Research Article

Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus

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Science  14 Jun 2019:
Vol. 364, Issue 6445, eaat2349
DOI: 10.1126/science.aat2349

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

Structured Abstract

INTRODUCTION

Saturn’s main ring system is associated with a family of small moons. Pan and Daphnis orbit within the A-ring’s Encke Gap and Keeler Gap, respectively, whereas Pandora and Prometheus orbit just outside the F-ring and Atlas just outside the A-ring. The latter three moons help to confine ring particles.

The moons Janus and Epimetheus are in closely spaced orbits that they exchange approximately every 4 years; these two objects may be collisional fragments of a larger body. All these moons have densities much less than 1000 kg/m3, indicating that they formed from ring debris that accumulated around a preexisting core.

RATIONALE

During the final stages of the Cassini mission, the spacecraft made a series of close observations of Saturn’s rings. Flybys of Pan, Daphnis, Pandora, Atlas, and Epimetheus were performed to investigate the geologic processes shaping their surfaces, their composition, their thermal and ultraviolet properties, their relationship to Saturn’s ring system, and their interactions with particles in Saturn’s magnetosphere.

RESULTS

The moons that orbit in ring gaps or are adjacent to the main rings have equatorial ridges of material consisting of accreted particles that are distinct from their rounded central cores. The cores are more structurally sound than ridges, with rougher surfaces and more impact craters. Complex patterns of grooves formed by tidal stresses crisscross the moons.

A visible-infrared reflectance spectrum of Pan, which is embedded in the rings, shows that it is redder than any of the other ring moons. The color of the moons becomes more red as the distance to Enceladus increases. This suggests that the optical properties of the moons are determined by the balance of two external effects: addition of a red coloring agent from the main rings, and accretion of neutral-colored icy particles or water vapor possibly from the E-ring, which is formed by Enceladus’s plume. The exact composition of the red material from the main ring system is unknown, although a mixture containing organic silicates and iron is likely. Differences in particle size also affect the moons’ spectra.

Measurements of the spectral slope of Epimetheus in the ultraviolet suggest that it is less affected by particles from the E-ring than are the mid-sized main moons farther from Saturn. Temperature maps were derived for both Atlas and Epimetheus, whose blackbody temperatures were 82 ± 5 K and 90 ± 3 K, respectively.

Carbon dioxide, which is present on the eight mid-sized saturnian moons, was not detected on the ring moons, nor were any volatiles other than water ice. Measurements showed a scarcity of high-energy ions in the vicinity of the ring moons and only transient energetic electron populations; in the ring gaps, no trapped electron or proton radiation was detected. Although particle bombardment alters both the albedo and color of the main moons’ surfaces, for the ring moons it appears to be unimportant.

CONCLUSION

Saturn’s ring moons record a complex geologic history with groove formation caused by tidal stresses and accretion of ring particles. The moons embedded within the rings or near their edges have solid cores with equatorial ridges of more weakly consolidated material. The finding of a porous surface further supports substantial accretion. High-resolution images strongly suggest exposures of a solid substrate distinct from the mobile regolith that frequently covers essentially all small Solar System objects. These exposures may eventually help to reveal systematic trends of the evolution of moons and their geologic structures for the whole of Saturn’s satellite system.

The A-ring “shepherd” moon Atlas showing ridges of accumulated ring particles.

(A) Visible-light image of Atlas showing the core and equatorial ridge. (B) Infrared image of the moon at 2.0 μm. (C) Thermal infrared scan of Atlas consistent with a mean temperature of 82 ± 5 K.

Abstract

Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Data on the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultraviolet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s main ring system and accretion of bright icy particles or water vapor from volcanic plumes originating on the moon Enceladus.

Saturn possesses a family of small inner irregular moons that orbit close to its rings. Two moons orbit in gaps within Saturn’s main ring system: Daphnis, which dwells in the A-ring’s Keeler Gap (1), and Pan, which is found in the Encke Gap, also in the A-ring (2). Three others, called shepherd moons, orbit at the edges of the A-ring (Atlas) or the F-ring (Pandora and Prometheus) (3) (fig. S2). The co-orbital moons Janus and Epimetheus share horseshoe orbits outside the F-ring and swap their positions every 4 years (fig. S2). Saturn’s rings are almost certainly tied to the origin and continued existence of these moons (1). It remains unclear whether the rings formed from the breakup of an inner moon, or whether the present ring moons formed from the consolidation of existing ring material, either primordial or impact-created. The alteration processes acting on these moons and the rings, past and present, are also unknown.

Prior to Saturn’s exploration by spacecraft, the main rings were thought to be unconsolidated primordial debris, unable to form a moon because of tidal forces (4, 5). Evidence from the two Voyager spacecraft suggested that the rings and inner moons constituted debris from the breakup of a single parent body, or perhaps several parent bodies, with the moons being the largest fragments (4). Measurement of the rings’ and moons’ bulk densities using Cassini data (5), along with dynamical studies and the existence of ridges around the equators of Atlas and Pan (5, 6), suggested a more complicated, multistage formation. The ring moons—from Pan out to Pandora, but possibly also Janus and Epimetheus—likely formed from the very early accretion of low-density debris around a denser seed, presumably a collisional shard from the breakup of a preexisting moon (5). In the cases of Atlas and Pan, this was followed by a second stage of accretion of material onto the equator, after the rings had settled into their present very thin disk (6, 7). In this scenario, the surfaces of these moons should be similar in composition to the rings.

