 and ground-and space-based telescope facilities have led to some fascinating discoveries, our understanding of Uranus' classical moons is severely limited by the absence of data collected during close-up observations made by a Uranus orbiter. Similarly, Uranus' small ring moons and irregular satellites remain unexplored, and their surface geologies and compositions are almost entirely unknown. New measurements made by modern instruments onboard an orbiting spacecraft are critical to investigate the surfaces and interiors of the classical moons and determine whether they are ocean worlds with present day subsurface liquid H 2 O layers. As described by multiple recent studies (e.g., Fletcher et al. 2020;Cohen et al. 2020;Leonard et al. 2021), a mission to Uranus would allow us to study interactions between Uranus' magnetosphere and its rings and moons, improve our understanding of how geologic processes operate in cold and distant ice giant systems, and enable a more complete investigation of organics in the outer Solar System. An orbiter would provide us with an unparalleled opportunity to study the origin and evolution of Uranus' ring-moon system. Furthermore, an orbiter could provide new insight into the origins of Uranus' irregular satellites, and whether they were sourced from the primordial Kuiper Belt or were captured from nearby heliocentric orbits (e.g., Jewitt & Haghighipour 2007).
Multiple close proximity flybys made by a Uranus orbiter would therefore address many outstanding science questions for the Uranian system (Table 1). A spacecraft mission to Uranus can be carried out with existing chemical propulsion technology by making use of a Jupiter gravity assist in the 2030 -2034 time frame, leading to a flight time of only ∼11 years, arriving in the early to mid 2040's (Hofstadter et al. 2019). Crucially, this arrival time frame in northern summer would allow us to observe the Uranian moons' northern hemispheres, which were shrouded by winter darkness at the time of the Voyager 2 flyby and have never been imaged. An orbiter could then continually collect data and search for evidence of ongoing geologic activity, as well as search for evidence of volatile migration of CO 2 and other species in response to seasonal changes as the Uranian system transitions into southern spring in 2050.
An orbiter equipped with several key instruments could determine whether the classical Uranian moons are ocean worlds. The highest priority instrument for identifying subsurface oceans is a magnetometer, which could detect and characterize induced magnetic fields emanating from briny liquid layers in the interiors of these moons. Visible (VIS, 0.4 -0.7 µm) and mid-infrared (MIR, 5 -250 µm) cameras would be vital for identifying recent geologic activity, hot spots, and possible communication between the interiors and surfaces of these moons. A VIS and NIR mapping spectrometer (0.4 -5 µm) would be critical for characterizing the spectral signature and distribution of volatile species that could result from recent outgassing of material, as well as endogenic salts that may have been exposed or emplaced by geologic activity. Ideally, an orbiter would get close enough to achieve spatial resolutions of 100 m/pixel for large regions of each classical moon's surface. If only a limited number of close flybys are possible, Ariel and Miranda likely represent the highest priority targets because they display the best evidence for geologic activity in the recent past. • Do the classical moons have tenuous atmospheres?
• Search for exospheres. VIS camera • Do volatile constituents migrate seasonally?
• Characterize the distributions and VIS/NIR mapping spectral signatures of spectrometer condensed volatiles.
Plasma spectrometer UV imaging spectrometer • Do magnetospheric charged particles weather the • Characterize magnetic field and Magnetometer surfaces of the ring moons and classical moons? charged particle populations Plasma spectrometer proximal to these moons.
Energetic particle • Search for evidence of detector irradiation darkening of surface ices VIS/NIR spectrometer and the presence of radiolytic products.
UV imaging spectrometer • Is the red material on the classical moons • Make an inbound flyby of an irregular VIS camera organic-rich, and did it originate from the satellite, and image the irregular VIS/NIR mapping irregular satellites? satellites while in Uranus orbit. spectrometer • Characterize the compositions of the Dust spectrometer classical moons and collect dust samples.
• Does the ring moon Mab sustain the µ-ring?
• Compare the surface compositions of VIS camera • Does µ-ring material coat Puck and Miranda?
