Abstract Index

  1. Keck Observations of Saturn’s Ring Plane Crossing I. de Pater, M. R. Showalter, J. J. Lissauer, J. R. Graham,

  2. Azimuthal Distribution of Arcs and Clumps in the F Ring of Saturn During August 1995 Ring Plane Crossing C. Ferrari, A. Brahic, S. Charnoz, P. Thebault, F. Roddier

  3. High-Resolution Imaging of Saturn’s G Ring from the Hubble Space Telescope R. G. French, J. Cuzzi, L. Dones, J. Lissauer

  4. F-Ring Model from Analysis of the Ring’s Radial Structure M. K. Gordon, C. D. Murray, S. M. Giuliatti Winter

  5. WHT Observations of the Saturn System, 2 & 3 August, 1995 M. K. Gordon, S. M. Larson, C. D. Murray, D. H. P. Jones, S. C. Greaves, O. Munoz

  6. Calar Alto Observations of Saturn’s 1995 Ring Plane Crossings L. M. Lanier, D. P. Hamilton, T. M. Herbst

  7. HST Observations of Small Saturnian Satellites and F Ring Clumps C. A. McGhee, P. D. Nicholson, R. G. French

  8. Multiwavelength Radio Observations of Saturn’s Rings at Low Inclinations L. A. Molnar, D. E. Dunn, J. D. Fix

  9. Saturn Ring Profiles from the August 1995 HST Data P. D. Nicholson

  10. Future Ring Plane Crossings: 1997-2047 P. D. Nicholson

  11. Inclined Features in Saturn’s Rings C. B. Olkin, A. S. Bosh

  12. Role of the F Ring during the Ring Plane Crossings in 1995-96 F. Poulet, B. Sicardy

  13. Photometric Analysis and Modeling of the August 1995 Saturn Ring HST Data C. Roddier, F. Roddier

  14. Adaptive Optics Observations of Saturn’s Ring Plane Crossing in August 1995 F. Roddier, C. Roddier, C. Dumas, J. E. Graves, M. J. Northcott, T. Owen, A. Brahic

  15. Ground-Based Observations of the Saturn Ring-Plane Crossings of August and November 1995 B. R. Scharringhausen, P. D. Nicholson, K. Matthews, P. McGregor, R. G. French, L. Lim

  16. Satellite Detection Limits in HST Data P. Seitzer

  17. Dynamics of Clumps in Saturn’s F Ring M. R. Showalter

  18. A Search for Saturnian Moonlets Using the WIYN Telescope M. R. Showalter, L. Dones, R. H. Durisen, R. K. Honeycutt, J. S. Jurcevic, R. Tripoli-Strom, C. Strom

  19. G Ring Particle Sizes Derived From Ring Plane Crossing Observations H. B. Throop, L. W. Esposito

Keck Observations of Saturn’s Ring Plane Crossing

I. de Pater (Berkeley), M. R. Showalter (Stanford), J. J. Lissauer (NASA Ames), J. R. Graham (Berkeley)

We observed Saturn’s ring plane crossing with the W.M. Keck telescope on Mauna Kea, Hawaii, May 21-23 and August 8-10, 1995. Most obervatoons were carried out in a nnarrow band filter at 2.26 µm, but we also obtained data at 1.65 µm and 1.24 µm. Part of the data have been published in Icarus: de Pater, I., M. R. Showalter, J. J. Lissauer, and J. R. Graham, 1996, Keck infrared observations of Saturn’s E and G rings during Earth’s 1995 ring plane crossing, Icarus Note 121, 195-198. At this meeting we will briefly discuss which data we have, and what the status of data reduction and analysis is.

