Search

The Cassini spacecraft discovered many close-in small satellites in Saturnian system, and several of them exhibit exotic orbital states due to interactions with Mimas and the oblateness of the planet. This work is devoted to Methone, which is currently involved in a 15:14 Mean-Motion Resonance with Mimas. We give an in deep study the current orbit of Methone by analyzing and identifying the short, resonant and long-term gravitational perturbations on its orbit. In addition, we perform numerical integrations of full equations of motion of ensembles of close-in small bodies orbiting the non-central field of Saturn. Spectral analyses of the orbits and interpretation of them in dynamical maps allow us to describe the orbit and the dynamics of Methone in view of resonant and long-term dynamics. We show that the current geometric orbit of Methone is aligned with Mimas' due to a forced resonant component in eccentricity, leading to simultaneous oscillations of several critical angles of the expanded disturbing function. Thus, we explain the simultaneous oscillations of four critical arguments associated to the resonance. The mapping of the Mimas-Methone resonance shows that the domains of the 15:14 Mimas-Methone resonance are dominated by regular motions associated to the Corotation resonance located at eccentricities lower than $\sim 0.015$ and osculating semi-major axis in the interval 194,660-194,730 km. Methone is currently located deeply within this site., Published on Astronomy and Astrophysics

The Saturnian satellite Tethys exhibits geological and spectral properties, whose appearance, nature and spatial distribution partly mirror those identified on the neighboring satellites Dione and Rhea or fit to the picture how spectral surface properties are expected to change from one satellite to the other within the inner Saturnian system. However, we also identified spectral variations that are unique in the Saturnian system. Whereas geologically young surface features are characterized by pure H2O-ice composition with relatively large particles, which match the particle sizes measured for fresh surface features also on Dione and Rhea, geologically old weathered regions are dominated by submicron-sized ice particles. Our investigations confirm that the Odysseus impact event did not cause the formation of Tethys' extended graben system Ithaca Chasma. On the contrary, Odysseus might be responsible for the N-S trending 'icy' bands that mark Tethys' surface in the center of its leading and trailing hemisphere.

We investigate the origin of the steep compositional gradient inferred for the Galilean satellites. We analyze N-body simulations of satellite accretion (Ogihara, M., Ida, S. [2012]. Astrophys. J. 753, 60) to: (1) determine the extent to which individual satellites accrete material from different semi-major axes (‘radial mixing’); and (2) calculate the change in rock:ice ratios due to vapor production or physical erosion during collisions. Because of inwards migration, satellites experience enough radial mixing that any initial compositional gradient is efficiently smoothed out. Mean-motion resonances generally prevent large proto-satellites from colliding with each other; as a result, neither vapor production nor physical erosion are capable of explaining the steep compositional gradient. According to the models presented here, even combining an initial compositional gradient with impact-related mass loss cannot reproduce the observed gradient. Some other physical process must have been responsible, perhaps tidally-driven volatile loss at Io and Europa. Impact-related mass loss was probably more important in the saturnian system, and may explain some of the observed satellite diversity there.

The Cassini Imaging Science Subsystem (ISS) acquired many high-resolution images (

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., 18 pages, 3 figures, Planetary and Space Science, in press

The Cassini Imaging Science Subsystem (ISS) acquired 370 high-resolution images ( http://ciclops.org/maps 〉 and the Planetary Data System 〈 http://pds.jpl.nasa.gov 〉. This atlas completes the series of the atlases of the Saturnian medium-sized satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Iapetus.

We describe a search for dust created in collisions between the Saturnian irregular satellites using archival \emph{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.

Among the icy satellites of Saturn, Iapetus shows a striking dichotomy between its leading and trailing hemispheres, the former being significantly darker than the latter. Thanks to the VIMS imaging spectrometer on-board Cassini, it is now possible to investigate the spectral features of the satellites in Saturn system within a wider spectral range and with an enhanced accuracy than with previously available data. In this work, we present an application of the G-mode method to the high resolution, visible and near infrared data of Phoebe, Iapetus and Hyperion collected by Cassini/VIMS, to search for compositional correlations. We also present the results of a dynamical study on the efficiency of Iapetus in capturing dust grains travelling inward in Saturn system to evaluate the viability of Poynting-Robertson drag as the physical mechanism transferring the dark material to the satellite. The results of spectroscopic classification are used jointly with the ones of the dynamical study to describe a plausible physical scenario for the origin of Iapetus' dichotomy. Our work shows that mass transfer from the outer Saturnian system is an efficient mechanism, particularly for the range of sizes hypothesised for the particles composing the newly discovered outer ring around Saturn. Both spectral and dynamical data indicate Phoebe as the main source of the dark material. However, we suggest a multi-source scenario where now extinct prograde satellites and the disruptive impacts that generated the putative collisional families played a significant role in supplying the original amount of dark material., Comment: 20 pages, 4 tables, 11 figures, major revision (manuscript extended and completed, figures added and corrected, new results added), minor revision and finalization of author list, moderate revision (update of the manuscript following reviewer's feedback and discovery of the new Saturnian outer ring)