Analysis of the optical properties of the moons, including color, albedo, and spectral properties in the visible and infrared between 0.35 and 5.2 μm, has shown that they resemble the ring systems in which they are embedded or which they abut (811). An unidentified low-albedo reddish material that could be organic molecules, silicates, or iron particles (912) appears to be abundant in the rings and has also tinged the moons (812), further supporting a common origin and implying continuing accretion of particles onto the moons’ surfaces. The interactions of the ring system with the inner moons may form two distinct zones: an inner region in the vicinity of the main ring system that is dominated by the red chromophore, and an outer region that is dominated by fresh, high-albedo icy particles from the E-ring. Complicating the picture, however, is the possible influence of interactions with magnetospheric particles, which have been shown to alter the color and albedo of the main moon system of Saturn (13, 14). It is unclear whether any volatiles other than water ice exist on the ring moons. The presence of molecules with higher volatility than water ice would indicate material originating in a colder region outside the saturnian system; for example, the discovery of CO2 ice on the irregular outer moon Phoebe suggested that it originated in the Kuiper Belt (15).

The last phase of Cassini’s mission began on 30 November 2016 and ended on 15 September 2017, with two distinct periods: the ring-grazing (or F-ring) orbits, in which 20 close passes to the F-ring were performed, and the proximal orbits (or “Grand Finale”), which executed 23 dives between the planet and the main ring system. During the ring-grazing orbits, Cassini performed its closest flybys of Pan, Daphnis, Atlas, Pandora, and Epimetheus (Table 1). A second flyby of Epimetheus was performed at a slightly greater distance. Data were obtained using several instruments on Cassini: the Imaging Science Subsystem (ISS) (16); the Visual Infrared Mapping Spectrometer (VIMS), taking medium-resolution spectra between 0.35 and 5.1 μm (17); the Cassini Composite Infrared Spectrometer (CIRS) (18); the Ultraviolet Imaging Spectrometer (UVIS) (19); the Cosmic Dust Analyzer (CDA) (20); and the Magnetospheric Imaging Instrument (MIMI) (21). The dust and plasma environment in the vicinity of the small inner moons was observed by CDA and MIMI during the subsequent proximal orbits.

Table 1 Summary of the close flybys of Saturn’s ring moons during the ring-grazing orbits.
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Geology and morphology

Previous images of the ring moons showed distinctive equatorial ridges on Pan and Atlas (5, 6) that were interpreted as likely formed by accretion of ring particles, whereas those of Daphnis were ambiguous. The small satellites are all in synchronous rotation, tidally locked to the planet (7). Prometheus’s and Pandora’s orbits straddle the F-ring, and although they exhibit different surface morphology, their densities are nearly identical (3) (table S1). The small (mean radius <5 km) satellites Aegaeon, Methone, and Pallene, which orbit in diffuse rings or ring arcs (22, 23), have smooth ellipsoidal shapes indicative of hydrostatic equilibrium (7). The co-orbital satellites Epimetheus and Janus, by far the largest of the inner small moons, have nearly identical mean densities (table S1), which are also the highest among the inner small moons. Grooves had been observed on Epimetheus (24), and there were suggestions of discrete crater-filling sediments on both Janus and Epimetheus (7). Epimetheus experiences a ~7° forced wobble (libration) around a purely synchronous rotation (25). Table S1 summarizes the shapes, volumes, and calculated mean densities of the small satellites of Saturn based on the images taken during the flybys (26, 27). Epimetheus and Janus have densities substantially above 500 kg m–3; the lowest density (and highest uncertainty) is that of Daphnis, at 274 ± 142 kg m–3. Surface accelerations vary substantially across each object because of their irregular shapes and tidal accelerations (table S1).

Main ring moons and ridges

The flyby images in Fig. 1 show that the equatorial ridges on Pan and Atlas are morphologically distinct from the more rounded central component of each moon. The ridges are different sizes on each moon: The fractional volumes of the ridges are Pan ~10%, Daphnis ~1%, and Atlas ~25%. Atlas’s ridge appears smooth in the highest-resolution image (76 m/pixel), with some elongate brighter albedo markings. The ridge contacts the central component that has distinct ridge and groove topography (Fig. 1C); it has a previously known slight polygonal equatorial profile (7). Pan’s ridge has a distinct boundary with the central component, a somewhat polygonal equatorial shape, some grooves, small ridges, and several small impact craters. The profile of Pan’s ridge varies considerably with longitude. Figure 2 shows Pan’s northern hemisphere, with calculated relative gravitational topography and surface slopes using existing techniques (3, 5, 7) (data S1). Unlike some equatorial ridges on small asteroids (28, 29), Pan’s ridge was not formed by material sliding toward lower gravitational potential areas generated by rotation and tides, because the slope directions are not latitudinally directed. The distinct boundary between the ridge and the central component core, the differing surface morphology on each, and the large differences in relative heights along the ridge require the ridge formation to be unrelated to surface, gravity-driven processes. These observations are consistent with formation of the ridge by the accretion of particles, with a distribution dictated by the relative orbital and rotational dynamics of the moon and ring particles (6).

Fig. 1 Grayscale images of the ring moons obtained with ISS during the Cassini flybys.

