P12B-01 

Properties of Enceladus' gas plumes from simulations of the their plasma interaction with Saturn's magnetosphere

* Saur, J (saur@geo.uni-koeln.de), Institute of Geophysics and Meteorology, University of Cologne, Cologne, 50923, Germany Schilling, N (schilling@geo.uni-koeln.de), Institute of Geophysics and Meteorology, University of Cologne, Cologne, 50923, Germany Neubauer, F M (neubauer@geo.uni-koeln.de), Institute of Geophysics and Meteorology, University of Cologne, Cologne, 50923, Germany Strobel, D F (strobel@jhu.edu), Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, United States Dougherty, M K), Imperial College, Blackett Lab, London, SW72BZ, United Kingdom Russell, C T (ctrussell@igpp.ucla.edu), Institute of Geophysics, UCLA, LA, CA 90095, United States

We present three-dimensional plasma simulations of the interaction of Saturn's magnetosphere with the gas plumes originating near Enceladus' south pole. The pronounced north-south asymmetry of Enceladus' gas environment drives an electric current system which couples both the northern and southern hemispheres of Enceladus. We updated our model to numerically include this feature. In our presentation we will compare our model results with the magnetic field and neutral gas observations of Cassini's flybys at Enceladus to further constrain properties of Enceladus' gas plumes.

P12B-02 

Coorbital Collision as the Source of Enceladus Heat

* Peale, S J (peale@physics.ucsb.edu), UCSB, Dept. of Physics, Santa Barbara, CA 93106, United States Greenberg, R (greenberg@lpl.arizona.edu), U. of Arizona, Lunar and Planetary Lab, Tucson, AZ 85721, United States

The source of energy for the observed plumes on Enceladus is a mystery. An explanation using tidal heating has difficulty in explaining the extreme localization of the plumes. That issue aside, the tides cannot produce enough dissipation to either account for the current energy loss or to store sufficient energy for later release, even given the current or likely past orbital resonances (Meyer and Wisdom, 2007). Here we consider the possibility that Enceladus has absorbed its coorbital in a collision gentle enough to preserve ancient cratering records, but with sufficient energy deposition to power the plumes for at least a short time. Numerical integration of Enceladus, its coorbital, Dione, and Titan, with Dione and Enceladus on converging orbits due to differential tidal expansion, leads to capture of Enceladus into the 2:1 orbital resonance, while destabilizing the coorbital into a collision with Enceladus. The relative velocity of the collision is only slightly above the escape velocity from Enceladus of 0.24 km/sec. One calculated collision had a relative velocity at impact of 0.26 km/sec with relative kinetic energy of 3.8× 10²⁹ ergs for m=0.01m_E. If most of this is deposited into Enceladus, it would power the energy lost in the plumes of 6 GW (Spencer et al. 2006) for about 200,000 years. The collision has the virtue of depositing the energy rather locally, and an underdense warm region after the satellite has equilibrated would naturally migrate to the pole to restore rotation about the axis of maximum moment of inertia.

P12B-03 INVITED 

Hyperion and its Cousins: Sponges, Landslides, Layers, and Ridges

* Thomas, P C (pct2@cornell.edu), Cornell University, 422 Space Sciences Cornell University, Ithaca, NY 14853,

Cassini's exploration of the Saturn system has given detailed views of small, irregularly-shaped satellites from ~10 to 135 km-mean radius that orbit within the rings to Phoebe nearly 100 times further from Saturn. The spongy appearance of Hyperion, reflecting a high density of well-preserved 2-10 km craters, may derive from a combination of effects of low porosity and Hyperion's size on the generation of impact crater ejecta. Smaller, porous moons in the Saturn system do not show similar effects with available data. Low gravity is not a deterrent to downlsope processes: these dominate the surface of Telesto which is essentially self-buried in debris. Ring- related satellite Atlas and (probably Pan) shows a distinct two-component surface that may represent different stages of accretion. These satellites, and other ring-related ones are shaped such that parts of their surfaces have almost zero escape velocity and thus may retard further accretion, leaving them in balance with hyper velocity impact effects. Phoebe is distinct from the more icy inner satellites, with higher density, probably much lower porosity, and some layering of icy and rocky components. Comparison to recent data on comets and small asteroids suggests much remains to be learned regarding crater formation and removal on low-gravity objects.

