Near-infrared studies of the satellites of Saturn and Uranus

New JHK photometry and spectrometry (1.4–2.6 μm) are presented for Enceladus, Hyperion, Phoebe, Umbriel, Titania, and Oberon. From spectral signatures, mainly in the 2-μm region, water ice is verified on Enceladus and identified on Hyperion and the three Uranian satellites. The JHK photometry shows that Phoebe is different from all other satellites and asteroids observed thus far. The new photometry corroborates the earlier conclusion by Cruikshank et al. (1977) Astrophys. J217, 1006–1010] that the Uranian satellites, as a class, have overall surface reflectances different from other water-ice-covered satellites, and the reason for the difference remains unclear. The diameters and the masses of the Uranian satellites are reviewed in light of the probable high albedo representative of ice-covered surfaces and the new dynamical studies by Greenberg, 1975, Greenberg, 1976, Greenberg, 1978.     

Hyperion: Analysis of Voyager observations

A total of 82 images of Hyperion was returned by the Voyager spacecraft; the most detailed views have a nominal resolution of 8.7 km/line pair. Hyperion had a rotation period of about 13 days and a spin vector lying close to its orbital plane at the time of the Voyager 2 encounter in 1981. The satellite's shape is very irregular, and cannot be approximated suitably by an ellipsoid. The largest cross section (A × C) is about 370 × 225 km; the B × C cross section is approximately 280 × 225 km. Most prominent among the surface features is a 120-km-diameter crater with an estimated depth of 10 km, and a series of arcuate scarps 300 km long that have relief in excess of 5 km. The density of large craters of Hyperion is smaller than that on other small Saturnian satellites and suggests the possibility that the last significant fragmentation of Hyperion occurred near the end of or after initial heavy bombardment. Voyager photometry yields an average normal reflectance of the surface material of 0.21 in the clear filter (0.47 μm) and evidence of slight albedo mottling over the surface. The disk-integrated phase coefficient between phase angles of 22° and 82° is 0.018 mag/de; there is little indication of a strong opposition effect in Voyager data down to phase angles of 3°. Hyperion's average color is definitely redder than that of Phobe, but matches that of the dark material on the leading hemisphere of Iapetus quite well. The satellite's albedo and color are consistent with those of contaminated water ice but since no mass determinations of Hyperion exist we do not know whether the bulk composition is icy or rocky.     

Saturn’s icy satellites investigated by Cassini-VIMS: II. Results at the end of nominal mission

We report the detailed analysis of the spectrophotometric properties of Saturn’s icy satellites as derived by full-disk observations obtained by visual and infrared mapping spectrometer (VIMS) experiment aboard Cassini. In this paper, we have extended the coverage until the end of the Cassini’s nominal mission (June 1st 2008), while a previous paper (Filacchione, G., and 28 colleagues [2007]. Icarus 186, 259-290, hereby referred to as Paper I) reported the preliminary results of this study.During the four years of nominal mission, VIMS has observed the entire population of Saturn’s icy satellites allowing us to make a comparative analysis of the VIS-NIR spectral properties of the major satellites (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus) and irregular moons (Atlas, Prometheus, Pandora, Janus, Epimetheus, Telesto, Calypso, Phoebe). The results we discuss here are derived from the entire dataset available at June 2008 which consists of 1417 full-disk observations acquired from a variety of distances and inclinations from the equatorial plane, with different phase angles and hemispheric coverage. The most important spectrophotometric indicators (as defined in Paper I: I/F continua at 0.55 μm, 1.822 μm and 3.547 μm, visible spectral slopes, water and carbon dioxide bands depths and positions) are calculated for each observation in order to investigate the disk-integrated composition of the satellites, the distribution of water ice respect to “contaminants” abundances and typical regolith grain properties. These quantities vary from the almost pure water ice surfaces of Enceladus and Calypso to the organic and carbon dioxide rich Hyperion, Iapetus and Phoebe. Janus visible colors are intermediate between these two classes having a slightly positive spectral slope. These results could help to decipher the origins and evolutionary history of the minor moons of the Saturn’s system. We introduce a polar representation of the spectrophotometric parameters as function of the solar phase angle (along radial distance) and of the effective longitude interval illuminated by the Sun and covered by VIMS during the observation (in azimuth) to better investigate the spatial distribution of the spectrophotometric quantities across the regular satellites hemispheres. Finally, we report the observed spectral positions of the 4.26 μm band of the carbon dioxide present in the surface material of three outermost moons Hyperion, Iapetus and Phoebe.     