(A) Pan, image number N1867606181, Clear/Clear filters, from 26°S, at a scale of 182 m/pixel. (B) Pan, N186704669, Clear/Clear filters, from 39°N; 147 m/pixel. (C) Atlas, N1870699087, Clear/IR3 filters, from 40°N; anti-Saturn point at lower left; 108 m/pixel. (D) Daphnis, N1863267232, Clear/Green filters, from 14°N; anti-Saturn point to left; 170 m/pixel. (E) Pandora, N1860790629, Clear/Green filters, 240 m/pixel. The sub-spacecraft point is 35°N, 98°W; Pandora’s north pole is close to two small craters above the large, bright-walled crater. (F) Epimetheus, N1866365809, Clear/UV3 filters, 99 m/pixel. Grooves and craters dominate the surface. All scale bars, 5 km. Images were chosen for scale and viewing geometry; different filters have little effect on visibility of morphologic detail.

Fig. 2 Relative topography and slopes on Pan.

(A) Grayscale image N1867604669 from 39°N, 217°W (rotated from view in Fig. 1). (B) Dynamic topography is the relative potential energy at the surface (due to mass, rotation, and tides) divided by an average surface acceleration (5, 7). A homogeneous interior density is assumed. (C) Slopes are the angles between the surface normals and the (negative) net acceleration vectors.

The calculated mean densities of Pan, Atlas, and Daphnis result in calculated surface accelerations near zero at the sub- and anti-Saturn points, which suggests that those points cannot accrete additional material. The rest of the surfaces have inward-directed net accelerations. The surfaces of these three moons may be crudely divided into three units on the basis of morphology, geography, and surface texture visible at the available resolutions (Fig. 3). The equatorial ridges generally are the smoothest terrain on each moon.

Fig. 3 Distribution of geological units on Pan, Atlas, Daphnis, and Pandora.

In each panel, the three main units are highlighted in color, with the uninterpreted grayscale image alongside for comparison. Cratered surfaces (blue) have numerous craters, relatively crisp surface relief, and regolith typical of other small bodies in the saturnian system. Smooth terrains (cyan) are distinctly smooth relative to typical small-body cratered surfaces; some is material collected in crater floors. Exposed substrates (yellow) are relatively bright with lineations, more typical of rigid materials than of loose regolith. Unclassified areas (gray) are those for which insufficient data are available to resolve ambiguities between terrain types. (A) Atlas, resolution 94 m/pixel. (B) Daphnis, 167 m/pixel. (C) Pan, 144 m/pixel (top), 279 m/pixel (bottom). (D) Pandora, 137 m/pixel (top), 200 m/pixel (bottom).

The central components have more impact craters than do the ridges on Pan and Atlas, which display a few sub-kilometer impact craters. Pan and Atlas’s central components show lineated topography indicative of structures such as faults or fractures. Pan has two distinct global sets of quasi-parallel faults. The first is roughly concentric to the long axis and exhibits conspicuous scarps and terracing, likely formed by equatorward displacements. The axial symmetry of this system suggests that tidal forces were involved in its development. The second system is oriented obliquely to the first and is visible in both the north and south hemispheres (Figs. 1A and 3C). In contrast, Atlas’s central component core exhibits patterns of elongated ridge and groove topography that do not have fault scarp morphologies; the core appears to be covered by at least tens of meters of loose material (regolith).

Pan’s equatorial ridge is thickest north-south at longitudes of approximately 220°, 310°, 135°, and 50°W, yet its radial extent peaks at longitudes of about 5°, 55°, 100°, 180°, 235°, and 310°W. The ridge supports grooves and small craters; their presence suggests some cohesion in this low-gravity environment (<2 mm s–2). Atlas’s equatorial profile is also somewhat polygonal but not as pronounced as Pan’s.

The classification of some material units on Pan’s southern hemisphere is ambiguous, in part because these are not directly illuminated by the Sun, only by light reflected off Saturn. These unclassified units in Fig. 3C include knobby streaks of hummocked material oriented approximately parallel to the equator, as well as hummocky deposits outlining a curvilinear depression on the Saturn-facing side.

The spatial resolution of the Daphnis imagery is 170 m/pixel, poorer than that of Pan (147 m/pixel) and Atlas (76 m/pixel). Daphnis is only about one-quarter the dimensions of the other ring moons. As a result, it is not clear whether its near-equatorial ridge is smoother or otherwise different from the rest of the surface. The equatorial ridge extends at least from 75°W to 185°W. An additional ridge at 22°N runs from ~60°W to 120°W. Both ridges are 300 to 400 m north-south and perhaps radially 300 m in extent. The core has an elongated (2.5 km) depression that is roughly aligned east-west.

F-ring moons

Prometheus and Pandora orbit inside and outside the F-ring, respectively. The images taken during the Pandora flyby show grooves and debris on the surface of this shepherd moon (Fig. 1E). Although many of the grooves form a pattern concentric to the major axis of the body, there is a slight offset between them, especially noticeable on the sub-Saturn side, which reflects orientations seen in previous observations (22).

Part of Pandora’s leading hemisphere is smooth in comparison to other regions on this moon (Figs. 1E and 3D). The smooth deposits are most continuous near the equator but become patchy at high latitudes, where they appear to be too thin to mute the coarse surface relief along protruding crater rims. The smooth deposits extend approximately ±60° in latitude, slightly more than the maximum latitude of the ridge on Atlas. This arrangement might indicate the accretion of material, as with the main ring moons. If so, the accretion efficacy on Pandora is at least two orders of magnitude smaller than on Pan and Atlas, and much broader latitudinally. However, variations in resolution, illumination, and viewing geometry make mapping of textural variations on Pandora ambiguous.