P12B-04 INVITED 

Fields and plasma in the wakes of the Saturnian inert moons: global hybrid simulation results

* Roussos, E (roussos@mps.mpg.de), Max Planck Institut fuer Sonnensystemforschung, Max Planck Str. 2, 37191, Germany, Katlenburg-Lindau, 37191, Germany Mueller, J (joa.mueller@tu-bs.de), Institut fuer Theoretische Physik, TU Braunschweig, Hans-Sommer-Strasse 66, Braunschweig, 38106, Germany Simon, S (sven.simon@tu-bs.de), Institut fuer Theoretische Physik, TU Braunschweig, Hans-Sommer-Strasse 66, Braunschweig, 38106, Germany Boesswetter, A (a.boesswetter@tu-bs.de), Institut fuer Theoretische Physik, TU Braunschweig, Hans-Sommer-Strasse 66, Braunschweig, 38106, Germany Motschmann, U (u.motschmann@tu-bs.de), Institut fuer Theoretische Physik, TU Braunschweig, Hans-Sommer-Strasse 66, Braunschweig, 38106, Germany Krupp, N (krupp@mps.mpg.de), Max Planck Institut fuer Sonnensystemforschung, Max Planck Str. 2, 37191, Germany, Katlenburg-Lindau, 37191, Germany Fraenz, M (fraenz@mps.mpg.de), Max Planck Institut fuer Sonnensystemforschung, Max Planck Str. 2, 37191, Germany, Katlenburg-Lindau, 37191, Germany Woch, J (woch@mps.mpg.de), Max Planck Institut fuer Sonnensystemforschung, Max Planck Str. 2, 37191, Germany, Katlenburg-Lindau, 37191, Germany Khurana, K (kkhurana@igpp.ucla.edu), Institute of Geophysics and Planetary Physics, University of California at Los Angeles, 6863 Slichter Halll, Los Angeles, 90095-1567, United States Dougherty, M (m.dougherty@ic.ac.uk), Blackett Laboratory, Imperial College London, Prince Consort Road, London, SW7 2BW, United Kingdom

The dynamics of magnetized, plasma vacuums is a fundamental physical problem and the wakes of Saturn's inert moons represent an ideal physical laboratory where this topic can be thoroughly investigated. With Cassini having already performed several close icy moon flybys, significant contribution to our understanding of the physical processes involved could come from combined data-simulation studies. Here we present such a case study where Rhea's magnetospheric interaction is simulated using a three dimensional, hybrid plasma simulation code, where ions are treated as particles and electrons as a massless, charge-neutralizing fluid. Many of the expected features of the interaction are reproduced, such as the magnetic field compression in the wake, the rarefaction in the wake sides and the extended plasma depletion along the magnetic field direction. The simulation successfully reproduces the signature in the Cassini magnetometer data, acquired during the close flyby to Rhea on November 2005, but only partly the particle distributions observed. Both agreements and disagreements of a simulation with the data can tell us a lot about the determining factors of the interaction. As Rhea is one of the many mass absorbing moons of Saturn, such a case study should be relevant for most lunar- type interactions at Saturn.