Saturn's icy satellites investigated by Cassini-VIMS: I. Full-disk properties: 350-5100 nm reflectance spectra and phase curves

Saturn's icy satellites are among the main scientific objectives of the Cassini-VIMS (Visual and Infrared Mapping Spectrometer) experiment. This paper contains a first systematic and comparative analysis of the full-disk spectral properties of Dione, Enceladus, Epimetheus, Hyperion, Iapetus, Mimas, Phoebe, Rhea and Tethys as observed by VIMS from July 2004 to June 2005. The disk integrated properties (350-5100 nm reflectance spectra and phase curves at 550-2232 nm) and images of satellites are reported and discussed in detail together with the observed geometry. In general, the spectra in the visible spectral range are almost featureless and can be classified according to the spectral slopes: from the bluish Enceladus and Phoebe to the redder Iapetus, Hyperion and Epimetheus. In the 1000-1300 nm range the spectra of Enceladus, Tethys, Mimas and Rhea are characterized by a negative slope, consistent with a surface largely dominated by water ice, while the spectra of Iapetus, Hyperion and Phoebe show a considerable reddening pointing out the relevant role played by darkening materials present on the surface. In between these two classes are Dione and Epimetheus, which have a flat spectrum in this range. The main absorption bands identified in the infrared are the 1520, 2020, 3000 nm H₂O/OH bands (for all satellites), although Iapetus dark terrains show mostly a deep 3000 nm band while the 1520 and 2020 nm bands are very faint. In this spectral range, the Iapetus spectrum is characterized by a strong reddening. The CO₂ band at 4260 nm and the Fresnel ice peak around 3100 nm are evident only on Hyperion, Phoebe and Iapetus. The phase curves at 550 and at 2232 nm are reported for all the available observations in the 0°-144° range; Rhea shows an opposition surge at visible wavelengths in the 0.5°-1.17° interval. The improvement on the retrieval of the full-disk reflectance spectra can be appreciated by a direct comparison with ground-based telescopic data available from literature. Finally, data processing strategies and recent upgrades introduced in the VIMS-V calibration pipeline (flat-field and destriping-despiking algorithm) are discussed in appendices.     

The Uranian satellites and hyperion: New spectrophotometry and compositional implications

New reflectance spectra at 3.5% resolution have been obtained for Ariel, Titania, Oberon, and Hyperion in the 0.8- to 1.6-μm spectral region. The new spectra show no absoptions other than the 1.5-μm water-ice feature (within the precision of the data), and demonstrate extension into the 0.8- to 1.6-μm region of the 1.5- to 2.5-μm spectral similarity of Ariel to Hyperion (R. H. Brown and D. P. Cruikshank (1983). Icarus55,93-92). The new data confirm the presence of dark, spectrally bland component on/in the water-ice surfaces of the Uranian satellites, which, with some reservations, has spectral similarities to the dark substance on the leading side of Iapetus and the dark material on/in the surface of Hyperion, as well as other dark, spectrally neutral substances such as charcoal. Attempts were made to match the spectra of Ariel, Titania, and Oberon with additive reflectance mixes (areal coverage) of fine-grained water frost and various dark components such as charcoal, lampblack, and charcoal-water-ice mixtures. The results were broad limits on the amounts of possible areal coverage of a charcoal-like spectral component on the surfaces of the Uranian satellites, but the data are not of sufficient precision to conclusively determine whether the dominant mode of contaminant dispersal is areal or voluminal. The effect of highly variegated albedos on the diameters derived by R. H. Brown, D. P. Cruikshank, and D. Morrison (1982a) (Nature300, 423–425) is found to be small.     

