Richard G. Jerousek

Stellar occultation measurements of Saturn’s rings from the Cassini Ultraviolet Imaging Spectrograph (UVIS) High Speed Photometer (HSP) reveal dozens of regions in the heart of the B ring that are opaque at the sensitivity level of the measurements (Colwell et al. 2021, 2024). We have identified 24 regions with an average transparency less than 0.25%, including 9 with an average transparency less than 0.1% and radial extents greater than 10 km. There are broad expanses within these opaque regions where the transparency is indistinguishable from zero. Occultations of the star Beta Centauri with photon count rates of up to 600 per 1 ms integration period provide the strongest constraints on transparency. The elevation angle B (sin(B)=0.918) and brightness of Beta Centauri make it best suited among the UVIS stellar occultations to probe high optical depth regions. The radial resolution of these occultations is about 8 meters, while the azimuthal resolution in the frame co-moving with the ring particles is ~20-30 meters. Each measurement samples an area in the frame of the particles of ~400 m2. The average transparency of each designated opaque region is > 0 because of the presence of narrow, transient features we dub “phantoms” that increase the average transparency of the region. Phantoms are analogous to the ghosts observed by Baillie et al. (2013) and Green et al. (2024) in the C ring and Cassini Division. While ghosts were defined by being indistinguishable from 100% transparent in an otherwise moderate optical depth background, phantoms are defined as narrow, non-axisymmetric features with non-zero transparency in an otherwise opaque background (Figure 1).Figure 1: Stacked and offset (by 5%) transparency profiles of the O1 opaque region in Saturn’s B2 ring region as measured by the Cassini UVIS HSP. The different curves are for different observations of the same star system: Beta Centauri. Narrow spikes in transparency are identified as phantoms. The vertical dashed lines indicate the boundaries of the opaque region. We observed phantoms that are only a single HSP measurement, indicating a radial extent of < 10 m, but most span several measurements. Phantom transparencies range from a few percent up, but in some cases the transparency exceeds 10% and can approach 50%. There are 15 measurements of the B ring with Beta Centauri occultations, and the phantoms do not repeat from one occultation to the next, highlighting their non-axisymmetric and/or temporally variable nature. The Beta Centauri system has two stars that contribute to the HSP signal, and the projected separation of these two components in the ring plane enables us to place limits on the azimuthal extent of some phantoms, which in some cases is < 100 m. At some locations there are clusters of phantom-like features that appear in all occultations at the same location. We identify these axisymmetric features “grassy regions” based on their appearance in plots of transparency (Figure 2). The average transparency of grassy regions is still low, typically less than 2%. The opaque regions are bordered by regions of higher optical depth (Figure 2).Figure 2: Stacked and offset (by 5%) transparency profiles of an opaque region in Saturn’s B2 ring region as measured by the Cassini UVIS HSP. The different curves are for different observations of the same star system: Beta Centauri. This opaque region is home to phantoms (isolated spikes in transparency) as well as several grassy regions, identifiable by their presence at the same location in all occultation profiles and marked by blue dashed lines. The measured optical depth of the opaque stretches between phantoms can exceed 6 in some cases (see for example the bottom curve in Figure 2 between 100,250-100,300 km). N-body simulations have struggled to reproduce such high optical depths. Individual ring particles in the simulations clump together due to their mutual gravity, leaving gaps which elevate the overal transparency of the region. We will present results of simulations with modified boundary conditions and particle properties to try to explain the existence of such high-optical-depth regions and the sub-structure of phantoms and grassy regions within them. Baillié, K., J. E. Colwell, L. W. Esposito, and M. C. Lewis 2013. Meter-sized Moonlet Population in Saturn’s C Ring and Cassini Division. Astron J. 145, 171, doi:10.1088/0004-6256/145/6/171. Colwell, J. E., M. Brooks, R. Jerousek, C. Coleman, M. S. Tiscareno, K.-M. Aye, M. Lewis, L. W. Esposito 2021. Irregular Structure in the Core of Saturn’s B Ring. 2021 AGU Fall Meeting P34A-01. Colwell, J. E., A. Goforth, C. Coleman, R. Jerousek, L. W. Esposito 2024. Saturn’s B Ring Phantoms: Frequency, Properties, and Distribution. 2024 Meeting of the American Geophysical Union. P33F-2931. Green, M. R., J. E. Colwell, L. W. Esposito, R. G. Jerousek 2024. Particle sizes in Saturn’s rings from UVIS stellar occultations 2. Outlier populatinos in the C ring and Cassini Division. Icarus 416, 116081.

