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The Jovian and Saturnian regular satellites are believed to have formed within circumplanetary disks of gas and solids produced during the end stages of nebular gas inflow to the planets [...], The origin of Saturn's rings has not been adequately explained. The current rings are more than 90 to 95 per cent water ice (1), which implies that initially they were almost pure ice because they are continually polluted by rocky meteoroids (2). In contrast, a half-rock, half-ice mixture (similar to the composition of many of the satellites in the outer Solar System) would generally be expected. Previous ring origin theories invoke the collisional disruption of a small moon (3,4), or the tidal disruption of a comet during a close passage by Saturn (5). These models are improbable and/or struggle to account for basic properties of the rings, including their icy composition. Saturn has only one large satellite, Titan, whereas Jupiter has four large satellites; additional large satellites probably existed originally but were lost as they spiralled into Saturn (6). Here I report numerical simulations of the tidal removal of mass from a differentiated, Titan-sized satellite as it migrates inward towards Saturn. Planetary tidal forces preferentially strip material from the satellite's outer icy layers, while its rocky core remains intact and is lost to collision with the planet. The result is a pure ice ring much more massive than Saturn's current rings. As the ring evolves, its mass decreases and icy moons are spawned from its outer edge (7) with estimated masses consistent with Saturn's ice-rich moons interior to and including Tethys.

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2006.03.005 Byline: Matija Auk, Brett J. Gladman Keywords: Origin; Solar System; Celestial mechanics; Resonances; Satellites of Saturn; Satellites of Uranus Abstract: The passage of Jupiter and Saturn through mutual 1:2 mean-motion resonance has recently been put forward as explanation for their relatively high eccentricities [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461] and the origin of Jupiter's Trojans [Morbidelli, A., Levison, H.F., Tsiganis, K., Gomes, R., 2005. Nature 435, 462-465]. Additional constraints on this event based on other small-body populations would be highly desirable. Since some outer satellite orbits are known to be strongly affected by the near-resonance of Jupiter and Saturn ('the Great Inequality'; Auk, M., Burns, J.A., 2004b. Astron. J. 128, 2518-2541), the irregular satellites are natural candidates for such a connection. In order to explore this scenario, we have integrated 9200 test particles around both Jupiter and Saturn while they went through a resonance-crossing event similar to that described by Tsiganis et al. [Tsiganis, K., Gomes, R., Morbidelli, A., Levison, H.F., 2005. Nature 435, 459-461]. The test particles were positioned on a grid in semimajor axes and inclinations, while their initial pericenters were put at just 0.01 AU from their parent planets. The goal of the experiment was to find out if short-lived bodies, spiraling into the planet due to gas drag (or alternatively on orbits crossing those of the regular satellites), could have their pericenters raised by the resonant perturbations. We found that about 3% of the particles had their pericenters raised above 0.03 AU (i.e. beyond Iapetus) at Saturn, but the same happened for only 0.1% of the particles at Jupiter. The distribution of surviving particles at Saturn has strong similarities to that of the known irregular satellites. If saturnian irregular satellites had their origin during the 1:2 resonance crossing, they present an excellent probe into the early Solar System's evolution. We also explore the applicability of this mechanism for Uranus, and find that only some of the uranian irregular satellites have orbits consistent with resonant pericenter lifting. In particular, the more distant and eccentric satellites like Sycorax could be stabilized by this process, while closer-in moons with lower eccentricity orbits like Caliban probably did not evolve by this process alone. Author Affiliation: Department of Physics and Astronomy, University of British Columbia, 6224 Agricultural Rd., Vancouver, B.C. V6T 1Z1, Canada Article History: Received 6 December 2005; Revised 28 February 2006