The results of the reduction, investigation, and comparison of the photographic observations of the major Saturnian satellites and CCD observations with an ST6 CCD camera obtained at the 264nch Pulkovo refractor in 1995–2007 are presented. A comparison of the observational results with the TASS 1.7 theory of motion of the Saturnian satellites has served as the basis for investigating and comparing the series of observations. The period-averaged (O-C) residuals and observational errors have been calculated. A comparison of the series of CCD and photographic observations has shown the same external accuracy of the observations at a higher internal accuracy of the CCD observations than that of the photographic ones. A comparison of the Pulkovo results with those of other authors has shown them to be close in accuracy. The accuracy of the theory has been estimated by comparing simultaneous (on the same night) CCD and photographic observations. The errors of the observations and the theory have been found from this comparison to be the following: 0.081“ and 0.067” for the observations and 0.077“ and 0.115” for the theory (inxandy, respectively). An analysis of the dependence of (O-C)x,y for three satellites (the sixth, seventh, and eighth) on the satellite positions in Saturn-centered orbits has revealed systemat ic deviations for the seventh satellite in both coordinates. The positions of Saturn have been determined from satellite observations without measuring its images on photographic plates with accuracies of 0.121“ and 0.105” in right ascension and declination, respectively.

著者人数：156名, Accepted: 2008-05-27, 資料番号: SA1000998000

The Cassini imaging science subsystem (ISS) acquired 449 high-resolution images ( http://ciclops.org/maps ].

The Cassini Imaging Science Subsystem (ISS) acquired 377 high-resolution images ( http://ciclops.org/maps ).

Images of the icy Saturnian satellites Mimas, Enceladus, Tethys, Dione, Rhea, Iapetus, and Phoebe, derived by the Voyager and Cassini cameras are used to produce new local high-resolution image mosaics as well as global mosaics [http://ciclops.org, http://photojournal.jpl.nasa.gov]. These global mosaics are valuable both for scientific interpretation and for the planning of future flybys later in the ongoing Cassini orbital tour. Furthermore, these global mosaics can be extended to standard cartographic products.

During its 2005 January opposition, the saturnian system could be viewed at an unusually low phase angle. We surveyed a subset of Saturn's irregular satellites to obtain their true opposition magnitudes, or nearly so, down to phase angle values of 0.01 deg. Combining our data taken at the Palomar 200-inch and Cerro Tololo Inter-American Observatory's 4-m Blanco telescope with those in the literature, we present the first phase curves for nearly half the irregular satellites originally reported by Gladman et al. [2001. Nature 412, 163-166], including Paaliaq (SXX), Siarnaq (SXXIX), Tarvos (SXXI), Ijiraq (SXXII), Albiorix (SXVI), and additionally Phoebe's narrowest angle brightness measured to date. We find centaur-like steepness in the phase curves or opposition surges in most cases with the notable exception of three, Albiorix and Tarvos, which are suspected to be of similar origin based on dynamical arguments, and Siarnaq.During its 2005 January opposition, the saturnian system could be viewed at an unusually low phase angle. We surveyed a subset of Saturn's irregular satellites to obtain their true opposition magnitudes, or nearly so, down to phase angle values of 0.01 deg. Combining our data taken at the Palomar 200-inch and Cerro Tololo Inter-American Observatory's 4-m Blanco telescope with those in the literature, we present the first phase curves for nearly half the irregular satellites originally reported by Gladman et al. [2001. Nature 412, 163-166], including Paaliaq (SXX), Siarnaq (SXXIX), Tarvos (SXXI), Ijiraq (SXXII), Albiorix (SXVI), and additionally Phoebe's narrowest angle brightness measured to date. We find centaur-like steepness in the phase curves or opposition surges in most cases with the notable exception of three, Albiorix and Tarvos, which are suspected to be of similar origin based on dynamical arguments, and Siarnaq.