P12B-05 

Rhea's interaction with Saturn's magnetosphere

* Jones, G H (ghj@mssl.ucl.ac.uk), Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, United Kingdom * Jones, G H (ghj@mssl.ucl.ac.uk), Centre for Planetary Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom Roussos, E (roussos@mps.mpg.de), Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany Krupp, N (krupp@mps.mpg.de), Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany Beckmann, U), Max Planck Institut fuer Kernphysik, Saupfercheckweg 1, Heidelberg, 69117, Germany Coates, A J (ajc@mssl.ucl.ac.uk), Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, United Kingdom Coates, A J (ajc@mssl.ucl.ac.uk), Centre for Planetary Sciences, University College London, Gower Street, London, WC1E 6BT, United Kingdom Crary, F (fcrary@swri.edu), Southwest Research Institute, Culebra Rd., San Antonio, TX 78238, Dandouras, I (iannis.dandouras@cesr.fr), Centre d'Etude Spatiale des Rayonnements, Paul Sabatier University, Centre National de la Recherche Scientifique, 9 Avenue du Colonel Roche, Toulouse, 31400, France Dikarev, V (dikarev@mps.mpg.de), Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany Dikarev, V (dikarev@mps.mpg.de), Max Planck Institut fuer Kernphysik, Saupfercheckweg 1, Heidelberg, 69117, Germany Dikarev, V (dikarev@mps.mpg.de), Astronomical Institute of St. Petersburg State University, Universitetskii pr. 28, Staryi Peterhof, St. Petersburg, 198504, Russian Federation Dougherty, M K (m.dougherty@imperial.ac.uk), Imperial College London, The Blackett Laboratory, London, SW7 2BW, United Kingdom Garnier, P (Philippe.Garnier@cesr.fr), Centre d'Etude Spatiale des Rayonnements, Paul Sabatier University, Centre National de la Recherche Scientifique, 9 Avenue du Colonel Roche, Toulouse, 31400, France Hansen, C J (cjhansen@jpl.nasa.gov), Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, Hendrix, A R (arh@jpl.nasa.gov), Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, Hospodarsky, G B (george-hospodarsky@uiowa.edu), The University of Iowa, Department of Physics and Astronomy, Iowa City, IA 52242, Johnson, R E (rej@virginia.edu), University of Virginia, Department of Materials Science and Engineering, PO Box 400745, 116 Engineer's Way, Charlottesville, VA 22904, Kempf, S (sascha.kempf@mpi-hd.mpg.de), Max Planck Institut fuer Kernphysik, Saupfercheckweg 1, Heidelberg, 69117, Germany Khurana, K (kkhurana@igpp.ucla.edu), University of California, Los Angeles, Institute of Geophysics and Planetary Physics, 6863 Slichter Hall, Los Angeles, CA 90095, Krimigis, S M (tom.krimigis@jhuapl.edu), The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, Krueger, H), Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany Kurth, W S (william-kurth@uiowa.edu), The University of Iowa, Department of Physics and Astronomy, Iowa City, IA 52242, Lagg, A (lagg@mps.mpg.de), Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany McAndrews, H J (hazelm@lanl.gov), Los Alamos National Laboratory, Space and Atmospheric Science Group, Los Alamos, NM 87545, Mitchell, D G (don.mitchell@jhuapl.edu), The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, Paranicas, C (chris.paranicas@jhuapl.edu), The Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, MD 20723, Postberg, F (frank.postberg@mpi-hd.mpg.de), Max Planck Institut fuer Kernphysik, Saupfercheckweg 1, Heidelberg, 69117, Germany Russell, C T (ctrussel@igpp.ucla.edu), University of California, Los Angeles, Institute of Geophysics and Planetary Physics, 6863 Slichter Hall, Los Angeles, CA 90095, Saur, J (saur@geo.uni-koeln.de), Universitaet zu Koeln, Institut fuer Geophysik und Meteorologie, Albertus-Magnus-Platz, Koeln, 50923, Germany Seiss, M), Universitaet Potsdam, AG Nichtlineare Dynamik, Postfach 601553, Potsdam, 14469, Germany Spahn, F (frank@agnld.uni-potsdam.de), Universitaet Potsdam, AG Nichtlineare Dynamik, Postfach 601553, Potsdam, 14469, Germany Srama, R (ralf.srama@mpi-hd.mpg.de), Max Planck Institut fuer Kernphysik, Saupfercheckweg 1, Heidelberg, 69117, Germany Strobel, D F (strobel@jhu.edu), The Johns Hopkins University, Department of Earth and Planetary Science, Baltimore, MD 21218, Tokar, R L (rlt@lanl.gov), Los Alamos National Laboratory, Space and Atmospheric Science Group, Los Alamos, NM 87545, Wahlund, J (jwe@irfu.se), Swedish Institute of Space Physics, Box 537, Uppsala, 751 21, Sweden Wilson, R J (rjw@lanl.gov), Los Alamos National Laboratory, Space and Atmospheric Science Group, Los Alamos, NM 87545, Woch, J (woch@mps.mpg.de), Max-Planck-Institut fuer Sonnensystemforschung, Max-Planck-Str. 2, Katlenburg-Lindau, 37191, Germany Young, D (dyoung@swri.edu), Southwest Research Institute, Culebra Rd., San Antonio, TX 78238,