Azimuthal Distribution of Arcs and Clumps in the F Ring of Saturn During

August 1995 Ring Plane Crossing

C. Ferrari, A. Brahic, S. Charnoz, P. Thebault (Europa, Univ. Paris 7), F. Roddier (IFA, Univ. of Hawaii)

Ground-based observations of the August 1995 Saturn’s ring plane crossing with the University of Hawaii Adaptive Optics System have led to the discovery of several new objects orbiting around the planet. Their orbits are known with variable accuracy but most of them are compatible with the F ring. Some of these objects are clearly azimuthally elongated structures, possible arcs, the others are not resolved. Planetary arcs are swarms of clumps, which largest particles are most probaly unresolved kilometer-sized objects (Ferrari and Brahic, 1992, 1994, 1997). Are these objects evolving with time ? On which timescale ? The F ring arcs have been first observed during the Voyager encounters. New constraints on the evolution of the brightests of them, on a two-weeks timescale, at the epoch of the Voyager 2 encounter, are presented. A revised orbit of the F ring is used to derive an azimuthal distribution of the newly discovered objects at August 1995 epoch. This is compared with the azimuthal profile of the F ring observed three months later during the Sun ring plane crossing (Nicholson et al., 1996). The possible nature and lifetime of these new objects is discussed.

High-Resolution Imaging of Saturn’s G Ring from the Hubble Space Telescope

R. G. French (Wellesley), J. Cuzzi (NASA Ames), L. Dones (San Jose State), J. Lissauer (NASA Ames)

Saturn’s tenuous G Ring was first detected in 1979 by Pioneer 11 from its absorption of high-energy protons (Van Allen et al. 1980, Science 207, 415) and was subsequently imaged by Voyager 2 in 1980-81 (Showalter and Cuzzi 1993, Icarus 103, 124). More recently, images of the G Ring were obtained in the near-IR (de Pater et al, 1996, Icarus 121, 195; Bauer et al 1997, Icarus 125, 440) and with the HST WFPC2 wide-field camera (Nicholson et al 1996, Science 272, 509) during the 10 August 1995 Earth crossing and the 17-21 November 1995 solar crossing of Saturn’s ring plane (RPX), when the nearly edge-on configuration enhanced the slant path optical depth of the ring to detectable levels. The picture that has emerged from these observations is of a faint ring centered at about 168,000 km radius, with a FWHM of about 4,000 km. The resolution of the Voyager images was limited to ≥ 1000 km by stellar smear and camera rotation during the exposures, and the 1995 HST images were binned over 2 x 2 wide-field camera pixels, resulting in a radial scale of ~1300 km per binned pixel. On 14 October 1996, we obtained a 400-s exposure of the G ring with the Planetary Camera of HST’s WFPC2, using the F555W filter. The radial resolution is ~285 km per pixel, providing the highest resolution image of the G Ring ever obtained. The radial scale of the image is well determined because Epimetheus was in the field of view and can be used as an astrometric reference. The radially integrated equivalent width of the ring is ~0.92 ± 0.07 m, comparable to the RPX results from HST. The ring opening angle was 3.8° at a phase angle of 1.9°. The radial µI/F profile shows an inner edge with a roughly linear ramp that increases from zero at a radius of r=165,000 km to a maximum of 2.1 x 10-7 at r=168,500 km, and then decreases more gradually to zero at r=174,000 km. The half-intensity points are at r=167,000 and 171,000 km. There is no evidence of a narrow central core; the overall structure is similar to that seen from Voyager and HST at RPX.

F-Ring Model from Analysis of the Ring’s Radial Structure

M. K. Gordon (USGS, Flagstaff), C. D. Murray (Queen Mary and Westfield College, London), S. M. Giuliatti Winter (UNESP, Guaratinguet, Brazil)

Several Voyager 2 images of the F ring show that it is composed of at least four separate, non-intersecting strands extending 45° in longitude; the two brightest components are the central pair. In contrast, some Voyager 1 images show apparent intersections of two bright strands giving rise to a “braided” appearance with a faint inner strand. A single high-resolution Voyager 1 image of the F ring shows two bright parallel strands with a faint outer strand; this configuration was also detectable in a sequence of low-resolution images.