The Cassini-Huygens arrival into the Saturnian system brought a large amount of data about the satellites and rings. Two diffuse rings were found in the region between the A ring and Prometheus. R/2004 S1 is coorbital to Atlas and R/2004 S2 is close to Prometheus. In this work we analysed the closest approach between Prometheus and both rings. As a result we found that the satellite removes particles from R/2004 S2 ring. Long-term numerical simulations showed that some particles can cross the F ring region . The well known region of the F ring, where small satellites are present and particles are being taking from the ring, gains a new insight with the presence of particles from R/2004 S2 ring. The computation of the Lyapunov Characteristic Exponent reveled that the R/2004 S2 ring lies in a chaotic region while R/2004 S1 ring and Atlas are in a stable region. Atlas is responsible for the formation of three regimes in the R/2004 S1 ring, as expected for a satellite embedded in a ring.

We have fitted numerically integrated orbits for the major Saturnian satellites to a large set of astrometric observations over the time interval 1966 September to 2003 December and to data obtained with the Pioneer 11, Voyager, and Cassini spacecraft. The results of the fit are new ephemerides for the satellites and a revised gravity field for the Saturnian system. We include an accuracy assessment for the ephemerides and the gravity parameters.

We have reanalyzed the Voyager 1 UVS solar occultations by Titan to expand upon previous analyses and to resolve inconsistencies that have been noted in the scientific literature. To do so, we have developed a detailed model of the UVS detector and improved both the data reduction methods and retrieval techniques. In comparison to the values previously determined by Smith et al. (1982, J. Geophys. Res. 87, 1351–1359) we find N2 densities that are 25–60% higher, CH4 densities that are smaller by a factor of 3–7, and C2H2 densities that are roughly two orders of magnitude smaller. Our values for the thermospheric temperature are 153–158 K, which are approximately 20–40 K colder than previous estimates. We also report the first-ever determination from Voyager UVS data of density profile information for C2H4, HCN, and HC3N. Finally, we present a simple engineering model that is consistent with our new results in the upper atmosphere and merges smoothly with the model of Yelle et al. (1997, in: HUYGENS Science, Payload and Mission, in: ESA SP, vol. 1177, pp. 243–256) in the lower atmosphere. Our results provide improved constraints for photochemical models and offer scientists a better understanding of Titan's upper atmosphere as we head into the Cassini era in the exploration of the saturnian system.

The Cassini mission to Saturn employs a Saturn orbiter and a Titan probe to conduct an intensive investigation of the Saturnian system. The orbiter flies a series of orbits, incorporating flybys of the Saturnian satellites, called the ‘satellite tour.’ During the tour, the gravitational fields of the satellites (mainly Titan) are used to modify and control the orbit, targeting from one satellite flyby to the next. The tour trajectory must also be designed to maximize opportunities for a diverse set of science observations, subject to mission-imposed constraints. Tour design studies have been conducted for Cassini over a period of several years to identify trades and strategies for achieving these sometimes conflicting goals. Concepts, strategies, and techniques previously developed for the Galileo mission to Jupiter have been modified, and new ones have been developed, to meet the requirements of the Cassini mission. A sample tour is presented illustrating the application of tour design strategies developed for Cassini.

International audience; Fundamental questions involving the origin, evolution, and history of both Titan and the broader Saturnian system can be answered by exploring this satellite from an orbiter and also in situ. We present the science case for an exploration of Titan and one of its lakes from a dedicated orbiter and a lake probe. Observations from an orbit-platform can improve our understanding of Titan׳s geological processes, surface composition and atmospheric properties. Further, combined measurements of the gravity field, rotational dynamics and electromagnetic field can expand our understanding of the interior and evolution of Titan. An in situ exploration of Titan׳s lakes provides an unprecedented opportunity to understand the hydrocarbon cycle, investigate a natural laboratory for prebiotic chemistry and habitability potential, and study meteorological and marine processes in an exotic environment. We briefly discuss possible mission scenarios for a future exploration of Titan with an orbiter and a lake probe. •We present the science case for Titan׳s exploration.•We present mission scenarios for a future exploration of Titan.•This paper is based on a white paper presented for 2013 ESA׳s Call for Large missions.

