Saturn's ring and nearby faint satellites

Observations of Saturn's rings during passage of the Earth through the ring plane, coupled with those of others, suggest a ring thickness of 1.3 ± 0.3 km. The wide disparity in the optical depth of Cassini's division found by other investigators is resolved, and for conservative isotropic single scattering, a normal optical depth for Cassini's division of 0.060 ± 0.006 is obtained. We find the mean normal optical depth of ring C to be 0.074 ± 0.007. Analysis of all available observations of faint objects near Saturn indicates the presence of at least one previously undiscovered satellite of Saturn. The orbit for Janus determined by Dollfus is supported. These satellites may be major members of an extended ring.     

An analysis of bending waves in Saturn's rings using voyager radio occultation data

The oblique geometry of the Voyager 1 radio occulation of Saturn's rings resulted in a strong coupling between the local slope of the ring midplane and the associated radio opacity (optical depth). We apply a model of this relationship to those regions of the rings where bending waves have been observed in the radio data. Using the Shu et al. linear model for a bending wave (F.H. Shu, J.N. Cuzzi, and J.J. Lissauer, 1983,Icarus53, 185–206), we obtain height profiles for the Mimas 5:3 and 7:4 bending waves. The first oscillation of the Mimas 5:3 bending wave has an amplitude of about 800 m, in agreement with the prediction of the Shu et al. model. However, the rest of the wave may be explained only by either a greatly decreased amplitude in the region beyond the second cycle, or by a significant enhancement in radio optical depth in the region of the bending wave. The shape of the enhancement necessary is similar to that of the enhancement at photopolarimetry wavelengths (L.W. Esposito, M. O'Callaghan, and R.A. West, 1983,Icarus56, 439–452), but differs in the region of the first cycle. Our solution gives 131,901±6 km as the resonance location, and a surface mass density of 35±6g cm⁻². The error bars on the resonance location do not include the uncertainty in the radial scale of the radio occultation data, which is approximately 10 km (R.A. Simpson, G.L. Tyler, and J.B. Holberg, 1983,Astron. J.88, 1531–1536). The Mimas 7:4 bending wave conforms more closely to the linear model, and requires no reduction in amplitude or enhancement in optical depth. We find a surface mass density of 30.5±9 g cm⁻², and resonance location at 127,765±7km.     

The vertical structure of the F ring of Saturn from ring-plane crossings

We present a photometric model of the rings of Saturn which includes the main rings and an F ring, inclined to the main rings, with a Gaussian vertical profile of optical depth. This model reproduces the asymmetry in brightness between the east and west ansae of the rings of Saturn that was observed by the Hubble Space Telescope (HST) within a few hours after the Earth ring-plane crossing (RPX) of 10 August 1995. The model shows that during this observation the inclined F ring unevenly blocked the east and west ansae of the main rings. The brightness asymmetry produced by the model is highly sensitive to the vertical thickness and radial optical depth of the F ring. The F-ring model that best matches the observations has a vertical full width at half maximum of 13 ± 7 km and an equivalent depth of 10 ± 4 km. The model also reproduces the shape of the HST profiles of ring brightness vs. distance from Saturn, both before and after the time of ring-plane crossing. Smaller asymmetries observed before the RPX, when the Earth was on the dark side of the rings, cannot be explained by blocking of the main rings by the F ring or vice versa and are probably instead due to the intrinsic longitudinal variation exhibited by the F ring.     