Puck and Miranda to the composition VIS/NIR mapping of Mab and the µ-ring. spectrometer • Collect µ-ring dust samples.
Dust spectrometer • What is the dynamic history of the moons?
• Measure eccentricities, inclinations, Radio science • Did the classical moons share orbital resonances tidal Q(ω) of Uranus, and Love numbers. subsystem in the past, and did these resonances drive tidal Analyze topography of geologic features VIS camera heating and endogenic activity?
to estimate paleo heat fluxes.
• Did the classical moons form in Uranus' • Measure crater frequency distributions. VIS camera circumplanetary disk or evolve from its rings?
• Compare surface compositions VIS/NIR mapping of ring moons and classical moons. spectrometer • Search for streamer channels in the rings.

BACKGROUND AND SCIENCE QUESTIONS
Uranus is orbited by 27 known moons (Figure 1), including its five largest moons, Miranda, Ariel, Umbriel, Titania, and Oberon, which have semi-major axes ranging between 5.1 to 23 Uranian radii (R U ) (Table 2, Figure 2). Interior to the classical moons, thirteen small ring moons orbit within Uranus' ring system, with semi-major axes ranging between 2 to 3.8 R U (Table 3, Figure 3). Far beyond the orbital zone of the classical moons, nine irregular satellites orbit Uranus on highly inclined and eccentric orbits, with semi-major axes ranging between 169 to 805 R U (Table 3, Figure 1). In the following subsections, we briefly describe the state of knowledge and some of the outstanding science questions for these 27 moons. The sizes of the five classical moons (white filled circles) are scaled correctly relative to each other but have been increased by a factor of 20 relative to the size of Uranus (cyan filled circle). The orbits and relative sizes of the ring moons are shown in Figure 3. Right: Diagram illustrating the orbits of Uranus' eight retrograde irregular satellites (red) and one prograde irregular satellite (blue). The orbital zone of the ring and classical moons is contained within the cyan colored box. The orbits of the irregular satellites were simulated using orbital elements provided on the Jet Propulsion Laboratory Horizons On-Line Ephemeris System ( https:// ssd.jpl.nasa.gov/ ?sat elem, originally generated by: Nrco0e -Own work, CC BY-SA 4.0, https:// commons.wikimedia.org/ w/ index.php? curid=98634845 ).

Geology of the Uranian Satellites
Classical Moons: The Imaging Science System (ISS) onboard Voyager 2 collected fascinating images of the five classical moons (Figure 2). Because the Voyager 2 flyby occurred during southern summer, when the subsolar point was ∼81 • S, the northern hemispheres of these moons were mostly hidden from view and were not imaged (Smith et al. 1986). The incomplete spatial coverage, and generally low spatial resolution of the available images, limits our understanding of different terrains and geologic features, in particular for the more distant moons Umbriel, Titania, and Oberon.
