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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2009.07.028 Byline: P.D. Nicholson, M.M. Hedman Keywords: Planetary rings; Saturn; rings; Occultations Abstract: An increasing body of evidence shows that, at the sub-km level, Saturn's main A and B rings are dominated by an ever-changing pattern of elongated, canted structures known as self-gravity wakes. Best known for causing azimuthal variations in the rings' reflectivity, these structures also have a profound influence on how the transmission of the rings varies with both longitude and opening angle, B (Colwell et al. [2006] Geophys. Res. Lett. 33, 7201; Colwell et al. [2007] Icarus 190, 127-144; Hedman et al. [2007] Astron. J. 133, 2624-2629). We use data from three stellar occultations observed by Cassini's Visual and Infrared Mapping Spectrometer (VIMS) to measure the transmission of the rings as a function of B, when viewed parallel to the wakes. These data are used to constrain properties of the self-gravity wakes as a function of radius across the A and B rings: specifically the fractional width of the gaps between the wakes, G/I', and the average normal optical depth in the gaps, I.sub.G. We find that the overall normal optical depth of the rings, I.sub.n is primarily controlled by G/I', which varies between Author Affiliation: Department of Astronomy, Cornell University, Ithaca, NY 14853-6801, USA Article History: Received 25 December 2008; Revised 22 July 2009; Accepted 23 July 2009

We review our understanding of Saturn's rings after nearly 6 years of observations by the Cassini spacecraft. Saturn's rings are composed mostly of water ice but also contain an undetermined reddish contaminant. The rings exhibit a range of structure across many spatial scales; some of this involves the interplay of the fluid nature and the self-gravity of innumerable orbiting centimeter- to meter-sized particles, and the effects of several peripheral and embedded moonlets, but much remains unexplained. A few aspects of ring structure change on time scales as short as days. It remains unclear whether the vigorous evolutionary processes to which the rings are subject imply a much younger age than that of the solar system. Processes on view at Saturn have parallels in circumstellar disks. 10.1126/science.1179118

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2008.12.024 Byline: A. Bellucci (a), B. Sicardy (a)(b), P. Drossart (a), P. Rannou (c)(d), P.D. Nicholson (e), M. Hedman (e), K.H. Baines (f), B. Burrati (f) Keywords: Titan; Occultations; Atmospheres; composition Abstract: A solar occultation by Titan's atmosphere has been observed through the solar port of the Cassini/VIMS instrument on January 15th, 2006. Transmission spectra acquired during solar egress probe the atmosphere in the altitude range 70 to 900 km at the latitude of 71[degrees] S. Several molecular absorption bands of CH.sub.4 and CO are visible in these data. A line-by-line radiative transfer calculation in spherical geometry is used to model three methane bands (1.7, 2.3, 3.3 [mu]m) and the CO 4.7 [mu]m band. Above 200 km, the methane 2.3 [mu]m band is well fit with constant mixing ratio between 1.4 and 1.7%, in agreement with in situ and other Cassini measurements. Under 200 km, there are discrepancies between models and observations that are yet fully understood. Under 480 km, the 3.3 [mu]m CH.sub.4 band is mixed with a large and deep additional absorption. It corresponds to the C-H stretching mode of aliphatic hydrocarbon chains attached to large organic molecules. The CO 4.7 [mu]m band is observed in the lower stratosphere (altitudes below 150 km) and is well fit with a model with constant mixing ratio of 33[+ or -]10 ppm. The continuum level of the observed transmission spectra provides new constraints on the aerosol content of the atmosphere. A model using fractal aggregates and optical properties of tholins produced by Khare et al. [Khare, B.N., Sagan, C., Arakawa, E.T., Suits, F., Callcott, T.A., Williams, M.W., 1984. Icarus 60, 127-137] is developed. Fractal aggregates with more than 1000 spheres of radius 0.05 [mu]m are needed to fit the data. Clear differences in the chemical composition are revealed between tholins and actual haze particles. Extinction and density profiles are also retrieved using an inversion of the continuum values. An exponential increase of the haze number density is observed under 420 km with a typical scale height of 60 km. Author Affiliation: (a) Laboratoire d'Etudes Spatiales (LESIA), Observatoire de Paris, Univserite Pierre et Marie Curie, Universite Paris-Diderot, 5 Place Jules Janssen, 92195 Meudon, France (b) Institut Universitaire de France, 103, bd Saint-Michel, 75005 Paris, France (c) GSMA (UMR 6089) Universite de Reims Champagne-Ardenne, 51687 Reims cedex, France (d) Service d'Aeronomie (UMR 7620) Universite de Versailles St. Quentin, 91370 Verrieres le Buisson, France (e) Cornell University, Department of Astronomy, Space Science Bldg., Ithaca, NY 14853, USA (f) Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA Article History: Received 5 June 2008; Revised 21 November 2008; Accepted 12 December 2008

Titan was once thought to have global oceans of light hydrocarbons on its surface (1-5), but after 40 close flybys of Titan by the Cassini spacecraft, it has become clear [...]