Orbits were fit to each strand by assuming that the two brightest strands in the Voyager 1 images corresponded to those in the Voyager 2 images. The maximum separation in semimajor axes of the four strands is 300 km. Their orbits suggest that they have comparable eccentricities and nearly aligned pericenters. We believe this configuration could be maintained if the main strand contains sufficient mass. Slight offsets from exact alignment of perichrones should lead to intersection of adjacent strands at some longitudes, while giving a parallel appearance at others. The semimajor axes of the strands are not associated with resonances with the ring’s shepherding satellites Prometheus and Pandora. Although differential precession should cause Prometheus to disrupt the F ring every 19 years, it is unclear how the ring re-organises itself without the influence of small, embedded satellites.

WHT Observations of the Saturn System, 2 & 3 August, 1995

M. K. Gordon (USGS, Flagstaff), S. M. Larson (LPL), C. D. Murray, D. H. P. Jones, S. C. Greaves (Queen Mary and Westfield College, London), O. Munoz (IAA)

We observed the Saturn system using the 4.2 m William Herschel Telescope on La Palma during the nights of 2 & 3 August, 1995. Our objectives included the reacquisition of previously identified small satellites, the search for additional (undiscovered) satellites and color determination of the E ring. For satellite and main ring system observations, we used an 890 nm narrow band filter and exposure times of 16 - 256 s. The observing conditions varied significantly due to partial cloud cover and the Sahara dust in the atmosphere; the seeing varied from 0.7” - 1.7”. The observations were made one week prior to the August ring plane crossing while the earth and sun were on opposite sides of the ring plane. The earth was within 0.2° of the ring plane, and the solar phase angle was ~4°. The spectrum of the E ring had a blue slope consistent with later HST results. This paper will describe the data and present the results of data analysis to date.

Calar Alto Observations of Saturn’s 1995 Ring Plane Crossings

L. M. Lanier, D. P. Hamilton (U. of Maryland), T. M. Herbst (MPI fuer Astronomie, Heidelberg)

We report on Calar Alto data from the Saturn Ring Plane Crossings (RPX) that occurred in 1995. Our data, which consists of infrared images and spectra taken from the 3.5 meter telescope at the Calar Alto Observatory in southern Spain, span the dates May 20-22, August 8-11, and November 20-23. Here we investigate images taken with a broad band filter covering the 2.1 to 2.4 micron region. Methane in Saturn’s atmosphere strongly absorbs at these wavelengths which greatly reduces scattered light from the planet. The camera’s plate scale is 0.33”/pixel, and in our May observations, Saturn’s diameter was 17”, the rings spanned 42”, atmospheric seeing was ~1”. The exposure times for our images varied from a few seconds to several minutes.

We have performed basic data reduction on our May data by subtracting a sky frame from each of our images and dividing by a flat field image. We also replaced all bad pixels with the median of the surrounding pixels in each image. Once the images were reduced, we developed IDL routines to calculate the flux from the ring ansae. Although Saturn’s rings are 3 to 4 pixels wide in our images due to atmospheric seeing, the physical width of the rings is less than a pixel; accordingly, we produced a one-dimensional profile of the rings. We then extracted the brightest seven pixel segments of each ring ansa from the profile. Averaging over seven pixels improved signal to noise and minimized interference from the planet and nearby satellites.

Our analysis of the reduced data shows an asymmetry between the east and west sides (ansae) of Saturn’s rings. Our ground-based infrared data indicates that the west ansa is brighter than the east ansa for 30 hours prior to the RPX, which occurred on May 22 at 5:35am. This result is in agreement with, and extends, the optical HST results of Bosh etal. (1997), which show that the west ansa is brighter than the east for three hours prior to the May RPX. We are currently refining our data analysis in order to better quantify the asymmetry. We plan to present plots of the ring flux vs. time for our May, August, and November data sets. We also hope to report on measurements of the positions of the small satellites visible in our data: Janus, Epimetheus, Prometheus, and Pandora.

