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ABSTRACT: Saturn's C ring contains multiple spiral patterns that appear to be density
waves driven by periodic gravitational perturbations. In other parts of
Saturn's rings, such waves are generated by Lindblad resonances with Saturn's
various moons, but most of the wave-like C-ring features are not situated near
any strong resonance with any known moon. Using stellar occultation data
obtained by the Visual and Infrared Mapping Spectrometer (VIMS) onboard the
Cassini spacecraft, we investigate the origin of six unidentified C-ring waves
located between 80,900 and 87,200 km from Saturn's center. By measuring
differences in the waves' phases among the different occultations, we are able
to determine both the number of arms in each spiral pattern and the speeds at
which these patterns rotate around the planet. We find that all six of these
waves have between 2 and 4 arms and pattern speeds between 1660 degrees/day and
1861 degrees/day. These speeds are too large to be attributed to any satellite
resonance. Instead they are comparable to the predicted pattern speeds of waves
generated by low-order normal-mode oscillations within the planet [Marley &
Porco 1993, Icarus 106, 508]. The precise pattern speeds associated with these
waves should therefore provide strong constraints on Saturn's internal
structure. Furthermore, we identify multiple waves with the same number of arms
and very similar pattern speeds, indicating that multiple m=3 and m=2 sectoral
(l=m) modes may exist within the planet.
04/2013;
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G. Filacchione,
F. Capaccioni,
R. N. Clark,
P. D. Nicholson,
D. P. Cruikshank,
J. N. Cuzzi,
J. I. Lunine,
R. H. Brown,
P. Cerroni,
F. Tosi,
M. Ciarniello,
B. J. Buratti, M. M. Hedman,
E. Flamini
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ABSTRACT: Over the last eight years, the Visual and Infrared Mapping Spectrometer
(VIMS) aboard the Cassini orbiter has returned hyperspectral images in the
0.35-5.1 micron range of the icy satellites and rings of Saturn. These very
different objects show significant variations in surface composition, roughness
and regolith grain size as a result of their evolutionary histories, endogenic
processes and interactions with exogenic particles. The distributions of
surface water ice and chromophores, i.e. organic and non-icy materials, across
the saturnian system, are traced using specific spectral indicators (spectral
slopes and absorption band depths) obtained from rings mosaics and
disk-integrated satellites observations by VIMS.
01/2013;
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ABSTRACT: The Encke Gap is a 320-km-wide opening in Saturn's outer A ring that contains
the orbit of the small moon Pan and an array of dusty features composed of
particles less than 100 microns across. In particular, there are three narrow
ringlets in this region that are not longitudinally homogeneous, but instead
contain series of bright clumps. Using images obtained by the Cassini
spacecraft, we track the motions of these clumps and demonstrate that they do
not follow the predicted trajectories of isolated ring particles moving under
the influence of Saturn's and Pan's gravitational fields. We also examine the
orbital properties of these ringlets by comparing images taken at different
longitudes and times. We find evidence that the orbits of these particles have
forced eccentricities induced by solar radiation pressure. In addition, the
mean radial positions of the particles in these ringlets appear to vary with
local co-rotating longitude, perhaps due to the combined action of drag forces,
gravitational perturbations from Pan, and collisions among the ring particles.
The dynamics of the dust within this gap therefore appears to be much more
complex than previously appreciated.
11/2012;
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ABSTRACT: Saturn's main rings exhibit variations in both their opacity and spectral
properties on a broad range of spatial scales, and the correlations between
these parameters can provide insights into the processes that shape the
composition and dynamics of the rings. The Visual and Infrared Mapping
Spectrometer (VIMS) instrument onboard the Cassini Spacecraft has obtained
spectra of the rings between 0.35 and 5.2 microns with sufficient spatial
resolution to discern variations on scales below 200 km. These relatively
high-resolution spectral data reveal that both the depths of the near-infrared
water-ice absorption bands and the visible spectral slopes are often correlated
with structural parameters such as the rings' optical depth. Using a simplified
model for the ring-particles' regolith properties, we have begun to disentangle
the trends due to changes in the gross composition of the ring particles from
those that may be due to shifts in the texture of the ring particles' regolith.