Co-orbital moons

The highest-resolution images of the flybys were of Epimetheus, the smaller of the co-orbital moons, reaching scales of 36 and 49 m/pixel. These data enabled enhanced mapping of grooves and sediment coverings seen in previous observations (24). The grooves are global in occurrence, largely beaded to straight, elongated depressions that appear to be formed in loose regolith. There are some exposures of brighter material apparently devoid of regolith cover (Fig. 1F) that also show elongated lineations, generally slight depressions. These align with the grooves nearby that appear to be regolith features and largely align with the regolith groove global patterns. This association appears to support a previously proposed relation of at least some regolith grooves with fractures or other structures in a more rigid underlying bedrock, although the variety of groove morphologies on many objects suggests a multiplicity of groove origins (24, 3032).

Colors of the small ring satellites and Pandora

The whole-disk colors of the ring satellites as measured in ISS broadband filters (33) follow trends with distance from Saturn similar to those found by the VIMS instrument (811). The ISS Narrow Angle Camera (NAC) uses paired broadband filters. The CL1:UV3 pair (0.341 μm) and CL1:IR3 pair (0.930 μm) span the spectral range of the camera, and IR3/UV3 ratios represent the observed brightness value in each CL1:UV3 broadband filter relative to the corresponding value in the CL1:IR3 filter (7). For reference, Enceladus, the presumed source of ice particles that mute colors on other satellites, has an effectively neutral IR3/UV3 ratio of 1.03 ± 0.02 (34).

The spatially resolved colors of Pan, Daphnis, and Atlas can be used to show the effects of material deposited from the rings (3) (table S2). Closest to Saturn, Pan’s average IR3/UV3 ratio is red at 2.5 ± 0.2 but is significantly smaller than the value of 3.3 ± 0.2 for the adjacent A-ring (i.e., Pan is less red than the rings). Farther out, the A-ring IR3/UV3 ratio decreases from 2.7 ± 0.2 on the inside of the Keeler Gap (which contains Daphnis) to 2.2 ± 0.3 on the outside. The mean value is not statistically different from that of Daphnis itself, 2.3 ± 0.3. The equatorial ridges on the ring satellites may be very old (5), but the colors most likely reflect a patina of material deposited from geologically recent and ongoing processes. Atlas, which falls just outside the A-ring, has an IR3/UV3 ratio of 2.4 ± 0.1. Pandora, which is near the F-ring and farther from Saturn, has a lower IR3/UV3 ratio of 1.9 ± 0.1. It lacks an equatorial ridge but possesses smooth deposits, which on the leading side extend from the equator to mid-latitudes.

Among the terrains shown in Fig. 3, color differences can be identified in the high-resolution images of all moons except Daphnis, for which the CL1:UV3 images were badly blurred by spacecraft motion. The IR3/UV3 ratio for cratered materials on Pan is about 19% higher than for its equatorial ridge and reaches approximately the average global value. Similarly, the ratio for cratered materials on Atlas is about 16% higher than for its ridge, but in this case, the global average value closely matches that of Atlas’s larger equatorial ridge. For Pandora, the cratered materials have an IR3/UV3 ratio that is 15% lower than for the smooth materials toward the equator. The global average ratio falls between that of the cratered material and the smooth deposits. Exposed substrate is visible as a scarp on Pan and a bright exposed crater wall on Pandora. On Pan, the IR3/UV3 ratio of exposed substrate is intermediate between the ridge materials and crater materials. However, on Pandora, the corresponding ratio for the exposed crater wall is not statistically distinguishable from that of the cratered material.

Composition

Compositional information on the surfaces of the moons was acquired using VIMS (17). Prior to the close flybys of the ring moons, spectra taken by VIMS from greater distances were obtained (811). Water ice was the only volatile identified, but the moons’ visible colors varied, especially in the 0.35- to 0.55-μm spectral region, which suggested contamination by a reddish chromophore that perhaps came from the ring system itself. This coloring agent is distinct from the low-albedo red material from the Phoebe ring that is deposited on the leading hemisphere of Iapetus and on Hyperion (8, 9).

The close flybys of the embedded moons Daphnis and Pan enabled the acquisition of spectra of these moons, although only an IR spectrum (1.0 to 5.0 μm) for Daphnis was successfully obtained. These data provide a test for the origin of the red chromophore in the inner saturnian system. They also provide rudimentary information on spatial variations in composition on the moon’s surfaces, although the spatial resolution is only about 1 to 2% (depending on the instrument mode) of ISS’s resolution (3) (fig. S1).

Figure 4 shows the spectrum of each moon from 0.35 to 4.2 or 3.6 μm (1 to 3.6 μm for Daphnis). The only absorption bands detectable are those of water ice at 1.25, 1.6, 2.0, and 3.0 μm. No other volatiles are detectable, including CO2, although its strongest absorption band in this spectral region is at 4.26 μm, in the noisy region of the spectrum beyond about 3.5 μm. There is a deep absorption band for crystalline water ice at 1.65 μm. This spectral band is sensitive to radiation damage (35); its unusual depth relative to previous Cassini spectra of icy moons (811) implies a lack of radiation damage in the ring environment, which is expected given the dearth of high-energy particles in the rings (see below). Water ice spectral bands are also sensitive to grain size, with deeper bands signifying larger grains (36). A larger particle size could signify larger regolith grains in the main ring system than in the E-ring, or it could simply be due to gravitational escape of the smaller particles, some of which could be formed by continual impacts.

