The surfaces of Larissa and Proteus

Topographic models of Neptune's small inner satellites Larissa and Proteus were derived from the shapes of limbs and terminators in Voyager images, modified locally to accomodate large craters and ridges. The models are presented here in tabular and graphic form, including the first map of Larissa and the first detailed relief map of Proteus. The shape of Larissa is approximated by a triaxial ellipsoid with axes of 208, 192 and 178 km, but is only weakly constrained by the single available view. The volume is estimated to be 3.5 ± 1.0 × 10⁶ km³. The surface is heavily cratered and may be crossed by one or two poorly seen linear ridges. Proteus is approximated by a triaxial ellipsoid with axes of 424, 390 and 396 km (the latter being the rotation axis dimension). The volume is estimated to be 3.4 ± 0.4 × 10⁷ km³. Its surface appears to be very heavily cratered and extensive evidence for linear fractures is observed despite very low image quality.     

Orbital resonances in the inner neptunian system: II. Resonant history of Proteus, Larissa, Galatea, and Despina

We investigate the orbital history of the small neptunian satellites discovered by Voyager 2. Over the age of the Solar System, tidal forces have caused the satellites to migrate radially, bringing them through mean-motion resonances with one another. In this paper, we extend our study of the largest satellites Proteus and Larissa [Zhang, K., Hamilton, D.P., 2007. Icarus 188, 386-399] by adding in mid-sized Galatea and Despina. We test the hypothesis that these moons all formed with zero inclinations, and that orbital resonances excited their tilts during tidal migration. We find that the current orbital inclinations of Proteus, Galatea, and Despina are consistent with resonant excitation if they have a common density . Larissa's inclination, however, is too large to have been caused by resonant kicks between these four satellites; we suggest that a prior resonant capture event involving either Naiad or Thalassa is responsible. Our solution requires at least three past resonances with Proteus, which helps constrain the tidal migration timescale and thus Neptune's tidal quality factor: 9000<QN<36,000. We also improve our determination of Qs for Proteus and Larissa, finding 36<QP<700 and 18<QL<200. Finally, we derive a more general resonant capture condition, and work out a resonant overlap criterion relevant to satellite orbital evolution around an oblate primary.     

The shapes and surface features of Prometheus and Pandora

Topographic models of Saturn's F-Ring shepherd satellites Prometheus and Pandora were derived from the shapes of limbs and terminators in Voyager images, modified locally to accommodate large craters and ridges. The models are presented here in tabular and graphic form, including the first published maps of the satellites. The shape of Prometheus is approximated by a triaxial ellipsoid with axes of 145, 85 and 60 km. The volume is estimated to be 3.9 ± 1.0 × 10⁵ km³, significantly smaller than previous estimates. A system of prominent ridges and valleys cross the north polar region. Prometheus appears to be less heavily cratered than the other small satellites near the edge of the rings, though this may be an artifact of the low resolution of available images. Pandora is approximated by a triaxial ellipsoid with axes of 114, 84 and 62 km. The volume is estimated to be 3.1 ± 1.0 × 10⁵ km³. Its surface appears to be very heavily cratered.     

Orbital resonances in the inner neptunian system: I. The 2:1 Proteus-Larissa mean-motion resonance

We investigate the orbital resonant history of Proteus and Larissa, the two largest inner neptunian satellites discovered by Voyager 2. Due to tidal migration, these two satellites probably passed through their 2:1 mean-motion resonance a few hundred million years ago. We explore this resonance passage as a method to excite orbital eccentricities and inclinations, and find interesting constraints on the satellites' mean density () and their tidal dissipation parameters (Qs>10). Through numerical study of this mean-motion resonance passage, we identify a new type of three-body resonance between the satellite pair and Triton. These new resonances occur near the traditional two-body resonances between the small satellites and, surprisingly, are much stronger than their two-body counterparts due to Triton's large mass and orbital inclination. We determine the relevant resonant arguments and derive a mathematical framework for analyzing resonances in this special system.     