Consistent with previous studies, this analysis finds that the C ring and the
Cassini Division possess enhanced concentrations of a contaminant that absorbs
light over a broad range of wavelengths. On the other hand, a second
contaminant that preferentially absorbs at short visible and near-ultraviolet
wavelengths is found to be more evenly distributed throughout the rings. The
optical activity of this short-wavelength absorber increases in the inner B
ring inwards of 100,000 km from Saturn center, which may provide clues to the
origin of this contaminant. The spectral variations identified as shifts in the
regolith texture are in some places clearly correlated with the ring's optical
depth, and in other locations they appear to be associated with the
disturbances generated by strong mean-motion resonances with Saturn's various
moons.
10/2012;
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G Filacchione,
F Capaccioni,
M Ciarniello,
P ~D Nicholson,
R ~N Clark,
D ~P Cruikshank,
J ~N Cuzzi, M ~M Hedman,
B ~J Buratti,
P Cerroni,
F Tosi,
R ~H Brown
AAS/Division for Planetary Sciences Meeting Abstracts; 10/2012
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G Filacchione,
F Capaccioni,
M Ciarniello,
R ~N Clark,
P ~D Nicholson,
D ~P Cruikshank,
J ~N Cuzzi, M ~M Hedman,
F Tosi,
B ~J Buratti,
P Cerroni,
R ~H Brown
EGU General Assembly Conference Abstracts; 04/2012
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G. Filacchione,
F. Capaccioni,
M. Ciarniello,
R. N. Clark,
J. N. Cuzzi,
P. D. Nicholson,
D. P. Cruikshank, M. M. Hedman,
B. J. Buratti,
J. I. Lunine,
L. A. Soderblom,
F. Tosi,
P. Cerroni,
R. H. Brown,
T. B. McCord,
R. Jaumann,
K. Stephan,
K. H. Baines,
E. Flamini
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ABSTRACT: In the last few years Cassini-VIMS, the Visible and Infared Mapping
Spectrometer, returned to us a comprehensive view of the Saturn's icy
satellites and rings. After having analyzed the satellites' spectral properties
(Filacchione et al. (2007a)) and their distribution across the satellites'
hemispheres (Filacchione et al. (2010)), we proceed in this paper to
investigate the radial variability of icy satellites (principal and minor) and
main rings average spectral properties. This analysis is done by using 2,264
disk-integrated observations of the satellites and a 12x700 pixels-wide rings
radial mosaic acquired with a spatial resolution of about 125 km/pixel. The
comparative analysis of these data allows us to retrieve the amount of both
water ice and red contaminant materials distributed across Saturn's system and
the typical surface regolith grain sizes. These measurements highlight very
striking differences in the population here analyzed, which vary from the
almost uncontaminated and water ice-rich surfaces of Enceladus and Calypso to
the metal/organic-rich and red surfaces of Iapetus' leading hemisphere and
Phoebe. Rings spectra appear more red than the icy satellites in the visible
range but show more intense 1.5-2.0 micron band depths. The correlations among
spectral slopes, band depths, visual albedo and phase permit us to cluster the
saturnian population in different spectral classes which are detected not only
among the principal satellites and rings but among co-orbital minor moons as
well. Finally, we have applied Hapke's theory to retrieve the best spectral
fits to Saturn's inner regular satellites using the same methodology applied
previously for Rhea data discussed in Ciarniello et al. (2011).
03/2012;
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American Geophysical Union, Fall Meeting; 12/2011
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ABSTRACT: Saturn's diffuse E ring consists of many tiny (micron and sub-micron) grains
of water ice distributed between the orbits of Mimas and Titan. Various
gravitational and non-gravitational forces perturb these particles' orbits,
causing the ring's local particle density to vary noticeably with distance from
the planet, height above the ring-plane, hour angle and time. Using
remote-sensing data obtained by the Cassini spacecraft in 2005 and 2006, we
investigate the E-ring's three-dimensional structure during a time when the Sun
illuminated the rings from the south at high elevation angles (> 15 degrees).