Fig. 4 VIMS spectra and colors of the five moons and the A- to C-rings.

(A to E) Spectra of Pan (A), Daphnis (B), Atlas (C), Pandora (D), and Epimetheus (E). Noisy data at the long wavelengths are not shown. I/F is the reflected intensity compared with the incident solar flux. (F) Color-color plot of Saturn’s main ring system and Enceladus (8, 9) compared with Epimetheus, Atlas, Pandora, and Pan.

In general, there is a gradient depending on the position of the moon with respect to the rings, with Pan, which is embedded in the Encke Gap, being the reddest and Epimetheus, which is farthest from the rings and closest to Enceladus, being the bluest. (The one exception to this pattern is Pandora’s bluer color in the 0.55- to 0.95-μm region.) This effect results from the countervailing processes of contamination by a red chromophore from the main rings and ice particles or water vapor from the E-ring, which originates from Enceladus’s plume.

The VIMS colors agree with those derived by ISS. The VIMS equivalent values at the same wavelengths as the effective wavelengths of the ISS filters yield IR3/UV3 ratios of 2.7 ± 0.3 for Pan, 2.2 ± 0.2 for Atlas, 1.7 ± 0.2 for Pandora, and 1.5 ± 0.1 for Epimetheus. (The VIMS spectrum extends to only 0.35 μm; the visible slopes of the spectra were linearly extrapolated to 0.34 μm to match the wavelength of the ISS UV3 filter.) The moons embedded in the rings show spectral differences with the surrounding rings; in general they are less red (Fig. 4F). The VIMS ratio image of Atlas (fig. S1) shows uniformity between the main body and its equatorial ridge, at least in water ice abundance, which implies the accumulation of particles away from the equator to provide a globally homogeneous surface. Color differences below the spatial resolution of VIMS exist, as detected by ISS in the visible spectrum.

The spectrum of Pan is redder in the 0.35- and 0.55-μm region than other saturnian moons. Atlas, the shepherd moon just outside the A-ring, is also red but less so than Pan, and Pandora, which is associated with the F-ring, even less. The color of Epimetheus is more like that of the medium-sized moons Enceladus and Mimas (810). Thus, there is a gradient in color with distance from Saturn’s ring system, with the embedded Pan being the reddest. Figure 4, A to E, shows that the slope of the visible spectrum increases as the distance to Saturn increases, and this is quantified in Fig. 4F, which shows the visible colors derived from the flybys with the colors of the main ring system of Saturn (9). These results imply that the red chromophore comes from the rings themselves. However, the differences in color between the moons and their adjacent rings—the small moons are consistently bluer than their surrounding rings—could be due to another contaminant: particles of almost pure water ice or vapor from the E-ring. This ring is a diffuse torus that is fed from the plume of Enceladus. The particles have a wide range of orbital elements and predominantly impact the leading sides of the main moons (and the trailing side of Mimas), altering their albedos and colors (3739). The ring moons’ leading hemispheres would tend to accrete more fresh grains of water ice than the surrounding ring particles.

The depth of the water ice band at 2.0 μm compared to the continuum at 1.8 μm (i.e., the 1.8/2.0 μm ratio) is 5.2 ± 0.1 for Pan, 5.0 ± 0.2 for Daphnis, 4.4 ± 0.1 for Atlas, 3.4 ± 0.1 for Pandora, and 2.4 ± 0.1 for Epimetheus. The band depths increase closer to Saturn, most likely as a result of the increasing particle sizes (36). This is consistent with the moons embedded in the ring (Pan and Daphnis) being coated with main ring particles rather than with smaller particles from the E-ring. (The absorption band at 1.6 μm shows a similar but weaker trend.) Because the main ring system provides a shield against the E-ring, particle size may be a factor in determining the color and reflectivity of these moons.

Interactions between moons and magnetospheric particles can also alter the moons’ colors and albedos (13, 14). However, there is a dearth of high-energy particles in the vicinity of these moons (see below). Another factor that may alter spectral slopes and band depths is the particle size of the accreted ring particles (36), which may not be the same as that of the native particles.

Ultraviolet and thermal infrared observations of the moons

During the ring-grazing orbits, the spacecraft was in a high-radiation, high-dust environment that produced high background levels in ultraviolet observations with UVIS. The only moon detected was Epimetheus, during the encounter on 21 February 2017 (3) (fig. S4), in which the signal is above the background for only the longest far-UV wavelengths, ~0.170 to 0.19 μm. However, this single UV measurement of reflectance places some constraints on surface composition and external effects on Epimetheus. At 72° solar phase angle (the angle between the spacecraft, Epimetheus, and the Sun), the derived normal reflectance (average over 0.17 to 0.19 μm) is 0.09 ± 0.02. For comparison, this is lower than the reflectance measured at Tethys under similar viewing geometry by a factor of roughly 1.5 to 2 (40); however, Tethys has a higher visible geometric albedo [~1.2, versus ~0.73 for Epimetheus (37)], which indicates that Epimetheus may have a roughly uniformly lower reflectance than Tethys in the UV-visible range. The UV-visible spectral slope and albedo are strongly driven by external effects, because this spectral range senses the uppermost layer of the regolith affected by processes including plasma and E-ring grain bombardment. The UVIS measurement combined with the visible albedo suggests that Epimetheus is not as affected by the brightening effects of the E-ring grains as Tethys is (37), or that there is some other darkening agent or process important at Epimetheus’s location. Thus, the UV-visible albedo of Epimetheus may simply reflect the relative importance of the alteration by the reddish lower-albedo chromophore and the icy E-ring particles at this moon’s distance.

