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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

Using adaptive optics on the W. M. Keck II telescope, we imaged Titan several times during 1999 to 2001 in narrowband near-infrared filters selected to probe Titan's stratosphere and upper troposphere. We observed a bright feature around the south pole, possibly a collar of haze or clouds. Further, we find that solar phase angle explains most of the observed east-west brightness asymmetry of Titan's atmosphere, although the data do not preclude the presence of a 'morning fog' effect at small solar phase angle. Key Words: Titan; satellites; atmospheres; infrared observations.

The near-infrared reflectance spectrum of Saturn's satellite Phoebe shows a broad absorption band at 2.0 [[micro]meter] and absorption at [Lambda] [greater than] 2.2 [[micro]meter], both characteristic of [H.sub.2]O ice. We have successfully modeled the surface of Phoebe with an intimate (granular) mix of [H.sub.2]O ice (3% by weight, grain size 500 [[micro]meter]) mixed with fine grains of [H.sub.2]O ice (0.25%) with amorphous carbon (grain size 900 [[micro]meter]) as the dominant component. This model reproduces the shape of the measured spectrum and the observed albedo of 0.10 for Phoebe, but it is not unique. The presence of ice establishes Phoebe as an original member of the outer Solar System rather than a renegade asteroid. Key Words: Phoebe; water ice; Kuiper Belt; irregular satellites.

New 4800-7200 [Angstrom] spectra of Saturn's rings, and the icy saturnian satellites Enceladus, Tethys, Dione, Rhea, and Iapetus, do not show the weak 5773 and 6275 [Angstrom] absorption bands due to high-density condensed 02 that are seen, with a maximum depth of 1.8%, on Ganymede's trailing side. The 5773 [Angstrom] band depth must be [less than or equal to]0.6% (and very probably [less than or equal to]0.3%) on the rings, Tethys, Dione, Rhea, and the trailing side of Iapetus, and [less than or equal to] 1.2% on the trailing side of Enceladus. The lack of detectable absorption does not rule out abundant [O.sub.2] molecules that are separated too well to produce these absorption bands. The observations show that the presence of [O.sub.3], recently detected on Rhea and Dione (Noll et al. 1997b, Nature 388, 45-47), does not require the presence of detectable quantities of high-density [O.sub.2]. I discuss the implications of the new observations for models of [O.sub.2] formation on Ganymede. Key Words: Satellites of Saturn; spectroscopy; satellites of Jupiter; radiation chemistry; surfaces, satellite.

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 [approximately]1 MHz, compared with around 3.1 for pure water ice. The loss tangents of ammonia-rich ices seem somewhat ([approximately]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, Icarus 86, 336-354) and Maetzler (1998, in Solar System Ices (B. Schmitt, C. DeBergh, and M. Festou, Eds.), pp. 241-257, Kluwer Academic, Dordrecht), but considerably higher than models by Chyba et 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.

The first near-infrared (0.9 [[micro]meter]) photometric observations of Saturn's five medium-sized satellites were obtained during the August 1995 ring plane crossing with Palomar Observatory's 60-in. telescope. The albedos of the satellites are very high, indicating that a very low fraction of dark opaque contaminants is present on their icy surfaces. The geometric albedos for the leading sides of the satellites are Enceladus: 1.02; Tethys: 0.90; Dione: 0.72; and Rhea: 0.76. The geometric albedo of the trailing side of Mimas is 0.72. The amplitudes of the satellites' rotational light curves are somewhat less than those measured at visible wavelengths. They range from 0.40 (Dione) to 0.10 (Rhea) to 0.05 (Tethys). Both Enceladus and Mimas are brighter on the trailing side, unlike the other three satellites. These results suggest that the optical properties of all five satellites are determined by interactions with Saturn's E-ring.