These observations show that the ring's vertical thickness grows with distance
from Enceladus' orbit and its peak brightness density shifts from south to
north of Saturn's equator plane with increasing distance from the planet. These
data also reveal a localized depletion in particle density near Saturn's
equatorial plane around Enceladus' semi-major axis. Finally, variations are
detected in the radial brightness profile and the vertical thickness of the
ring as a function of longitude relative to the Sun. Possible physical
mechanisms and processes that may be responsible for some of these structures
include solar radiation pressure, variations in the ambient plasma, and
electromagnetic perturbations associated with Saturn's shadow.
11/2011;
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ABSTRACT: In August 2009 the Sun illuminated Saturn’s rings from almost exactly edge-on, revealing a subtle corrugation that extends
across the entire C ring. This corrugation’s amplitude is 2 to 20 meters and its wavelength is 30 to 80 kilometers. Radial
trends in the corrugation’s wavelength indicate that this structure—like a similar corrugation previously identified in the
D ring—results from differential nodal regression within a ring that became tilted relative to Saturn’s equator plane in 1983.
We suggest that this initial tilt arose because interplanetary debris struck the rings. The corrugation’s radial extent implies
that the impacting material was a dispersed cloud of debris instead of a single object, and the corrugation’s amplitude indicates
that the debris’ total mass was ~1011 to 1013 kilograms.
Science 05/2011; 332(6030):708-711. · 31.20 Impact Factor
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[show abstract]
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ABSTRACT: In August 2009 the Sun illuminated Saturn's rings from almost exactly edge-on, revealing a subtle corrugation that extends across the entire C ring. This corrugation's amplitude is 2 to 20 meters and its wavelength is 30 to 80 kilometers. Radial trends in the corrugation's wavelength indicate that this structure--like a similar corrugation previously identified in the D ring--results from differential nodal regression within a ring that became tilted relative to Saturn's equator plane in 1983. We suggest that this initial tilt arose because interplanetary debris struck the rings. The corrugation's radial extent implies that the impacting material was a dispersed cloud of debris instead of a single object, and the corrugation's amplitude indicates that the debris' total mass was ~10(11) to 10(13) kilograms.
Science 03/2011; 332(6030):708-11. · 31.20 Impact Factor
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ABSTRACT: Stellar occultations by Saturn's rings observed with the Visual and Infrared
Mapping Spectrometer (VIMS) onboard the Cassini spacecraft reveal that dusty
features such as the F ring and the ringlets in the Encke and the Laplace Gaps
have distinctive infrared transmission spectra. These spectra show a narrow
optical depth minimum at wavelengths around 2.87 microns. This minimum is
likely due to the Christiansen Effect, a reduction in the extinction of small
particles when their (complex) refractive index is close to that of the
surrounding medium. Simple Mie-scattering models demonstrate that the strength
of this opacity dip is sensitive to the size distribution of particles between
1 and 100 microns across. Furthermore, the spatial resolution of the
occultation data is sufficient to reveal variations in the transmission spectra
within and among these rings. For example, in both the Encke Gap ringlets and F
ring, the opacity dip weakens with increasing local optical depth, which is
consistent with the larger particles being concentrated near the cores of these
rings. The strength of the opacity dip varies most dramatically within the F
ring; certain compact regions of enhanced optical depth lack an opacity dip and
therefore appear to have a greatly reduced fraction of grains in the few-micron
size range.Such spectrally-identifiable structures probably represent a subset
of the compact optically-thick clumps observed by other Cassini instruments.
These variations in the ring's particle size distribution can provide new
insights into the processes of grain aggregation, disruption and transport
within dusty rings. For example, the unusual spectral properties of the F-ring
clumps could perhaps be ascribed to small grains adhering onto the surface of
larger particles in regions of anomalously low velocity dispersion.