A spectroscopic study of the surfaces of Saturn's large satellites: H2O ice, tholins, and minor constituents

We present spectra of Saturn's icy satellites Mimas, Enceladus, Tethys, Dione, Rhea, and Hyperion, 1.0-2.5 μm, with data extending to shorter (Mimas and Enceladus) and longer (Rhea and Dione) wavelengths for certain objects. The spectral resolution (R=λ/Δλ) of the data shown here is in the range 800-1000, depending on the specific instrument and configuration used; this is higher than the resolution (R=225 at 3 μm) afforded by the Visual-Infrared Mapping Spectrometer on the Cassini spacecraft. All of the spectra are dominated by water ice absorption bands and no other features are clearly identified. Spectra of all of these satellites show the characteristic signature of hexagonal H₂O ice at 1.65 μm. We model the leading hemisphere of Rhea in the wavelength range 0.3-3.6 μm with the Hapke and the Shkuratov radiative transfer codes and discuss the relative merits of the two approaches to fitting the spectrum. In calculations with both codes, the only components used are H₂O ice, which is the dominant constituent, and a small amount of tholin (Ice Tholin II). Tholin in small quantities (few percent, depending on the mixing mechanism) appears to be an essential component to give the basic red color of the satellite in the region 0.3-1.0 μm. The quantity and mode of mixing of tholin that can produce the intense coloration of Rhea and other icy satellites has bearing on its likely presence in many other icy bodies of the outer Solar System, both of high and low geometric albedos. Using the modeling codes, we also establish detection limits for the ices of CO₂ (a few weight percent, depending on particle size and mixing), CH₄ (same), and NH₄OH (0.5 weight percent) in our globally averaged spectra of Rhea's leading hemisphere. New laboratory spectral data for NH₄OH are presented for the purpose of detection on icy bodies. These limits for CO₂, CH₄, and NH₄OH on Rhea are also applicable to the other icy satellites for which spectra are presented here. The reflectance spectrum of Hyperion shows evidence for a broad, unidentified absorption band centered at 1.75 μm.     

Saturn's rings: 3-mm low-inclination observations and derived properties

Recent 3-mm observations of Saturn at low ring inclinations are combined with previous observations of E. E. Epstein, M. A. Janssen, J. N. Cuzzi, W. G. Fogarty, and J. Mottmann (Icarus41, 103–118) to determine a much more precise brightness temperature for Saturn's rings. Allowing for uncertainties in the optical depth and uniformity of the A and B rings and for ambiguities due to the C ring, but assuming the ring brightness to remain approximately constant with inclination, a mean brightness temperature for the A and B rings of 17 ± 4°K was determined. The portion of this brightness attributed to ring particle thermal emission is 11 ± 5°K. The disk temperature of Saturn without the rings would be 156 ± 6°K, relative to B. L. Ulich, J. H. Davis, P. J. Rhodes, and J. M. Hollis' (1980, IEEE Trans. Antennas Propag.AP-28, 367–376) absolutely calibrated disk temperature for Jupiter. Assuming that the ring particles are pure water ice, a simple slab emission model leads to an estimate of typical particle sizes of ≈0.3 m. A multiple-scattering model gives a ring particle effective isotropic single-scattering albedo of 0.85 ± 0.05. This albedo has been compared with theoretical Mie calculations of average albedo for various combinations of particle size distribution and refractive indices. If the maximum particle radius (≈5 m) deduced from Voyager bistatic radar observations (E. A. Marouf, G. L. Tyler, H. A. Zebker, V. R. Eshleman, 1983, Icarus54, 189–211) is correct, our results indicate either (a) a particle distribution between 1 cm and several meters radius of the form r^?s with 3.3 ? s ? 3.6, or (b) a material absorption coefficient between 3 and 10 times lower than that of pure water ice Ih at 85°K, or both. Merely decreasing the density of the ice Ih particles by increasing their porosity will not produce the observed particle albedo. The low ring brightness temperature allows an upper limit on the ring particle silicate content of ≈10% by mass if the rocky material is uniformly distributed; however, there could be considerably more silicate material if it is segregated from the icy material.     

