Special program of observations of Jovian and Saturnian satellites for 2009

Noteworthy phenomena, viz., mutual occultations and eclipses in the system of Jupiter’s Galilean satellites and in the system of Saturn’s principal satellites, will occur in 2009. The relatively simple photometry of these phenomena makes it possible to obtain positional data at a higher accuracy than can be achieved in regular astrometric observations. The visibility conditions for the satellites are described here and observational recommendations are given. The ephemerides of these phenomena are available via the Internet from the MULTI-SAT ephemerides server at http:/www.sai.msu.ru/neb/nss/index.htm.     

The origin of Jovian and Saturnian satellites in accretion disks

The properties of gas-dust disks that surrounded Jupiter and Saturn during the final stage of their formation are analyzed. The sizes of the disks are determined by the total planetocentric angular momentum of the matter accreted by planets and correspond to the sizes of the orbits of their largest satellites. The mass of the solid component of disks is limited from below by the total mass of the Galilean satellites of Jupiter (no less than 4 × 10²⁶ g) and the mass of the largest Saturnian satellites (1.4 × 10²⁶ g), whereas the mass of the gaseous component is limited from above by the amount of hydrogen and helium that could have been later lost by the disks. Our analysis of the known mechanisms of dissipation of gas showed that its simultaneous content in the disks relative to the solid component was much lower than the corresponding gas-to-solid ratio in the Sun. A certain amount of solid compounds (including ice) could have been brought into the disks with planetesimals, which had undergone mutual collisions in the neighborhood of giant planets and served as germs of satellites. The bulk of solid matter appears to have been captured into disks when the latter were crossed by smaller and intermediate-sized planetesimals, which then became parts of the satellites.     

Occurrence of central peak craters on the Uranian satellites: Implications for surface structure and comparison with the Jovian and Saturnian systems

Craters with central peaks occur on the Uranian satellites Ariel, Umbriel, Titania, and Oberon; but do not occur on Miranda. The inelastic surface of Miranda is apparently due to the heavy tectonic reworking of its surface. A theory of expansion/contraction is proposed to explain the tectonic history of Miranda. The existence of central peak craters on the four largest satellites of Uranus implies that they have surface strengths similar to those of the Saturnian satellites and silicate bodies of the inner solar system which all have central peak craters. The absence of central peak craters on Miranda implies that it has an inelastic surface similar to those of the Jovian ice satellites Ganymede and Callisto whose surfaces do not contain central peak craters.     

Voyager disk resolved photometry of the Saturnian satellites

Photometric analysis of Voyager images of the medium-sized icy satellites of Saturn shows that their surfaces exhibit a wide range of scattering properties. At low phase angles, Rhea and Dione closely follow lunar behavior with almost no limb darkening. Mimas, Tethys, and especially Enceladus shiw significant limb darkening at low phase angles, which suggests multiple scattering is important for their surfaces. A simple photometric function of the form I/F = f(α)Aμ₀/(μ + μ₀) + (1 ? A)μ₀ has been fit to the observations. For normal reflectances <0.6, we find lunar-like scattering properties (A = 1). No satellite's surface can be described by Lambert's Law (A = 0). Dione exhibits the widest albedo variations (about 50%). A longitudinal dark stripe which represents a 15% decrease in albedo is situated near the center of the trailing side of Tethys. A correlation is found between the albedo and color of the satellites: the darker objects are redder. Similarly, darker areas of each satellite are redder. Spectral reflectances of Mimas and Enceladus can be derived for the first time. After the proper calibrations to the Voyager color images are made, it is found that both satellites have remarkably flat spectra into the ultraviolet.     

Mutual phenomena of Jovian satellites

Results of the observations of mutual eclipses of Galilean satellites observed from the Vainu Bappu Observatory during 1985 are presented. Theoretical models assuming a uniform disc, Lambert’s law and Lommel-Seeliger’s law describing the scattering characteristics of the surface of the eclipsed satellite were used to fit the observations. Light curves of the 1E2 event on 1985 September 24 and the 3E1 event on 1985 October 24 observed from VBO and published light curves of the 1E2 event on 1985 September 14, the 3E1 event on 1985 September 26 and the 2E1 event on 1985 October 28 (Arlotet al 1989) were fitted with theoretical light curves using Marquardt’s algorithm. The best fitting was obtained using Lommel-Seeliger’s law to describe the scattering over the surface of Io and Europa. During the fitting, a parameter^δxshift which shifts the theoretical light curve along the direction of relative motion of the eclipsed satellite with respect to the shadow centre, on the sky plane (as seen from the Sun) was determined along with the impact parameter. In absence of other sources like prominent surface features or non perfect sky conditions which could lead to asymmetric light curves,^δxshift would be a measure of the phase correction (Aksnes, Franklin & Magnusson 1986) with an accuracy as that of the midtime. Heliocentric Δα cos (δ) and gDδ at mid times derived from fitted impact parameters are reported     

The galilean satellites and the Jovian magnetic field

Alfvén and Arrhenius have proposed that satellites may form from the condensation of plasma suspended in the dipole magnetic field of a central body. Further, they predict that the condensed material will appear at an orbit distance of two-thirds the distance of the plasma. Recent spacecraft measurements have defined the radial extent of the stable quasi-dipole Jovian magnetic field. In support of the Alfvén and Arrhenius hypothesis, we note that all the Galilean satellites lie within two-thirds of this distance.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978.     

