Orbits of Saturn's F ring and its shepherding satellites

The shape and orientation of Saturn's F ring and the orbits of its two shepherding satellites have been determined from Voyager images. The data and processing are described, and orbital parameter estimates and associated uncertainties are presented. In addition, evidence that suggests that the F-ring braids are formed very near the conjunctions of the shepherding satellites is presented.     

Triaxiality of satellites and small bodies in the Solar System

The triaxial figures are very common shape of most of planetary satellites as well as of smaell bodies as asteroids. There are 21 satellites in the Solar System triaxial figures of which were detected in situ evidently (Davies et al., 1995). However, the total number of triaxiaxial satellites in the Solar System should be in fact larger. In this paper the general theory of triaxiality due to tidal forces is discussed in regard to the very recent numerical data. Since they orbit synchronously, as a rule: their orbital periods are equal to the rotational periods, the tidal forces may be responsible for their triaxial figures. On the other hand the origin of triaxiality of asteroids due to another process and the of their figures cannot be axplained by the tidal effects.     

Sizes, shapes, and albedos of the inner satellites of Neptune

Based on 87 resolved Voyager images of the five innermost satellites of Neptune, their shapes were measured and fit by tri-axial ellipsoids with the semi-axes of 48 × 30 × 26 km for Naiad, 54 × 50 × 26 km for Thalassa, 90 × 74 × 64 km for Despina, 102 × 92 × 72 km for Galatea, and 108 × 102 × 84 km for Larissa. Thomas and Veverka published a similar shape for Larissa (104 × 89 km, J. Geophys. Res. 96, 19261-19268, 1991). The other satellites had no published shapes. Using Voyager photometry of the six inner satellites by the same authors and the revised sizes, including the published size of Proteus, the reflectivity within this inner system was found to vary by about 30%. Geometric albedos in the visible are estimated between 0.07 for Naiad and 0.10 for Proteus. The rotational lightcurves of these satellites seem to be due to satellite shapes.     

Tidal dissipation,orbital evolution,and the nature of Saturn's inner satellites

Estimates of tidal damping times of the orbital eccentricities of Saturn's inner satellites place constraints on some satellite rigidities and dissipation functions Q. These constraints favor rock-like rather than ice-like properties for Mimas and probably Dione. Photometric and other observational data are consistent with relatively higher densities for these two satellites, but require lower densities for Tethys, Enceladus, and Rhea. This leads to a nonmonotonic density distribution for Saturn's inner satellites, apparently determined by different mass fractions of rocky materials. In spite of the consequences of tidal dissipation for the orbital eccentricity decay and implications for satellite compositions, tidal heating is not an important contributor to the thermal history of any Saturnian satellite.     

伽利略卫星的平衡形状参数和潮汐耗散因子

木星的四颗大卫星都是同步轨旋卫星，常被称为伽利略卫星，美国发射的伽利略飞船自1995年12月抵达木星系统后，的几年来对木星的四颗伽利略卫星进行了一系列的探测，利用飞船探测的最新资料作为约束条件，建立了伽利略卫星的一组内部结构模型，然后按照同步轨旋卫星的形态理论公式计算了它们的平衡形太参数及潮汐耗散因子等。     

Shapes of the saturnian icy satellites and their significance

The sizes and shapes of six icy saturnian satellites have been measured from Cassini Imaging Science Subsystem (ISS) data, employing limb coordinates and stereogrammetric control points. Mimas, Enceladus, Tethys, Dione and Rhea are well described by triaxial ellipsoids; Iapetus is best represented by an oblate spheroid. All satellites appear to have approached relaxed, equilibrium shapes at some point in their evolution, but all support at least 300 m of global-wavelength topography. The shape of Enceladus is most consistent with a homogeneous interior. If Enceladus is differentiated, its shape and apparent relaxation require either lateral inhomogeneities in an icy mantle and/or an irregularly shaped core. Iapetus supports a fossil bulge of over 30 km, and provides a benchmark for impact modification of shapes after global relaxation. Satellites such as Mimas that have smoother limbs than Iapetus, and are expected to have higher impact rates, must have relaxed after the shape of Iapetus was frozen.     