A time-dependent one-dimensional model of Saturn's ionosphere has been developed as an intermediate step towards a fully coupled Saturn Thermosphere-Ionosphere Model (STIM). A global circulation model (GCM) of the thermosphere provides the latitude and local time dependent neutral atmosphere, from which a globally varying ionosphere is calculated. Four ion species are used ([H.sup.+], [H.sup.+.sub.2], [H.sup.+.sub.3], and [He.sup.+]) with current cross-sections and reaction rates, and the SOLAR2000 model for the Sun's irradiance. Occultation data from the Voyager photopolarimeter system (PPS) are adapted to model the radial profile of the ultraviolet (UV) optical depth of the rings. Diurnal electron density peak values and heights are generated for all latitudes and two seasons under solar minimum and solar maximum conditions, both with and without shadowing from the rings. Saturn's lower ionosphere is shown to be in photochemical equilibrium, whereas diffusive processes are important in the topside. In agreement with previous 1-D models, the ionosphere is dominated by [H.sup.+] and [H.sup.+.sub.3], with a peak electron density of ~[10.sup.4] electrons [cm.sup.-3]. At low- and mid-latitudes, [H.sup.+] is the dominant ion, and the electron density exhibits a diurnal maximum during the mid-afternoon. At higher latitudes and shadowed latitudes (smaller ionizing fluxes), the diurnal maximum retreats towards noon, and the ratio of [[H.sup.+]]/[[H.sup.+.sub.3]] decreases, with [H.sup.+.sub.3] becoming the dominant ion at altitudes near the peak (~ 1200-1600 km) for noon-time hours. Shadowing from the rings leads to attenuation of solar flux, the magnitude and latitudinal structure of which is seasonal. During solstice, the season for the Cassini spacecraft's encounter with Saturn, attenuation has a maximum of two orders of magnitude, causing a reduction in modeled peak electron densities and total electron column contents by as much as a factor of three. Calculations are performed that explore the parameter space for charge-exchange reactions of [H.sup.+] with vibrationally excited [H.sub.2], and for different influxes of [H.sub.2]O, resulting in a maximum diurnal variation in electron density much weaker than the diurnal variations inferred from Voyager's Saturn Electrostatic Discharge (SED) measurements. Peak values of height-integrated Pedersen conductivities at high latitudes during solar maximum are modeled to be ~42 mho in the summer hemisphere during solstice and ~18 mho during equinox, indicating that even without ionization produced by auroral processes, magnetosphere-ionosphere coupling can be highly variable. Keywords: Ionospheres; Planetary rings; Saturn; Saturn, atmosphere

We present near-infrared (1.24-2.26 [micro]m) images of Saturn's E and G rings which were taken with the W.M. Keck telescope in 1995 August 9-11, during the period that Earth crossed Saturn's ring plane. Our data confirm that the E ring is very blue. Its radial and vertical structure are found to be remarkably similar to that apparent in the HST ringplane crossing data at visible wavelengths, reinforcing models of the ring's peculiar narrow or very steep particle size distribution. Our data show unambiguously that the satellite Tethys is a secondary source of material for the E ring. The G ring is found to be distinctly red, similar in color to Jupiter's main ring, indicative of a (more typical) broad particle size distribution. Keywords: Infrared observations; Planetary rings; Saturn

We have obtained numerically integrated orbits for Saturn's coorbital satellites, Janus and Epimetheus, together with Saturn's F-ring shepherding satellites, Prometheus and Pandora. The orbits are fit to astrometric observations acquired with the Hubble Space Telescope and from Earth-based observatories and to imaging data acquired from the Voyager spacecraft. The observations cover the 38 year period from the 1966 Saturn ring plane crossing to the spring of 2004. In the process of determining the orbits we have found masses for all four satellites. The densities derived from the masses for Janus, Epimetheus, Prometheus, and Pandora in units of [gcm.sup.-3] are [0.61.sup.+0.07.sub.-0.06], [0.64.sup.+0.13.sub.-0.09], [0.42.sup.+0.27.sub.-.013], and [0 54.sup.+0.30.sub.-0.15], respectively. Keywords: Satellites, general; Satellites of Saturn; Orbits

All of the large impact features of the middle-sized icy satellites of Saturn and Uranus that were clearly observed by the Voyager spacecraft are described. New image mosaics and stereo-and-photoclinometrically-derived digital elevation models are presented. Landforms related to large impact features, such as secondary craters and possible antipodal effects are examined and evaluated. Of the large impacts, Odysseus on Tethys appears to have had the most profound effect on its 'target' satellite of any of the impact features we examined. Our modeling suggests that the Odysseus impact may have caused the prompt formation of Ithaca Chasma, a belt of tectonic troughs that roughly follow a great circle normal to the center of Odysseus, although other hypotheses remain viable. We identify probable secondary cratering from Tirawa on Rhea. We attribute a number of converging coalescing crater chains on Rhea to a putative, possibly relatively fresh, ~350 km-diameter impact feature. We examine the antipodes of Odysseus, the putative ~350 km-diameter Rhean impact feature, and Tirawa, and conclude that evidence from Voyager data for damage from seismic focusing is equivocal, although our modeling results indicate that such damage may have occurred. We propose a number of observations and tests for Cassini that offer the opportunity to differentiate among the various explanations and speculations reviewed and evaluated in this study. Keywords: Satellites of Saturn; Satellites of Uranus; Impact processes; Surfaces, satellite