02/2011;
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EPSC-DPS Joint Meeting abstracts; 01/2011
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EGU General Assembly (abstract); 01/2011
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ABSTRACT: The so-called "Charming Ringlet" (R/2006 S3) is a low-optical-depth, dusty ringlet located in the Laplace gap in the Cassini Division. This ringlet is particularly interesting because its radial position varies systematically with longitude relative to the Sun in such a way that the ringlet's geometric center appears to be displaced away from Saturn's center in a direction roughly toward the Sun. In other words, the ringlet is always found at greater distances from the planet's center at longitudes near the sub-solar longitude than it is at longitudes near Saturn's shadow. This "heliotropic" behavior indicates that the dynamics of the particles in this ring are being influenced by solar radiation pressure. In order to investigate this phenomenon, which has been predicted theoretically but has never been observed this clearly, we analyze multiple image sequences of this ringlet obtained by Cassini in order to constrain its shape and orientation. These data can be fit reasonably well with a model in which both the eccentricity and the inclination of the ringlet have "forced" components (that maintain a fixed orientation relative to the Sun) as well as "free" components (that drift around the planet at steady rates determined by Saturn's oblateness). While the magnitude of the forced eccentricity is roughly consistent with theoretical expectations for radiation pressure acting on 10-to-100-micron-wide icy grains, the existence of significant free eccentricities and inclinations poses a significant challenge for models of low-optical-depth dusty rings. Comment: 31 pages, 6 figures, accepted for publication in Icarus. Slight edits made to match various proof corrections
06/2010;
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J. N. Cuzzi,
J. A. Burns,
S. Charnoz,
R. N. Clark,
J. E. Colwell,
L. Dones,
L. W. Esposito,
G. Filacchione,
R. G. French, M. M. Hedman, [......],
P. D. Nicholson,
C. C. Porco,
J. Schmidt,
M. R. Showalter,
L. J. Spilker,
J. N. Spitale,
R. Srama,
M. Sremčević,
M. S. Tiscareno,
J. Weiss
[show abstract]
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ABSTRACT: 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.
Science 03/2010; 327(5972):1470-1475. · 31.20 Impact Factor
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J N Cuzzi,
J A Burns,
S Charnoz,
R N Clark,
J E Colwell,
L Dones,
L W Esposito,
G Filacchione,
R G French, M M Hedman, [......],
P D Nicholson,
C C Porco,
J Schmidt,
M R Showalter,
L J Spilker,
J N Spitale,
R Srama,
M Sremcević,
M S Tiscareno,
J Weiss
[show abstract]
[hide abstract]
ABSTRACT: 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.
Science 03/2010; 327(5972):1470-5. · 31.20 Impact Factor
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Matthew S. Tiscareno,
N. Albers,
A. Brahic,
S. M. Brooks,
J. A. Burns,
C. Chavez,
J. E. Colwell,
J. N. Cuzzi,
I. de Pater,
L. Dones, [......],
R. T. Pappalardo,
H. Salo,
J. Schmidt,
M. R. Showalter,
F. Spahn,
L. J. Spilker,
R. Srama,
M. Sremcevic,
G. R. Stewart,
P. Yanamandra-Fisher
[show abstract]
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ABSTRACT: The study of planetary ring systems is a key component of planetary
science for several reasons: 1) The evolution and current states of
planets and their satellites are affected in many ways by rings, while
2) conversely, properties of planets and moons and other solar system
populations are revealed by their effects on rings; 3) highly structured
and apparently delicate ring systems may be bellwethers, constraining
various theories of the origin and evolution of their entire planetary
system; and finally, 4) planetary rings provide an easily observable
analogue to other astrophysical disk systems, enabling real "ground
truth” results applicable to disks much more remote in space
and/or time, including proto-planetary disks, circum-stellar disks, and
even galaxies.
Significant advances have been made in rings science in the past decade.