The innermost moon Miranda displays abundant evidence for endogenic geologic activity, including three large polygonal shaped regions called coronae, which were likely formed by tectonic and/or cryovolcanic processes (Figure 1a-c) (e.g., Smith et al. 1986;Croft & Soderblom 1991;Greenberg 1991;Schenk 1991;Kargel 1995;Pappalardo et al. 1997;Hammond & Barr 2014;Beddingfield et al. 2015;Beddingfield & Cartwright 2020). The origin and time scale of activity on Miranda is not well understood, and it is unknown if this activity is associated with a subsurface ocean, either now or in the past. Searching for and characterizing induced magnetic fields, plumes, and surface heat anomalies, as well as analyzing geologic surface features interpreted to be cryovolcanic in origin, is paramount to determine if Miranda is an ocean world. Tidal heating of Miranda from past orbital resonances (Tittemore & Wisdom 1990;Ćuk et al. 2020) may have been an important driver of resurfacing in the recent past. Additionally, Miranda displays ancient cratered terrain pockmarked with 'subdued' craters, which have smooth floors and subtle rims that have been mantled by an unknown source of material (e.g., Smith et al. 1986;Beddingfield & Cartwright 2020;Cartwright et al. 2020a). These subdued craters are reminiscent of lunar highlands and could be blanketed by impact-generated regolith (Croft 1987), but they are also reminiscent of the plume-mantled craters on the ocean world Enceladus (e.g., Kirchoff & Schenk 2009), hinting that a similar plume-driven mantling process may have occurred on Miranda. Miranda's neighbor Ariel also displays widespread evidence for resurfacing that could have been spurred by ocean world activity. Sections of Ariel's imaged surface are dominated by large canyons called 'chasmata' (Figure 2d, 2e) that were likely formed by tectonic processes (e.g., Smith et al. 1986). The smooth floors of some of these chasmata are bowed up with two parallel medial ridges that are separated by a topographic low, reminiscent of fissure style volcanism on Earth (Schenk 1991). Large fracture systems cut across other parts of Ariel's surface, and clusters of curvilinear features referred to as 'flow bands' could be cryovolcanic features (e.g., Plescia 1987;Croft & Soderblom 1991;Schenk 1991;Beddingfield & Cartwright 2021). Furthermore, Ariel's surface geology is consistent with high heat fluxes in its interior, which could drive extrusion of material (Peterson et al. 2015). Regions of Ariel's surface are relatively young (∼1-2 Ga) (Zahnle et al. 2003), but the fairly low resolution of the ISS images ( 1 km/pixel) makes investigation of the processes that formed these younger terrains more difficult.
Although Umbriel has the darkest and oldest surface of the five classical moons (e.g. Smith et al. 1986;Zahnle et al. 2003), it has some large craters with bright floors, like Wunda crater, which has a bright annulus of material surrounding its central peak (e.g., Smith et al. 1986) (Figure 2f). Wunda crater is located near the center of Umbriel's trailing hemisphere, where the abundance of CO 2 ice is highest (Grundy et al. 2006;Cartwright et al. 2015). Consequently, the bright annulus of material mantling this crater may represent a large deposit of CO 2 ice, possibly originating from post-impact cryovolcanic infilling (Helfenstein et al. 1989), or from the accumulation of radiolytically-produced CO 2 molecules that get cold trapped in Wunda (Sori et al. 2017). Furthermore, large polygonal basins are present on Umbriel, hinting at global-scale resurfacing in the past (Helfenstein et al. 1989). However, the poor resolution of the available data ( 5 km/pixel) severely limits analyses of these features and our ability to determine whether they resulted from ocean world activity.
The surfaces of the outer moons Titania and Oberon exhibit evidence for tectonism, with large chasmata and linear surface features, as well as smooth plains that may have formed from cryovolcanic processes (e.g., Smith et al. 1986;Croft & Soderblom 1991) (Figure 2g, 2h). Additionally, Oberon has an ∼11 km tall 'limb mountain' that could be the central peak for a relaxed complex crater (Smith et al. 1986;McKinnon et al. 1991;Croft & Soderblom 1991;Moore et al. 2004). Similar to Umbriel, the poor resolution of the ISS images for Titania and Oberon ( 3 and 6 km/pixel, respectively) limits our ability to investigate possible ocean world activity on these two moons.