We present a comparative occultation analysis of narrow, eccentric ringlets in the Saturnian and Uranian ring systems, focusing on how mesoscale structure and particle size distributions vary with orbital phase (true anomaly). Our study combines the full suite of Cassini stellar and radio occultation profiles of Saturn’s narrow ringlets, obtained by Cassini’s UVIS (λ = 150 nm), VIMS (λ = 2.84 μm), and RSS (Ka-, X-, S-band experiments). We compare our results with a reanalysis of Voyager 2 PPS and RSS occultations of Uranus’s ε, α, β, η, and δ rings.We group Saturn ringlet profiles into four orbital quadrants—periapsis, apoapsis, and the streamline divergence/convergence sectors—to isolate phase-dependent structure (Figure 1). In each quadrant, we model viewing-geometry dependent normal optical depth using the irregularly spaced, three component, granola bar self-gravity wake model of Esposito et al. (2025) (Figure 2). The model fits the dimensional ratios and cant angles of aligned structures such as self-gravity wakes, particle-depleted lanes, or both. Following Green et al. (2024), we compute the normalized central moments of UVIS HSP occultation transparencies to constrain the size of the largest ring particles (amax) and to characterize the clumpiness of the ringlets with orbital phase. We compare these moments to predictions from Esposito et al. (2025) for wake-dominated rings. Simultaneously, we perform multi-wavelength particle size distribution fits by selecting UVIS, VIMS, and RSS profiles at similar orbital phase and with similar viewing geometries to constrain the power-law index (q) and minimum particle radius (amin) (Figure 3).For the Uranian rings, we use the σ-Sagittarii and β-Persei Voyager 2 PPS occultations to extract radial optical depth profiles of the dense narrow rings, supplemented by RSS S/X-band differential attenuation. While the geometry does not allow for true-anomaly-resolved comparisons, we compare Voyager 2 PPS occultation statistics and RSS S and X band optical depths to enable structural comparison.Our approach incorporates a self-consistent model of wavelength and viewing-geometry dependent optical depths with an analysis of photon counting statistics (Cassini UVIS HSP and Voyager 2 PPS). We find that Saturn’s ringlets exhibit significant variation in both self-gravity wake dimensions and inferred size distributions between quadrants (Figure 4).By applying a common framework across two planetary ring systems, we aim to identify techniques to constrain the phase-dependence of the particle size distribution, surface densities, and structure of dense narrow rings from in-situ occultations. This approach is intended not only to advance current understanding but also to inform the design of future occultation experiments at Uranus.Figure 1. Ringlet edge radii from UVIS, VIMS, and RSS occulations colored by sine of the ring opening angle for the Maxwell Ringlet. Occultations are separated into quadrants (Groups 1 – 4) in true anomaly for orbital phase-dependent analysis.Figure 2. The irregularly spaced granola bar self-gravity wake model of Esposito et al. (2025). 𝐸[𝜆T] is the expected value of the Toomre critical wavelength or the largest unstable wavelength toward gravitational collapse (Julian and Toomre 1966): λΤ = 4π2GΣ/κ2 where Σ is the local surface mass density and κ is the local epicyclic frequency. The black dashed arrow is the line of sight from Cassini to the occulted star (or to Earth in the case of radio occultations. The red square is the area over which light is collected during a single integration of the UVIS instrument. (see Jerousek et al. (2024) for details on the integration area).Figure 3. The Maxwell ringlet contains a prominent spiral density wave driven by an outer Lindblad resonance (OLR). We fit optical depths in the first trough of the wave. Structure nearly disappears at apoapsis when particle streamlines are most widely separated. At other orbital phases, the model best matches a structure with broad radial spacing and variable vertical extent.Figure 4. Example of particle size distribution fits for the Maxwell ringlet at two orbital phases.. Best-fitting parameters are consistent with Jerousek et al. (2020) for the background C ring near periapsis where q is much larger. Ring particle radii generally range from several millimeters up to 5 – 10 m.References:Green, M. R., Colwell, J. E., Esposito, L. W., & Jerousek, R. G. (2024). Particle sizes in Saturn’s rings from UVIS stellar occultations 2. Outlier Populations in the C ring and Cassini Division. Icarus, 416, 116081.Esposito, L. W., Colwell, J. Ε., Eckert, S., Green, M. R., Jerousek, R. G., & Madhusudhanan, S. (2025). Statistics of Saturn's ring occultations: Implications for structure, dynamics, and origins. Icarus, 429, 116386.Jerousek, R. G., Colwell, J. E., Esposito, L. W., Tiscareno, M. S., Lewis, M. C., Pohl, L., & Benavides, D. A. (2024). The smallest structures in Saturn’s rings from UVIS stellar occultations. Icarus, 415, 116069.