High-Resolution 0.33-0.92 μm Spectra of Iapetus, Hyperion, Phoebe, Rhea, Dione, and D-Type Asteroids: How Are They Related?

New high-resolution spectra in the 0.33 to 0.92 μm range of Iapetus, Hyperion, Phoebe, Dione, Rhea, and three D-type asteroids were obtained on the Palomar 200-inch telescope and the double spectrograph. The spectra of Hyperion and the low-albedo hemisphere of Iapetus can both be closely matched by a simple model that is the linear admixture of the spectrum of a medium-sized, high-albedo icy saturnian satellite and D-type material. Our results support an exogenous origin to the dark material on Iapetus; furthermore, this material may share a common origin and a similar means of transport with material on the surface of Hyperion. The recently discovered retrograde satellites of Saturn (Gladman et al., Nature412, 163-166) may be the source of this material. The leading sides of Callisto and the Uranian satellites may be subjected to a similar alteration mechanism as that of Iapetus: accretion of low-albedo dust originating from outer retrograde satellites. Phoebe does not appear to be related to either Iapetus or Hyperion. Separate spectra of the two hemispheres of Phoebe show no identifiable global compositional differences.     

Saturn's satellites: Nuar-infrared spectrophotometry (0.6–2.5 μm) of the leading and trailing sides and compositional implications

Near-infrared spectra, 0.65–2.5 μm, are presented for Tethys, Dione, Rhea, Iapetus, and Hyperion. Water ice absorptions at 2.0, 1.5, and 1.25 μm are seen in the spectra of all five objects (except the 1.25-μm band was not detected in spectra of Hyperion) and the weak 1.04-μm ice absorption is detected on the leading and trailing sides of Rhea, and the trailing side of Dione. Upper limits to the 1.04-μm ice band depth are <0.3% for the leading side of Dione; <0.7% for the leading side of Iapetus, and the trailing side of Tethys; <1% on the trailing side of Iapetus; and <5% on the leading side of Tethys. The leading-trailing side ice band depth differences on Saturn's satellites are similar to those for the Galilean satellites, indicating possible surface modification by magnetospheric charged particle bombardment. Limits are determined for the amount of particulates, trapped gases, and amonium hydroxide on the surface. The surfaces of Saturn's satellites (except the dark side of Iapetus) are nearly pure water ice, with probably less than about 1 wt% particulate minerals. The ice could be clathrates with as much as a few weight percent trapped gases. The upper limit of amonium hydroxide depends on the spectral data precision and varies from ～ 1 wt% NH₃ for the leading side of Rhea to ～ 10 wt% NH₃ for Dione.     

Color photometry of surface features on Ganymede and Callisto

Voyager imaging data demonstrate that the scattering properties (“phase curves”) of all major terrain types on Ganymede and callisto are not significantly wavelength dependent between 0.4 and 0.6 μm. Our data suggest that the phase curves may be slightly steeper at the shorter wavelengths, consistent with the trend of telescopic observations near opposition. However, the differences are small and entirely within the uncertainties of our analysis. Our result indicates that the phase integrals (0.8 for Ganymede and 0.6 for Callisto) derived by S. W. Squyres and J. Veverka [Icarus46, 137–155 (1981)] from the abundant Voyager clear filter observations are reliable measures of the radiometric phase integrals. The corresponding values of the Bond albedo turn out to be 0.35 for Ganymede and 0.11 for Callisto.     

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).     

Are the polarization data consistent with constant flux models of Titan's atmosphere?