High-resolution CASSINI-VIMS mosaics of Titan and the icy Saturnian satellites

The Visual Infrared Mapping Spectrometer (VIMS) onboard the CASSINI spacecraft obtained new spectral data of the icy satellites of Saturn after its arrival at Saturn in June 2004. VIMS operates in a spectral range from 0.35 to 5.2 μm, generating image cubes in which each pixel represents a spectrum consisting of 352 contiguous wavebands.As an imaging spectrometer VIMS combines the characteristics of both a spectrometer and an imaging instrument. This makes it possible to analyze the spectrum of each pixel separately and to map the spectral characteristics spatially, which is important to study the relationships between spectral information and geological and geomorphologic surface features.The spatial analysis of the spectral data requires the determination of the exact geographic position of each pixel on the specific surface and that all 352 spectral elements of each pixel show the same region of the target. We developed a method to reproject each pixel geometrically and to convert the spectral data into map projected image cubes. This method can also be applied to mosaic different VIMS observations. Based on these mosaics, maps of the spectral properties for each Saturnian satellite can be derived and attributed to geographic positions as well as to geological and geomorphologic surface features. These map-projected mosaics are the basis for all further investigations.     

Multicolor photometry of outer Jovian satellites

Multicolor photometry was obtained of satellites J6 Himalia, J7 Elara, and J10 Lysithea in the prograde cloud of outer Jovian satellites, and of J8 Pasiphae, J9 Sinope, and J11 Carme in the retrograde cloud. Our data for J9 are fragmentary; otherwise, the satellites all look like C-class asteroids, except J11, which shows a remarkable brightness in the ultraviolet. The absence of D-class spectra among the outer Jovian satellites suggests that they were not derived from the same population as the outer-belt and Trojan asteroid populations.     

Regions of possible motions for new Jovian satellites

We present the results of our simulation and study of the regions of possible motions for 46 newly discovered Jovian satellites. We show that the orbits of some satellites (such as S/2003 J02, S/2003 J03, S/2003 J04, S/2003 J10, S/2003 J12, and S/2003 J23) presently cannot yet be determined with an acceptable accuracy for planning observations, because the amount of observational information is insufficient.     

Ground-based observations of the Jovian ring and inner satellites

Ground-based 0.9-μm observations of the Jovian ring and inner satellites are reported. The ring observations substantially confirm those obtained by the Voyager spacecraft. The first ground-based detection of 1979J2 suggests a geometric albedo of ～0.10 and a new value for its orbit period of 16 hr 11 min 23.5 ± 0.5 sec.     

Ion irradiation experiments and nitrogen bearing species on Jovian and Saturnian icy surfaces

Implantation of reactive ions into targets of planetary interest is a relevant subject to be studied in the laboratory. It could in fact produce new molecular species that are not native to those surfaces. Presented here are new laboratory results obtained by nitrogen implantation (15–30 keV N⁺) on frozen mixtures of H₂O:CH₄ (2:1). These species have been chosen in view of their possible presence on the surface of Jovian and Saturnian satellites and rings. In fact these surfaces are exposed to intense irradiation by magnetospheric and/or solar energetic particles. The laboratory investigation utilizes IR spectroscopy. The main objectives of the present study are to identify newly produced species and to verify if these (or at least if the profile of their IR bands) are different from those produced by unreactive ions impinging on targets in which nitrogen is already present, occurring in the form of frozen NH₃ (Strazzulla and Palumbo, 1998) or N₂ (Palumbo et al., 1999). I find that CN-bearing group is in fact formed and its IR feature has a profile (peak position and band profile) that differs from that obtained after irradiation or frozen gases containing nitrogen. The relevance the results might have to elucidate the origin of some species observed on Jovian icy moons or predicted to be observed on Saturnian satellites are outlined.     

Mapping of the icy Saturnian satellites: First results from Cassini-ISS

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.     

Occurrence of central peak craters on the Saturnian satellites: Implications for surface structure

The presence of craters with central peaks on the ice satellites of Saturn implies that their surface elastic strength is comparable to that of the Moon, Mars, and Mercury which have central peak craters, rather than that of the Jovian ice satellites Ganymede and Callisto which do not have central peak craters.     

Evolutionary characteristics of the orbits of outer Saturnian,Uranian, and Neptunian satellites

We present the results of our systematic study of the long-period orbital evolution of all of the outer Saturnian, Uranian, and Neptunian satellites known to date. The plots of the orbital elements against time give a clear idea of the pattern of the orbital evolution of each satellite. The tabular data allow us to estimate the basic parameters of the evolving orbits, including the ranges of variation in the semimajor axes, eccentricities, and ecliptical inclinations as well as the variation periods and mean motions of the arguments of pericenters and the longitudes of the nodes. We compare the results obtained by numerically integrating the rigorous equations of the perturbed motion of the satellites with the analytical and numerical-analytical results. The satellite orbits with a librational pattern of variation in the arguments of pericenters are set apart.     