Thermal infrared observations with CIRS detected two moons: Epimetheus and Atlas (Fig. 5) (3). The results were modeled with a blackbody fitted to the observed radiance over all wavelengths. Both Epimetheus and Atlas are visible above the background sky. The mean surface temperatures are 90.1 ± 2.7 K on Epimetheus and 82.4 ± 4.7 K on Atlas.

Fig. 5 CIRS thermal infrared and ISS visible-light observations of Atlas and Epimetheus.

(A) Blackbody temperature distributions of the two moons, determined by fitting a blackbody curve to the full CIRS radiance spectrum at each location. The axes are offsets in right ascension and declination, with the origin at the center of the target (note that the IR images do not fall on this location because only the hotter side is detected). (B) ISS observations of both targets taken immediately before and after the CIRS scan, on the same orientation (3).

Particle observations

Throughout the ring-grazing orbits, the particle and electromagnetic field instruments CDA and MIMI measured Saturn’s plasma and dust environment, including the regions around the small inner moons. During this period, Cassini passed close to the orbits of the co-orbital moons Janus and Epimetheus. During 11 of the 20 ring plane crossings, the CDA’s High Rate Detector (HRD) detected a total of about 2000 dust grains with radii larger than 0.8 μm (Fig. 6). The vertically integrated number density of grains smaller than 1.6 μm does not depend on the radial distance to Saturn, whereas the density of larger grains drops by about 50% over a radial distance of approximately 3500 km. Because the larger particles are less susceptible to nongravitational forces, large particles ejected from the moons stay closer to their parent bodies and form a more confined ring, which was previously detected by the Cassini camera (41). Fitting a Gaussian distribution to the HRD data, and accounting for the dust background from the F- and G-rings, constrains the radial full width at half maximum (FWHM) of the Janus-Epimetheus ring to about 4300 km. This implies a total number of 2 (±1) × 1019 ring particles larger than 1.6 μm.

Fig. 6 Radial dust density distribution obtained from CDA-HRD measurements.

The density of the ≥0.8-μm particles indicates a constant profile (red dashed line), whereas the density of the ≥1.6-μm particles decreases inward from the orbit of Janus and Epimetheus (vertical gray line). The dust distribution of the larger particles is modeled by a Gaussian distribution (blue dashed line) with a maximum at the mean radial position of Janus and Epimetheus, plus a constant background density. Error bars are derived from Poisson statistics.

Many dust rings are formed by ejecta from high-velocity impacts of interplanetary micrometeoroids eroding the surfaces of satellites without atmospheres. The measured particle number in the Janus-Epimetheus ring constrains the poorly known parameters of the impact-ejection dust creation model (42, 43) at Saturn. Using an unfocused flux of >3.6 × 10–16 kg m–2 s–1 with a mean impact speed of 4.3 km s–1 (44), the dust production rate from both moons totals about 0.81 kg s–1 (0.57 kg s–1 from Janus and 0.24 kg s–1 from Epimetheus). This corresponds to 9.1 × 1011 particles larger than 1.6 μm per second (6.4 × 1011 s–1 from Janus and 2.7 × 1011 s–1 from Epimetheus), assuming a cumulative power-law size distribution for a dust diameter dd–α with α = 2.4 and a maximal ejecta mass of 1 × 10–8 kg, consistent with observations of impact-generated dust clouds around the Galilean moons (42, 45).

To explain the measured number of ring particles, this comparably high production rate requires a shallow slope of the cumulative ejecta velocity v distribution ∝ v–γ (γ = 1) and a kinetic energy dissipation at the higher end of the values predicted by laboratory experiments (46, 47). The kinetic energy ratio of ejecta to impactor is 5%. This points to a highly dissipative and porous (snow or regolith) surface. We find that most ejecta are gravitationally bound to the moons and fall back to their surface, whereas only about 5% of them for Janus and 7% for Epimetheus escape to the surrounding ring. Numerical simulations (3) show that most of the ring particles are recaptured by the source moons after an average lifetime of 60 years, resulting in an estimate of 9.8 × 1019 ring particles larger than 1.6 μm. This value is in rough agreement with the observed value of 2 (±1) × 1019 particles, which in turn constrains the poorly known parameters of the impact-ejection model, which can vary by orders of magnitude.

The CDA Chemical Analyzer (9) recorded mass spectra of sub–micrometer-sized dust particles (0.1 to 0.4 μm). The compositional analysis of these spectra recorded near the ring plane shows mostly ice grains but also about 3% pure silicate grains or ice-silicate mixtures (3) (fig. S5). The source of the icy particles could be either the inner edge of the E-ring or surface ejecta of the nearby small ice moons. Because silicate-rich grains of this size have not been detected in the E-ring (48), these must originate from a different source, possibly the nearby moons Janus and Epimetheus or the F- and G-rings.