Two-dimensional model calculations (altitude and solar zenith angle) are performed to investigate the impact of electron chemistry on the composition and structure of Io's atmosphere. The calculations are based upon the model of Wong and Smyth (2000, Icarus 146, 60-74) for Io's S[O.sub.2] sublimation atmosphere with the addition of new electron chemistry, where the interactions of the electrons and neutrals are treated in a simple fashion. The model calculations are presented for Io's atmosphere at western elongation (dusk ansa) for both a low-density case (subsolar temperature of 113 K) and a high-density case (subsolar temperature of 120 K). The impact of electron-neutral chemistry on the composition and structure of Io's atmosphere is confined primarily to an interaction layer. The penetration depth of the interaction layer is limited to high altitudes in the thicker dayside atmosphere but reaches the surface in the thinner dayside and/or nightside atmosphere at larger solar zenith angles. Within most of the thicker dayside atmosphere, the column density of S[O.sub.2] is not significantly altered by electrons, but in the interaction layer all number densities are significantly altered: S[O.sub.2] is reduced, O, SO, S, and [O.sub.2] are greatly enhanced, and O, SO, and S become comparable to S[O.sub.2] at high altitudes. For the thinner nightside atmosphere, the species number densities are dramatically altered: S[O.sub.2] is drastically reduced to the least abundant species of the S[O.sub.2] family, SO and [O.sub.2] are significantly reduced at all altitudes, and O and S are dramatically enhanced and become the dominant species at all altitudes except near the surface. The interaction layer also defines the location of the emission layer for neutrals excited by electron impact and hence determines the fraction of the total neutral column density that is visible in remote observation. Electron chemistry may also impact the ratio of the equatorial to polar S[O.sub.2] column density deduced from Lyman-[alpha] images and the north-south alternating and System III longitude-dependent asymmetry observed in polar O and S emissions. eywords: Io; Satellites, atmospheres; Atmospheres, structure; Atmospheres, composition; Satellites of Jupiter

Lithospheric dilation on Europa has occurred at ridges, bands, and various hybrid lineaments on a global scale over a large part of the geological age of the surface. Dilational ridges (Class 2 in the R. Greenberg et al. (1998, Icarus 135, 64-78) taxonomy) are elevated, are usually a few kilometers across, and may have a lineated or hummocky interior and a pronounced medial groove. Bands are lower and usually wider than Class 2 ridges, and may have a lineated interior with no prominent medial groove. Some lineaments have characteristics of both ridges and bands. The character of Class 2 ridges, bands, and hybrid forms suggests that they are dilational gaps in the lithosphere, filled from below, and that they constitute a morphological continuum with Class 2 ridges and bands as end-members. These relationships may be explained by a model in which external forcing superimposes a secular dilation on the tidal cycle that opens and closes cracks each Europan day, resulting in incomplete closure with accumulation and possible extrusion of new ice fill. Where the lineament ultimately falls on the morphological continuum--especially how much it is elevated above ambient terrain--depends upon the ratio of daily secular dilation to the amplitude of the cyclic tidal separation. We call this ratio the 'dilation quotient.' Changes in the dilation quotient during the active life of the lineament will create variable lineament forms. One driver for dilation is tidal 'walking' of strike-slip faults, which dilates linked nonparallel cracks. That process is prominent in the 800-km-long strike-slip fault Astypalaea Linea. A subsurface liquid water ocean allows the decoupling needed for horizontal displacements and is the source for the ice that fills the dilated lineaments. Key Words: Europa; tectonics; satellites of Jupiter; satellite; surfaces.

The compensation state of 49 intermediate size (120 to 600 km diameter) lunar craters are investigated using the most recent spherical harmonic models of the lunar topography and gravity, truncated at degree n = 110. The total mass anomalies per unit area (i.e., the lateral variations of the vertically integrated density perturbations per unit area) within an otherwise uniform crust of 60 km thickness are determined such that, together with the surface topography, give rise to the model gravity anomalies. Crustal thicknesses of 40 and 80 km are also considered, but the general results of this study are not significantly affected. Excess mass anomalies are obtained by subtracting from the total mass anomalies the mass anomalies that are required for the isostatic compensation of the surface topography. The excess mass anomaly of a crater denotes its particular state of compensation. Dependencies of the excess mass anomalies on crater location, size, and age are investigated, but in general few discernable trends are evident. Although the vast majority of craters indicate some compensation, no correlation exists between age or size and the state of compensation. Roughly 16% of the craters show no compensation, and in some cases have mass deficiencies most likely due to the shock fractured bedrock: the breccia lens of lower density. The crust in these regions was likely cold and rigid enough at the time of impact to rigidly support the stress caused by crater excavation. These features are seen throughout different geological periods, demonstrating that the lunar crust cooled quickly and strengthened soon after formation. A comparison of the compensation state of craters Apollo, Korolev, and Hertzsprung suggests that the thermal and mechanical properties of the crust prior to impact had an appreciable effect on the compensation, and that crustal thickness may be the single most important factor controlling the compensation of intermediate size craters. The characteristics of the excess mass anomaly profiles of the eight well-known near side mascon basins are used to identify new mascon-like craters. Ten newly found mascons are confirmed: Humboldtianum, Moscoviense, Mendel-Rydberg, Lorentz, Hertzsprung, Korolev, Schrodinger, Freundlich-Sharonov, Coulomb-Sarton, and Schiller-Zucchius, while two more, Deslandres and Dirichlet-Jackson, are very plausible. These results show that mare flow is not necessarily required to produce mascon-like characteristics. Key Words: Moon; crater compensation; lunar mascons; lunar crust.