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Saturnian Satellites
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Title: Cassini ISS Mutual Event Astrometry of the Mid-sized Saturnian Satellites 2005-2012
Author: N.J. Cooper, C.D. Murray, V. Lainey, R. Tajeddine, M.W. Evans, G.A. Williams

We present astrometric observations of the Saturnian satellites Mimas, Enceladus, Tethys, Dione and Rhea from Cassini Imaging Science Subsystem (ISS) narrow-angle camera (NAC) images. Image sequences were designed to observe mutual occultations between these satellites.
The positions of satellite centres were estimated by fitting ellipsoidal shape models to the measured limbs of the imaged satellites. Spacecraft pointing corrections were computed using the UCAC2 star catalogue.
We provide a total of 2303 astrometric observations, resulting in 976 pairs, the remainder consisting of observations of a single satellite. Mean residuals for the individual satellite positions relative to the SAT360 ephemeris were 4.3 km in the line direction and -2.4 km in the sample direction, with standard deviations of 5.6 and 7.0 km respectively, an order of magnitude improvement in precision compared to published HST observations. By considering inter-satellite separations, uncertainties in camera pointing and spacecraft positioning along with possible biases in the individual positions of the satellites can be largely eliminated, resulting in an order-of-magnitude increase in accuracy compared to that achievable using the individual satellite positions. We show how factors relating to the viewing geometry cause small biases in the individual positions of order 0.28 pixel to become systematic across the dataset as a whole and discuss options for reducing their effects . The reduced astrometric data are provided in the form of individual positions for each satellite, together with the measured positions of reference stars, in order to allow more flexibility in the processing of the observations, taking into account possible future advances in limb-fitting techniques as well as the future availability of more accurate star catalogues, such as those from the GAIA mission.

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Saturn moons
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Chips off the Mantle: Saturn's moons explained

Many years ago, astronomy students were taught that Saturn was, in essence, a mini-Jupiter. The truth, it seems is far more puzzling.
When you look closer, you find that Jupiter has four large moons (Ganymede, Callisto, Io, and Europa), while Saturn has only one, Titan.
Titan, however, is truly enormous.
In a presentation this week at a meeting of the American Astronomical Society's Division for Planetary Sciences, Erik Asphaug, a planetary scientist from the University of California, proposed a solution for both processes. Titan, he suggested, initially had several large moons, just like Jupiter, each with a rocky core and an icy mantle - counterparts to Earth's iron core and rocky mantle.

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RE: 60 Saturn moons
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Saturn's middle-sized moons spawned during formation of Titan

The middle-sized moons of Saturn, a half-dozen icy bodies dwarfed by Saturn's massive moon Titan are among the mysteries of the outer solar system. Now, a new model for the origin of the Saturn system has indicated that these middle-sized moons were spawned during giant impacts in which several major satellites merged to form Titan. Erik Asphaug, professor of Earth and planetary sciences at the University of California, Santa Cruz, and his colleague Andreas Reufer of the University of Bern, Switzerland, propose that the Saturn system started with a family of major satellites comparable to the four large moons of Jupiter (known as the Galilean moons, discovered by Galileo in 1610).
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Icy Moons through Cassini's Eyes

Enceladus Terrain Portrait of Janus Enceladus Crescent Enceladus Plume Icy Dione Dione Close-Up

These raw, unprocessed images of Saturn's moons Enceladus, Janus and Dione were taken on March 27 and 28, 2012, by NASA's Cassini spacecraft.
Cassini passed Enceladus first on March 27, coming within about 46 miles (74 kilometers) of the moon's surface. The encounter was primarily designed for Cassini's ion and neutral mass spectrometer, which "tasted" the composition of Enceladus' south polar plume. Other instruments, including the Cassini plasma spectrometer and composite infrared spectrometer, also took measurements.

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Saturn Trojan satellites
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Title: Influence of the coorbital resonance on the rotation of the Trojan satellites of Saturn
Authors: Philippe Robutel (IMCCE), Nicolas Rambaux (IMCCE), Maryame El Moutamid (IMCCE, LESIA)

The Cassini spacecraft collects high resolution images of the saturnian satellites and reveals the surface of these new worlds. The shape and rotation of the satellites can be determined from the Cassini Imaging Science Subsystem data, employing limb coordinates and stereogrammetric control points. This is the case for Epimetheus (Tiscareno et al. 2009) that opens elaboration of new rotational models (Tiscareno et al. 2009; Noyelles 2010; Robutel et al. 2011). Especially, Epimetheus is characterised by its horseshoe shape orbit and the presence of the swap is essential to introduce explicitly into rotational models. During its journey in the saturnian system, Cassini spacecraft accumulates the observational data of the other satellites and it will be possible to determine the rotational parameters of several of them. To prepare these future observations, we built rotational models of the coorbital (also called Trojan) satellites Telesto, Calypso, Helene, and Polydeuces, in addition to Janus and Epimetheus. Indeed, Telesto and Calypso orbit around the L_4 and L_5 Lagrange points of Saturn-Tethys while Helene and Polydeuces are coorbital of Dione. The goal of this study is to understand how the departure from the Keplerian motion induced by the perturbations of the coorbital body, influences the rotation of these satellites. To this aim, we introduce explicitly the perturbation in the rotational equations by using the formalism developed by Erdi (1977) to represent the coorbital motions, and so we describe the rotational motion of the coorbitals, Janus and Epimetheus included, in compact form.