I report preliminary measurements of the complex permittivity of frozen aqueous ammonia solutions at liquid nitrogen temperatures, representative of those in the saturnian system. The real part of the dielectric constant of 30% ammonia ice is around 4.5 at near-DC frequencies and at ∼1 MHz, compared with around 3.1 for pure water ice. The loss tangents of ammonia-rich ices seem somewhat (∼50%) higher than those for water ice, for which the few low-temperature experiments to date indicate values comparable with predictions by Thompson and Squyres (1990,Icarus86, 336–354) and Maetzler (1998, inSolar System Ices(B. Schmitt, C. DeBergh, and M. Festou, Eds.), pp. 241–257, Kluwer Academic, Dordrecht), but considerably higher than models by Chybaet al. (1998,Icarus, in press). Ammonia-rich ice may reconcile the radar and optical appearance of Titan's surface: the detectability of water–ammonia ice on Titan by the Cassini mission and the implications for Titan's origin and evolution are discussed.

An evolutionary model of the G Ring incorporating theoretical results from R. M. Canup and L. W. Esposito (1995,Icarus113, 331–352) yields a complete particle size distribution that is consistent with existing observations. Results from numerical modeling demonstrate that a G Ring origin from the disruption of a 1.5–3 km progenitor satellite can match all known properties of the ring. In addition, we estimate the population of unseen macroscopic material from both observational upper limits and our theoretical model for the region surrounding the G Ring, where the Cassini spacecraft will likely make its innermost passes in the saturnian system. For models that fit all available data, the probability of Cassini striking a hazardous ring particle is less than 1%.

In October 1997 a unique mission to the Saturnian system will be launched by NASA, the CASSINI mission. One goal of this mission is to study the Saturnian dust environment, and for this task, the COSMIC DUST ANALYZER (CDA) has been developed and is currently under test. Impact ionization is used to determine the speed (1 – 100 km/s) and the mass (1×10 −15 – 1×10 −9 g) of impinging particles. furthermore, the electric charge (1×10 −15 – 1×10 −12 C) of the particles can be measured via the induction principle, whilst an integrated time-of-flight mass spectrometer will analyze the chemical composition of the dust. In order to achieve sufficient sensitivity for dust fluxes as low as 1 particle/month, the sensor has a large sensitive area of 0.1 m 2 . This paper will describe the measurement signals of the engineering model of the CDA and their interpretation.

Context. The Saturnian satellite system has been observed in detail by the Cassini-Huygens mission. These satellites present different surface features, including impact craters caused by small objects probably coming from the trans-Neptunian region. Aims. In this paper we calculate the production of craters on the mid-sized Saturnian satellites produced by Centaurs from the scattered disk (SD) and plutinos in order to determine this contribution, and we compare our estimations with the Cassini observations. Methods. We used a method developed in a previous paper that uses a numerical investigation of the dynamical evolution of Centaur objects to calculate the production of craters. We used a size-frequency distribution (SFD) of scattered disk objects (SDOs) as a power law with a break at diameters d = 60 km considering two cases for the differential power-law index: s2 = 2.5 and s2 = 3.5 for d < 60 km. Results. We calculated the number of craters, the greatest crater produced by Centaurs from the SD and plutinos, and the present cratering rate on each of the mid-sized satellites, for both cases of the SFD of SDOs considered. The contribution of plutinos is negligible compared to SDOs. From our calculations and the comparison with observations we note that the calculated number of craters for s2 = 3.5 is in general nearer the observed number. However, in general for smaller craters, the observed number is less than the calculated one. This trend can be explained by at least two mechanisms. On the one hand, this could be caused by an erasing process that gradually buries the craters, which does not affect large craters. On the other hand, the comparison of the calculated and observed crater size-frequency distribution for different size ranges implies that for d < 60 km, the SFD of SDOs is consistent with the assumed index s2 = 3.5, for d ≥ 0.2-1.4 kmand for d ≤ 0.2-1.4 km, it is consistent with s2 = 2.5. Then in the range d ~ 0.2-1.4 km, the SFD of SDOs could have a new break. This change of slope could explain the reduction of small craters, at least for some cases. Conclusions. We found a good agreement when comparing our results with observations. However, independent determination of surface ages and geological processes are needed to determine if there is a new break on the SFD of SDOs, if there is a planetocentric source of craters in the Saturnian system, and which craters are primordial., Facultad de Ciencias Astronómicas y Geofísicas, Instituto de Astrofísica de La Plata