Relatively less is known about the interiors of the classical moons, and measuring magnetic induction with a magnetometer is critical for determining whether they possess subsurface oceans (Cochrane et al. 2021;Weiss et al. 2021). Other measurements are needed to investigate the internal structures and bulk compositions of these moons, which is critical for improving our understanding of their formation and evolution. The densities of Ariel, Umbriel, Titania, and Oberon are 1.66 ± 0.15 g cm −3 , 1.39 ± 0.16 g cm −3 , 1.71 ± 0.05 g cm −3 , and 1.63 ± 0.05 g cm −3 , respectively, indicating that these moons are made of at least 50% silicate material by mass, whereas Miranda's density is only 1.2 ± 0.14 g cm −3 , indicating a larger H 2 O ice fraction (Jacobson et al. 1992). Measuring the non-spherical gravity field would shed light on the differentiation state and the nature of the endogenic activity exhibited by these moons. Measuring libration amplitudes (Steinbrügge et al. 2019) could also be used to investigate whether these moons have subsurface liquid layers. Furthermore, searching for thermal anomalies across the surfaces of these moons, using a MIR camera equipped with a suite of multiple narrow and wide filters spanning 5 to 250 µm, would improve our understanding of their heat budgets and the long-term survivability of liquid water in their interiors. Thus, characterizing the surfaces and interiors of the classical moons to determine whether they are, or were, ocean worlds requires high resolution datasets, which can only be collected by a Uranus orbiter making multiple close flybys of each moon. Ring Moons and Irregular Satellites: Uranus possesses the most densely packed group of moons in the Solar System, with nine ring moons (Bianca to Perdita) orbiting between 59,000 to 77,000 km from the planet's center (e.g., Showalter 2020). Voyager 2 initially discovered ten ring moons: Cordelia, Ophelia, Bianca, Cressida, Desdemona, Juliet, Portia, Rosalind, Belinda, and Puck (e.g., Smith et al. 1986). Perdita was discovered later through reanalysis of Voyager 2 images (Karkoschka 2001). Cupid and Mab were discovered using the Hubble Space Telescope (Showalter & Lissauer 2006;De Pater et al. 2006). Mab orbits within the outermost and dusty µ-ring (blue colored region in Figure 3), which might be sustained by material ejected from the surface of this tiny moon (Showalter & Lissauer 2006;De Pater et al. 2006;Sfair & Winter 2012). Furthermore, µ-ring material could spiral inward and coat the leading hemisphere of Puck (French et al. 2017), and perhaps it also spirals outward and mantles Miranda, possibly contributing to its substantial regolith cover (Cartwright et al. 2020a,b). Little is known about the surface geologies of the ring moons as only Puck was spatially resolved by Voyager 2/ISS (∼4.5 km/pixel) (Figure 3). These ISS images revealed a heavily cratered surface, suggesting that Puck may have been collisionally disrupted and then reaccreted into a rubble pile (Smith et al. 1986;Croft & Soderblom 1991). Voyager 2 did not detect any of Uranus' nine known irregular satellites, which were discovered by ground-based observers (Gladman et al. 1998(Gladman et al. , 2000Kavelaars et al. 2004;Sheppard et al. 2005). Thus, the geologies of Uranus' ring moons and irregular satellites remain unexplored, and new observations made by an orbiter would dramatically expand our knowledge of these objects.

Surface Compositions of the Uranian Satellites
Classical Moons: Ground-based, NIR telescope observations (∼1 -2.5 µm) determined that the classical moons have surface compositions dominated by a mixture of H 2 O ice and a dark, spectrally neutral material of unknown origin (e.g., Cruikshank et al. 1977;Brown & Cruikshank 1983;Brown & Clark 1984). Laboratory experiments indicate that the dark material has a spectral signature similar to amorphous carbon and/or silicates (Clark & Lucey 1984). More recent NIR telescope observations have determined that the Uranian moons display leading/trailing and planetocentric asymmetries in their compositions (Figure 4). For example, 'pure' CO 2 ice (i.e., segregated from other constituents in concentrated deposits) has been detected on the trailing hemispheres of Ariel, Umbriel, Titania, and Oberon, with larger abundances on the inner moons, Ariel and Umbriel, compared to the outer moons, Titania and Oberon ( Figure  5). (Grundy et al. 2003(Grundy et al. , 2006Cartwright et al. 2015). CO 2 ice on these moons could be generated via irradiation of native H 2 O ice and C-rich material by magnetospheric charged particles (Grundy et al. 2006;Cartwright et al. 2015). Supporting this hypothesis, trapped OH has possibly been detected on these moons (Roush et al. 1997), which is a strong catalyst for radiolytic generation of CO 2 molecules from substrates composed of H 2 O ice and carbon-rich species (e.g., Mennella et al. 2004;Raut et al. 2012). H 2 O ice bands are weaker on the trailing hemispheres of these moons (Figure 5), perhaps in part due to large deposits of CO 2 ice masking the NIR spectral signature (∼1 -2.5 µm) of H 2 O ice (Cartwright et al. 2015(Cartwright et al. , 2020b. Another possibility is that heliocentric dust impacts promote regolith overturn and expose 'fresh' H 2 O ice, primarily on these moons' leading hemispheres (Cartwright et al. 2018). Spectrally red material (∼0.4 -1.3 µm) has also been detected, primarily on the leading hemispheres of the outer moons, Titania and Oberon ( Figure 5) (Buratti & Mosher 1991;Bell III & McCord 1991;Helfenstein et al. 1991;Cartwright et al. 2018). The distribution of red material could result from the accumulation of infalling planetocentric dust from retrograde irregular satellites (Buratti & Mosher 1991;Tamayo et al. 2013;Cartwright et al. 2018), which are spectrally redder than the classical moons (e.g., Grav et al. 2004;Maris et al. 2007).