HST Observations of Small Saturnian Satellites and F Ring Clumps

C. A. McGhee, P. D. Nicholson (Cornell), R. G. French (Wellesley)

Hubble Space Telescope (HST) observations of Saturn’s ring and satellite system during the August 1995 Earth ring plane crossing and during the November 1995 solar crossing are presented. Very low ring brightnesses during these periods allowed detections of faint satellites Pandora and Prometheus and lagrangian satellites Telesto, Calypso and Helene, and identification of objects determined to be in or near the F ring (Nicholson et. al [1996], Science 272, 509). In August, three objects identified as 1995S5-S7 were seen in multiple HST orbits, while November’s data show at least two elongated “clumps.” Attempts to correlate these objects with those found in the May Earth-crossing HST data of Bosh and Rivkin ([1996], Science 272, 518) (1995S1-S4) have resulted in the possible linking of 1995S3 with 1995S6, if both objects are assumed to be exactly within the F ring. While 1995S1 is not seen convincingly in subsequent data sets, this object may be present in some three August HST images and two November images. Data taken with the Canada- France-Hawaii Telescope (CFHT) with an adaptive optics system during the August crossing resulted in the identification of several new objects (Roddier et al [1996] IAUC 6407); object 1995S9 has been recovered in five August HST images.

The puzzling question of Prometheus’ 18.8° lag has spurred the development of new analysis and measurement techniques for better determination of orbits of these bodies, and for the first time these techniques are applied consistently to all three HST data sets. The new techniques include subtraction of templates from the images, as opposed to the ring-pair subtraction used formerly to identify and measure the new objects, and the inclusion of orbital smear in modelling the image shapes, rather than using a simple centroid procedure to measure the positions. The template subtraction has several advantages, including a better ability to distinguish among the many small objects that are very close together in some of the frames. Our goals are to fit the remaining objects that were seen in the HST data, as well as to link the objects from all the HST data sets together with ground-based observations.

Multiwavelength Radio Observations of Saturn’s Rings at Low Inclinations

L. A. Molnar, D. E. Dunn, and J. D. Fix (U. Iowa)

We present final brightness temperature maps of Saturn as observed at its equinox (20 November 1995) with the Very Large Array at 0.7, 2.0, 3.6, and 6.1 cm wavelength. The ring inclination angle was +2.7°. The western ansa was brighter than the eastern one at every wavelength, an effect seen by de Pater and Dickel (1991) at higher inclinations. The asymmetry cannot be understood in terms of a single particle scattering phase function which should be mirror symmetric on average. We suggest instead the effect is caused by multiple scattering in an anisotropic particle distribution, such as a gravitational wake.

We also present preliminary brightness temperature maps of Saturn at a second epoch, February 1997, at 1.3, 2.0, 3.6, 6.1, 18, and 20 cm wavelength. The ring inclination angle was -4.9°. The data at this epoch have a higher sensitivity and finer resolution than the earlier data, in addition to havin a more uniform spatial coverage.