Saturn's icy satellites are among the main scientific objectives of the Cassini-VIMS (Visual and Infrared Mapping Spectrometer) experiment. This paper contains a first systematic and comparative analysis of the full-disk spectral properties of Dione, Enceladus, Epimetheus, Hyperion, Iapetus, Mimas, Phoebe, Rhea and Tethys as observed by VIMS from July 2004 to June 2005. The disk integrated properties (350-5100 nm reflectance spectra and phase curves at 550-2232 nm) and images of satellites are reported and discussed in detail together with the observed geometry. In general, the spectra in the visible spectral range are almost featureless and can be classified according to the spectral slopes: from the bluish Enceladus and Phoebe to the redder Iapetus, Hyperion and Epimetheus. In the 1000-1300 nm range the spectra of Enceladus, Tethys, Mimas and Rhea are characterized by a negative slope, consistent with a surface largely dominated by water ice, while the spectra of Iapetus, Hyperion and Phoebe show a considerable reddening pointing out the relevant role played by darkening materials present on the surface. In between these two classes are Dione and Epimetheus, which have a fiat spectrum in this range. The main absorption bands identified in the infrared are the 1520, 2020, 3000 nm [H.sub.2]O/OH bands (for all satellites), although Iapetus dark terrains show mostly a deep 3000 nm band while the 1520 and 2020 nm bands are very faint. In this spectral range, the Iapetus spectrum is characterized by a strong reddening. The C[O.sub.2] band at 4260 nm and the Fresnel ice peak around 3100 nm are evident only on Hyperion, Phoebe and Iapetus. The phase curves at 550 and at 2232 nm are reported for all the available observations in the 0[degrees]-144[degrees] range; Rhea shows an opposition surge at visible wavelengths in the 0.5[degrees]-1.17[degrees] interval. The improvement on the retrieval of the full-disk reflectance spectra can be appreciated by a direct comparison with ground-based telescopic data available from literature. Finally, data processing strategies and recent upgrades introduced in the VIMS-V calibration pipeline (flat-field and destriping-despiking algorithm) are discussed in appendices. Keywords: Saturn, satellites; Spectroscopy; Infrared observations; Image processing

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2005.03.002 Byline: G. Neugebauer (a)(b)(c), K. Matthews (a)(b), P.D. Nicholson (d), B.T. Soifer (b), I. Gatley (b), S.V.W. Beckwith (b) Keywords: Satellites of Saturn; Infrared observations; Planetary rings; Saturn; Eclipses Abstract: Measurements of Iapetus as seen at 20 and 2.2 [mu]m in the shadow of Saturn's ring are given, providing the thermal response to a rapidly varying heat input. The 20 [mu]m thermal emission follows the 2.2 [mu]m flux input closely. The observations, plus a simple diffusion calculation, imply that the surface of Iapetus is made of material having a very small thermal inertia, probably I[less than or approximately equal to]10Jm.sup.-2s.sup.-1/2K.sup.-1. Author Affiliation: (a) Hale Observatories, California Institute of Technology, Carnegie Institution of Washington, 813 Santa Barbara Street, Pasadena, CA 91101, USA (b) California Institute of Technology, Pasadena, CA 91125, USA (c) 7519 S. Eliot Lane, Tuscon, AZ 85747, USA (d) Department of Astronomy, Cornell University, Ithaca, NY 14853, USA Article History: Received 1 November 2004; Revised 3 March 2005

The Visual and Infrared Mapping Spectrometer (VIMS) observed the Galilean satellites during the Cassini spacecraft's 2000/2001 flyby of Jupiter, providing compositional and thermal information about their surfaces. The Cassini spacecraft approached the jovian system no closer than about 126 Jupiter radii, about 9 million kilometers, at a phase angle of < 90[degrees], resulting in only sub-pixel observations by VIMS of the Galilean satellites. Nevertheless, most of the spectral features discovered by the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft during more than four years of observations have been identified in the VIMS data analyzed so far, including a possible [sup.13]C absorption. In addition, VIMS made observations in the visible part of the spectrum and at several new phase angles for all the Galilean satellites and the calculated phase functions are presented. In the process of analyzing these data, the VIMS radiometric and spectral calibrations were better determined in preparation for entry into the Saturn system. Treatment of these data is presented as an example of the VIMS data reduction, calibration and analysis process and a detailed explanation is given of the calibration process applied to the Jupiter data. Keywords: Satellites; Galilean satellites; Jupiter system: Satellite surfaces: Surface composition; Space mission