The Low Energy Magnetospheric Measurements System (LEMMS) of the MIMI energetic charged particle detector surveyed the planet’s radiation belts inward of Saturn’s G-ring and monitored the energetic particle environment of the five small moons. LEMMS measures energetic electrons and ions above 18 and 27 keV, respectively, reaching into the MeV energy range. The region inward of Saturn’s G-ring has been sampled in the past on several occasions with Pioneer 11 and Cassini (4951). It contains the location where both Saturn’s proton and electron radiation belts have their highest intensities, which lies between the G-ring and Janus and Epimetheus’s orbits. Inward of that maximum, intensities drop gradually up to the outer edge of Saturn’s A-ring, which absorbs all energetic particles. Superimposed on the radial profile of radiation belt fluxes are localized dropouts originating from Saturn’s moons and rings (52). Although several of these features can be attributed to specific moons [e.g., Janus and Epimetheus (53)], any influences by Pandora, Prometheus, and Atlas (orbiting within the radiation belt boundaries) are less clear. These moons orbit close to Saturn’s A- and F-rings, complicating the separation of the different contributions. Understanding how effectively these moons sweep out particle radiation also determines the radiation environment to which their surfaces are exposed.

Figure 7A shows count rates of >12-MeV and >25-MeV protons as a function of the L-shell [expressed as multiples (L) of Saturn’s radius of 60,268 km] averaged over the proximal orbits. The L-shell is defined as the distance from Saturn that a magnetic field line intersects the magnetic equator. The spacecraft L-shell at the time of data collection is determined by mapping along Saturn’s magnetic field using a third-order multipole model for Saturn’s internal magnetic field (54). Figure 7 shows the previously established sectorization of the MeV proton radiation belts, due to the moons and rings that absorb any protons diffusing across their orbits (55, 56). Among these different sectors, the least well characterized by previous observations is the “Minor Belt,” centered at approximately L = 2.29. The gap immediately outside the Minor Belt is centered near the F-ring (L ~ 2.32); we find that the gap boundaries coincide with the L-shells of Prometheus and Pandora (Fig. 7A). Pandora and Prometheus are therefore absorbing protons at a rate that is high enough to counter the diffusive influx of protons from the surrounding belt sectors. Effectively, the two moons and the F-ring form an extended obstacle to proton radiation. The net result is that the weathering of Pandora’s and Prometheus’s surfaces by energetic protons is negligible because they orbit within the proton radiation gaps they create. Atlas’s effects cannot be distinguished from those of the A-ring, but that moon is also exposed to very low proton fluxes. Overall, almost all of Saturn’s inner moons (except Dione, Rhea, or minor moons such as Anthe or Pallene) orbit in regions free from energetic ions (5759). This is unlike Jupiter’s satellites, whose surface chemistry and thin atmospheric properties are strongly affected by irradiation from high fluxes of keV and MeV particles (60, 61).

Fig. 7 Average count rates for protons and electrons, as measured by MIMI/LEMMS.

The channels are >12 and >25 MeV for protons (A) and >800 keV for electrons (B); both sets of data are shown as a function of L-shell, with 1σ error bars. Absence of error bars indicates an uncertainty larger than the corresponding mean value. The orbits of several of Saturn’s large icy moons are also marked. The inset in (A) zooms into the region of the Minor Belt, highlighting the absorbing effects of Atlas, Pandora, Prometheus, and the A- and F-rings. The inset in (B) shows a high–time resolution series of observations (1 sample per 0.3125 s) from LEMMS obtained during the second proximal orbit on 2 May 2017. The blue arrow marks an electron microsignature within one of the MeV electron spikes seen consistently during Cassini’s outbound crossings near the L-shell of the A-ring’s outer edge.

Figure 7B shows the proximal orbit averages of electron count rates from LEMMS channel E5 (>0.8 MeV) as a function of the L-shell. Electron radiation levels are more variable than those of protons, as the large error bars indicate, because moons and rings are not effective in sweeping out electrons from their orbits (52, 62). Inside L = 2.4 (inward of the Janus and Epimetheus orbits), electron rates fall slowly toward the outer edge of the A-ring (L = 2.27). This drop is interrupted by an enhancement of the mean electron rates near the L-shells of the F-ring, Pandora, and Prometheus. In the absence of a MeV electron source, such an enhancement, which was absent from past observations at the same L-shell (53, 63), is unexpected. The 1σ error bars in that location span more than two orders of magnitude in amplitude, indicating much higher variability than in the surrounding regions. This large scatter is attributed to spikes of enhanced MeV electron flux observed in 18 of the 22 outbound crossings outward of the A-ring’s edge and between L = 2.31 and L = 2.35. The radial extent of an individual spike is less than 1800 km along the equatorial plane, and the electron intensity within them can be enhanced by as much as a factor of 300 relative to their surroundings. The inset of Fig. 7B shows one such resolved spike, captured by the high–time resolution measurements of LEMMS priority channel E4 (0.8 to 4.2 MeV) on 2 May 2017. Because most measurements in the inbound portion of Cassini’s orbit showed no evidence of similar spikes in the same L-shell range, we deduce that these features are usually located a few hours after local noon, and their longitudinal extent ranges between 22° and 37° in the clockwise direction, starting from a magnetospheric local time of 13:20. The longitudinal extent cannot be constrained in the anticlockwise direction. Most of these enhancements were seen around the L-shells of the F-ring, Prometheus, and Pandora. This electron belt component is therefore limited in local-time range. As a result, energetic electron bombardment of the three moons is variable in intensity, is episodic, and occurs only for a fraction of their orbit around Saturn. Material interaction signatures of energetic electrons are seen as localized depletions (microsignatures) within the electron spikes. These may be due to Atlas, Prometheus, Pandora, or F-ring clumps (63); an example is shown in the inset of Fig. 7B and could have formed only after the electron enhancement developed.