The highest-priority rings research recommendations of the last
Planetary Science Decadal Survey were to operate and extend the Cassini
orbiter mission at Saturn; this has been done with tremendous success,
accounting for much of the progress made on key science questions, as we
will describe. Important progress in understanding the rings of Saturn
and other planets has also come from Earth-based observational and
theoretical work, again as prioritized by the last Decadal Survey.
However, much important work remains to be done. At Saturn, the Cassini
Solstice Mission must be brought to a successful completion. Priority
should also be placed on sending spacecraft to Neptune and/or Uranus,
now unvisited for more than 20 years. At Jupiter and Pluto,
opportunities afforded by visiting spacecraft capable of studying rings
should be exploited. On Earth, the need for continued research and
analysis remains strong, including in-depth analysis of rings data
already obtained, numerical and theoretical modeling work, laboratory
analysis of materials and processes analogous to those found in the
outer solar system, and continued Earth-based observations.
11/2009; 41.
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ABSTRACT: The Cassini Division in Saturn's rings contains a series of eight named gaps, three of which contain dense ringlets. Observations of stellar occultations by the Visual and Infrared Mapping Spectrometer onboard the Cassini spacecraft have yielded ~40 accurate and precise measurements of the radial position of the edges of all of these gaps and ringlets. These data reveal suggestive patterns in the shapes of many of the gap edges: the outer edges of the 5 gaps without ringlets are circular to within 1 km, while the inner edges of 6 of the gaps are eccentric, with apsidal precession rates consistent with those expected for eccentric orbits near each edge. Intriguingly, the pattern speeds of these eccentric inner gap edges, together with that of the eccentric Huygens ringlet,form a series with a characteristic spacing of 0.06 degrees/day. The two gaps with non-eccentric inner edges lie near first-order Inner Lindblad Resonances (ILRs) with moons. One such edge is close to the 5:4 ILR with Prometheus. The other resonantly confined edge is the outer edge of the B ring, which lies near the 2:1 Mimas ILR. Detailed investigation of the B-ring-edge data confirm the presence of an m=2 perturbation on the B-ring edge, but also suggest that this pattern moves or librates relative to Mimas. The B-ring edge also has an m=1 component that rotates around the planet at a rate close to the expected apsidal precession rate. The pattern speeds of the eccentric edges in the Cassini Division can potentially be generated from various combinations of the pattern speeds of structures observed on the edge of the B ring. We therefore suggest that the locations of most of the gaps in the Cassini Division may be determined by resonances involving a combination of perturbations from Mimas and the massive edge of the B ring. Comment: 47 pages, 12 figures, accepted for publication in the Astronomical Journal, Some small typographical and wording errors fixed
11/2009;
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[show abstract]
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ABSTRACT: Aegaeon (Saturn LIII, S/2008 S1) is a small satellite of Saturn that orbits
within a bright arc of material near the inner edge of Saturn's G ring. This
object was observed in 21 images with Cassini's Narrow-Angle Camera between
June 15 (DOY 166), 2007 and February 20 (DOY 51), 2009. If Aegaeon has similar
surface scattering properties as other nearby small Saturnian satellites
(Pallene, Methone and Anthe), then its diameter is approximately 500 m. Orbit
models based on numerical integrations of the full equations of motion show
that Aegaeon's orbital motion is strongly influenced by multiple resonances
with Mimas. In particular, like the G-ring arc it inhabits, Aegaeon is trapped
in the 7:6 corotation eccentricity resonance with Mimas. Aegaeon, Anthe and
Methone therefore form a distinctive class of objects in the Saturn system:
small moons in co-rotation eccentricity resonances with Mimas associated with
arcs of debris. Comparisons among these different ring-arc systems reveal that
Aegaeon's orbit is closer to the exact resonance than Anthe's and Methone's
orbits are. This could indicate that Aegaeon has undergone significant orbital
evolution via its interactions with the other objects in its arc, which would
be consistent with the evidence that Aegaeon's mass is much smaller relative to
the total mass in its arc than Anthe's and Methone's masses are.
11/2009;