Spectrophotometry of the small satellites of Saturn and their relationship to Iapetus, Phoebe, and Hyperion

We have obtained broadband spectrophotometric observations of four of the recently discovered small satellites of Saturn (Gladman et al., 2001, Nature 412, 163-166). The new data enable an understanding of the provenance, composition, and interrelationships among these satellites and the other satellites of Saturn, particularly Iapetus, Phoebe, and Hyperion. Temporal coverage of one satellite (S21 Tarvos) was sufficient to determine a partial rotational lightcurve. Our major findings include: (1) the satellites are red and similar in color, comparable to D-type asteroids, some KBOs, Iapetus, and Hyperion; (2) none of the satellites, including those from the “Phoebe Group” has any spectrophotometric relationship to Phoebe; and (3) S21 Tarvos exhibits a rotational lightcurve, although the data are not well-constrained and more observations are required to fit a definitive period. Dust created by meteoritic impacts and ejected from these satellites and additional undiscovered ones may be the source of the exogenous material deposited on the low-albedo side of Iapetus. Recent work which states that the small irregular satellites of Saturn have impacted Phoebe at least 6-7 times in the age of the Solar System (Nesvorny et al., 2003, Astron. J. 126, 398-429), suggests that such collisions may have propelled additional material from both Phoebe and the small irregular satellites toward Iapetus. The accretion of material from outer retrograde satellites may be a process that also occurs on Callisto and the uranian satellites.     

Tetrahedron formation of nanosatellites with single-input control

The present paper studies the formation flight of four nanosatellites forming a tetrahedron. The main goal of this research is to find the relative orbits of these satellites that, at least in the linear Hill–Clohessy–Wiltshire model, ensure finite relative motion and keep the volume and shape of the tetrahedron configuration. Since real motions of these satellites will differ from the linear ones, especially under the influence of the \(J_{2}\) perturbation, active control is necessary. In addition, the limited size of the satellites does not allow us to use a complex 3-axis attitude control system. In the present paper we consider the passive magnetic attitude control system and suppose that the thrust direction is always aligned with the local geomagnetic field. In order to increase mission lifetime the control algorithm that minimizes the propellant consumption and keeps the tetrahedron volume and shape is investigated.     

Noncircular outer disks in unbarred S0 galaxies: NGC 502 and NGC 5485

Highly noncircular outer stellar disks have been detected in two SA0 (unbarred) galaxies by comparing the spectroscopic data on the rotation of stars and the photometric data on the shape and orientation of isophotes. In NGC 502, the oval distortion of the disk is manifested in the shape of the inner and outer elliptical rings occupying wide radial zones between the bulge and the disk and at the outer disk edge; such a structure can be a consequence of the so-called “dry minor merger,” multiple cannibalization of gas-free satellites. In NGC 5485, the stellar kinematics is absolutely unrelated to the orientation of isophotes in the disk region, and for this galaxy the conclusion about its global triaxial structure is unavoidable.     

G-MODE CLASSIFICATION OF SPECTROSCOPIC DATA

We present the results of the application of the G-mode method to the spectral classification of the icy satellites of the giant planets. G-mode is a multivariate statistical technique for the classification of samples depending on many variables. Here this method is tested on the infrared spectra acquired by the Cassini/VIMS instrument onboard the Cassini spacecraft. This work demonstrates the suitability of automatic spectral classification methods for the study of fair resolution spectra, such as those from VIMS. Our data set is composed by two different kinds of data: observations of point targets (Galilean satellites data) and observations with medium spatial resolution (Phoebe data). In both situations, the G-mode classification performed well. In the first case, of a large number of subpixel observations of the Galilean satellites, through the G-mode it was possible to find statistically meaningful spectral groups of observations. In the case of Phoebe, of some spatially resolved observations, the G-mode classification of␣the infrared spectra of the surface led to several types, dominated by the different illumination geometry of the pixels, because, due to the irregular shape of the satellite, a proper illumination correction was not trivial to apply. Nevertheless, the decrease of the confidence level of the test as well as the re-application of the G-mode on the main type found, led to further types, whose statistical distance can be related to different chemical abundances. We plan to use the G-mode also on the data coming from ongoing and future observations of the icy Saturnian satellites.In the helioseismology literature, G-modes are gravity wave modes of the frequencies of oscillations of the Sun. Here we are dealing with a clustering method, which is essentially different.*E-mail: federico.tosi@rm.iasf.cnr.it     