Saturn’s wayward shepherds: the peregrinations of Prometheus and Pandora

Saturn’s narrow F ring is flanked by two nearby small satellites, Prometheus and Pandora, discovered in Voyager images taken in 1980 and 1981 (Synnott et al., 1983, Icarus 53, 156-158). Observations with the Hubble Space Telescope (HST) during the ring plane crossings (RPX) of 1995 led to the unexpected finding that Prometheus was ∼19° behind its predicted orbital longitude, based on the Synnott et al. (1983) Voyager ephemeris (Bosh and Rivkin, 1996 Science 272, 518-521; Nicholson et al., 1996, Science 272, 509-515). Whereas Pandora was at its predicted location in August 1995, McGhee (2000, Ph.D. thesis, Cornell University) found from the May and November 1995 RPX data that Pandora also deviates from the Synnott et al. (1983) Voyager ephemeris. Using archival HST data from 1994, previously unexamined RPX images, and a large series of targeted WFPC2 observations between 1996 and 2002, we have determined highly accurate sky-plane positions for Prometheus, Pandora, and nine other satellites found in our images. We compare the Prometheus and Pandora measurements to the predictions of substantially revised and improved ephemerides for the two satellites based on an extensive analysis of a large set of Voyager images (Murray et al., 2000, Bull. Am. Astron. Soc. 32, 1090; Evans, 2001 Ph.D. thesis, Queen Mary College). From December 1994 to December 2000, Prometheus’ orbital longitude lag was changing by −0.71° year⁻¹ relative to the new Voyager ephemeris. In contrast, Pandora is ahead of the revised Voyager prediction. From 1994 to 2000, its longitude offset changed by +0.44° year⁻¹, showing in addition an ∼585 day oscillatory component with amplitude Δλ_CR0 = 0.65 ± 0.07° whose phase matches the expected perturbation due to the nearby 3:2 corotation resonance with Mimas, modulated by the 71-year libration in the longitude of Mimas due to its 4:2 resonance with Tethys. We determine orbital elements for freely precessing equatorial orbits from fits to the 1994-2000 HST observations, from which we conclude that Prometheus’ semimajor axis was 0.31 km larger, and Pandora’s was 0.20 km smaller, than during the Voyager epoch. Subsequent observations in 2001-2002 reveal a new twist in the meanderings of these satellites: Prometheus’ mean motion changed suddenly by an additional −0.77° year⁻¹, equivalent to a further increase in semimajor axis of 0.33 km, at the same time that Pandora’s mean motion changed by +0.92° year⁻¹, corresponding to a change of −0.42 km in its semimajor axis. There is an apparent anticorrelation of the motions of these two moons seen in the 2001-2002 observations, as well as over the 20-year interval since the Voyager epoch. This suggests a common origin for their wanderings, perhaps through direct exchange of energy between the satellites as the result of resonances, possibly involving the F ring.     

The control networks of the satellites of Uranus

Control networks of the five large satellites of Uranus have been established photogrammetrically from pictures taken by the Voyager 2 spacecraft. The control networks cover the illuminated southern hemisphere of each satellite. Coordinates are listed for 103 points on Miranda, 52 points on Ariel, 43 points on Umbriel, 46 points on Titania, and 34 points on Oberon; some points are identified on the U.S. Geological Survey maps of these satellites. Miranda is ellipsoidal in shape with radii of 241, 235 and 232 km. Mean radii are 579 km for Ariel, 586 km for Umbriel, 790 km for Titania, and 762 km for Oberon.     

The topography of Epimetheus

A topographic model of Saturn's smaller co-orbital satellite Epimetheus was derived from the shapes of limbs, terminators and shadows of the F Ring in Voyager images, modified locally to accommodate large craters and ridges. The model is presented here in tabular and graphic form, including the first detailed shaded relief maps of the satellite. The shape is approximated by a triaxial ellipsoid with axes of 144, 108 and 98 km. The volume is estimated to be 8 ± 1 × 10⁵ km³. A prominent valley and several grooves are found on the heavily cratered surface.     

A model and map of Amalthea

A topographic model of Amalthea (JV) was derived from the shapes of limbs and terminators in Voyager images, modified locally to accommodate large craters and ridges. The model is presented in tabular and graphic form, including the first detailed shaded relief maps of the satellite. The shape is very irregular, with radii varying between about 53 and 151 ± 5 km. The minimum value occurs in a deep crater at the south pole. The volume is estimated to be 2.5 ± 0.5 × 10⁶km³. A prominent groove or valley extends some 150 km across the trailing side. High albedo, spectrally distinct markings are mapped and found to have a less obvious relationship with relief than previously suggested.     

Saturn's dynamic D ring

The Cassini spacecraft has provided the first clear images of the D ring since the Voyager missions. These observations show that the structure of the D ring has undergone significant changes over the last 25 years. The brightest of the three ringlets seen in the Voyager images (named D72), has transformed from a narrow, <40-km wide ringlet to a much broader and more diffuse 250-km wide feature. In addition, its center of light has shifted inwards by over 200 km relative to other features in the D ring. Cassini also finds that the locations of other narrow features in the D ring and the structure of the diffuse material in the D ring differ from those measured by Voyager. Furthermore, Cassini has detected additional ringlets and structures in the D ring that were not observed by Voyager. These include a sheet of material just interior to the inner edge of the C ring that is only observable at phase angles below about 60°. New photometric and spectroscopic data from the ISS (Imaging Science Subsystem) and VIMS (Visual and Infrared Mapping Spectrometer) instruments onboard Cassini show the D ring contains a variety of different particle populations with typical particle sizes ranging from 1 to 100 microns. High-resolution images reveal fine-scale structures in the D ring that appear to be variable in time and/or longitude. Particularly interesting is a remarkably regular, periodic structure with a wavelength of ∼30 km extending between orbital radii of 73,200 and 74,000 km. A similar structure was previously observed in 1995 during the occultation of the star GSC5249-01240, at which time it had a wavelength of ∼60 km. We interpret this structure as a periodic vertical corrugation in the D ring produced by differential nodal regression of an initially inclined ring. We speculate that this structure may have formed in response to an impact with a comet or meteoroid in early 1984.     