There is no discernible signal of trapped electron or proton radiation at the orbits of the Keeler and Encke Gaps, where Daphnis and Pan are orbiting (54).

Summary and conclusions

The low densities of the small moons of Saturn, measured during the flybys, are consistent with a multistage formation scenario involving accretion of ring material (5, 6). The colors of the moons embedded in the A-ring are more consistent with the rings the closer the moons are to Saturn. This suggests the ongoing accretion of a reddish chromophore, possibly a mixture of organics and iron (912), onto the surfaces of the moons. The difference in color between the moons and their adjacent ring may be explained by the accretion of bright, icy particles or, more likely, water vapor from the E-ring. Each moon’s surface is subjected to a balance between these two ongoing processes, with their distance from Saturn and Enceladus determining the resulting color, as illustrated in Fig. 4F. The detection of abundant ice grains by CDA supports this view. The bluer core of Atlas is also explained by the accretion of E-ring particles, which have a wider range of inclinations than main ring particles. If the ring moons formed from the same material as the rings, they would have been the same color, and the color gradient may be solely due to contamination by the E-ring. The size of particles on the moons’ surfaces also plays a role, especially for the moons embedded in the main ring system, which would shield these moons from the E-ring.

The dearth of high-energy ions close to the moons lessens the alteration processes caused by bombardment with magnetospheric particles. The strong crystalline water ice band at 1.65 μm also suggests low radiation damage. This low-energy plasma environment is unlike the main moons of Saturn, especially Dione and Rhea, as they dwell in a region where alteration by ions is substantial. Particle radiation would tend to darken and redden the surfaces, so the red chromophore on the trailing hemispheres of the main moons may be unrelated to the red material contributing to the colors of the ring moons (64). Contamination of Saturn’s rings by bright icy particles or water vapor offers counterevidence to previous arguments that the observed brightness of the rings indicates recent formation (65).

The moons’ geology records a complex history, including groove formation caused by tidal stresses and accretion of ring particles. The CDA finding of a porous surface further supports substantial accretion. Although the topography and surface slopes strongly suggest that the equatorial ridges of Pan and Atlas are accreted from the rings and are not formed by normal surface transport, the ridges on these objects appear in a variety of forms. The flyby images strongly suggest exposures of a solid substrate distinct from the mobile regolith that covers many small Solar System objects.

Supplementary Materials

science.sciencemag.org/content/364/6445/eaat2349/suppl/DC1

Materials and Methods

Supplementary Text

Figs. S1 to S5

Tables S1 to S4

Data S1

References (6786)

References and Notes

  1. See supplementary materials.
Acknowledgments: The authors are grateful to the Cassini project engineers and staff for their dedicated service that led to the success of the final stages of the mission. Funding: This paper was funded by the Cassini Project. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract to NASA. Also supported by Deutsches Zentrum für Luft- und Raumfahrt grants OH 1401 and 1503 and by Deutsche Forschungsgemeinschaft (DFG) grants Ho5720/1-1 and OH 1401 (M.Se., R.S., H.H., M.Sa., F.S., T.A., S.K., N.Kh., G.M.-K., F.P., and J.Si.), 50OH1501 (J.Si., G.M.-K., and T.A.), and 50OH1503 (T.D. and H.R.); the Italian Space Agency (G.F. and M.C.); and DFG projects PO 1015/2-1, /3-1, /4-1 and ERC Consolidator Grant 724908-Habitat OASIS (N.Kh. and F.P.). Author contributions: B.J.B., P.C.T., E.R., C.H., M.Se., A.R.H., P.H., H.H., N.Kh., H.-W.H., T.W.M.: planning observations, data analysis, and writing; R.H.B., R.S., T.A., K.H.B., S.K., S.M.K., D.M., G.M.-K., P.D.N., C.C.P., H.R., J.Si., L.A.S.: instrument development and planning observations; R.N.C., T.D., M.Sa., F.S., J.Sp., N.Kr., F.P., C.P., G.H.J., P.K., J.L., H.-W.H.: data analysis and planning observations; G.F., M.C., T.E.: data analysis. Competing interests: None. Data and materials availability: All data used in this paper are archived in NASA’s Planetary Data System (PDS). The ISS, VIMS, CIRS, and UVIS data can be found at https://pds-rings.seti.org/cassini/. Relevant periods of data acquisition (in Universal Time): Pan, 7 March 2017, 16:35 to 19:05; Daphnis, 16 January 2017, 11:33 to 14:03; Atlas, 12 April 2017, 11:30 to 14:10; Pandora, 18 December 2016, 19:59 to 21:54; Epimetheus, 30 January 2017, 19:22 to 21:12, and 21 February 2017, 09:33 to 10:43. The CDA and MIMI data were acquired continuously throughout the F-ring orbital period, lasting from 30 November 2016 to 22 April 2017 and during the proximal orbits, which lasted from the end of the F-ring orbits until the end of the mission on 15 September 2017. CDA observations can be found at https://pds.nasa.gov/ds-view/pds/viewDataset.jsp?dsid=CO-D-CDA-3/4/5-DUST-V1.0, and MIMI data at https://pds-ppi.igpp.ucla.edu/search/view/?f=yes&id=pds://PPI/CO-S-MIMI-4-LEMMS-CALIB-V1.0. The Pan model slope and topography shown in Fig. 2 are provided in data S1. The software for the CDA modeling in the supplementary materials can be found at https://github.com/hohoff/ddx (66).
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