There is substantial evidence for the existence of small satellites in the saturnian system in addition to those with well-determined orbits derived from ground-based and Voyager observations. Charged particle data from Pioneer 11 and Voyager 2 provide specific evidence for the existence of material coorbital with Mimas. This work is an attempt to identify small satellites coorbital with Mimas which may be responsible for the charged particle results. The research uses a combination of the NAIF/SPICE Toolkit, IRAF, and locally developed software to analyze images in the Voyager archive. We also develop a new approach to the problem of blemish removal. Four possible new satellites are detected in Voyager 2 images after the identification and elimination of blemishes and background stars. Details of the search criteria and probable orbits are discussed for each of the candidate objects. Three of the candidates may be coorbital with Mimas. If there is such a collection of small satellites in the orbit of Mimas it could explain the depletions seen in the charged particle data. One of the candidates, if it is in the ring plane, would be on an orbit at the outer edge of the faint G ring. If so, it may provide a source for the ring material. Due to the limited coverage of Voyager images in the search region it was not possible to find additional images of the candidates. As a consequence the features identified in this work must remain candidate objects until confirmed either by ground-based or spacecraft observations.

International audience; Context. The Cassini mission in the Saturnian system is an outstanding opportunity to improve our knowledge of the satellites of Saturn. The data obtained thanks to this mission must be balanced with theoretical models. Aims: This paper aims at modelling the rotation of Mimas with respect to its possible internal structure. Methods: We first built different interior models, considering Mimas to be composed of two rigid layers with different porosity. Then we simulated the rotational behaviour of these models in a three-degree of freedom numerical code, considering the complete ephemerides of a Mimas whose rotation is disturbed by Saturn. We also estimated the deviation of its longitudinal orientation caused by tides. Results: We expect a signature of the internal structure up to 0.53° in the longitudinal librations and an obliquity between 2 and 3 arcmin, the exact values depend on the interior. Conclusions: The longitudinal librations should be detectable, but inverting them to arrive at clues on the internal structure of Mimas is challenging.

If the solar radiation pressure effect is taken into consideration, the structure of the atomic hydrogen cloud emitted from Titan would be significantly modified. Instead of a circular torus characterized by azimuthal symmetry and with density peak near the orbit of Titan, the atomic hydrogen cloud will take on a pear-shaped structure with the long-lived H atoms distributed through the whole Saturnian system.

Saturn is the only known planet to have coorbital satellite systems. In the present work we studied the process of mass accretion as a possible mechanism for coorbital satellites formation. The system considered is composed of Saturn, a proto-satellite and a cloud of planetesimals distributed in the coorbital region around a triangular Lagrangian point. The adopted relative mass for the proto-satellite was 10^-6 of Saturn's mass and for each planetesimal of the cloud three cases of relative mass were considered, 10^-14, 10^-13 and 10^-12 masses of Saturn. In the simulations each cloud of planetesimal was composed of 10^3, 5 x 10^3 or 10^4 planetesimals. The results of the simulations show the formation of coorbital satellites with relative masses of the same order of those found in the saturnian system (10^-13 - 10^-9). Most of them present horseshoe type orbits, but a significant part is in tadpole orbit around L_4 or L_5. Therefore, the results indicate that this is a plausible mechanism for the formation of coorbital satellites.

The Cassini Imaging Science Subsystem (ISS) acquired 282, 258, and 513 high-resolution images ( http://ciclops.org/maps ] and the Planetary Data System [ http://pds.jpl.nasa.gov ].

International audience; In the context of the Cassini/Huygens mission, we performed supporting ground-based observations to complement the results from the NASA/ESA/ASI space mission to the Saturnian system with particular focus on Titan. On the nights of 18 and 19 December 2004, we conducted adaptive optics observations with VLT/NACO to search for and map the distribution of CO2 ice deposits on the spatially resolved surface of Titan (65 mas resolution). This experiment became possible because (1) solid CO2 possesses two narrow and strong absorption lines at 2012 nm and 2070 nm that fall into the 2.05 mum window of Titan's atmosphere and (2) the width of these bands matches the band pass of the Fabry-Perot instrument installed in NACO. We do not detect this chemical compound, but we can derive firm limits on the abundance of CO2 ice on the surface of Titan at sub-Earth longitudes 284°W and 307°W. With a spatial sampling of 65 mas, we conclude that a partial surface coverage of segregated CO2 ice does not exceed 7% or 14% for bright or dark surface regions, respectively.