During the Cassini-Jupiter flyby, VIMS observed Io at different phase angles, both in full sunlight and in eclipse. By using the sunlight measurements, we were able to produce phase curves in the visual through all the near infrared wavelengths covered by the VIMS instrument (0.85-5.1 [micro]m). The phase angle spanned from ~2[degrees] to ~120[degrees]. The measurements, done just after Io emerged from Jupiter's shadow, show an increase of about 15% in Io's reflectance with respect to what would be predicted by the phase curve. This behavior is observed at wavelengths > 1.2 [micro]m. Moreover, just after emergence from eclipse an increase of about 25% is observed in the depth of S[O.sub.2] frost bands at 4.07 and 4.35 [micro]m. At 0.879 < [lambda] < 1.04 [micro]m the brightening is 10-24%. Below [lambda] = 0.879 [micro]m the brightening, if present, should be less than the precision of our measurements (~5%). Apparently, these observations are not explained neither by a diverse spatial distribution of S[O.sub.2] on the Io' surface nor by atmospheric S[O.sub.2] condensation on the surface during the eclipse. Keywords: Io; Atmosphere; Infrared observations: Satellites of Jupiter

Cassini's Imaging Science Subsystem (ISS) instrument took nearly 1200 images of the Jupiter ring system during the spacecraft's 6-month encounter with Jupiter (Porco et al., 2003, Science 299, 1541-1547). These observations constitute the most complete data set of the ring taken by a single instrument, both in phase angle (0.5[degrees]-120[degrees] at seven angles) and wavelength (0.45-0.93 [micro]m through eight filters). The main ring was detected in all targeted exposures; the halo and gossamer rings were too faint to be detected above the planet's stray light. The optical depth and radial profile of the main ring are consistent with previous observations. No broad asymmetries within the ring were seen; we did identify possible hints of 1000 km-scale azimuthal clumps within the ring. Cassini observations taken within 0.02[degrees] of the ring plane place an upper limit on the ring's full thickness of 80 km at a phase angle of 64[degrees]. We have combined the Cassini ISS and VIMS (Visible and Infrared Mapping Spectrometer) observations with those from Voyager, HST (Hubble Space Telescope), Keck, Galileo, Palomar, and IRTF (Infrared Telescope Facility). We have fit the entire suite of data using a photometric model that includes microscopic silicate dust grains as well as larger, long-lived 'parent bodies' that engender this dust. Our best-fit model to all the data indicates an optical depth of small particles of [[tau].sub.s] = 4.7 x [10.sup.-6] and large bodies [[tau].sub.l] = 1.3 x [10.sup.-6]. The dust's cross-sectional area peaks near 15 [micro]m. The data are fit significantly better using non-spherical rather than spherical dust grains. The parent bodies themselves must be very red from 0.4-2.5 [micro]m, and may have absorption features near 0.8 and 2.2 [micro]m. Keywords: Planetary rings; Jupiter; Photometry; Dust

We report the discovery of four new uranian irregular satellites in our deep, [m.sub.R] ~ 25.4, optical search around that planet. The orbital properties of these satellites are diverse. There is some grouping of inclinations and one of the satellites appears to be inside the Kozai resonant zone of Uranus. Further, we find that the differential size distribution of satellites is rather shallow compared to objects in the asteroid and Kuiper belts, going as ~ [r.sup.-2.4]. We also report a strong coupling between semi-major axis and orbital eccentricity. We comment on the apparent paradox between the inc|ination grouping, shal|ow size distribution, and orbital correlation as they relate to the likelihood of a collisional origin for the uranian irregulars. The currently observed irregulars appear to be consistent with a disruptive formation process and a collisional origin for Uranus' obliquity. Keywords: Irregular satellites; Uranus