The instruments aboard the Cassini orbiter continue to provide a wealth of invaluable information on plasma interactions at the icy moons of Saturn. 1528 km-wide Rhea is the largest of Saturn's icy satellites. Here, we present our analysis of data from the two closest flybys of this moon to date, in November 2005 and August 2007. Data from six Cassini instruments are presented; MIMI, CAPS, MAG, RPWS, UVIS, and CDA, covering the plasma, neutral gas, and dust environments of the moon. The complementary information provided by these instruments allows us to draw a picture of this moon's intriguing interaction with Saturn's magnetospheric plasma. We compare the results at Rhea to those obtained at other moons, and we present our arguments for the causes of variations observed in the magnetospheric electron population.

P12B-06 

Geology and Geomorphology of Rhea: a First Look at the High-Resolution Cassini ISS Images from the Targeted Flyby on Aug. 30, 2007.

* Wagner, R J (Roland.Wagner@dlr.de), DLR, Institute of Planetary Research, Rutherfordstrasse 2, Berlin, D-12489, Germany Neukum, G (gneukum@zedat.fu-berlin.de), Dept. of Earth Sciences, Inst. of Geosciences, Freie Universitaet Berlin, Malteserstrasse 74-100, Berlin, D-12249, Germany Giese, B), DLR, Institute of Planetary Research, Rutherfordstrasse 2, Berlin, D-12489, Germany Roatsch, T), DLR, Institute of Planetary Research, Rutherfordstrasse 2, Berlin, D-12489, Germany Denk, T), Dept. of Earth Sciences, Inst. of Geosciences, Freie Universitaet Berlin, Malteserstrasse 74-100, Berlin, D-12249, Germany