The color of the Uranian rings

Observations of the Uranian rings were made in several color filters by the Voyager Imaging Science experiment in January 1986 for the purpose of determining the color of the rings. Selected images were taken through the Violet (λ = 0.41 μm), Clear (λ = 0.48 μm), and Green (λ = 0.55 μm) filters of the Voyager 2 narrow angle camera. The results of the analysis are consistent with the α, β, η, γ, δ, and ϵ rings being very dark, with flat spectra throughout the visible, and are comparable to the latest Voyager results showing a lack of color for the Uranian satellites. The general lack of color in the ring/satellite system of Uranus is remarkably different than the case of the distinctly reddish systems of Jupiter and Saturn. The unique combination of low absolute reflectivity and flat spectrum which characterizes the Uranian rings supports the concept that the Uranian ring material is compositionally distinct from either the Si- and S-rich Jovian ring and inner satellites, or the water-ice-rich rings and inner satellites of Saturn. Of all cosmically abundant materials, the candidate which best matches the low brightness and flat spectrum of the Uranian rings is carbon.     

Orbits of the small inner satellites of Jupiter

Orbital parameter estimates and associated uncertainties determined from Voyager imaging data are presented for the newly discovered small inner satellites of Jupiter.     

Revised exponential distance relation for the Uranian system after the Voyager 2 fly-by

The rings and satellites discovered by Voyager 2 in the Uranian system confirm the existence of small satellites and rings foreseen before their discoveries and close to the locations suggested by an exponential distance relation found by the method of comparison of magnitude order of successive distances ratios of the large Uranian satellites. A new exponential distance relation is deduced, fitting the distances of the large satellites and groups of rings and new small satellites with a spacing ratio of 1.456. Some characteristics of an hypothetical parent body for the new inner satellites are also deduced.     

The topography of Janus

A topographic model of Saturn's larger co-orbital satellite Janus was derived from the shapes of limbs and terminators in Voyager images, modified locally to accommodate large craters and ridges. The model is presented here in tabular and graphic form, including the first detailed shaded relief maps of the satellite. The shape is approximated by a triaxial ellipsoid with axes of 196, 192 and 150 km. The volume is estimated to be 3.0 ± 0.5 × 10⁶ km³, leading to a density estimate of 0.67 ± 0.10 g/cm³. The surface is heavily cratered. Several possible crater chains of uncertain significance are observed, but few prominent linear ridges and no narrow grooves.     

Voyager observations of 1985U1

1985U1 is an irregularly shapedsatellite about 75 km in average radius that orbits Uranus between the ϵ ring and Miranda. It is the only one of the 10 satellites of Uranus discovered by Voyager that was resolved in sufficient detail to permit determinations of the size, shape, and photometric properties. The albedo (0.07–0.09) is considerably lower than those of the large Uranian satellites and slightly higher than that of Saturn's Phoebe. The satellite's opposition magnitude should be about +20.5 in the V filter.     

Ground-based observations of the Jovian ring and inner satellites

Ground-based 0.9-μm observations of the Jovian ring and inner satellites are reported. The ring observations substantially confirm those obtained by the Voyager spacecraft. The first ground-based detection of 1979J2 suggests a geometric albedo of ～0.10 and a new value for its orbit period of 16 hr 11 min 23.5 ± 0.5 sec.     

Disentangling Saturn's F Ring. I. Clump orbits and lifetimes

A comprehensive analysis of the Voyager images reveals the kinematics and lifetimes of clumps in the F Ring. At any given time, the ring has 2-3 major clumps, each several times brighter than the typical ring, plus numerous smaller features. A total of 34 individual clumps have been tracked over periods of 1-7 weeks. The clumps orbited at measurably different rates, implying a 100-km range of semimajor axes centered on 140,220 km. Most are centered around the nominal mean motion of the ring's core, but a few outliers may be associated with a different strand, or with no strand at all. Most clumps change very little over the ∼30 days that they can be detected; however, no clump persisted for the nine-month interval between the two Voyager encounters. The brightest Voyager 2 clump is unusual in that it travels at a rapid mean motion and seems to be associated with the formation of several other clumps.     