Saturn’s F ring as seen by Cassini UVIS: Kinematics and statistics

We present a new orbital model of Saturn’s F ring core based on 93 occultations by the Cassini Ultraviolet Imaging Spectrograph (UVIS) and the Voyager radio and stellar occultations. We demonstrate that the core, despite its intrinsic variability, is well-described as an inclined, freely precessing ellipse. We find that post-fit residuals with a root-mean-square of 24 km are genuine, representing the well-known non-Keplerian features observed in the ring. Over the nearly 4 years of UVIS observations we find the residual variance to increase, coincident with the apse anti-alignment of Prometheus and F ring core in December 2009. This increase in dynamical F ring core temperature most likely reflects the ever-stronger perturbations by Prometheus. Our results are in good agreement with Earth-based and HST observations as well as Voyager imaging.Cassini UVIS stellar occultations resolve the F ring at unprecedented resolutions of a few meters and we identify the F ring core and inner and outer strands. We infer their normal optical depth and full width at half maximum (FWHM) and show that core and strands form distinct morphological groups. Typically, a strand is about ten times wider than the core (average FWHM is ～10 km) while having a ten times smaller optical depth. Unlike in pre-Cassini occultations the F ring core displays significant optical depth with in some cases >3. In many cases we find a narrow optically thick component (～ few km and τ > 0.5) embedded in the F ring core. Entertaining the possibility that this is the actual, “true” F ring core then UVIS results suggest that this “true” core is highly non-continuous. In addition, we report the detection of a previously unknown structure – dubbed the “secondary” as it visually resembles the F ring core. Its morphology is similar to that of the core in optical depth and FWHM and it displays individual opaque features. Despite its core-like appearance, we show that its kinematics is consistent with that of strands. We conclude that it is the most prominent strand seen to date. It represents a striking example of strand creation resulting in what could be called a morphological “small-scale” version of the F ring core. This extraordinary object should be one of the prime targets of future F ring studies.     

On the origin of the wide HI absorption line towards Sgr A*

We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of V_1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s⁻¹. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.     

The evolution of spokes in Saturn's B ring

The optical appearance of spokes was studied in high resolution (?200 km/lp) images obtained by Voyager 2. Spokes are classified into three categories. (1) Extended spokes are seen in the distance interval of 100,000 to 112,000 km from Saturn's center. They have diffuse edges and are slightly wedge shaped. Their width at the base (towards Saturn) is about 20,000 km. Their active times (during which they increase in width) range from 4000 to 12,000 sec. (2) Narrow spokes are found in the distance range 104,000 to 116,000 km, have sharply defined edges, and are narrowest at the corotation distance (112, 300 km). Their typical radial extension and width is 6000 and 2,000 km, respectively. (3) Filamentary spokes are found outside 110,000 km mostly joined with a wider spoke further in. They are typically 3000 km in length and 500 km in width. Their active time is less than 1000 sec. Several narrow spokes were observed during formation along radial lines in the sunlit portion of the ring. The formation time is typically ?5 min for a 6000-km-long spoke. The rate of spoke formation is highest at the morning ansa outside Saturn's shadow. Several spokes have been found where one edge revolves with Keplerian speed whereas the other edge stays radial. Recurrent spoke patterns have been observed at the period of Saturn's rotation. From edge-on views of the ring system, an upper limit for the height of spokes of 80 km is derived.     

Jupiter's ring system: New results on structure and particle properties

A reexamination of the Voyager images has yielded a refined understanding of Jupiter's diffuse ring system. The system is composed of a relatively bright narrow ring and inner toroidal halo, in addition to the exterior “gossamer” ring discussed elsewhere (Showalter et al., 1985, Nature 316, 526–528). The previously suspected inner disk is absent. The main ring is ∼7000 km wide and has an abrupt outer boundary at a radius of 129,130 ± 100 km. Visible in the ring are several narrow bright features, which may bear some relationship to Adrastea and Metis; these features appear to be narrower and relatively brighter in backscatter. The smallest ring particles obey a power law size distribution, and have an optical depth of 1–6 × 10⁻⁶ for grains up to 100 μm in radius. The largest bodies are dark, rough, and red, and of comparable total optical depth. The halo arises at the bright ring's inner boundary and rapidly expands inward to a ∼20,000-km full thickness, but remains symmetric about the ring plane. It disappears from sight at a radius of 90,000 km, roughly halfway between the main ring and the planet's cloudtops. The halo particles are not predominantly Rayleigh scatterers; they appear to obey a size distribution similar to that of the micron-sized population in the main ring, and comprise a similar optical depth.     