Estimates of the physical parameters of remote planetary satellites

Since 1998, 97 new remote satellites of Jupiter, Saturn, Uranus, and Neptune have been discovered. Since their brightness is rather weak, it is difficult to perform their photometry and determine their physical parameters. For 27 satellites from this group, different authors carried out special photometric investigations. For the other 70 satellites, the magnitudes accompanying the results of astrometric observations published in Minor Planet Circulars (MPC) are the only photometric data. In the present study, the photometric model parameters for all of the 97 remote satellites have been determined. From the hypothetic values of albedo and material density of the satellites, their sizes and gravitation parameters have been estimated. The whole volume of the obtained results is available in the database of the natural planetary satellites (NSDB) (Arlot and Emelyanov, 2009) published on the Internet ().     

Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements: 2003

Every three years the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements revises tables giving the directions of the north poles of rotation and the prime meridians of the planets, satellites, and asteroids. This report introduces a system of cartographic coordinates for asteroids and comets. A topographic reference surface for Mars is recommended. Tables for the rotational elements of the planets and satellites and size and shape of the planets and satellites are not included, since there were no changes to the values. They are available in the previous report (Celest. Mech. Dyn. Astron., 82, 83–110, 2002), a version of which is also available on a web site.     

Are plasma satellites present among chromospheric lines?

From a study of the chromospheric lines listed by Pierce (1968, 1973) indications have been found that first-order plasma satellites appear sometimes as a pair of faint chromospheric emission lines in such places where forbidden lines, mainly of metals, should be located. Second-order satellites are indicated sometimes by an asymmetry in the emission wings of stronger lines, either with respect to intensity or with respect to distance from the line centre and, if so, with the remote wing of diffuse shape. Finally diffuse faint lines, appearing in somewhat greater distances from strong chromospheric lines, suggest the appearance sometimes of fourth-order plasma satellites. All the three types of satellites give electron densities of the order of 10¹² cm^?3. Traces of possible absorption satellites are found also for some of the lines in the solar spectrum. In this connection special attention is drawn to a few faint solar lines located near the wavelengths of corona emission lines. The observational evidence is discussed on the bases of recent theoretical research by Drawin and Truong-Bach (1982a, b).     

Cassini VIMS observations of the Galilean satellites including the VIMS calibration procedure

The Visual and Infrared Mapping Spectrometer (VIMS) observed the Galilean satellites during the Cassini spacecraft's 2000/2001 flyby of Jupiter, providing compositional and thermal information about their surfaces. The Cassini spacecraft approached the jovian system no closer than about 126 Jupiter radii, about 9 million kilometers, at a phase angle of <90°, resulting in only sub-pixel observations by VIMS of the Galilean satellites. Nevertheless, most of the spectral features discovered by the Near Infrared Mapping Spectrometer (NIMS) aboard the Galileo spacecraft during more than four years of observations have been identified in the VIMS data analyzed so far, including a possible ¹³C absorption. In addition, VIMS made observations in the visible part of the spectrum and at several new phase angles for all the Galilean satellites and the calculated phase functions are presented. In the process of analyzing these data, the VIMS radiometric and spectral calibrations were better determined in preparation for entry into the Saturn system. Treatment of these data is presented as an example of the VIMS data reduction, calibration and analysis process and a detailed explanation is given of the calibration process applied to the Jupiter data.     