Over longer wavelengths (∼3 -6.5 µm), spectrophotometric data collected by the Infrared Array Camera (Fazio et al. 2004) on the Spitzer Space Telescope (Werner et al. 2004) indicate that the spectral signature of CO 2 ice is strangely absent from all of these moons (Cartwright et al. 2015(Cartwright et al. , 2020b. One possible explanation is that the regoliths of the classical moons are mantled by a thin layer of small H 2 O ice grains (≤ 2 µm diameters), which obscures the longer wavelength spectral signature of CO 2 ice retained beneath this topmost layer (Cartwright et al. 2015(Cartwright et al. , 2018(Cartwright et al. , 2020b. Supporting this possibility, visible wavelength polarimetry data suggest that these moons have porous regoliths, dominated by small grains (Afanasiev et al. 2014). Similarly, over longer mid-infrared wavelengths (20 -50 µm), data collected by the Infrared Interferometer Spectrometer (IRIS) on Voyager 2 suggest that Miranda and Ariel have regoliths with unusual microstructures, possibly dominated by dark, isotropically scattering grains (Hanel et al. 1986). These different datasets all suggest that the regoliths of the Uranian moons are notably different from both H 2 O ice-rich and dark material-rich Galilean and Saturnian moons and may be more comparable to H 2 O ice-rich trans-Neptunian objects (Afanasiev et al. 2014;Cartwright et al. 2020b;Detre et al. 2020).
Although these results are intriguing, new spacecraft measurements are needed to better understand the processes modifying the surface compositions of the Uranian moons. For example, the distribution of CO 2 ice is only longitudinally constrained, limiting our ability to determine whether this volatile is generated by charged particle radiolysis, or whether it is a native constituent sourced from their interiors. Unlike the other classical moons, CO 2 ice and leading/trailing longitudinal asymmetries in composition are absent on Miranda (Bauer et al. 2002;Grundy et al. 2006;Gourgeot et al. 2014;Cartwright et al. 2018Cartwright et al. , 2020bDeColibus et al. 2020), adding to the mystery surrounding this moon. Because of the large obliquity of the Uranian system, and the associated seasonal effects, CO 2 ice exposed or Figure 5. Top: Example NIR spectra of Ariel's leading and trailing hemisphere (blue and orange 1σ error bars, respectively), collected with the SpeX Spectrograph on NASA's Infrared Telescope Facility (IRTF) (Rayner et al. 2003), offset vertically for clarity. These data demonstrate the strong leading/trailing longitudinal asymmetry in the distribution of CO2 ice observed on these moons (wavelength positions of prominent CO2 bands are indicated with black dashed lines). The observed CO2 ice bands are compared to an example laboratory spectrum of 'pure' CO2 ice (green) (Hansen 1997). Bottom: Example IRTF/SpeX spectra of Titania's leading and trailing hemisphere (blue and orange 1σ error bars, respectively). These data highlight the NIR wavelength region where spectrally red material has been detected on these moons (0.7 -1.25 µm, red box), with more red material on their leading hemispheres. These data also show the leading/trailing longitudinal asymmetry in H2O ice band strengths, which are stronger on the leading hemispheres of Ariel, Umbriel, Titania, and Oberon. Gray-toned zones represent wavelength regions with lower atmospheric transparency and increased telluric contamination. generated at polar latitudes on these moons should sublimate, migrate in tenuous exospheres to their low latitudes, and condense in cold traps (Grundy et al. 2006;Sori et al. 2017;Cartwright et al. 2021). Spatially resolved spectra collected by an orbiter are needed to more completely understand the origin and nature of CO 2 ice. Additionally, the regolith properties of the classical moons could result from interactions with the surrounding space environment, which cannot be properly assessed without data collected by an orbiter.