The consequences of the assumption that the mass flux through Titan's atmosphere is independent of height, when it is applied to the model of M. G. Tomasko and P. H. Smith (1982, Icarus51, 65–95) for the Voyager IPP polarization measurements, is explored. It is found that one of the basic assumptions of the constant flux model must be altered in order to make it consistent with both the polarization data and the observed equivalent widths of the methane bands.     

Hyperion: Collisional disruption of a resonant satellite

Hyperion is an irregularly shaped object of about 285 km in mean diameter, which appears as the likely remmant of a catastrophic collisional evolution. Since the peculiar orbit of this satellite (in $\text{4}\text{3}$ resonance locking with Titan) provides an effective mechanism to prevent any reaccretion of secondary fragments originated in a breakup event, the present Hyperion is probably the “core” of a disrupted precursor. This contrasts with the other, regularly shaped small satellites of Saturn, which, according to B.A. Smith et al. [Science215, 504–537 (1982)], were disrupted several times but could reaccrete from narrow rings of collisional fragments. The numerical experiments performed to explore the region of the phase space surrounding the present orbit show that most fragments ejected with a relative velocity $\text{?0.1}\text{km}\text{/}\text{sec}$ rapidly attain chaotic-type orbits, having repeated close encounters with Titan. Ejection velocities of this order of magnitude are indeed expected for a collision at a velocity of ～ 10 km/sec with a projectile-to-target mass ratio of the order of 10^?3; similar effects could be produced by less energetic but nearly grazing collisions. Such events are not likely to displace the largest remnant (i.e., the present Hyperion) outside the stable region of the phase space associated with the resonance, but could be responsible for the large amplitude of the observed orbital libration.     

Application of a radiative transfer model to bright icy satellites

A radiative transfer model, derived largely from the work of B.W. Hapke (1981, J. Geophys. Res.86, 3039–3054) and J.D. Goguen (1981, Ph.D. thesis, Cornell University, Ithaca, N.Y.), is fit to Voyager imaging observations of Europa, Mimas, Enceladus, and Rhea. It is possible to place constraints on the single-scattering albedo, the porosity of the optically active upper regolith, the single-particle phase functions, and, in the cases of Europa and Mimas, the mean slope angle of macroscopic surface features. The texture of the surfaces of the Saturnian satellites appears to be similar to the Earth's moon. However, Europa is found to have a distinctly more compact regolith and a more forward-scattering single-particle phase function.     

Near-infrared spectra of the Galilean satellites: Observations and compositional implications