Radii and Albedos of four Trojan asteroids and Jovian satellites 6 and 7

Thermal radiation has been detected from four asteroids of the Trojan group, and J6 and J7, the brightest of the outer satellites of Jupiter. The six objects all have exceedingly low geometric albedos of 0.02 or 0.03 according to calculations based on their known visual brightness and the measured thermal fluxes. 624 Hektor, the largest object studied here, has a radius of 110 ± 20 km, though the exact shape of this body is in question. While the sample observed in this work is small (the total number of Trojans larger than 0.25 km in radius is about 1000), the fact that all four studied have similarly low albedos suggests that this property is characterisic of the Trojans and at least two of the outer members of Jupiter's retinue of satellites. The low surface albedo of the Trojans may preclude the proposed origin of the Jovian group of comets among these bodies according to E. Rabe. Updated tables of the dimensions of all the Jovian satellites are given.     

Comparison of central pit craters on Mars,Mercury, Ganymede,and the Saturnian satellites

We report on the first results of a large‐scale comparison study of central pit craters throughout the solar system, focused on Mars, Mercury, Ganymede, Rhea, Dione, and Tethys. We have identified 10 more central pit craters on Rhea, Dione, and Tethys than have previously been reported. We see a general trend that the median ratio of the pit to crater diameter (D_p/D_c) decreases with increasing gravity and decreasing volatile content of the crust. Floor pits are more common on volatile‐rich bodies while summit pits become more common as crustal volatile content decreases. Uplifted bedrock from below the crater floor occurs in the central peak upon which summit pits are found and in rims around floor pits, which may or may not break the surface. Peaks on which summit pits are found on Mars and Mercury share similar characteristics to those of nonpitted central peaks, indicating that some normal central peaks undergo an additional process to create summit pits. Martian floor pits do not appear to be the result of a central peak collapse as the median ratio of the peak to crater diameter (D_pk/D_c) is about twice as high for central peaks/summit pits than D_p/D_c values for floor pits. Median D_pk/D_c is twice as high for Mars as for Mercury, reflecting differing crustal strength between the two bodies. Results indicate that a complicated interplay of crustal volatiles, target strength, surface gravity, and impactor energy along with both uplift and collapse are involved in central pit formation. Multiple formation models may be required to explain the range of central pits seen throughout the solar system.     

The Galilean satellites as a source of CO in the Jovian upper atmosphere

Material from the Galilean satellites of Jupiter ejected by energetic particles in the Jovian magnetosphere may provide large sources of oxygen to the parent planet. Formation of a CO molecule is the ultimate fate of an oxygen atom in the upper Jovian atmosphere. This high altitude source of CO supports Beer and Taylor's (1978, Astrophys. J.221) observations and analysis, provided that the globally averaged O atom input flux is ～10⁷ cm^?2 sec^?1 and the eddy diffusion coefficient at the tropopause is ～10³ cm² sec^?1. Implications for the possible presence of other atoms and molecules derived from the satellites are discussed.     

Fluffy layers obtained by ion bombardment of frozen methane: Experiments and applications to Saturnian and Uranian satellites

We present experimental results on some physical properties of thick organic residues obtained by bombarding frozen methane with 1.5 MeV protons. After proton fluences of ～1E + 16 protons cm^?2 the synthesized layers appear to be amorphous and fluffy, and to have low density. Their IR transmission spectrum (2.5–10 μm) is typical of long-chain polymerlike substances. At higher fluences (～1E + 17 protons cm^?2) the residues evolve into a carbonlike dark material, the density as well as the stoichiometric ratio C:H increases and the IR features are decreased in strengths. Their reflectance spectrum (0.6?2.5 μm) resembles that of charcoal. We suggest that the new materials can be present on/in the surfaces of the Uranian satellites, of Hyperion, and the dark side of lapetus. We show in fact that they could be synthesized in large quantities during the T Tau phase of the Sun when a copious emission of mega-electronvolt protons is plausible. This assumes that Saturnian and Uranian satellites were partially methane covered in the first evolutive stage of the Solar System.     

An explanation for the light curve of Jupiter's and Saturn's satellites

Several satellites of Jupiter and Saturn show an asymmetric reflectance between the leading hemisphere (which is generally brighter for the inner satellites of both systems) and the trailing one (which is brighter for the outer satellites Callisto and Iapetus). In order to seek a unified explantation of these observational data we assume that, during the final phase of the satellite accumulation process, the surfaces were subjected to a heavy meteoroidal bombardment by the residual bodies in the circumplanetary protosatellite swarms. With suitable hypotheses about the orbital elements of these bodies, the resulting collision rate is anisotropic in an opposite way for inner and outer satellites, with a difference between the two hemispheres of the order of 10–20% for all satellites except Iapetus (for which the anisotropy is larger). We conclude that the model can qualitatively account for the observed effect, even if it is difficult to propose a detailed mechanism for changing the albedo properties of the satellite surfaces by means of meteoroidal collisions.