We report unusual and somewhat unexpected observations of the jovian satellite Io, showing strong methane absorption bands. These observations were made by the Cassini VIMS experiment during the Jupiter flyby of December/January 2000/2001. The explanation is straightforward: Entering or exiting from Jupiter's shadow during an eclipse, Io is illuminated by solar light which has transited the atmosphere of Jupiter. This light, therefore becomes imprinted with the spectral signature of Jupiter's upper atmosphere, which includes strong atmospheric methane absorption bands. Intercepting solar light refracted by the jovian atmosphere, Io essentially becomes a 'mirror' for solar occultation events of Jupiter. The thickness of the layer where refracted solar light is observed is so large (more than 3000 km at Io's orbit), that we can foresee a nearly continuous multi-year period of similar events at Saturn, utilizing the large and bright ring system. During Cassini's 4-year nominal mission, this probing technique should reveal information of Saturn's atmosphere over a large range of southern latitudes and times. Keywords: Occultations; Jupiter; Infrared observations

The Cassini Visual and Infrared Mapping Spectrometer (VIMS) is an imaging spectrometer coveting the wavelength range 0.3-5.2 [micro]m in 352 spectral channels, with a nominal instantaneous field of view of 0.5 mrad. The Cassini flyby of Jupiter represented a unique opportunity to accomplish two important goals: scientific observations of the jovian system and functional tests of the VIMS instrument under conditions similar to those expected to obtain during Cassini's 4-year tour of the saturnian system. Results acquired over a complete range of visual to near-infrared wavelengths from 0.3 to 5.2 [micro]m are presented. First detections include methane fluorescence on Jupiter, a surprisingly high opposition surge on Europa, the first visual-near-IR spectra of Himalia and Jupiter's optically-thin ring system, and the first near-infrared observations of the rings over an extensive range of phase angles (0-120[degrees]). Similarities in the center-to-limb profiles of [H.sup.+.sub.3] and C[H.sub.4] emissions indicate that the [H.sup.+.sub.3] ionospheric density is solar-controlled outside of the auroral regions. The existence of jovian N[H.sub.3] absorption at 0.93 [micro]m is confirmed. Himalia has a slightly reddish spectrum, an apparent absorption near 3 [micro]m, and a geometric albedo of 0.06 [+ or -] 0.01 at 2.2 [micro]m (assuming an 85-km radius). If the 3-[micro]m feature in Himalia's spectrum is eventually confirmed, it would be suggestive of the presence of water in some form, either free, bound, or incorporated in layer-lattice silicates. Finally, a mean ring-particle radius of 10 [micro]m is found to be consistent with Mie-scattering models fit to VIMS near-infrared observations acquired over 0-120[degrees] phase angle. Keywords: Jupiter; Jovian satellites; Jupiter's ring; Spectroscopy; Planetary atmospheres; Planetary surfaces; Cassini mission

The occultation of GSC5249-01240 by Saturn's rings was observed in a spectrally resolved mode using the Faint Object Spectrograph on the Hubble Space Telescope. By combining these data with other occultation data, we have determined the inclination of the F ring to be 0.0065 [+ or -] 0.0014 deg. Our inclined F ring orbit model explains an abrupt decrease in flux at the west ansa of the ring that was observed during the November 1995 ring-plane crossing as well as the nondetection of 1995 S5 during one set of observations in November 1995. The F ring's equivalent depth is found to have no significant dependence on wavelength between 0.27 and 0.74 [micro]m, indicating the presence of a population of ring particles larger in size than ~10 [micro]m. This contrasts with the results from a previous analysis of Voyager images at various phase angles, in which the particles were determined to be predominantly submicron in size. The difference may be due to temporal or longitudinal variability within the ring. Key Words: planetary rings, Saturn; occultations; orbits.

Due to its extremely prolate figure, Asteroid (433) Eros may exhibit an unusual nonprincipal axis rotation state. As a result of a relatively small difference between the maximum and intermediate moments of inertia, a small perturbation such as the gravitational torques experienced during a close planetary encounter could have induced a rather large amplitude libration about the longest axis. Observation of such a state will permit measurement of the moment of inertia ratios; this task may be possible once the NEAR spacecraft enters orbit around Eros in early 2000 to begin an extensive study of its composition and structure. Key Words: Eros; rotation; asteroids, rotation.

The first near-infrared (0.9 [[micro]meter]) photometric observations of Saturn's five medium-sized satellites were obtained during the August 1995 ring plane crossing with Palomar Observatory's 60-in. telescope. The albedos of the satellites are very high, indicating that a very low fraction of dark opaque contaminants is present on their icy surfaces. The geometric albedos for the leading sides of the satellites are Enceladus: 1.02; Tethys: 0.90; Dione: 0.72; and Rhea: 0.76. The geometric albedo of the trailing side of Mimas is 0.72. The amplitudes of the satellites' rotational light curves are somewhat less than those measured at visible wavelengths. They range from 0.40 (Dione) to 0.10 (Rhea) to 0.05 (Tethys). Both Enceladus and Mimas are brighter on the trailing side, unlike the other three satellites. These results suggest that the optical properties of all five satellites are determined by interactions with Saturn's E-ring.