The 1 May 1982 stellar occultation by Uranus and the rings: Observations from Mount Stromlo Observatory

The 1 May 1982 occultation of KME 15 by Uranus and its rings was observed at λ = 2.2 μm using the 1.9-m telescope of the Mount Stromlo Observatory. From model fits to the immersion and emersion ring profiles, accurate midtimes for rings 6, 5, 4, α, β, η, γ, σ, and ϵ, and ring widths, equivalent widths, and normal optical depths for all but ring 6 were obtained. The recently discovered ring 1986 U1R is not detectable in the data, setting an upper limit on the product of ring width and normal optical depth of ≤0.4 km at λ = 2.2 μm. From the immersion and emersion atmosphere occultations, vertical temperature profiles were obtained by numerical inversion. Both profiles show mean temperatures near 130°K and a local maximum near the 8-μbar pressure level.     

The 7 and 25 June 1985 Neptune occultations: Constraints on the putative Neptune “arc”

Stellar occultations by Neptune on 7 and 25 June 1985 were observed in the K band from Sutherland (SAAO) to search for conforming evidence of the ring-like “arc” reported by Hubbard et al. (W.B. Hubbard, A. Brahic, B. Sicardy, L.-R. Elicer, F. Roques, and F. Vilas (1986). Nature 319, 636–640). A binary star was occulted on 7 June 1985, and since both components were occulted by the planet, their relative positions could be precisely determined. A single sharp dip, of high signal-to-noise ratio, was observed in the post-emersion occultation trace. If this feature were caused by material near Neptune, its corresponding projected equatorial plane radius is either 62,600 ± 160 km or 63,760 ± 120 km, depending on which of the binary star pair was occulted. The equatorial radius, width, and optical depth of the 7 June feature are similar to those described by Hubbard et al. The absence of a corresponding post-emersion dip due to the occultation of the companion star suggests that the ring-like material is discontinuous over a scale of several thousand kilometers in ring circumference. No ring-like features were observed during pre-immersion. The 25 June 1985 occultation was also successfully observed, including atmospheric occultation profiles for both immersion and emersion. No evidence for ring-like material was found in the region probed by this occultation during post-emersion, which included the entire range of equatorial radii over which “arc” events have been previously reported.     

The brightening of Saturn’s F ring

Image photometry reveals that the F ring is approximately twice as bright during the Cassini tour as it was during the Voyager flybys of 1980 and 1981. It is also three times as wide and has a higher integrated optical depth. We have performed photometric measurements of more than 4800 images of Saturn’s F ring taken over a 5-year period with Cassini’s Narrow Angle Camera. We show that the ring is not optically thin in many observing geometries and apply a photometric model based on single-scattering in the presence of shadowing and obscuration, deriving a mean effective optical depth τ ≈ 0.033. Stellar occultation data from Voyager PPS and Cassini VIMS validate both the optical depth and the width measurements. In contrast to this decades-scale change, the baseline properties of the F ring have not changed significantly from 2004 to 2009. However, we have investigated one major, bright feature that appeared in the ring in late 2006. This transient feature increased the ring’s overall mean brightness by 84% and decayed with a half-life of 91 days.     