Influence of the coorbital resonance on the rotation of the Trojan satellites of Saturn

The Cassini spacecraft collects high resolution images of the Saturnian satellites and reveals the surface of these new worlds. Tiscareno et?al. succeeded to determine the Epimetheus rotation from the Cassini Imaging Science Subsystem data, initiating studies on the rotation of Epimetheus and its companion Janus (Tiscareno et?al., Icarus 204:254?C261, 2009; Noyelles, Icarus 207:887?C902, 2010; Robutel et?al., Icarus 211:758?C769, 2011). Especially, Epimetheus is characterized by its horseshoe shape orbit and the presence of the swap has to be introduced 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 érdi (Celest Mech 15:367?C383, 1977) to represent the coorbital motions, and so we describe the rotational motion of the coorbitals, Janus and Epimetheus included, in compact form.     

Mutual occultations and eclipses of the Galilean satellites of Jupiter in 1997: Astrometric results of observations

In 2006, a complete database of the international campaign on photometric observations of the Galilean satellites of Jupiter in the 1997 epoch of mutual occultations and eclipses was published. Only two thirds of the observations were considered by other authors beforehand. In this study, we have processed the whole observational database with an original technique in order to obtain the astrometric data. We determined 301 relative positions of the satellites from photometric observations performed at 50 observatories around the world. The results are put into a common database of all observations of the natural planetary satellites called the Natural Satellites Data Center (NSDC) available on the Internet site http://www.sai.msu.ru/neb/nss/index.htm. The influence of random and systematic errors on the accuracy of determining the coordinates of satellites has been analyzed. It has been shown that the largest systematic errors are caused by inaccurate elimination of the background of the photometric measurements and by the erroneous data on the albedo of satellites. The actual accuracy of astrometric results is 0.05″ and 0.07″ in right ascension and declination, respectively. New recommendations for photometric observations of satellites during the considered phenomena have been developed in order to avoid the systematic errors.     

Characteristic features in the spectra of Europa, Ganymede, and Callisto

The results of ground-based spectrophotometry of the icy Galilean satellites of Jupiter—Europa, Ganymede, and Callisto—are discussed. The observations were carried out in the 0.39–0.92 μm range with the use of the CCD spectrometer mounted on the 1.25-m telescope of the Crimean laboratory of the Sternberg Astronomical Institute in March 2004. It is noted that the calculated reflectance spectra of the satellites mainly agree with the analogous data of the earlier ground-based observations and investigations in the Voyager and Galileo space missions. The present study was aimed at identifying new weak absorption bands (with the relative intensity of ～3–5%) in the reflectance spectra of these bodies with laboratory measurements (Landau et al., 1962; Ramaprasad et al., 1978; Burns, 1993; Busarev et al., 2008). It has been ascertained that the spectra of all of the considered objects contain weak absorption bands of molecular oxygen adsorbed into water ice, which is apparently caused by the radiative implantation of O⁺ ions into the surface material of the satellites in the magnetosphere of Jupiter. At the same time, spectral features of iron of different valence (Fe²⁺ and Fe³⁺) values typical of hydrated silicates were detected on Ganymede and Callisto, while probable indications of methane of presumably endogenous origin, adsorbed into water ice, were found on Europa. The reflectance spectra of the icy Galilean satellites were compared to the reflectance spectra of the asteroids 51 Nemausa (C-class) and 92 Undina (X-class).     

Mutual occultations and eclipses of the Galilean satellites of Jupiter in 2002–2003: final astrometric results

The photometry of mutual occultations and eclipses of natural planetary satellites can be used to infer very accurate astrometric data. This can be achieved by processing the light curves of the satellites observed during international campaigns of photometric observations of these mutual events.
This work focuses on processing the complete data base of photometric observations of the mutual occultations and eclipses of the Galilean satellites made during the international campaign in 2002–2003. The final goal is to derive new accurate astrometric data.
We propose the most accurate photometric model of mutual events based on all the data available to date about the satellites, and develop the corresponding method for extracting astrometric data. This method is applied to derive astrometric data from photometric observations of mutual occultations and eclipses of the Galilean satellites.
We process the 371 light curves obtained during the international campaign of photometric observations of the Galilean satellites in 2002–2003. As compared with the theory, the rms 'O-C' residuals with respect to theory is equal to 0.055 and 0.064 arcsec in right ascension and declination, respectively, for the 274 best observations. Topocentric or heliocentric angular differences for satellite pairs are obtained for 119 time instants during the time period from 2002 October 10 to 2003 July 17.     