Saturn's moon Enceladus emits a plume of water vapour and micro-metre-sized ice particles from a series of warm fissures located near its south pole (1-10). This geological activity could be powered or controlled by variations in the tidal stresses experienced by Enceladus as it moves around its slightly eccentric orbit. The specific mechanisms by which these varying stresses are converted into heat, however, are still being debated (11-16). Furthermore, it has proved difficult to find a clear correlation between the predicted tidal forces and measured temporal variations in the plume's gas content (17-19) or the particle flux from individual sources (20,21). Here we report that the plume's horizontally integrated brightness is several times greater when Enceladus is near the point in its eccentric orbit where it is furthest from Saturn (apocentre) than it is when near the point of closest approach to the planet (pericentre). More material therefore seems to be escaping from beneath Enceladus' surface at times when geophysical models predict its fissures should be under tension (12,15,16) and therefore may be wider open., This analysis focuses on 252 images of the Enceladus plume at wavelengths of 0.88-1.56 µm obtained by the Cassini spacecraft's Visual and Infrared Mapping Spectrometer (VIMS) (22) between 2005 and [...]

Using the SpectroCam-10 instrument on the Hale 200' telescope at Palomar Observatory, we obtained an extensive set of images and spectra of Mars in the thermal infrared during the 1993 and 1995 oppositions. Images were taken using a set of seven wide-band filters centered at 7.9, 8.8, 9.8, 10.3, 11.7, 12.5, and 17.9 [[micro]meter]. Spatially resolved slit spectra with a spectral resolving power of [Lambda]/[Delta][Lambda] = 100 between 7.5 and 13.5 [[micro]meter] were also obtained. Initial analysis of data from the 1993 opposition led to the identification of an absorption feature at 9.2 [[micro]meter] in the Acidalia region. At the time the feature was first identified, it was impossible to determine whether it was caused by a local enhancement of atmospheric dust over Acidalia, or by nonunit emissivity of surface materials in that region. Subsequent observations taken during the 1995 opposition showed that the feature in Acidalia persisted, indicating that it was of surficial origin. Because of its spectral position and shape, we interpret the feature to be the result of the emissivity spectrum of coarse (sand-sized or greater) silicate materials on the surface in Acidalia. While the spectral range available in our data is probably insufficient to make a specific mineralogical interpretation, we find it to be consistent with the emissivity spectrum of a terrestrial basalt sample measured in the laboratory.

The world-wide observing campaign of the Comet P/Shoemaker-Levy 9 (SL9) impacts into Jupiter during July 1994 has led to an unprecedented quantity of imaging data, predominantly in the visible to near-infrared wavelength regime. In an effort to provide reference brightness values for Jupiter at all latitudes, we present a compilation of absolute reflectivity scans along the jovian central meridian during the impact epoch. Images from both Earth-orbiting and ground-based telescopes were photometrically calibrated and analyzed to provide as much spatial resolution and wavelength coverage as possible. This fundamental reference will be made available to the entire planetary science community as a means to photometrically reduce uncalibrated data from the SL9 impacts via the Planetary Data System (PDS). Until the entire data set is ingested into the PDS, it will be available through a World Wide Web site and the AAS CD-ROM series.

A study shows that the instantaneous rotation rate of Hyperion, an irregular satellite with an independent orbit, is 72 + 3/-4 deg/day, or 4.25 times the synchronous rate. The body's axis of minimum inertia and instantaneous spin axis are nearly coincident, with an obliquity of about 62 degs. A wobble of a period of seven days is prominently seen in the satellite's lightcurve. These are the results obtained using the observations made during Voyager 2 encounter in 1981.

A numerical analysis shows that rotational motion of Hyperion, an irregular satellite with an independent orbit, is a combination of a free wobble, forced precession and rotation about the long axis. The wobbling period of about seven days and the rotation period of about five days are in a similar range. This makes the rotational motion very complex. Results obtained using the observations made by Voyager 2 in 1981 are discussed.

Author(s): D. P. Cruikshank (corresponding author) [1]; J. B. Dalton [3]; C. M. Dalle Ore [3]; J. Bauer [4]; K. Stephan [5]; G. Filacchione [6]; A. R. Hendrix [4]; C. [...]