Titan, Saturn's largest moon, was found to have a thick atmosphere containing nitrogen and methane. Methane was almost saturated at the moon\s tropopause, which led to the hypothesis that it may experience a methane analogue of the Earth's hydrological cycle. However recent data suggests large areas of super-saturated methane, which indicates a dry and stagnant environment. Near-infrared spectroscopic observations detected significant flux enhancements, indicating the presence of reflective methane condensation clouds.

Evidence for a marginal detection of the Titan ionosphere has been obtained froma new analysis of the dual-frequency Doppler data recorded during the Voyager 1occultation in 1980. The original report by Lindal et al. (1983, Icarus 53,348-363) gave only upper bounds on the peak electron density of 3000 cm 3 duringingress (evening terminator) and 5000 [cm.sup.-3] during egress (morningterminator). The dual-frequency ingress data imply a maximum electron density of2400 [+ or -] 1100 [cm.sup.-3] for Titan's upper ionosphere at an altitude of 1180[+ or -] 150 km. The egress data were determined to be of limited use for thisanalysis because the X-band signal was received for only a few seconds.Nevertheless, a distinct ionospheric peak is revealed in the S-band data for bothingress and egress. The height and peak density of this ionized layer are in goodagreement with expectations from numerical models that invoke photoionization andenergetic electron impacts. Key Words: Titan; radio observations; planetary ionospheres.

Images of Titan were acquired with the ADONIS adaptive optics system installed at the ESO 3.6-m telescope in La Silla (Chile). The near-infrared range from 1-2.5 [[micro]meter] was covered in 1994 and 1995 in the methane atmospheric windows at 1.3, 1.6, and 2.0 [[micro]meter] with narrow-band filters and CVF. Diffraction-limited images were obtained (0.14 arcsec at 2 [[micro]meter]), thus allowing us to resolve the Titan disk. We have corrected the images for systematic effects, removed center-to-limb variation, and applied deconvolution processes to our data. The contrast on the images is on the order of 30%. Both the leading and trailing hemispheres were observed. The 2-[[micro]meter] images presented in this paper show a broad bright feature on the surface of Titan's leading hemisphere (near 114 [degrees] LCM), which is in agreement with the HST images and with spectroscopy performed at these longitudes from the ground that show a higher geometric albedo on Titan's leading side. High-quality flat fielding of our data has highlighted the presence of at least two main bright spots within the equatorial region. The trailing hemisphere of Titan's surface is not completely dark but shows brightness near the poles, with some north-to-south asymmetry. The northern latitudes are uniformly brighter than the lower equatorial and southern latitudes (factor of about 2). Our results bring new constraints on the models of the satellite's surface.

It is argued that Hyperion, an irregularly shaped distant Saturn satellite, may act as a reasonably effective source of dust in the saturnian system. Several mechanisms (e.g., hyper-velocity impacts of interplanetary grains or dust particles coming from the outermost saturnian moon Phoebe) should produce impact ejecta from Hyperion's surface, an appreciable share of which would escape into the planetocentric orbits initially close to that of Hyperion. Three forces of major importance influence the motion of the dust grains: Titan's gravity, solar radiation pressure, and plasma drag force. The first acting alone would lock the particles ejected at small initial speeds in a strong 4:3 mean motion resonance with Titan, so that they would form a stable dust belt around Hyperion's orbit. However, solar radiation pressure and especially plasma drag force effectively destroy the resonant locking in an interval ranging from one to several saturnian years for 10 to 100 [[micro]meter]-sized particles and at an even faster rate for smaller grains. Initial speeds of about 100 m [sec.sup.-1] and greater (measured at the boundary of Hyperion's action sphere) are also sufficient to unlock particles from the resonance stability zone. Once the resonance is broken, the orbits of dust particles become unstable and experience multiple close approaches to Titan. Eventually they can either collide with Titan, spread into the inner part of the saturnian system, or escape to the interplanetary space. Using numerical integrations, we performed a statistical study of the grain trajectories to determine the probabilities of various scenarios and to construct a relative distribution of dust in the Hyperion-Titan system. We found that most of the grains larger than ~5[[micro]meter] will finally collide with Titan, smaller particles down to ~1 to 2 [[micro]meter] will escape out of the system, and still smaller (submicrometer) particles will rapidly collide with Saturn. Of-order estimates of the dust influx to Titan are also made; they show that the upper limit of the income rate of the Hyperion particles is comparable with the direct influx of interplanetary grains.