We have obtained reflectivity spectra of the trailing and leading sides of all four Galilean satellites with circular variable filter wheel spectrometers operating in the 0.7- to 5.5-μm spectral interval. These observations were obtained at an altitude of 41,000 ft from the Kuiper Airborne Observatory. Features seen in these data include a 2.9-μm band present in the spectra of both sides of Callisto; the well-known 1.5-μm and 2.0-μm combination bands and the previously more poorly defined 3.1-μm fundamental of water ice observed in the spectra of both sides of Europa and Ganymede; and features centered at 1.35 ± 0.1, 2.55 ± 0.1, and 4.05 ± 0.05 μm noted in the spectra of both sides of Io. In an effort to interpret these data, we have compared them with laboratory spectra as well as synthetic spectra constructed with a simple multiple-scattering theory. We attribute the 2.9-μm feature of Callisto's spectra primarily to bound water, with the product of fractional abundance of bound water and mean grain radius in micrometers equaling approximately 3.5 × 10^?1 for both sides of the satellite. The fractional amounts of water ice cover on the trailing side of Ganymede, its leading side, and the leading side of Europa were found to be 50 ± 15, 65 ± 15, and 85% or greater, respectively. The bare ground areas on Ganymede have reflectivity properties in the 0.7- to 2.5-μm spectral region comparable to those of Callisto's surface and also have significant quantities of bound water, as does Callisto. Interpretation of the spectrum for the trailing side of Europa is complicated by magnetospheric particle bombardment which causes a perceptible broadening of strong bands, but the ice cover on this side is probably comparable to that on the leading side. These irradiation effects may be responsible for much of the difference in the visual geometric albedos of the two sides of Europa. Minor, but significant, amounts of ferrous-bearing material (either ferrous salts or alkali feldspars but not olivines or pyroxenes) account for the 1.35-μm feature of Io. The two longer wavelength bands are most likely attributable to nitrate salts. Ferrous salts and nitrates can jointly also account for much of the spectral variation in Io's visible reflectivity, thereby eliminating the need to postulate large quantities of sulfur. The absence of noticeable features near 3-μm wavelength in Io's spectra leads to upper bounds of 10% on the fractional cover of water and ammonia ice and 10^?3 on the relative abundance of bound water and hydroxylated material on Io. The two sides of Io have similar compositions. We suggest that the systematic increase in fractional water ice cover from Callisto to Ganymede to Europa is bought about by variations in efficiencies of recoating the satellite's surface by interior water brought to the surface, and by the deposition of extrinsic dust. The most important component of the latter is debris, derived from the outer irregular satellites of Jupiter, which impacts the Galilean satellites at relatively low velocities. Europa has the largest water ice cover because its crust is thinnest and thus the frequency of water recoating is the greatest, and because it is farthest from the sources of low-velocity dust. While models which depict Io's surface as consisting primarily of very fine-grained ice are no longer viable, we are unable to definitively distinguish between the salt assemblage and alkali feldspar models. The salt model can better account for Io's reflectivity spectrum from 0.3 to 5 μm, but the absence of appreciable quantities of bound water and hydroxylated material may not be readily understood within the context of that model.     

Cassini spectra and photometry 0.25-5.1 μm of the small inner satellites of Saturn

The nominal tour of the Cassini mission enabled the first spectra and solar phase curves of the small inner satellites of Saturn. We present spectra from the Visual Infrared Mapping Spectrometer (VIMS) and the Imaging Science Subsystem (ISS) that span the 0.25-5.1 μm spectral range. The composition of Atlas, Pandora, Janus, Epimetheus, Calypso, and Telesto is primarily water ice, with a small amount (∼5%) of contaminant, which most likely consists of hydrocarbons. The optical properties of the “shepherd” satellites and the coorbitals are tied to the A-ring, while those of the Tethys Lagrangians are tied to the E-ring of Saturn. The color of the satellites becomes progressively bluer with distance from Saturn, presumably from the increased influence of the E-ring; Telesto is as blue as Enceladus. Janus and Epimetheus have very similar spectra, although the latter appears to have a thicker coating of ring material. For at least four of the satellites, we find evidence for the spectral line at 0.68 μm that Vilas et al. [Vilas, F., Larsen, S.M., Stockstill, K.R., Gaffley, M.J., 1996. Icarus 124, 262-267] attributed to hydrated iron minerals on Iapetus and Hyperion. However, it is difficult to produce a spectral mixing model that includes this component. We find no evidence for CO₂ on any of the small satellites. There was a sufficient excursion in solar phase angle to create solar phase curves for Janus and Telesto. They bear a close similarity to the solar phase curves of the medium-sized inner icy satellites. Preliminary spectral modeling suggests that the contaminant on these bodies is not the same as the exogenously placed low-albedo material on Iapetus, but is rather a native material. The lack of CO₂ on the small inner satellites also suggests that their low-albedo material is distinct from that on Iapetus, Phoebe, and Hyperion.     