Background: Rhea, 1528 km in diameter, is Saturn's second-largest satellite. Its low average density of 1.233 gcm^-3 and gravitational data imply that Rhea is more or less an undifferentiated, homogeneous body composed preferentially of water ice [1][2]. Rhea's major geologic units are vast expanses of densely cratered plains, featuring craters in a wide range of degradation stages and impact basins up to several 100 km in diameter [3][4][5]. The Cassini spacecraft has been in orbit about Saturn since July 1, 2004, and the onboard ISS cameras have taken images of Rhea during several non-targeted flybys [6]. During one targeted flyby on Aug. 30, 2007 (orbit 049RH), the cameras acquired more than 270 images of the anti-saturnian hemisphere at resolutions of up to 35 m/pxl. Results: A regional mapping image sequence with an average resolution of 300 m/pxl shows the densely cratered plains between about 90 to 250° W. These data can be combined with images from the same region taken in earlier flybys (e.g. 018RH) to derive stereo information. Large, old, heavily degraded impact features can be mapped in these data and are used to derive the size-frequency distribution of large impact structures. One remarkable high-resolution target was a bright, 48-km large bright, fresh ray crater. Stereo data reveal the topography of this crater and its surroundings in high detail. The new images show that the frequency of small craters on the ray crater floor and in the continuous ejecta blanket is very low, inferring that this crater is stratigraphically very young. Secondary chains and bright rays extend several hundreds of kilometers outward. One intriguing feature in this crater is the occurrence of small clusters, resembling small secondaries, within the crater floor and in the continuous ejecta, but restricted to the eastern part of the crater. Since there is no crater younger than the ray crater at close range as a possible source, two explanations remain: (1) the small clusters were created by material ejected at a steep angle which re-impacted a part of the crater floor and continuous ejecta [7]. (2) The crater clusters may originate from material disintegrated from the major projectile which impacted upon the formation of the larger primary crater. References: [1] Thomas P. C. et al., LPSC 37th, abstr. No. 1639, 2006. [2] Anderson J. D. & G. Schubert, GRL 34, L02202, 2007. [3] Moore J. M. et al., JGR 90 (suppl.), C785-C795, 1985. [4] Moore J. M. et al., Icarus 171, 421-443, 2004. [5] Wagner R. J. et al., LPSC 38th, abstr. No. 1958, 2007. [6] Porco C. C. et al., SSR 115, 363-497, 2004. [7] Greeley R. et al., Satellites of Jupiter, (D. Morrison, ed.) UofA Press Tucson, Az., 340-378, 1982.

P12B-07 

The Topographies of Rhea and Iapetus in Comparison

* Giese, B (bernd.giese@dlr.de), DLR, Institute of Planetary Research, Rutherfordstraße 2, Berlin, 12489, Wagner, R), DLR, Institute of Planetary Research, Rutherfordstraße 2, Berlin, 12489, Roatsch, T), DLR, Institute of Planetary Research, Rutherfordstraße 2, Berlin, 12489, Denk, T), Institut für Geologische Wissenschaften,Freie Universität, Malteser Str.74-100, Berlin, 12249, Neukum, G), Institut für Geologische Wissenschaften,Freie Universität, Malteser Str.74-100, Berlin, 12249,

Rhea and Iapetus are similar-sized satellites of Saturn but their densities are different. Rhea has a higher rock mass ratio and, associated with it, more radiogenic heating. This implies a thinner lithosphere on Rhea than on Iapetus during their thermal evolution and is expected to have a topographic expression at the surface of these bodies: the thicker the lithosphere the more topography can be supported. We derived the topography of Rhea from Cassini stereo images and compared it with the topography of Iapetus derived earlier [1]. We found that large impact basins on Rhea have relief of up to 7 km whereas those on Iapetus reach relief of up to 14 km. This observation is consistent with a thinner lithosphere on Rhea compared to Iapetus during the early period of heavy bombardment. In particular, while Iapetus has retained impact basins with diameters of up to 800 km, the largest basin on Rhea found so far has a diameter of 440 km only. Larger basins, which are expected to have been formed on Rhea too, have obviously been relaxed rapidly or alternatively, could possibly not develop in a thin lithosphere at all. Depth-to-diameter ratios for craters < 100 km on Rhea fit those determined on Iapetus but the basins on Rhea show a higher degree of relaxation than those on Iapetus. [1] B. Giese at al., The topography of Iapetus' leading side, Icarus (2007),doi:10.1016/j.icarus.2007.06.005, in press.