Some ground-based spectra of the classical moons show a 2.2-µm absorption band ( Figure 6) (Bauer et al. 2002;Cartwright et al. 2018Cartwright et al. , 2020c, which is similar to a 2.2-µm feature attributed to NH 3 and NH 4 -bearing species on Pluto and its moons (e.g., Brown & Calvin 2000;Grundy et al. 2016;Cook et al. 2018;Dalle Ore et al. 2019;Cruikshank et al. 2019;Protopapa et al. 2020) and the Saturnian moon Enceladus (Emery et al. 2005;Verbiscer et al. 2006). NH 3 and NH 4 are highly efficient anti-freeze agents when mixed with liquid H 2 O that could promote the retention of subsurface oceans if present within the interiors of these moons (e.g., Spohn & Schubert 2003;Hussmann et al. 2015;Nimmo & Pappalardo 2016). Because NH 3 -rich planetesimals were likely incorporated into the proto-Uranian moons as they formed in the Uranian subnebula (e.g., Lewis 1972), large quantities of NH 3 could be present in their interiors. Furthermore, unlike H 2 O ice and CO 2 ice, the 2.2-µm band does not display leading/trailing longitudinal asymmetries in its distribution (Figure 6), which suggests that NH 3 -bearing and NH 4 -bearing species are native to these moons and are exposed by impact events, tectonism, and mass wasting, and/or emplaced in cryovolcanic deposits (Cartwright et al. 2020c). Other species like carbonates (-CO 3 group), including ammonium carbonate ((NH 4 ) 2 CO 3 ), could be contributing to the 2.2-µm band as well (Cartwright et al. 2020c). Higher spatial resolution spectra are needed to measure the spectral signature and spatial distribution of the 2.2-µm band to determine whether these moons are rich in NH 3 and other species that may have been sourced from liquid layers in their interiors.  Stryk & Stooke 2008).The cyan dots represent the mid-observation longitude and latitude for ground-based spectra that show 2.2-µm bands, and the black dots represent spectra that do not display 2.2-µm bands (modified from Cartwright et al. 2020c). These spectra are disk-integrated and average over an entire hemisphere of Ariel. Spectra for the dots labeled '1' and '2' are shown on the right. Right: Example IRTF/SpeX spectra of Ariel that display 2.2-µm bands, offset vertically for clarity. Dot-dashed lines highlight the wavelength range of the 2.2-µm band, and the dotted lines highlight the wavelength range of its band center.
The composition and origin of the widespread dark material and the spectrally red material on the Uranian moons remains poorly understood. These materials could be rich in hydrated silicates and organic constituents delivered to and/or native to these moons. Prior spacecraft missions have assessed the nature of organics in the Jupiter, Saturn, and Pluto systems, as well as on comets (e.g., McCord et al. 1997;Irvine et al. 2003;Clark et al. 2005;Waite Jr et al. 2009;Capaccioni et al. 2015;Grundy et al. 2016;Cruikshank et al. 2020). Measuring the spectral signature of organics (e.g., C-H stretching modes between 3.2 and 3.5 µm) in the Uranian system represents a key heliocentric link for improving our understanding of the nature and overall distribution of organic matter in the Solar System, as well as for investigating whether organics formed within the protoplanetary disk or were delivered as interstellar matter. Thus, new measurements made by an orbiter are critical for determining the spatial distribution and spectral signature of CO 2 ice and NH 3 -bearing species on the classical moons, and whether these constituents were exposed/emplaced on their surfaces by ocean world activity, as well as for investigating the origin and evolution of organic material in the Uranian System.