The structure and morphological properties of polymers produced photochemically from the UV irradiation of cyanoacetylene and cyanoacetylene mixtures have been examined to evaluate their possible contribution to the haze layers found on Titan. A structural analysis of these polymers may contribute to our understanding of the data returned from the Huygens probe of the Cassini mission that will pass through the atmosphere of Titan in the year 2004. Infrared analysis, elemental analysis, and thermal methods (thermogravimetric analysis, thermolysis, pyrolysis) were used to examine structures of polycyanoacetylenes produced by irradiation of the gas phase H[C.sub.3]N at 185 and 254 nm. The resulting brown to black polymer, which exists as small particles, is believed to be a branched chain of conjugated carbon-carbon double bonds, which, on exposure to heat, cyclizes to form a graphitic structure. Similar methods of analysis were used to show that when H[C.sub.3]N is photolyzed in the presence of Titan's other atmospheric constituents (C[H.sub.4], [C.sub.2][H.sub.6], [C.sub.2][H.sub.2], and CO), a copolymer is formed in which the added gases are incorporated as substituents on the polymer chain. Of special significance is the copolymer of H[C.sub.3]N and acetylene ([C.sub.2][H.sub.2]). Even in experiments where [C.sub.2][H.sub.2] was absorbing nearly all of the incident photons, the ratio of [C.sub.2][H.sub.2] to H[C.sub.3]N found in the resulting polymer was only 2:1. Scanning electron microscopy was used to visually examine the polymer particles. While pure polyacetylene particles are amorphous spheres roughly 1 [[micro]meter] in diameter, polycyanoacetylenes appear to be strands of rough, solid particles slightly smaller in size. The copolymer of H[C.sub.3]N and [C.sub.2][H.sub.2] exhibits characteristics of both pure polymers. This is particularly important as pure polyacetylenes do not match the optical constants measured for Titan's atmospheric hazes. The copolymers produced by the incorporation of other minor atmospheric constituents, like H[C.sub.3]N, into the polyacetylenes are expected to have optical constants more comparable to those of the Titan haze.

New measurements of Titan's hemispheric brightness asymmetry from HST images from 260 to 1040 nm show that the contrast is strongest near 450 nm (blue) and, with the opposite sign, at 889 nm (methane band). Comparison with a full Titan year of disk-integrated albedo data indicates that the seasonal cycle in asymmetry is smooth, but is insufficient to explain the variation in albedo, and a twice-per-year global albedo enhancement 50% larger than the hemispheric asymmetry amplitude is required, as noted by other workers. We also report measurements of limb-darkening (strongest at red wavelengths) and note that the transition zone between the 'hemispheres' lies in the northern, brighter hemisphere at present. Comparison of models to the HST data set indicates that a change in aerosol number density above 70 km, and largely below 120 km, is the likely mechanism and is probably driven by aerosol transport by meridonal and vertical winds.

We report new observations of the (1-0) line of HCN at 88.63 GHz, emitted by Titan, made in May 1995 with the IRAM 30-m radiotelescope located at Pico Veleta (Spain). Compared to previous observations of Tanguy et al. (1990, Icarus 85, 43-57), the signal-to-noise ratio has been improved by a factor of 5, permitting us to retrieve the vertical distribution of HCN in the 80-350 km altitude range. Using the production rate of HCN predicted from photochemical models, we derive a profile of the eddy diffusion coefficient up to 350 km altitude consistent with the observed HCN vertical distribution. This coefficient increases with height more rapidly than the standard [n.sup.-0.5] dependence in the stratosphere. The (4-3) line of [H.sup.13]CN at 345.34 GHz was subsequently observed with the JCMT on Mauna Kea (Hawaii) in June 1995. The analysis of the two spectra yields a 12C/13C ratio between 70 and 120, consistent with the terrestrial value. Taking into account possible fractionation processes, we show that the C-bearing species which formed Titan should have had a carbon isotopic ratio in the same range.