Self-gravity wakes and radial structure of Saturn's B ring

We analyze stellar occultations by Saturn's rings observed with the Cassini Ultraviolet Imaging Spectrograph and find large variations in the apparent normal optical depth of the B ring with viewing angle. The line-of-sight optical depth is roughly independent of the viewing angle out of the ring plane so that optical depth is independent of the path length of the line-of-sight. This suggests the ring is composed of virtually opaque clumps separated by nearly transparent gaps, with the relative abundance of clumps and gaps controlling the observed optical depth. The observations can be explained with a model of self-gravity wakes like those observed in the A ring. These trailing spiral density enhancements are due to the competing processes of self-gravitational accretion of ring particles and Kepler shear. The B ring wakes are flatter and more closely packed than their neighbors in the A ring, with height-to-width ratios <0.1 for most of the ring. The self-gravity wakes are seen in all regions of the B ring that are not opaque. The observed variation in total B ring optical depth is explained by the amount of relatively empty space between the self-gravity wakes. Wakes are more tightly packed in regions where the apparent normal optical depth is high, and the wakes are more widely spaced in lower optical depth regions. The normal optical depth of the gaps between the wakes is typically less than 0.5 and shows no correlation with position or overall optical depth in the ring. The wake height-to-width ratio varies with the overall optical depth, with flatter, more tightly packed wakes as the overall optical depth increases. The highly flattened profile of the wakes suggests that the self-gravity wakes in Saturn's B ring correspond to a monolayer of the largest particles in the ring. The wakes are canted to the orbital direction in the trailing sense, with a trend of decreasing cant angle with increasing orbital radius in the B ring. We present self-gravity wake properties across the B ring that can be used in radiative transfer modeling of the ring. A high radial resolution (∼10 m) scan of one part of the B ring during a grazing occultation shows a dominant wavelength of 160 m due to structures that have zero cant angle. These structures are seen at the same radial wavelength on both ingress and egress, but the individual peaks and troughs in optical depth do not match between ingress and egress. The structures are therefore not continuous ringlets and may be a manifestation of viscous overstability.     

A close look at Saturn's rings with Cassini VIMS

Soon after the Cassini-Huygens spacecraft entered orbit about Saturn on 1 July 2004, its Visual and Infrared Mapping Spectrometer obtained two continuous spectral scans across the rings, covering the wavelength range 0.35-5.1 μm, at a spatial resolution of 15-25 km. The first scan covers the outer C and inner B rings, while the second covers the Cassini Division and the entire A ring. Comparisons of the VIMS radial reflectance profile at 1.08 μm with similar profiles at a wavelength of 0.45 μm assembled from Voyager images show very little change in ring structure over the intervening 24 years, with the exception of a few features already known to be noncircular. A model for single-scattering by a classical, many-particle-thick slab of material with normal optical depths derived from the Voyager photopolarimeter stellar occultation is found to provide an excellent fit to the observed VIMS reflectance profiles for the C ring and Cassini Division, and an acceptable fit for the inner B ring. The A ring deviates significantly from such a model, consistent with previous suggestions that this region may be closer to a monolayer. An additional complication here is the azimuthally-variable average optical depth associated with “self-gravity wakes” in this region and the fact that much of the A ring may be a mixture of almost opaque wakes and relatively transparent interwake zones. Consistently with previous studies, we find that the near-infrared spectra of all main ring regions are dominated by water ice, with a typical regolith grain radius of 5-20 μm, while the steep decrease in visual reflectance shortward of 0.6 μm is suggestive of an organic contaminant, perhaps tholin-like. Although no materials other than H₂O ice have been identified with any certainty in the VIMS spectra of the rings, significant radial variations are seen in the strength of the water-ice absorption bands. Across the boundary between the C and B rings, over a radial range of ∼7000 km, the near-IR band depths strengthen considerably. A very similar pattern is seen across the outer half of the Cassini Division and into the inner A ring, accompanied by a steepening of the red slope in the visible spectrum shortward of 0.55 μm. We attribute these trends—as well as smaller-scale variations associated with strong density waves in the A ring—to differing grain sizes in the tholin-contaminated icy regolith that covers the surfaces of the decimeter-to-meter sized ring particles. On the largest scale, the spectral variations seen by VIMS suggest that the rings may be divided into two larger ‘ring complexes,’ with similar internal variations in structure, optical depth, particle size, regolith texture and composition. The inner complex comprises the C and B rings, while the outer comprises the Cassini Division and A ring.     

Observations of the Saturn E ring and a new satellite

The faint E ring of Saturn appears as a narrow ring 246,000 ± 4,000 km from the center of Saturn on photographs taken when the ring-plane inclination was 5°.4. The apparent brightness of the ring was uniform at all observed orbital longitudes and permits an estimate of the normal optical thickness. A faint satellite (1981S1) was observed near the L₄ triangular libration point of Tethys and is probably the same object as 1980S13.     