Saturn Ring Profiles from the August 1995 HST Data

P. D. Nicholson (Cornell)

During the second crossing of Saturn’s ring plane by the Earth in August 1995, HST observations were obtained with WFPC2 from August 10, 13:15 UT through August 11, 1:30 UT (Nicholson et al., [1996] Science 272, 509). On each HST orbit, a pair of 300 sec exposures was taken with the Wide Field camera which cover the entire ring system, plus a series of up to five 100 sec exposures with the Planetary Camera targeted to one ansa. All exposures were made with the FQCH4N 890 nm methane filter to suppress scattered light from Saturn. Prior to the Earth’s crossing the plane at 21 hr UT, the radial brightness profiles of both ring ansae are relatively flat and smooth, increasing slowly outwards until they terminate abruptly at the radius of the F Ring (r=140,200 km). 30-50% increases in brightness at radii corresponding to the C Ring and the Cassini Division 7 hr before the crossing indicate a modest contribution from light diffusely transmitted through the main rings, but closer to the crossing time the profiles are essentially featureless and most of the edge-on ring brightness seems to be attributable to the F Ring. The average vertically-integrated reflectivity (I/F) of the rings at this time was found to be 1.37 ± 0.15 km, with the the west ansa being ~25% brighter than the east. Within 45 min after the Earth had crossed the plane the central portion of the west ansa had brightened by a factor of two, while the east ansa had increased by less than 50%. By 23:50 UT, however, this asymmetry had largely disappeared, and in our final set of images the west ansa is ~15% fainter than the east ansa, an observation which it would be very interesting to confirm with ground-based observations. Linear fits to the pre- and post-crossing brightness over different radial regions yield crossing times which are significantly earlier on the west ansa, with the largest asymmetry occuring in the B Ring region, where the difference between east and west amounts to 56 ± 13 min. An effect of this amplitude is much too large to be accounted for by the rings’ Laplace plane warp of 140 m, and is most likely due to the F Ring’s small inclination (Olkin & Bosh [1996] BAAS, 28, 1125).

Future Ring Plane Crossings: 1997-2047

P. D. Nicholson (Cornell)

Saturnian ring plane crossings occur in series at alternating intervals of 13 3/4 and 15 3/4 years, due to Saturn’s orbital eccentricity. Roughly half involve a single crossing by the Earth and one by the Sun, and generally occur close to conjunction. These events are thus rarely observed. Slightly more than half, on the other hand, involve three Earth crossings, at least one of which occurs close to opposition. These latter crossing seasons are clearly more favorable for Earth-based observations. Late twentieth century astronomers have been particularly fortunate in this respect, as the crossings in 1966, 1979/80 and 1995/96 have all been triple. However, we are now in for a drier period for Saturn observers. The Earth crossings of 2009 Sept 4 and 2025 Mar 23 are both singles, and occur when Saturn is within 12° and 9° of the sun, respectively. The accompanying solar crossings are on 2007 Aug 11 and 2025 May 6. The next series of favorable triple ring plane crossings will not occur until 2038/39, when the second Earth crossing on 2039 April 1 falls only 2 weeks after opposition. The solar crossing on 2039 Jan 22 and the third Earth crossing on 2039 July 9 should also be observable.

It may be of some interest to note here the next few crossings for the other ringed planets. On 1997 Oct 22 the Earth will approach the jovian ring plane to within 0.001°, but not actually cross it. Keck observations are scheduled for this event. Looking farther downstream, Uranus provides the next series of ring plane crossings in 2007/8, in circumstances very similar to the recent sequence of Saturn events. The Earth crosses the uranian ring plane on 2007 May 3, 2007 Aug 16 (3 weeks before opposition) and 2008 Feb 20. Observations near these times could provide sensitive probes for low optical depth material analogous to Saturn’s E Ring, and for vertically-extended halos. Finally, we note that Neptune’s slow orbital motion permits quintuple crossings, the next of which will occur in 2045-2047. The earth crosses the neptunian ring plane on 2045 July 7 and Oct 10 (4 weeks before opposition), 2046 May 4, and 2047 Jan 23 and Feb 6.

All calculations courtesy the PDS Ring-Moon Systems Node and JPL Horizons software.

Inclined Features in Saturn’s Rings

C. B. Olkin, A. S. Bosh (Lowell Observatory)

On 21 November 1995 just as the Sun was completing its passage through Saturn’s ring plane, the occultation of the star GSC5249-01240 by Saturn and its rings was observed with the Faint Object Spectrograph on the Hubble Space Telescope (HST) and from the Infrared Telescope Facility (IRTF). The low opening angle of Saturn s rings (B ~ 3°) made these occultation observations unusually sensitive to ring inclinations. We used these data along with previous occultation data to determine a kinematic model for the F ring and established a non-zero value for the inclination of the F ring with a 4-sigma significance level. This is the first detection of an inclined ring feature in the Saturn system. The inclined ring model predicts an eclipse of the F ring near the West ansa. HST images taken immediately after the stellar occultation show an abrupt decline in flux very near the predicted location, providing support for the inclined ring model. This model also predicts an eclipse of the F ring on the East ansa closer to Saturn. We will examine images for evidence of any dimming on the East ansa. We will also present results of a search for other inclined features in Saturn’s rings.