Impact cratering records of the mid-sized, icy saturnian satellites

Resolution of Voyager 1 and 2 images of the mid-sized, icy saturnian satellites was generally not much better than 1 km per line pair, except for a few, isolated higher resolution images. Therefore, analyses of impact crater distributions were generally limited to diameters (D) of tens of kilometers. Even with the limitation, however, these analyses demonstrated that studying impact crater distributions could expand understanding of the geology of the saturnian satellites and impact cratering in the outer Solar System. Thus to gain further insight into Saturn’s mid-sized satellites and impact cratering in the outer Solar System, we have compiled cratering records of these satellites using higher resolution CassiniISS images. Images from Cassini of the satellites range in resolution from tens m/pixel to hundreds m/pixel. These high-resolution images provide a look at the impact cratering records of these satellites never seen before, expanding the observable craters down to diameters of hundreds of meters. The diameters and locations of all observable craters are recorded for regions of Mimas, Tethys, Dione, Rhea, Iapetus, and Phoebe. These impact crater data are then analyzed and compared using cumulative, differential and relative (R) size-frequency distributions. Results indicate that the heavily cratered terrains on Rhea and Iapetus have similar distributions implying one common impactor population bombarded these two satellites. The distributions for Mimas and Dione, however, are different from Rhea and Iapetus, but are similar to one another, possibly implying another impactor population common to those two satellites. The difference between these two populations is a relative increase of craters with diameters between 10 and 30 km and a relative deficiency of craters with diameters between 30 and 80 km for Mimas and Dione compared with Rhea and Iapetus. This may support the result from Voyager images of two distinct impactor populations. One population was suggested to have a greater number of large impactors, most likely heliocentric comets (Saturn Population I in the Voyager literature), and the other a relative deficiency of large impactors and a greater number of small impactors, most likely planetocentric debris (Saturn Population II). Meanwhile, Tethys’ impact crater size-frequency distribution, which has some similarity to the distributions of Mimas, Dione, Rhea, and Iapetus, may be transitional between the two populations. Furthermore, when the impact crater distributions from these older cratered terrains are compared to younger ones like Dione’s smooth plains, the distributions have some similarities and differences. Therefore, it is uncertain whether the size-frequency distribution of the impactor population(s) changed over time. Finally, we find that Phoebe has a unique impact crater distribution. Phoebe appears to be lacking craters in a narrow diameter range around 1 km. The explanation for this confined “dip” at D = 1 km is not yet clear, but may have something to do with the interaction of Saturn’s irregular satellites or the capture of Phoebe.     

Models,figures, and gravitational moments of the Galilean satellites of Jupiter and icy satellites of Saturn

A general theory for the figures of satellites, which are synchronously rotating in the gravitational field of a planet, is developed to the first approximation. Love numbers, figure parameters, and gravitational moments for two- and three-layer models of the Galilean satellites, Titan, and Saturn's icy satellites are calculated. With the assumed accuracy for flyby measurements of gravitational moments it should be possible to determine the degree of differentiation of Ganymede. The differences between equatorial a and polar c semiaxes, as derived from the observational data, appear to be exaggerated for Io and Mimas (although better agreement between calculated and observed values of (a?c) could be obtained if this satellite had a larger mass). For Enceladus the observed value of (a?c) is in satisfactory agreement with calculations, based on different types of trial models. However, in order to discriminate between different Enceladus trial models, it is necessary to determine the figure parameters more precisely.