Riley A. DeColibus

The Uranian moons Ariel, Umbriel, Titania, and Oberon are enriched in CO ₂ mixed with CO, but the origin(s) of these carbon oxides, be they primarily native or radiolytic, remain(s) uncertain. Using data collected by NIRSpec on the James Webb Space Telescope (JWST), we measured the spectral signature of CO ₂ and other carbon oxides to help disentangle these hypotheses. Through comparison to laboratory data, we find that many of the detected spectral features are consistent with CO ₂ ice, including ¹² CO ₂ scattering peaks (4.15–4.26 μ m), multilobe ¹³ CO ₂ bands (4.35–4.43 μ m), and CO ₂ biphonon and triphonon modes (4.80–5.25 μ m). Our measurements show that CO ₂ and CO are concentrated on the trailing hemispheres of the inner moons Ariel and Umbriel, potentially supporting a radiolytic production hypothesis, consistent with prior ground-based results. However, many of the identified spectral features are only observed in thick crystalline ice deposits measured in the laboratory, which may be difficult to form via radiolysis of carbon-bearing material mixed in icy regoliths. Similarly, the data exhibit weak 4.02 and 4.40 μ m bands, hinting at the presence of carbonate minerals and ¹³ CO ₂ clathrates, respectively, possibly formed in the interiors of these moons. Furthermore, JWST has revealed that CO ₂ is widespread at Uranus, present in its system of rings, ring moons, and irregular satellites, consistent with its largest moons accreting CO ₂ and other carbon oxides from the Uranian subnebula. We conclude that exposed carbon oxides are potentially native, with their surface distributions shaped by charged particle irradiation and seasonal sublimation–condensation cycles.

Weywot is the largest satellite of the candidate dwarf planet Quaoar, which also hosts a remarkable ring system located beyond its Roche limit—a configuration that challenges conventional understanding of ring dynamics. In this work, we predicted three stellar occultations by Weywot occurring in 2023 May and June, of which one was successfully observed on 2023 June 22. This multichord event took place over North America, where observers were deployed across the shadow path from Maryland to California. Five positive detections and two near misses allowed us to accurately constrain Weywot’s projected size and shape at the time of the occultation. We constrained the equivalent diameter to be between 116 and 172 km (with 95% confidence) and estimate a geometric albedo between 0.024 and 0.078—substantially lower than that of Quaoar. These results offer new insights into the physical properties of Quaoar’s system and provide valuable constraints for models of its formation and evolution.

We present an analysis of ground-based visible and near-infrared wavelength (VIS, ∼0.4–0.9 μ m) reflectance spectra of the four largest Uranian satellites, utilizing observations with multiple telescopes, collected near Uranian equinox and in northern summer (2002–2024). These data have much higher spectral resolution and signal-to-noise ratios compared to previously published optical spectra of the Uranian moons, collected primarily in the 1980s and 1990s. The higher signal-to-noise ratios and longitudinal coverage of these new data allowed us to determine that the Uranian satellites exhibit distinct spectral slope variations across the VIS, supporting previous findings that the leading hemispheres of Titania and Oberon are redder than their respective trailing hemispheres. We find that the northern leading hemisphere of Titania appears to be redder than Oberon, and that Titania may have a larger leading/trailing asymmetry in reddening on its northern hemisphere compared to the other three large moons. We also conducted a sensitive search for subtle absorption features, but found no reliable evidence of broad mineral absorption bands, condensed O ₂ , or other possible products of radiolytic processing; aside from the spectral slope, the spectra of the Uranian moons appear to be featureless in the visible wavelength range.