Recent spectroscopic observations of the saturnian system by the Hubble Space Telescope have shown the surprising results that the icy satellites and the rings are copious sources of neutral gas as a result of magnetospheric particle sputtering and meteoroid bombardment. Not only are the composition and structure of the saturnian magnetosphere significantly influenced by the continuous injection of water-group neutrals and ions, but the dynamics and mass distribution of the main ring as well as the tenuous E ring are also subject to the physical processes occurring in this unique plasma-dust-gas complex. By coupling the orbital motion of the neutral gas cloud with the photochemical effects and plasma chemistry, a numerical scheme is developed to analyze the possible contributions from different source mechanisms to the OH emission detected by the Hubble Space Telescope. It is found that, even if the maximum ion sputtering rate estimated by M. Shi, A. Baragiola, D. E. Grosjean, R. E. Johnson, S. Jurac, and J. Schou (1995, J. Geophys. Res. 100, 26387-26395) is used, the number density of the hydroxyl molecules still falls short by a factor of about 2. The discrepency can be partially elevated if additional source from meteoroid-impact vapor production at Enceladus is introduced. The collisional interaction of Enceladus with the E-ring particles of micron size first proposed by D. P. Hamilton and J. A. Burns (1993, Nature, 365, 498) is of particular promise. One corollary of the present model consideration is that the impact of the E-ring particles in highly eccentric orbits with the A ring could lead to a dense ring atmosphere.

Six previously unseen Pan wakes are found interior and exterior to the Encke gap in Saturn's A ring, one in the Voyager 2 photopolarimeter (PPS) stellar occultation data and five in the Voyager 1 radio science (RSS) earth occultation data. Pan orbits at the center of the Encke gap and maintains it. Originally it was hypothesized that a wake would be completely damped by the time it reached a longitude of 360 [degrees] relative to Pan. However, five of the six newly detected wakes are at longitudes in excess of 360 [degrees] and are a result of earlier encounters with Pan. The sixth is the first detection of the RSS outer Pan wake. The new PPS inner wake is at a longitude of 389.8 [degrees]. The new RSS inner wakes are at longitudes of 519.4 [degrees] [+ or -] 1.6 [degrees] and 879.4 [degrees] [+ or -] 1.6 [degrees]. The RSS outer wakes are at longitudes of 200.6 [degrees] [+ or -] 1.6 [degrees], 560.6 [degrees] [+ or -] 1.6 [degrees], and 920.6 [degrees] [+ or -] 1.6 [degrees]. Because of the time needed for a wake to develop after encountering Pan, the higher order wakes (longitude > 360 [degrees]) can be more prominent than their lower order counterparts which are superimposed at the same location. The radial dispersion behavior of the Pan wakes are characterized using a Burg autoregressive power spectral algorithm. The wake radial wavelength behavior is compared to a simple model which ignores collisions and self-gravity. The four wakes with longitudes below 360 [degrees] show an average deviation of 0-3% from the predicted wavelengths, indicative of the strength of collective effects. The detection of Pan wakes at longitudes greater than 360 [degrees] demonstrates that wakes persist for much longer than originally hypothesized and may interact with one another. The presence and characteristics of these wakes will provide an important test of kinetic theory models.