Saturn's nonaxisymmetric ring edges at 1.95 Rs and 2.27 Rs

The outer edges of Saturn's A and B rings, at 2.27 R_s and 1.95 R_s, have been examined using data acquired by four Voyager experiments. The shapes and kinematics of these features are influenced by their proximity to strong low-order Lindblad resonances. The data for the A-ring edge are consistent with a seven-loded radial distortion of amplitude 6.7 ± 1.5 km which rotates with the mass-weighted mean angular velocity of the coorbital satellite system. The B-ring edge has essentially a double-lobed figure of radial amplitude 74 ± 9 km which rotates with the mean motion of Mimas, though there is an indication that it is not completely described withe a simple Saturn-centered ellipse. An upper limit of 10 m has been placed on the vertical thickness in the unperturbed region of the B ring.     

VLA observations of Saturn at 1.3, 2, and 6 cm

Radio maps with a resolution of 1″.5 were made of Saturn at 1.3, 2, and 6 cm. The inclination of the ring plane was ?5°.4. A fraction of 0.49 ± 0.08 of Saturn's emitted light is transmitted through the rings at λ2 and λ6 cm. This value converts to an effective head-on or normal optical depth of 0.07 ± 0.02. The transparency at this small inclination angle must be provided entirely by the regions of very low optical depth, e.g., the Cassini and Encke's Divisions, and to achieve our number this requires either a relatively large fraction of gaps or a strong forward scattering by the ring particles. The planetary disk appears to be much less limb darkened in the N-S than the E-W direction, while cuts across the planet, averaged over all directions, agree with the theoretical limb-darkening curves for a planet with a uniform atmosphere and solar abundances for all chemical elements.     

Keck observations of the 2002-2003 jovian ring plane crossing

We present new observations of Jupiter's ring system at a wavelength of 2.2 μm obtained with the 10-m W.M. Keck telescopes on three nights during a ring plane crossing: UT 19 December 2002, and 22 and 26 January 2003. We used conventional imaging, plus adaptive optics on the last night. Here we present detailed radial profiles of the main ring, halo and gossamer rings, and interpret the data together with information extracted from radio observations of Jupiter's synchrotron radiation. The main ring is confined to a 800-km-wide annulus between 128,200 and 129,000 km, with a ∼5000 km extension on the inside. The normal optical depth is 8×¹⁰⁻⁶, 15% of which is provided by bodies with radii a?5 cm. These bodies are as red as Metis. Half the optical depth, τ≈4×¹⁰⁻⁶, is attributed to micron-sized dust, and the remaining τ≈3×¹⁰⁻⁶ to grains tens to hundreds of μm in size. The inward extension consists of micron-sized (a?10 μm) dust, which probably migrates inward under Poynting-Robertson drag. The inner limit of this extension falls near the 3:2 Lorentz resonance (at orbital radius r=122,400 km), and coincides with the outer limit of the halo. The gossamer rings appear to be radially confined, rather than broad sheets of material. The Amalthea ring is triangularly shaped, with a steep outer dropoff over ∼5000 km, extending a few 1000 km beyond the orbit of Amalthea, and a more gradual inner dropoff over 15,000-20,000 km. The inner edge is near the location of the synchronous orbit. The optical depth in the Amalthea ring is ∼5×¹⁰⁻⁷, up to 20% of which is comprised of macroscopic material. The optical depth in the Thebe ring is a factor of 3 smaller.     