JWST NIRCam images provide low-resolution spectra of the rings and inner moons orbiting Uranus and Neptune. These data reveal systematic variations in spectral parameters like the strength of the strong OH absorption band around 3 μm and the spectral slopes at continuum wavelengths. Neptune’s rings show an extremely weak 3 μm band, which is likely due to the small particle sizes in these dusty rings. Neptune’s small inner moons also have weaker 3 μm bands and redder continua than Uranus’ small inner moons, indicating that Neptune’s moons have a lower water-ice fraction. There are also clear spectral trends across the inner Uranian system. The strength of the 3 μm band clearly increases with distance from Uranus, with the rings having a noticeably weaker 3 μm band than most of the small inner moons, which have a weaker 3 μm band than the larger moons like Miranda. While the rings and most of the small moons have neutral spectra between 1.4 and 2.1 μm, the outermost small moon Mab exhibits a blue spectral slope comparable to Miranda, indicating that Mab’s surface may also be relatively water ice rich. The next moon interior to Mab, Puck, exhibits a stronger 3 μm band and bluer continuum slope than any of the moons orbiting interior to it, perhaps indicating that it is being covered by water-ice-rich material derived from Mab via the μ ring. Finally, the small moon Rosalind has a redder spectral slope than its neighbors, possibly due to being coated with material from the dusty ν ring.

The Uranian moon Ariel exhibits a diversity of geologically young landforms, with a surface composition rich in CO₂ ice. The origin of CO₂ and other species, however, remains uncertain. We report observations of Ariel’s leading and trailing hemispheres, collected with NIRSpec (2.87–5.10 μm) on the James Webb Space Telescope. These data shed new light on Ariel's spectral properties, revealing a double-lobed CO₂ ice scattering peak centered near 4.20 and 4.25 μm, with the 4.25 μm lobe possibly representing the largest CO₂ Fresnel peak yet observed in the solar system. A prominent 4.38 μm ¹³CO₂ ice feature is also present, as is a 4.90 μm band that results from ¹²CO₂ ice. The spectra reveal a 4.67 μm ¹²CO ice band and a broad 4.02 μm band that might result from carbonate minerals. The data confirm that features associated with CO₂ and CO are notably stronger on Ariel’s trailing hemisphere compared to its leading hemisphere. We compared the detected CO₂ features to synthetic spectra of CO₂ ice and mixtures of CO₂ with CO, H₂O, and amorphous carbon, finding that CO₂ could be concentrated in deposits thicker than ∼10 mm on Ariel’s trailing hemisphere. Comparison to laboratory data indicates that CO is likely mixed with CO₂. The evidence for thick CO₂ ice deposits and the possible presence of carbonates on both hemispheres suggests that some carbon oxides could be sourced from Ariel’s interior, with their surface distributions modified by charged particle bombardment, sublimation, and seasonal migration of CO and CO₂ from high to low latitudes.

We present three-dimensional (3D) maps of Jupiter’s atmospheric circulation at cloud-top level from Doppler-imaging data obtained in the visible domain with JIVE, the second node of the JOVIAL network, which is mounted on the Dunn Solar Telescope at Sunspot, New Mexico. We report on 12 nights of observations between 2018 May 4 and May 30, representing a total of about 80 hr. First, the average zonal wind profile derived from our data is compatible with that derived from cloud-tracking measurements performed on Hubble Space Telescope images obtained in 2018 April from the Outer Planet Atmospheres Legacy program. Second, we present the first ever 2D maps of Jupiter’s atmospheric circulation from Doppler measurements. The zonal velocity map highlights well-known atmospheric features, such as the equatorial hot spots and the Great Red Spot (GRS). In addition to zonal winds, we derive meridional and vertical velocity fields from the Doppler data. The motions attributed to vertical flows are mainly located at the boundary between the equatorial belts and tropical zones, which could indicate active motion in theses regions. Qualitatively, these results compare well to recent Juno data that have unveiled the 3D structure of Jupiter’s wind field. To the contrary, the motions attributed to meridional circulation are very different from what is obtained by cloud tracking, except at the GRS. Because of limitations with data resolution and processing techniques, we acknowledge that our measurements of the vertical or meridional flows of Jupiter are still to be confirmed.