In the frame of a program of systematic studies to determine or reassess the absolute intensities of gas phase organics of interest to Titan, we have experimentally studied the IR spectra in the gas phase of diazomethane, methyl azide, and methyl isocyanide, at room temperature, in the 250-4300 [cm.sup.-1] region. The absolute intensities, [S.sub.v], of the most intense vibrational bands of these compounds were determined experimentally together with the associated uncertainties. In order to estimate the detectability of the selected compounds by infrared spectroscopy in Titan's atmosphere, our data were compared with Voyager IRIS spectra of Titan. Upper limits of, respectively, 5, 5.4, and 1.3 ppb on the mean stratospheric abundances are inferred. These values are consistent with photochemical modeling of Titan's atmosphere. They appear auspicious for possible detection in Titan using upcoming space missions.

A linear mixing model has been used to separate the high-resolution visible/near-infrared reflectance spectrum of the Iapetus dark material from the spectra of the leading and trailing sides of Iapetus. A distinct absorption feature centered at 0.67 [[micro]meter] is apparent in the spectrum. A slight inflection covering 0.4 to 0.6 [[micro]meter] and a change in slope near 0.73 [[micro]meter] suggesting the lower wavelength edge of an absorption are also present. These absorption features are attributed to the 6[A.sub.1] [right arrow] 4[T.sub.2](G) and 6[A.sub.1] [right arrow] 4[T.sub.1](G) charge transfer transitions in minerals such as goethite and hematite that are products of the aqueous alteration of anhydrous silicates. We confirm the strong, positive spectral slope attributed to organic material.

We examine various mechanisms of erosion on Titan and their likely effect on Titan's geomorphology. Specifically we evaluate the solubility of likely crustal ices (water and ammonia) in liquid methane and ethane, and compare these with the solubility of rocks on Earth. Aeolian and diurnal weathering can be neglected, and glacial erosion is unlikely unless the atmosphere has frozen out in the past. Erosion due to rainfall is at most comparable to periglacial environments on Earth. We find that additional laboratory experiments are required if Cassini observations are to be adequately interpreted.

The albedo asymmetry of Iapetus is unique in the Solar System. Many models have been proposed to explain why the leading hemisphere has a reflectance 10-20 times lower than the trailing hemisphere and the poles. Compositional and observational constraints appear to rule out many of these models. Little attempt has been made to explain why, of all the moons in the Solar System, only Iapetus displays such properties. The photometric differences between Phoebe and the dark material on Iapetus, and the expectation that accreted dust from Phoebe would darken the poles more than the antapex of orbital motion on Iapetus, make the Phoebe dust model inconsistent with observations. Other models that instead have Iapetus accreting circumsaturnian material coming from either Hyperion or Iapetus itself are also beset by an inability to explain the bright poles. The endogenous extrusion of dark organics, by itself, is ruled out, since it is extremely unlikely that this model would produce elliptical albedo contours centered on the apex of orbital motion. The modern impact flux does not deliver enough kinetic energy to Iapetus's surface to vaporize significant quantities of water ice and produce a lag deposit. Also, impact vaporization is expected to form a similar lag deposit on the trailing hemisphere. Our most optimistic estimate of the amount of organic material that accumulates on the surface from in situ radiation synthesis of solid organics from methane clathrate is more than sufficient to explain the albedo and spectrum. This process, however, is more effective on the trailing than leading hemisphere, so the reverse asymmetry is expected. Iapetus's asymmetry is best explained by a thick primordial low-albedo subsurface layer of organics exhumed by impact erosion. The layer may be the consequence of ancient thermochemistry in Iapetus's interior, processing methane and/or HCN into dark organic matter which was then extruded over the entire surface. The model requires a thin layer of nearly pure water ice, either exogenous or endogenous, deposited over the dark organics. Subsequent impacts would have preferentially eroded the ice on the leading hemisphere, revealing the underlying dark material. The uniqueness of Iapetus's albedo asymmetry can be understood by its impact-induced erosion rate, size, and formation distance from the Sun and Saturn - a combination of parameters duplicated nowhere else in the Solar System. Tests of these conclusions can be made by Cassini.