International audience; In order to improve the orbit of Phoebe, the ninth satellite of the Saturnian system, 101 new observations were made by our research team in 2003, using a CCD detector of large size (2048×2048 pixels) mounted on the 1.56 m astrometric reflector at the Sheshan Station of Shanghai Astronomical Observatory. We fitted a numerical integration of its orbit to all of the collected Earth-based astrometric observations from 1904 to 2003, including the newest precise data sets from Qiao & Tang and from Peng et al. A new set of initial conditions of Phoebe has been obtained, leading to an improved orbit of this satellite.

International audience; The Cassini visual and infrared mapping spectrometer (VIMS) investigation is a multidisciplinary study of the Saturnian system. Visual and near-infrared imaging spectroscopy and high-speed spectrophotometry are the observational techniques. The scope of the investigation includes the rings, the surfaces of the icy satellites and Titan, and the atmospheres of Saturn and Titan. In this paper, we will elucidate the major scientific and measurement goals of the investigation, the major characteristics of the Cassini VIMS instrument, the instrument calibration, and operation, and the results of the recent Cassini flybys of Venus and the Earth Moon system.

As Voyager 1 sailed through Saturn’s system of moons and rings last November 1980 it revealed new worlds not seen by man before. For centuries, since Galileo’s first telescopic observations in 1610, the satellites of Saturn had been no more than pin points of light, whilst the structure of the rings was barely resolved beyond 3 principal bands. Yet, within the space of a few hours, that picture changed dramatically as the images of these objects grew through Voyager’s cameras from mere specks into full and wondrous worlds. These pictures contained features that were not only intricate and astonishing in detail but which were, in many cases, unfamiliar and unexpected. A composite view of the Saturnian system as seen by Voyager 1 appears in Figure 1. Saturn’s rings, once thought to be broad belts of particles spread uniformly thin through billions of years of evolution and interparticle collisions, were found to be divided into hundreds of individual ringlets (Figure 2). And Cassini’s Division, a region which had been previously thought to be empty because of a ‘sweeping’ influence of Mimas, was found to contain many ringlets itself! The appearance of light and dark radial spokes in the B ring, which rotated with a velocity contrary to the law expected of Keplerian orbits, was baffling. And the F ring (Figure 3) was found to contain knots, kinks and braids which probably indicated the presence of electro-magnetic forces as well as gravitational forces (Smith et al. 1981).

We searched for H2O 6(1,6)-5(2,3) maser emission at 22.235 GHz from several Saturnian satellites with the Nobeyama 45m radio telescope in May 2009. Observations were made for Titan, Hyperion, Enceladus and Atlas, for which Pogrebenko et al. (2009) had reported detections of water masers at 22.235 GHz, and in addition for Iapetus and other inner satellites. We detected no emission of the water maser line for all the satellites observed, although sensitivities of our observations were comparable or even better than those of Pogrebenko et al.. We infer that the water maser emission from the Saturnian system is extremely weak, or sporadic in nature. Monitoring over a long period and obtaining statistical results must be made for the further understanding of the water maser emission in the Saturnian system., Comment: 8 pages, 2 figures, accepted for publication in PASJ (Letter)

In July 2004 the Cassini-Huygens mission reached the Saturnian system and started its orbital tour. A total of 75 orbits will be carried out during the primary mission until August 2008. In these four years Cassini crosses the ring plane 150 times and spends approx. 400 h within Titan's orbit. The Cosmic Dust Analyser (CDA) onboard Cassini characterises the dust environment with its extended E ring and embedded moons. Here, we focus on the CDA results of the first year and we present the Dust Analyser (DA) data within Titan's orbit. This paper does investigate High Rate Detector data and dust composition measurements. The authors focus on the analysis of impact rates, which were strongly variable primarily due to changes of the spacecraft pointing. An overview is given about the ring plane crossings and the DA counter measurements. The DA dust impact rates are compared with the DA boresight configuration around all ring plane crossings between June 2004 and July 2005. Dust impacts were registered at altitudes as high as 100 000 km above the ring plane at distances from Saturn between 4 and 10 Saturn radii. In those regions the dust density of particles bigger than 0.5 mu m can reach values of 0.001 m(-3). (c) 2006 Published by Elsevier Ltd.