Certain spiral density waves in Saturn's rings are generated through resonances with planetary normal modes, making them valuable probes of Saturn's internal structure. Previous research has primarily focused on the rotation rates of these waves. However, other characteristics of these waves also contain valuable information about the planet's interior. In this work, we investigate the amplitudes of the waves across the C‐ring by analyzing high signal‐to‐noise profiles derived from phase‐corrected averages of occultation profiles obtained by Cassini's Visual and Infrared Mapping Spectrometer (VIMS). By fitting these wave profiles to linear density wave models, we estimate the ring's surface mass density, mass extinction coefficient, and effective kinematic viscosity at 34 locations in the C‐ring, as well as the amplitude of the gravitational potential perturbations associated with 6 satellite resonances and 28 planetary normal mode resonances. Our estimates of the C‐ring's mass extinction coefficient indicate that the typical particle mass density is around 0.3 g/cm³ interior to 84,000 km, but can get as low as 0.03 g/cm³ exterior to 84,000 km. We also find the ring's viscosity is reduced in the outer C‐ring, which is consistent with the exceptionally high porosity of the particles in this region. Meanwhile, we find the amplitudes of Saturn's normal modes are complex functions of frequency, and , implying that multiple factors influence how efficiently these modes are excited. This analysis identified two primary sources of these normal‐mode oscillations: a deep source located close to Saturn's core, and a shallow source residing near the surface.

Cassini's last look at Saturn's rings: During the final stages of the Cassini mission, the spacecraft flew between the planet and its rings, providing a new view on this spectacular system (see the Perspective by Ida). Setting the scene, Spilker reviews the numerous discoveries made using Cassini during the 13 years it spent orbiting Saturn. Iess et al. measured the gravitational pull on Cassini, separating the contributions from the planet and the rings. This allowed them to determine the interior structure of Saturn and the mass of its rings. Buratti et al. present observations of five small moons located in and around the rings. The moons each have distinctive shapes and compositions, owing to accretion of ring material. Tiscareno et al. observed the rings directly at close range, finding complex features sculpted by the gravitational interactions between moons and ring particles. Together, these results show that Saturn's rings are substantially younger than the planet itself and constrain models of their origin. Science , this issue p. 1046 , p. eaat2965 , p. eaat2349 , p. eaau1017 ; see also p. 1028

The Cassini spacecraft's Ultraviolet Imaging Spectrograph (UVIS) includes a high-speed photometer (HSP) that has observed more than 100 stellar occultations by Saturn's rings. Here, we document a standardized technique applied to the UVIS-HSP ring occultation datasets delivered to the Planetary Data System as higher level data products. These observations provide measurements of ring structure that approaches the scale of the largest common ring particles (∼5 m). The combination of multiple occultations at different viewing geometries enables reconstruction of the three-dimensional structure of the rings. This inversion of the occultation data depends on accurate calibration of the data so that occultations of different stars taken at different times and under different viewing conditions can be combined to retrieve ring structure. We provide examples of the structure of the rings as seen from several occultations at different incidence angles to the rings, illustrating changes in the apparent structure with viewing geometry.