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.

The Fourier transform spectrometer at the CFH Telescope in Hawaii was used to observe the near-infrared spectrum of Titan, Saturn's largest moon. The observations, which cover the planet's Greatest Western Elongation and opposite Conjunctions, revealed similar geometric albedos in Titan's 1.28- and 1.075-micrometer methane windows. All in all, they highlight the heterogeneous nature of Titan's surface and the presence of two distinct regions in the spectra of its surface albedo.

The dark material on the dark hemisphere of the Saturnian moon Iapetus originated from exogenous materials from the satellite Phoebe, which released them after being hit by a comet or asteroid. This theory is supported by observations of archived images taken by the Voyager spacecraft, which revealed that the interface between the satellite's light and dark hemispheres is characterized by a gradual change in color and albedo.

Near-infrared spectroscopic observations of Titan's surface through the haze of the satellite reveal that the satellite is brighter near its eastern elongation than near its western elongation. The brightness varies by a factor of 23% at 1:6 micrometers and 32% at 2 micrometers. Variations are caused by the changes in surface albedo and the duration of rotation period. The variation in brightness is recorded as a light-curve in the near-infrared spectra.

The newly developed photochemical model of Titan's atmosphere incorporates all the chemical compounds and their reactions from the surface to an altitude of 1240 kms. The methane photodissociation process, which is not incorporated in earlier models, is included in the new model. The model also computes the solar radiation transfer in the atmosphere of Titan using the Monte Carlo code and provides results that are consistent with the observations of Voyager spacecrafts.

The depth and existence of hydrocarbon oceans in the satellite Titan are constrained by tidal dissipations in the satellite. Tidal dissipations are caused by the presence of a large free eccentricity in the ocean. A hydrodynamical-numerical 2 degree ocean tide model proposes that an ocean consisting of hydrocarbon should be more than 500 meters deep.

One hundred and ninety eight mutual eclipses and occultations will occur between the major satellites of Saturn when the Earth and Sun pass through the ringplane of the planet between January 1995 and December 1996. The data are obtained from photometric observation of occultations and events which occurred in 1979 and 1980.

A group of observable phenomena of Saturnian satellites when they will be eclipsed by the planet during the years 1995 and 1996 are presented. Phenomena such as the eclipse of Saturn and its rings, mutual occultations and mutual eclipses will be observed. During these phenomena, campaigns will be organized and observers will be called for. The data provided by the observers will be incorporated to develop a dynamical model.

A zero net tidal torque is experienced by a satellite on an eccentric orbit in the absence of significant permanent asymmetry. Theoretical calculations show that this period for satellite Titan is 15.82 to 15.93 days, and the value depends on the tidal dissipation model. Radar observations also yield the same results. Radar data and asymmetry constraints show a large highland plateau of water-ice with a diameter of about 18 kilometers. The plateau is surrounded by hydrocarbon ocean.

Determination of temperature dependence of near-infrared spectrums of alpha and beta nitrogen ice shows that spectrums of beta ice have a complex 2.15 micrometer band influenced by temperature. The spectrums consist of two different components. alpha ice has complex spectrums and patterns with weak features which are influenced by temperature. The weak lines are caused by a double phonon transition. Results of the study are applied in the modelling of telescopic spectrums of Triton.

Rototranslational band of collision induced absorption spectrums of N2-CH4 pairs, at frequencies around 550 per centimeter with a temperature range of 70 to 300 kelvin, is calculated using a simple model. Adjustment of classical, multipole induced dipolar components by addition of semi-experimental quantum corrections yields a result in agreement with the latest experimental data. A new induction term, Q6, is incorporated in calculations. This is necessary for the agreement between the results. Temperature dependence of the absorption coefficient as a function of frequency is provided by this method at a temperature of 70 kelvin. The calculation will find applications in infrared opacity of Titan's atmosphere.