Role of the F Ring during the Ring Plane Crossings in 1995-96

F. Poulet (Obs. Paris), B. Sicardy (Obs. Paris, Univ./IUF Paris 6)

We will examine the role played by the Saturn F ring during the plane crossings in 1995-96. Emphasis is given on the ring thickness and the small inner objects.

We model the photometric data obtained during the Earth ring plane crossings and the solar crossing. The data from the Earth ring plane crossing could give lower limit on the physical height of the F ring (Poulet and Sicardy, BAAS 28, 1124). New constraints from the solar crossing in November 1995 are obtained, in order to explain both the August and November 1995 profiles.

New detections of four small inner objects were made, allowing us to cover several revolutions of two objects. We use the photometric model to infer the nature of these new objects. We also propose that they are composed of regolith ejecta and result from collisions between small moonlets. From the number and optical depth, we derive the lifetimes of the new objects and we estimate the total cross-sectionnal area and sizes of the unseen parent objects to be consistent with observations of the F ring region.

Photometric Analysis and Modeling of the August 1995 Saturn Ring HST Data

C. Roddier, F. Roddier (Institute for Astronomy, U. Hawaii)

Abstract: A photometric analysis of the August 1995 HST data is presented. A model developed from adaptive optics observations is compared with the photometric profiles observed along the rings. The model is found to be in good agreement with the data apart from the East-West asymmetry.

For the asymmetry, two possible effects are discussed. One is the occultation of the main rings by an inclined F ring. The other is a large scale distortion of the ring surface. An analytic model for a plausible distortion is fitted to the data. The implications of the result are discussed.

Adaptive Optics Observations of Saturn’s Ring Plane Crossing in August 1995

F. Roddier, C. Roddier, C. Dumas, J. E. Graves, M. J. Northcott, T. Owen (Institute for Astronomy, U. Hawaii), A. Brahic (Observatoire de Paris and Universite de Paris 7)

Abstract: An adaptive optics system developed at the University of Hawaii was mounted on the 3.6-m CFH telescope on Mauna Kea and used to observe Saturn during the August 95 ring-plane crossing. Near IR images of the ring system were obtained with a resolution of 0.1 arc-sec. These images have since been processed and analyzed both astrometrically and photometrically.

Evidence was found for a dozen of objects, probably clumps, orbiting at the distance of the F-ring. Among these objects two of them have been identified with objects 1995 S5 and S7 observed by the Hubble Space Telescope.

An eclipse of Epimetheus was observed over about 10 minutes. We have determined that Epimetheus was crossing the shadow produced by a region extending from the F ring to the Encke division. The observed positions of Epimetheus during the eclipse give constraints on its orbit, and on the optical depth of the rings.

Photometric profiles have been obtained along the rings, and their time evolution has been modeled. Values have been derived for the rings brightness (compared to a Lambert diffuser). The gap between the A ring and the F ring is found to contribute to the profiles with a transverse optical thickness of the order of 0.0001. Evidence is also found for a composite structure in the F ring with a geometrically thick component of very low particle density.