The Differential Speckle Survey Instrument (DSSI) was relocated to the Astrophysical Research Consortium 3.5 m telescope at Apache Point Observatory (APO) in early 2022. Here we present results from the first year of observations along with an updated instrument description for DSSI at APO, including a detailed description of a new internal slit mask assembly used to measure the instrument plate scale from first principles. Astrometric precision for DSSI at APO during this time was measured to be 2.06 ± 0.11 mas, with a photometric precision of 0.14 ± 0.04 mag. Results of 40 resolved binary systems are reported, including two that were previously unknown to be binaries: HIP 7535 and HIP 9603. We also present updated orbital fits for two systems: HIP 93903 and HIP 100714. Finally, we report updated or confirmed dispositions for five Kepler Objects of Interest (KOIs) that were previously explored in Colton et al., using speckle imaging to discern common proper motions pairs from line of sight companions: KOI-270, KOI-959, KOI-1613, KOI-1962, and KOI-3214AB.

Published near-IR spectra of the four largest classical Uranian satellites display the presence of discrete deposits of CO₂ ice, along with subtle absorption features around 2.2 μm. The two innermost satellites, Miranda and Ariel, also possess surfaces heavily modified by past endogenic activity. Previous observations of the smallest satellite, Miranda, have not detected the presence of CO₂ ice, and a report of an absorption feature at 2.2 μm has not been confirmed. An absorption feature at 2.2 μm could result from exposed or emplaced NH₃- or NH₄-bearing species, which have a limited lifetime on Miranda’s surface, and therefore may imply that Miranda’s internal activity was relatively recent. In this work, we analyzed near-IR spectra of Miranda to determine whether CO₂ ice and the 2.2 μm feature are present. We measured the band area and depth of the CO₂ ice triplet (1.966, 2.012, and 2.070 μm), a weak 2.13 μm band attributed to CO₂ ice mixed with H₂O ice, and the 2.2 μm band. We confirmed a prior detection of a 2.2 μm band on Miranda, but we found no evidence for CO₂ ice, either as discrete deposits or mixed with H₂O ice. We compared a high signal-to-noise-ratio spectrum of Miranda to synthetic and laboratory spectra of various candidate compounds to shed light on what species may be responsible for the 2.2 μm band. We conclude that the 2.2 μm absorption is best matched by a combination of NH₃ ice with NH₃ hydrates or NH₃–H₂O mixtures. NH₄-bearing salts like NH₄Cl are also promising candidates that warrant further investigation.

Many tidally locked icy satellites in the outer Solar System show leading/trailing hemispherical asymmetries in the strength of near-infrared (NIR) H$_2$O ice absorption bands, in which the absorption bands are stronger on the leading hemisphere. This is often attributed to a combination of magnetospheric irradiation effects and impact gardening, which can modify grain size, expose fresh ice, and produce dark contaminating compounds that reduce the strength of absorption features. Previous research identified this leading/trailing asymmetry on the four largest classical Uranian satellites but did not find a clear leading/trailing asymmetry on Miranda, the smallest and innermost classical moon. We undertook an extensive observational campaign to investigate variations of the NIR spectral signature of H$_2$O ice with longitude on Miranda's northern hemisphere. We acquired 22 new spectra with the TripleSpec spectrograph on the ARC 3.5m telescope and 4 new spectra with GNIRS on Gemini North. Our analysis also includes 3 unpublished and 7 previously published spectra taken with SpeX on the 3m IRTF. We confirm that Miranda has no substantial leading/trailing hemispherical asymmetry in the strength of its H$_2$O ice absorption features. We additionally find evidence for an anti-Uranus/sub-Uranus asymmetry in the strength of the 1.5-$\mu$m H$_2$O ice band that is not seen on the other Uranian satellites, suggesting that additional endogenic or exogenic processes influence the longitudinal distribution of H$_2$O ice band strengths on Miranda.