The blue and yellow geometric albedo changes observed in the satellite of Titan is simulated using two different Titan haze models. The range of variation in production rate, essential for interpretations of the observations, are obtained from Steady-state calculations. The range was a factor of two to four. Time varying production rate with the same range, used for calculating blue and yellow geometric albedo changes, yields results which are less than the observed results by a factor of 10. This decrease is caused by long haze residence. Functional representation of the production has no influence on the results obtained.

Saturn's moon Titan has a large atmosphere, unlike the other satellites. A thick stratospheric haze hides the surface of Saturn from view. The emission from Titan in the infrared is predominantly nitrogen, hydrogen and methane. A difference was observed in Titan's albedo when the satellite was at eastern and western elongation with respect to Saturn. Although these observations could be taken to show that Titan's surface is heterogenous, they could also be explained by transient clouds. These observations show that Titan is locked in a synchronous orbit around Saturn.

Ring features of planets exhibiting nonaxisymmetry are enhanced by the presence of shepherd satellites. The ring features are studied for the simulation of direct particles, with the aid of Aarseth's N-body integrator and estimations of particle-particle impacts. The techniques are used for the verification of the capability of the external satellites to generate clumps. The smoothening of the sharp features by interparticle collisions and the interaction parameters significant to Saturn's F-ring are investigated. The F-ring parameters show that the clumps cover the total azimuthal length, although azimuthal overlapping is not observed.

Newly obtained observations of the Saturnian coorbital satellites Janus and Epimetheus using a Caltech Cassegrain infrared camera were presented. Janus had a mass of (1.98 + or - 0. 12)x10(super 21) while Epimetheus had (0.55 + or - 0.03)x10(super 21) g. These values indicated that their mean yield densities were 0.65 + or - 0.08 and 0. 63 + or - 0.11 g cm(super -3), respectively. These densities indicated that both satellites were made up of mostly pure water ice.

An investigation of the physical properties of model Titan aerosols involved experiments in which acetylene, ehtylene and hydrogen cyanide were photolysed separately and as a mixture by ultraviolet light. The particles were found to be spherical, amorphous and quite sticky.

The surface characteristics of Mimas, a moon of Saturn, were modeled using imaging observations provided by Voyager. Hapke's equation was modified to accommodate anistropic multiple scattering and was used to analyze disk-integrated and disk-resolved Voyager clear filter photometric data. As near-opposition observations were limited, it was assumed that Mimas had a Rhea-like opposition surge. Photometric roughness was found to be great.

Observations of one of Saturn's moons from the two Voyager probes and the Hubble Space Telescope (HST) were compared. The observations were analyzed and it has been concluded that the variations in brightness may be explained by the seasonal model implying seasonal changes in the atmosphere of Titan. The images from the HST were subjected to the Fourier deconvolution technique and the Lucy iterative method of image deconvolution in order to clarify the images.

The formation of aerosols in Titan's atmosphere is discussed. There are two stages in the formation of aerosols. Elementary particles join to form roughly spheroid monomers which then collide with other monomers to form aggregates. These aerosols may explain the polarization of haze in Titan's atmosphere. The ideal altitude for the formation and growth of the aerosols is between 350 to 450 km.

