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TOPIC: Saturn's rings


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Encke Gap
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Title: Of Horseshoes and Heliotropes: Dynamics of Dust in the Encke Gap
Authors: M.M. Hedman, J.A. Burns, D.P. Hamilton, M.R. Showalter

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.

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RE: Saturn's rings
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Title: A multilayer model for thermal infrared emission of Saturn's rings. III: Thermal inertia inferred from Cassini CIRS
Authors: Ryuji Morishima, Linda Spilker, Keiji Ohtsuki

The thermal inertia values of Saturn's main rings (the A, B, and C rings and the Cassini division) are derived by applying our thermal model to azimuthally scanned spectra taken by the Cassini Composite Infrared Spectrometer (CIRS). Model fits show the thermal inertia of ring particles to be 16, 13, 20, and 11 Jm^{-2}K^{-1}s^{-} for the A, B, and C rings, and the Cassini division, respectively. However, there are systematic deviations between modelled and observed temperatures in Saturn's shadow depending on solar phase angle, and these deviations indicate that the apparent thermal inertia increases with solar phase angle. This dependence is likely to be explained if large slowly spinning particles have lower thermal inertia values than those for small fast spinning particles because the thermal emission of slow rotators is relatively stronger than that of fast rotators at low phase and vise versa. Additional parameter fits, which assume that slow and fast rotators have different thermal inertia values, show the derived thermal inertia values of slow (fast) rotators to be 8 (77), 8 (27), 9 (34), 5 (55) Jm^{-2}K^{-1}s^{-} for the A, B, and C rings, and the Cassini division, respectively. The values for fast rotators are still much smaller than those for solid ice with no porosity. Thus, fast rotators are likely to have surface regolith layers, but these may not be as fluffy as those for slow rotators, probably because the capability of holding regolith particles is limited for fast rotators due to the strong centrifugal force on surfaces of fast rotators. Other additional parameter fits, in which radii of fast rotators are varied, indicate that particles less than \sim 1 cm should not occupy more than a half of the cross section for the A, B, and C rings.

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Title: Vertical structures induced by embedded moonlets in Saturn's rings: the gap region
Authors: Holger Hoffmann, Frank Spahn, Martin Seis

We study the vertical extent of propeller structures in Saturn's rings. Our focus lies on the gap region of the propeller and on non-inclined propeller moonlets. In order to describe the vertical structure of propellers we extend the model of Spahn and Sremcevic (2000) to include the vertical direction. We find that the gravitational interaction of ring particles with the non-inclined moonlet does not induce considerable vertical excursions of ring particles, but causes a considerable thermal motion in the ring plane. We expect ring particle collisions to partly convert the lateral induced thermal motion into vertical excursions of ring particles. For the gap region of the propeller, we calculate gap averaged propeller heights on the order of 0.7 Hill radii, which is of the order of the moonlet radius. In our model the propeller height decreases exponentially until viscous heating and collisional cooling balance. We estimate Hill radii of 370m and 615m for the propellers Earhart and Bleriot. Our model predicts about 120km for the azimuthal extent of the Earhart propeller at Saturn's 2009 equinox, being consistent with values determined from Cassini images.

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Saturn's Rings are Back

pia14918-640.jpg

Cassini's recent return of ring images has started to pay off. A group of scientists has restarted the team's studies of propeller-shaped gaps. These gaps are cleared out by objects that are smaller than known moons but larger than typical ring particles. Cassini scientists haven't seen propellers in two years. Matt Tiscareno, a Cassini imaging team associate at Cornell University, Ithaca, N.Y., and colleagues have been following these objects for several years. Because some of the propellers are exactly where models predicted they would be, scientists believe they are seeing some old friends again.
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Title: Saturn's icy satellites and rings investigated by Cassini - VIMS. III. Radial compositional variability
Authors: 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

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 analysed 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 analysed, 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).

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Forensic Sleuthing Ties Ring Ripples to Impacts

Like forensic scientists examining fingerprints at a cosmic crime scene, scientists working with data from NASA's Cassini, Galileo and New Horizons missions have traced telltale ripples in the rings of Saturn and Jupiter back to collisions with cometary fragments dating back more than 10 years ago.
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Ring 'ripples' in Saturn and Jupiter linked to comets

Scientists say that strange ripples observed in the ring systems of Saturn and Jupiter were caused by comets.
The ripples, which the researchers say resemble the undulations of corrugated metal, were detected in both Saturn's rings and in Jupiter's lesser-known rings.

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Wobbles in the rings of Saturn and Jupiter preserve a record of past impacts.

The rings of Saturn and Jupiter contain ripples caused by comets that hit them decades ago. Monitoring how the rings wobble could reveal how common comet impacts are - and may also help astronomers map the planets' cores.
Matthew Hedman, an astronomer at Cornell University in Ithaca, New York, and his colleagues spotted the ripple in one of Saturn's rings in images taken by the Cassini spacecraft in 2009. Sunlight striking the rings edge-on revealed previously unseen bright and dark bands in the planet's C ring, which lies between about 74,600 and 92,000 kilometres from the planet's centre.

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Saturn's rings formed by destruction of giant moon
5 October 2010

Saturn's rings may have been formed from the death of an early Titan-sized moon whose upper layers were ripped off as it spiralled into the infant Saturn.
One of the problems in working out where Saturn's rings came from is their composition, says planetary scientist Robin Canup of the Southwest Research Institute in Boulder, Colorado. The rings are 90%-95% water ice - odd because the primordial solar system would have been comprised of about equal parts ice and rock.

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One of the solar systems most evocative mysteries - the origin of Saturns rings - may be a case of cosmic murder, new research suggests.
The victim: an unnamed moon of Saturn that disappeared about 4.5 billion years ago.
The suspect: a disk of hydrogen gas that once surrounded Saturn when its dozens of moons were forming, but has now fled the crime scene.
The cause of death: A forced plunge into Saturn.

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