Modeling the morphological diversity of impact craters on icy satellites

Impact craters on icy satellites display a wide range of morphologies, some of which have no counterpart on rocky bodies. Numerical simulation studies have struggled to reproduce the diversity of features, such as central pits and transitions in crater depth with increasing diameter, observed on the icy Galilean satellites. The transitions in crater depth (at diameters of about 26 and 150 km on Ganymede and Callisto) have been interpreted as reflecting subsurface structure. Using the CTH shock physics code, we model the formation of craters with diameters between 400 m and about 200 km on Ganymede using different subsurface temperature profiles. Our calculations include recent improvements in the model equation of state for H₂O and quasi-static strength parameters for ice. We find that the shock-induced formation of dense high-pressure polymorphs (ices VI and VII) creates a gap in the crater excavation flow, which we call discontinuous excavation. For craters larger than about 20 km, discontinuous excavation concentrates a hot plug of material (>270 K and mostly on the melting curve) in the center of the crater floor. The size and occurrence of the hot plug are in good agreement with the observed characteristics of central pit craters, and we propose that a genetic link exists between them. We also derive depth versus diameter curves for different internal temperature profiles. In a 120 K isothermal crust, calculated craters larger than about 30 km diameter are deeper than observed and do not reproduce the transition at about 26 km diameter. Calculated crater depths are shallower and in good agreement with observations between about 30 and 150 km diameter using a warm thermal gradient representing a convective interior. Hence, the depth-to-diameter transition at about 26 km reflects thermal weakening of ice. Finally, simulation results generally support the hypothesis that the anomalous interior morphologies for craters larger than 100 km are related to the presence of a subsurface ocean.     

Hyperion: Analysis of Voyager observations

A total of 82 images of Hyperion was returned by the Voyager spacecraft; the most detailed views have a nominal resolution of 8.7 km/line pair. Hyperion had a rotation period of about 13 days and a spin vector lying close to its orbital plane at the time of the Voyager 2 encounter in 1981. The satellite's shape is very irregular, and cannot be approximated suitably by an ellipsoid. The largest cross section (A × C) is about 370 × 225 km; the B × C cross section is approximately 280 × 225 km. Most prominent among the surface features is a 120-km-diameter crater with an estimated depth of 10 km, and a series of arcuate scarps 300 km long that have relief in excess of 5 km. The density of large craters of Hyperion is smaller than that on other small Saturnian satellites and suggests the possibility that the last significant fragmentation of Hyperion occurred near the end of or after initial heavy bombardment. Voyager photometry yields an average normal reflectance of the surface material of 0.21 in the clear filter (0.47 μm) and evidence of slight albedo mottling over the surface. The disk-integrated phase coefficient between phase angles of 22° and 82° is 0.018 mag/de; there is little indication of a strong opposition effect in Voyager data down to phase angles of 3°. Hyperion's average color is definitely redder than that of Phobe, but matches that of the dark material on the leading hemisphere of Iapetus quite well. The satellite's albedo and color are consistent with those of contaminated water ice but since no mass determinations of Hyperion exist we do not know whether the bulk composition is icy or rocky.     

Saturn satellite observations and orbits from the 1980 ring plane crossing

The ground-based observations of the recently discovered Saturnian satellites, obtained during the 1980 apparition, have been collected from the IAU Circulars and identified with and fit to four orbital groups: (1) the inner pair of coorbital librating satellites, (2) the satellite known as “Dione B” near the L₄ point of Dione-Saturn, (3) the satellites associated with the L₄ and L₅ points of Tethys-Saturn or, alternatively, one satellite unconfortably near the orbit of Tethys, and (4) the F-ring satellites observed by Voyager I.     

Surface composition and radius of hyperion

New spectrophotometric data for Hyperion in the region 1.5–2.6 um obtained in 1981 confirm the presence of water ice bands reported by D.P. Cruikshank (1980, Icarus 41, 246–258). The bands are now shown with sufficient clarity to permit improved comparisons with other ice-bearing satellites of Jupiter and Saturn and with laboratory samples. Comparisons with Ganymede and Rhea are shown, and Hyperion is found to differ from both satellites in terms of depth and width of the water ice bands. The sense of the difference is the same as noted earlier from broadband infrared photometry, but the physical cause is not fully understood. The effective radius of Hyperion (considered circular in cross section) derived from a 20-um flux measurement and a revised value of V(1,0) = 4.62 is r = 140 ± 19 km. This result is in better accord with both preliminary and refined values of the radius derived from Voyager images; the Voyager result supercedes that deduced from infrared observations.