Ground-Based Observations of the Saturn Ring-Plane Crossings of August and

November 1995

B. R. Scharringhausen, P. D. Nicholson (Cornell), K. Matthews (Caltech), P. McGregor (MSSSO), R. G. French (Wellesley), L. Lim (MIT)

We present two sets of ground-based observations of the 1995 Saturn ring plane crossings. Observations on the 5-meter Hale Telescope at Palomar were made on August 13 - 17 (UT) and November 21, 22, and 23. A 2.276 µm filter with a bandpass of 0.17 µm was used. This is in a methane absorption band where the albedo of Saturn is low, reducing scattered light from the planet. The 256 x 256 InSb Cassegrain Infrared Camera has a plate scale of 0.125” per pixel. The August observations followed the Earth crossing on August 10, and the November observations were at the end of the November 17-21 Sun crossing with a ring opening angle of 2.7°. Additional observations were made on August 9, 10, and 11 (UT) with the CASPIR camera on the ANU 2.3 meter telescope at Siding Spring Observatory (SSO), Australia. A 2.34 µm narrow band filter was used, and the plate scale was 0.25” per pixel. The August 10 SSO observations run up to 21:00 UT, just before the ring crossing.

The principal goal of the Palomar observations was to acquire accurate orbital data on both the ring moons and the classical satellites by observing their eclipses and occultations by Saturn, as well as several mutual events between larger satellites. This program was very successful, resulting in good data for a total of 20 events involving Mimas, Enceladus, Tethys, Dione, Rhea, Titan, and Hyperion, as well as 4 eclipses of Janus and Epimetheus. [Nicholson, et al. BAAS 23, 1073 (1996)] We present here radial profiles of ring brightness at 2.3 µm, their temporal evolution through the August ring plane crossing, and a search for any east-west asymmetries such as are seen in the HST data [Nicholson, et al. Science 272, 509 (1996)]. The Palomar observations on November 21 were made under conditions of excellent (0.3”) seeing, and provide additional astrometric data for Janus, Epimetheus, Pandora and Prometheus, as well as high-quality profiles of brightness for the unlit side of the rings.

Satellite Detection Limits in HST Data

P. Seitzer (Dept. of Astronomy, U. Michigan)

We analyze the completeness of detecting satellites in the 1995/1996 Saturn WFPC2 data sets. Artificial satellites of known magnitude and angular motion are introduced into pairs of WFPC2 images, and the frequency of recovery is determined as a function of angular motion, total magnitude, and whether the satellite is projected on or off the rings. This recovery frequency is then used to place limits on the sizes and distance from Saturn of undiscovered satellites.

Dynamics of Clumps in Saturn’s F Ring

M. R. Showalter (Stanford)

An exhaustive analysis of the Voyager image data sets reveals the F Ring to be the most dynamic ring in the solar system. Principal properties are as follows.

  1. At any given time, the ring holds 2-3 extremely bright clumps (each several times brighter than the local average for the ring) and perhaps 20-40 identifiable smaller clumps. In practice, the closer one looks at the rings, the finer detail one finds.

  2. No major clumps persist for the nine months between the Voyager encounters, but most survive for the ~6 weeks they can be detected during a single encounter.

  3. A few major clumps are seen to appear or disappear very quickly, on time scales as brief as days. One clump appears to spread longitudinally after it first appears, but other clumps stay roughly fixed in longitudinal extend. The processes behind clump formation and destruction are unknown.

  4. Clumps propagate at different mean motions; there is no evidence for a discrete set of rates that might correspond to the individual “strands” described in some models. Motions range from 581.81° /day to 582.41° /day, implying that the F Ring’s clumps span a semimajor axis range of ~80 km.

  5. Some sections of the ring show a distinct 3.2° periodicity in clump spacing, as expected from the gravitational perturbations by Prometheus. Others do not.

  6. In the Voyager 2 images, a single prominent clump seems to eject smaller clumps behind it on time scales of ~2 weeks. However, nothing analogous is observed in Voyager 1 data.