In May, August, and November 1995, Hubble Space Telescope (HST) observations during Saturn's ring-plane crossings allowed us to view saturnian satellites normally hidden to Earth-based observers in the glare of the rings. New measurements of Janus, Epimetheus, Prometheus, and Pandora have been combined to form revised orbital solutions using all three HST data sets. These measurements and orbit fits are presented, as well as similar fits for the brighter satellites Mimas, Tethys, Enceladus, Dione, and Rhea. Observations of the Lagrangian satellites Telesto, Calypso, and Helene are also reported. While most satellites were found to be close to their expected positions based on previous orbital solutions (Nicholson et al. 1992, Icarus 100, 464-484; Jacobson 1996, Bull. Amer. Astron. Soc. 28, 1185; and Harper and Taylor 1993, Astron. Astrophys. 268, 326-349). Prometheus lagged behind its predicted longitude by 18.85 [degrees] [+ or -] 0.04 [degrees]. A systematic drift in Pandora's longitude of -1.85 [degrees] relative to the Voyager ephemeris was observed between May and November. The new data on the coorbital satellites Janus and Epimetheus have resulted in a revised mass for Janus, ~6% smaller than the previous value (Jacobson 1995, Bull. Amer. Astron. Soc. 27, 1202). Subtraction of light from the edge-on rings has led to additional detections of objects S/1995-S1 and S3 (Bosh and Rivkin 1995, Science 272, 518-521) in the May data, and S/1995-S5, S6, S7 (Nicholson et al. 1995, Bull. Amer. Astron. Soc. 27, 1202) and S/1995-S9 (Roddier et al. 1996) in the August images. S1 is identified with Atlas but leads its predicted position by ~25 [degrees]. S3 has an orbit consistent with that of the narrow F ring, but S5, S6, S7, and S9 now appear to orbit ~530-950 km interior to this ring. S7 and S9 may even be coorbital with Prometheus. An object in the May images, possibly corresponding to S7, is also found to lie very close to Prometheus' orbit (~800 km interior to the F ring), but no convincing detections of an analogous object in this orbit at this longitude were made in the November data set. In addition to these previously reported objects, eight new objects were identified during multiple HST visits in May and August; all have orbits consistent with that of the F ring and are likely to have been short-lived clumps within this ring, similar to those seen in Voyager images (Showalter 1997, Bull. Amer. Astron. Soc. 29, 999). Key Words: planetary rings; Saturn; satellites of Saturn.

HNC is a major product of the dissociative recombination reaction of the important ionospheric ion [HCNH.sup.+], although to date this neutral has not been identified within Titan's atmosphere, nor considered as a component in ionospheric or photochemical models. We have studied a simple pseudo-steady-state model for the formation and removal of HNC, in which the loss processes considered are HNC protonation by reaction with H-bearing ions; reaction with H atoms, yielding HCN; reaction with [CH.sub.3] radicals, to form [CH.sub.3]CN; and reaction with a population of unidentified X radicals to yield further hypothesized products. Using the ion abundances of C. N. Keller et al. (1998, Planet. Space Sci. 46, 1157-1174) and of M. Banaszkiewicz et al. (2000, Icarus 147, 386-404), we find that the most important loss processes are the reactions with [CH.sub.3] and with other unidentified radicals. According to our calculations, the HNC concentration reaches a peak of [10.sup.4]-[10.sup.5] molecules per cubic centimeter at an altitude of 1000-1100 km (i.e., close to the ionospheric peak), but is very much reduced at lower altitudes. We find also that the HNC/HCN ratio in Titan's atmosphere may approach unity at the ionization peak altitude and above. We discuss prospects for the detection of HNC, or its reaction products, during the Cassini mission. Key Words: Titan; planetary ionospheres; molecular processes.

We present new spectra of the leading and trailing hemispheres of Iapetus from 2.4 to 3.8 [micro]m. We have combined the leading hemisphere spectra with previous observations by others to construct a composite spectrum of the dark side (leading) hemisphere from 0.3 to 3.8 [micro]m. We review attempts to deduce the composition of the dark material from previously available spectrophotometry. None of them (numbering more than 20 million!) leads to a synthetic spectrum that matches the new data. An intimate mixture of water ice, amorphous carbon, and a nitrogen-rich organic compound (modeled here as Triton tholin) can fit the entire composite dark side spectrum. Observations in this spectral region have not revealed this mix of material on any other object observed thus far. We propose that this dark material may have originated on Titan, where atmospheric photochemistry has been producing nitrogen-rich organic compounds for 4.5 GY. [C] 2001 Academic Press Key Words: Iapetus; organic chemistry; satellites of Saturn; spectrophotometry.