Ring Moons and Irregular Satellites: Far less is known about the compositions of Uranus' smaller ring moons and irregular satellites, which are too faint for spectroscopic observations using available telescopes (apparent magnitudes ranging between 19.8 and 25.8 at visible wavelengths). Photometric datasets indicate that the ring moons are dark (except possibly Mab, Showalter & Lissauer 2006), with neutral spectral slopes and slight reductions in albedo at 1.5 and 2.0 µm, consistent with the presence of H 2 O ice (e.g., Karkoschka 2001). More recent photometric measurements indicate that the ring moons may display latitudinal variations in albedo, possibly resulting from interactions with Uranus' magnetosphere (Paradis et al. 2019). Other photometric studies determined that Uranus' irregular satellites are dark and red (e.g., Grav et al. 2004;Maris et al. 2007), with possibly redder colors than the irregular satellites of the other giant planets (Graykowski & Jewitt 2018). Spectra of the largest Uranian irregular satellite Sycorax suggest that H 2 O ice is present (Romon et al. 2001), but no spectra exist for the other, fainter irregular satellites. Therefore, the compositions of these objects are essentially unknown, and new observations made by an orbiter are needed.

CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE EXPLORATION
Data returned by Voyager 2 and ground-and space-based telescopes have revealed tantalizing glimpses of the Uranian moons' geologic histories, compositions, and interactions with the surrounding space environment. However, Uranus' 27 satellites remain poorly understood. Only six Uranian moons were spatially resolved by ISS, and the flyby nature of the Voyager 2 encounter, the lack of a mapping spectrometer, and the low spatial resolution of collected datasets left many unanswered questions and limit our ability to determine whether the classical moons are, or were, ocean worlds. Future telescope facilities like the Extremely Large Telescopes (ELTs) and the proposed Large UV/Optical/IR Surveyor (LUVOIR) space telescope (e.g., Roberge et al. 2019) will be able to collect high quality images and spectra of the classical moons (Cartwright et al. 2019;Wong et al. 2020), providing new information about their surfaces (Figure 7). Although these telescope datasets will undoubtedly increase our knowledge of these moons, they will not be able to achieve sufficient spatial resolutions to identify linkages between geologic features and their surface compositions, nor probe their internal structures, or investigate moon-magnetosphere interactions (e.g., Kollmann et al. 2020). Furthermore, these telescope datasets will not be able to spatially resolve the ring moons and irregular satellites to assess their surface geologies and origins.  (2008)). Resampled images simulate what these moons would look like as seen by LUVOIR from the Earth-Sun L2 Lagrange point in 2040 with an 8 m (left) and 15 m (center) aperture. The simulated angular diameters are ∼0.08" and ∼0.12" for Ariel and Titania, respectively.
An orbiting spacecraft equipped with a magnetometer could search for induced magnetic fields emanating from briny oceans in the interiors of the classical moons. An orbiter could also search for plumes on the classical moons and other signs of recent endogenic geologic activity. An orbiter making multiple close flybys of Uranus' rings would dramatically improve our understanding of the ring moons and could investigate whether Mab is the source of the µ-ring. An orbiter could spend time looking outward, making key observations of the distant irregular satellites, similar to Cassini's observations of Saturn's irregular satellites (Denk & Mottola 2019). A close pass of an irregular satellite inbound to Uranus, like Cassini's inbound flyby of Phoebe (Porco et al. 2005), would represent an unparalleled opportunity to investigate the nature and origin of these likely captured objects. Thus, new datasets collected by an