A Search for Saturnian Moonlets Using the WIYN Telescope

M. R. Showalter (Stanford), L. Dones (San Jose State), R. H. Durisen, R. K. Honeycutt, J. S. Jurcevic, R. Tripoli-Strom, and C. Strom (Indiana)

During and just after the crossing of the Sun through Saturn’s ring plane in November 1995, we obtained 165 CCD images of Saturn’s satellite and ring system over a period of five nights at the 3.5-meter Wisconsin-Indiana-Yale- NOAO (WIYN) telescope at Kitt Peak. Our observations used a coronagraphic mask to reduce scattered light. Most images were taken in R-band or a narrowband 890-nm methane filter under sub-arcsecond seeing conditions.

The WIYN images are well-suited to detecting known small satellites of Saturn and for detecting possible new satellites, especially objects which might lie within the faint E Ring, which extends from about 3 to 8 Saturnian radii (RS). For example, images in R-band with exposure times of 1-5 minutes show Tethys’ tiny coorbital satellite Calypso (R magnitude ~18 and mean radius ~10 km) with signal-to-noise ranging from 30 to 80. Since we took several sequences of roughly ten images with identical viewing geometry, moving objects are easy to detect. Our orbital coverage is nearly complete in the range 2.5-5 RS, allowing us to detect moons with radii larger than 3 km with signal-to-noise of 10. The range 2.5-5 RS contains a number of satellite candidates seen in Voyager images [1,2]. These moons could not be confirmed because of the limited Voyager coverage at these distances from Saturn, so the WIYN data complement Voyager well. In the region of the main rings, the methane images provide our best data set because this passband minimizes scattered light from Saturn. These images show clear detections of Janus, Epimetheus, Prometheus, and Pandora. (Selected WIYN images and an MPEG movie showing the orbital motion of several ring moons are available at http://astrowww.astro.indiana.edu/personnel/strom/saturn/.)

We will present our analysis of several image sequences, primarily in R-band, from which we will be able to constrain the population of moonlets within the E Ring. We blink the images to provide a initial set of moving objects in the frames. In order to detect faint objects, we then subtract median frames for the sequence from each image, revealing moving objects in the difference frames [3, 4]. As a byproduct of this analysis, we may detect clumps or arcs in the F Ring, some of which have integrated brightnesses comparable to or greater than that of Atlas [5].

References:
[1] Synnott S. P. (1986). Icarus 67, 186-204.
[2] Gordon M. K. et al. (1996). Icarus 121, 114-125.
[3] Bosh A. S. and Rivkin A. S. (1996). Science 272, 518-521.
[4] Nicholson P. D. et al. (1996). Science 272, 509-515.
[5] Roddier C. et al. (1996). IAU Circular 6515.

G Ring Particle Sizes Derived From Ring Plane Crossing Observations

H. B. Throop, L. W. Esposito (LASP, U. Colorado)

The Saturn ring plane crossings in 1995-96 allowed observers using the Hubble Space Telescope and the W. M. Keck telescope to image the planet’s diffuse rings from 0.3 µm - 2.2 µm at a scattering angle of θ ≈ 175 °. We calculate the G ring reflectance for size distributions of dust to km-sized bodies derived from a physical, evolutionary model. The model tracks the evolution of the G ring from its initial formation following the disruption of a progenitor satellite (Canup & Esposito 1997), until a steady state distribution is reached. We calculate the total particle scattering from contributions due to Mie scattering, isotropic scattering, and Lambert scattering, and compare the spectra, phase curves, and RMS particle mass from our physical model to that observed by HST, Keck, and Voyager.

A range of particle size distributions from the models are consistent with the observations. These distributions have a dust component that can be described by the differential power law exponent qdust, in the range 1.5-3.5. A Gaussian size distribution centered at 15 µm also matches the observations, although is not predicted by the evolutionary model. Distributions with qdust > ~4, such as that proposed by Showalter & Cuzzi (1993) based on Voyager G ring photometry, are too blue to match the spectrum.

In order to fit the optical depth, many of the models require longer particle lifetimes against plasma drag than Voyager plasma measurements imply. This may suggest that plasma densities are overestimated, that the ring has unaccounted-for dust sources, or that the ring is not in steady-state and we are seeing it at a particularly bright moment.

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