P12B-08 

VIS-NIR Spectrophotometric Study of the Saturnian icy Satellites by Cassini-VIMS

* Filacchione, G (gianrico.filacchione@iasf-roma.inaf.it), INAF-IASF, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy Capaccioni, F (fabrizio.capaccioni@iasf-roma.inaf.it), INAF-IASF, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy Tosi, F (federico.tosi@iasf-roma.inaf.it), INAF-IFSI, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy Coradini, A (angioletta.coradini@ifsi-roma.inaf.it), INAF-IFSI, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy Cerroni, P (priscilla.cerroni@iasf-roma.inaf.it), INAF-IASF, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy Adriani, A (alberto.adriani@ifsi-roma.inaf.it), INAF-IFSI, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy McCord, T B (mccordtb@aol.com), Space Science Institute - Bear Fight Center, P.O. Box 667, Winthrop, WA 98862, United States Baines, K H (kbaines@pop.jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Bellucci, G (giancarlo.bellucci@ifsi-roma.inaf.it), INAF-IFSI, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy Brown, R H (rhb@lpl.arizona.edu), Lunar and Planetary Laboratory, University of Arizona, 1629 E. University Blvd., Tucson, AZ 85721-0092, United States Bibring, J (jean-pierre.bibring@ias.fr), Institut d'Astrophysique Spatiale, Universite' de Paris, Bâtiment 121, Université Paris Sud 1, Orsay, 91405, France Buratti, B J (bonnie.j.buratti@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Clark, R N (rclark@usgs.gov), U.S. Geological Survey, Denver Federal Center, West 6th Ave. & Kipling St., Lakewood, CO 80225, United States Combes, M (michel.combes@obspm.fr), Observatoire de Paris, 5, place Jules Janssen, Meudon, 92195, France Cruikshank, D P (Dale.P.Cruikshank@nasa.gov), NASA Ames Research Center, Moffett Field, Moffett Field, CA 94035, United States Formisano, V (vittorio.formisano@ifsi-roma.inaf.it), INAF-IFSI, via del Fosso del Cavaliere, 100, Rome, RM 00133, Italy Jaumann, R (Ralf.Jaumann@dlr.de), Deutsches Zentrum für Luft- und Raumfahrt (DLR), Rutherfordstraße 2, Berlin, 12489, Germany Langevin, Y G (yves.langevin@ias.fr), Institut d'Astrophysique Spatiale, Universite' de Paris, Bâtiment 121, Université Paris Sud 1, Orsay, 91405, France Matson, D L (Dennis.L.Matson@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Mennella, V (mennella@na.astro.it), INAF, Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, Napoli, 80131, Italy Nelson, R M (robert.m.nelson@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Nicholson, P D (nicholson@astrosun.tn.cornell.edu), Cornell University, Astronomy Department, 610 Space Science Building, Ithaca, NY 14853, United States Sicardy, B (bruno.sicardy@obspm.fr), Observatoire de Paris, 5, place Jules Janssen, Meudon, 92195, France Sotin, C (Christophe.Sotin@jpl.nasa.gov), Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, United States Sotin, C (Christophe.Sotin@jpl.nasa.gov), Laboratoire de Planetologie et Geodynamique, Universite' de Nantes, 2, rue de la Houssinière, Nantes, 44322, France

After the first three years of the nominal mission aboard the Cassini probe the VIMS (Visual and Infrared Mapping Spectrometer) experiment has collected more than one thousand useful full-disk observations of both regular (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, Phoebe) and minor (Atlas, Prometheus, Pandora, Janus, Epimetheus, Telesto, Calypso) icy moons of Saturn. These data, acquired from a variety of distances and inclinations from the equatorial plane, with different phase angles and hemispheric coverage, are analyzed by using several spectroscopic indicators (I/F continuum level, slopes, bands strengths) in order to identify analogies and differences in the compositional units of satellites and derive the phase curves at different longitudes; many observations acquired close to zero phase angle allow us to measure the opposition surge effect on several satellites. Concerning the composition we have derived the distribution of the water ice abundance and grain size from the almost pure icy surfaces of Enceladus and Calypso to the organic rich Hyperion, Iapetus and Phoebe. We report about the differences observed in the CO₂ band position which is shifted at shorter wavelengths on Hyperion respect to Phoebe and Iapetus; this effect is probably related to a different distribution of clathrates on these icy surfaces. This research was completed thanks to the support of the Italian Space Agency (ASI).

Author(s) (2007), Title, Eos Trans. AGU, 88(52), Fall Meet. Suppl., Abstract #####-##.