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60 Saturn moons
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Cassini Presents Saturn Moon Quintet

pia14573-640.jpg

With the artistry of a magazine cover shoot, NASA's Cassini spacecraft captured this portrait of five of Saturn's moons poised along the planet's rings.
From left to right are Janus, Pandora, Enceladus, Mimas and finally Rhea, bisected by the right side of the frame. The view was acquired at a distance of approximately 1.1 million kilometres from Rhea and 1.8 million kilometres from Enceladus.
The image was taken in visible green light with the Cassini spacecraft narrow-angle camera on July 29, 2011. Image scale is about 7 kilometres per pixel on Rhea and 11 kilometres per pixel on Enceladus.

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Saturn's Irregular Satellites
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Title: Searching for Saturn's Dust Swarm: Limits on the size distribution of Irregular Satellites from km to micron sizes
Authors: Grant M. Kennedy, Mark C. Wyatt, Kate Y. L. Su, John A. Stansberry

We describe a search for dust created in collisions between the Saturnian irregular satellites using archival Spitzer MIPS observations. Although we detected a degree scale Saturn-centric excess that might be attributed to an irregular satellite dust cloud, we attribute it to the far-field wings of the PSF due to nearby Saturn. The Spitzer PSF is poorly characterised at such radial distances, and we expect PSF characterisation to be the main issue for future observations that aim to detect such dust. The observations place an upper limit on the level of dust in the outer reaches of the Saturnian system, and constrain how the size distribution extrapolates from the smallest known (few km) size irregulars down to micron-size dust. Because the size distribution is indicative of the strength properties of irregulars, we show how our derived upper limit implies irregular satellite strengths more akin to comets than asteroids. This conclusion is consistent with their presumed capture from the outer regions of the Solar System.

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Saturnian System
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Title: Giant impacts in the Saturnian System: a possible origin of diversity in the inner mid-sized satellites
Authors: Yasuhito Sekine, Hidenori Genda

It is widely accepted that Titan and the mid-sized regular satellites around Saturn were formed in the circum-Saturn disk. Thus, if these mid-sized satellites were simply accreted by collisions of similar ice-rock satellitesimals in the disk, the observed wide diversity in density (i.e., the rock fraction) of the Saturnian mid-sized satellites is enigmatic. A recent circumplanetary disk model suggests satellite growth in an actively supplied circumplanetary disk, in which Titan-sized satellites migrate inward by interaction with the gas and are eventually lost to the gas planet. Here we report numerical simulations of giant impacts between Titan-sized migrating satellites and smaller satellites in the inner region of the Saturnian disk. Our results suggest that in a giant impact with impact velocity > 1.4 times the escape velocity and impact angle of ~45 degree, a smaller satellite is destroyed, forming multiple mid-sized satellites with a very wide diversity in satellite density (the rock fraction = 0-92 wt%). Our results of the relationship between the mass and rock fraction of the satellites resulting from giant impacts reproduce the observations of the Saturnian mid-sized satellites. Giant impacts also lead to internal melting of the formed mid-sized satellites, which would initiate strong tidal dissipation and geological activity, such as those observed on Enceladus today and Tethys in the past. Our findings also imply that giant impacts might have affected the fundamental physical property of the Saturnian mid-sized satellites as well as those of the terrestrial planets in the solar system and beyond.

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RE: 60 Saturn moons
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Saturn's Mysterious Moons



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Saturn moons
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Scientists using data from NASA's Cassini spacecraft have learned that distinctive, colourful bands and splotches embellish the surfaces of Saturn's inner, mid-size moons. The reddish and bluish hues on the icy surfaces of Mimas, Enceladus, Tethys, Dione and Rhea appear to be the aftermath of bombardments large and small.
A paper based on the findings was recently published online in the journal Icarus. In it, scientists describe prominent global patterns that trace the trade routes for material exchange between the moons themselves, an outer ring of Saturn known as the E ring and the planet's magnetic environment. The finding may explain the mysterious Pac-Man thermal pattern on Mimas, found earlier this year by Cassini scientists, said lead author Paul Schenk, who was funded by a Cassini data analysis program grant and is based at the Lunar and Planetary Institute in Houston.

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