The evolution of the lunar Nectaris multiring basin

Nectaris is an 820-km-diameter, multiring impact basin located on the near side of the Moon. The transient cavity is estimated to have been less than 90 km in depth and materials were excavated from a depth of less than 30 km. About 2 km thickness of impact melt is believed to line the cavity center. The impact event probably took place at about 3.98 ± 0.03 × 10⁹ years ago. Nectaris ejecta forms a substantial proportion of the surface materials at the Apollo 16 site. Inter-ring plains deposits were deposited after the formation of the Nectaris basin. The most persuasive origin for the smooth plains is one of extrusives overlain by a thin veneer of ejecta. Basaltic fragments within Apollo 16 samples are believed to have been largely derived from Nectaris. A titanium-rich Apollo 16 mare basalt fragment has an age of 3.79 ± 0.05 × 10⁹ years but, although some relatively titanium-enriched basalts occur in southern Nectaris, titanium-rich basalts are nowhere seen at the surface of the mare. The earliest recognized eruptives appear to be low-titanium (perhaps VLT) basalts found as pyroclastic materials on Daguerre and in the Gaudibert region. The majority of the surface basalts are of intermediate composition (possibly similar to Apollo 12 basalts) and have an age of approximately 3.6 × 10⁹ years. The basalt fill is estimated to have a minimum thickness of 3 km. Flood-style eruptions appear to have been the main form of extrusion. Mare ridges exhibit a strong north-south preferential alignment and appear to postdate basalt emplacement. The lack of basin-related graben in Nectaris is consistent with a thick lithosphere. The basin ring structure is best preserved in the southwest and least preserved in the northeast. This is believed to result from horizontal variations in the crust and lithosphere thicknesses and from the influence of the preexisting Fecunditatis and Tranquillitatis basins; the ring structure is best preserved where the lithosphere was thickest. Floor-fractured craters within Nectaris are intimately associated with the basalt fill both in terms of age and location. Theophilus ejecta, small craters, and Tycho rays, combined with subsidence and mare ridge development, were the only modifying influences on Nectaris since the termination of basalt eruptions.     

Venera 9, 10: Is there a dust ring around Venus?

Four surveys in which the geometrical parameters were suitable for observations on weak scattering objects were carried out by the Venera 9, 10 orbiters using 3000–8000 Å spectrometers. The results of one survey can be explained by a dust layer at the height of sighting h = 100–700 km. Its absence in other sessions suggests a ring structure. The spectrum of dust scattering is a power function of the wavelength with the index varying from ?2.1 at 100km to ?1.3 at 500km. A method is proposed for obtaining the optical thickness, density and size distribution of dust particles from the scattering spectra. For m > 10^?14 g the number of dust particles with a mass higher than m is proportional to m^?1.3. The radial optical thickness τ is 0.7 × 10^?5 at 5000 Å assuming the geometric thickness δ to be 100 km. The maximum optical thickness along the normal to the plane of the ring is τ_n = 4 × 10^?6. The mass of the ring is 20 tons or 5 × 10^?3 g cm^?1 per unit circumference length; the maximum mass in a column normal to the ring plane is 10^?10g cm^?2; the maximum density (for δ = 100 km) is 10^?17 g cm^?3. A satellite of Venus gradually destroyed by temperature effects and by meteorite streams and plasma fluxes is suggested as the source of dust in the ring. One of 1 km radius could sustain such a ring for a billion years. The zodiacal light intensity near Venus is estimated.     

Negative diffusion in planetary rings with a nearby moon

This paper analyzes a process that has been observed in simulations of numerous systems where ring material is strongly perturbed by a nearby moon. If the ring particles can be imparted with a forced eccentricity on the order of 10⁻⁵ in a single pass by the moon, particle orbits are observed to move towards regions of higher density as a result of the organized collisions that occur in the dense peaks of the satellite wake. The width of the ring can decrease by as much as 90% if the forced eccentricity is greater than 3 × 10⁻⁵ and the unperturbed geometric optical depth is greater than 0.03. The fractional change in ring width is relatively insensitive to the particle size so long as the particle radius is much less than the product of the semimajor axis and the forced eccentricity. Including a power law particle size distribution with slope of −2.8 spanning a decade in particle radius reduces the fractional width change by about 10% compared to the uniform particle-size case. Adding gravitational interactions between ring particles only has a significant effect on ring confinement if the unperturbed geometric optical depth exceeds .03, but a 40% reduction in ring width is still achieved in a self-gravitating ring of geometric optical depth 0.3 if the forced eccentricity exceeds 3 × 10⁻⁵. This process does not require the material to be in resonance with the moon, nor does it have any minimum mass constraints because particle self-gravity is not required. The collisional damping of satellite wakes therefore provides a simple mechanism by which a single moon can reduce the radial extent of any ringlet that is close to it and has sufficient optical depth for collisions to be significant.