Model of explosive eruptions of water vapor and dust on icy satellites

The possibility of generating water vapor and other gaseous products during nonvolcanic explosive eruptions in lithospheres of icy satellites is discussed. Explosive eruptions of ice, with its fragmentation into micro-and nanofragments, can occur in the extensive deep layers of such icy satellites as Europa, Ganymede, Enceladus, etc., if giant cracks are episodically formed in the lithospheres of these satellites. Such cracks can be produced by tidal forces, synchronous resonances of satellites, or especially powerful impacts. The model is based on the recently obtained experimental evidence that explosive ice instability (Bridgman effect) is formed at a strong nonuniform compression in the regions of high pressures and low temperatures. Water films, the thicknesses of which reach several microns, can be formed during the process of the mutual friction of ice fragments during their quasi-liquid flow at the instant of an explosive eruption. About 1–10 dm³ of a water film can be produced in 1 m³ of erupted ice fragments. Water vapor can be formed from a water film when this water boils up after a rapid pressure drop as a result of an ice-water mixture eruption from cracks. A certain amount of gaseous products in the form of hydrogen, oxygen, and ammonia molecules and radicals on their basis can be generated during the sputtering induced by electrons and ions and the dissociation of nanofragments of ice during the process of ice explosive fragmentation as a result of fracto-, tribo-, and secondary emission. The estimates indicate that the volume of water vapor erupted on satellites can be larger than that of discharged ionized gases by a factor of not less than 10⁵–10⁷. Water vapor and microscopic ice fragments can be erupted from cracks in the lithospheres of small Enceladus-type satellites at velocities higher than the second cosmic velocity. Gaseous products generated in such episodic processes can, most probably, substantially contribute to the density of the atmosphere that exists on small icy satellites, but can only insignificantly contribute to this density on large satellites. The stick-slip motions of the most condensed plumes of water vapor and dust, normal to the satellite surface, along the mouths of gigantic cracks may indicate that the proposed model is realistic. Such wanderings of water vapor plumes can result in the synchronous motions of thermal patches on the satellite surface along crack mouths at velocities of about 10 km/h.     

2.7- to 4.1-μm spectrophotometry of icy satellites of Saturn and Jupiter

We present spectrophotometry in the 27–41 μm spectral region for icy satellites of Saturn (Tethys, Dione, Rhea, Iapetus, and Hyperion) and Jupiter (Europa, Ganymede, and Callisto). The 3.6-μm reflectance peak characteristic of fine-grained water ice is observed prominently on the satellites of Saturn, faintly on the leading side of Europa, and not all on Ganymede, Callisto, or the dark side of Iapetus. The spectral reflectances of these icy satellites may be affected by their equilibrium surface temperatures and magnetospheric effects.     

Implications of Jupiter's early contraction history for the composition of the Galilean satellites

Graboske et al. (1973) have shown that Jupiter's luminosity was orders of magnitude larger during its initial contraction phase than it is today. As a result, during Jupiter's earliest contraction history, ices would have preferentially been prevented from condensing within the region containing the orbits of the inner satellites. The observed variation of the mean density of the Galilean satellites with distance from Jupiter implies that the satellite formation process was operative on a time scale of about five million years. Another consequence of the high luminosity phase is that water should be the only ice present in significant proportions in any of the Galilean satellites.     

Models of the Io Torus

Three-dimensional models of the Io torus are employed to analyze the spectroscopic data reported by J.S. Morgan (1985, Icarus62, 389–414). These models are used to compare Morgan's ground-based spectroscopic data with R.J. Oliversen's (1983, The Io Plasma Torus: Its Structure and Sulfur Emission Spectra. Ph.D. thesis, University of Wisconsin-Madison) nearly simultaneous [SII] images and with the in situ measurements made by Voyager 1. The models are also used to investigate whether the observed [SII] longitudinal intensity variations were caused by intrinsic or geometric effects, and to test the hypothesis that the observed optical east-west variations are consistent with the convective motions suggested by D.D. Barbosa and M.G. Kivelson (1983, Geophys. Res. Lett.10, 210–213) and W.-H. Ip and C. K. Goertz (1983, Nature302, 232–233). Oliversen's images are found to be in good agreement with Morgan's spectroscopic measurements. Three significant differences exist between these data and the torus described in the Voyager 1 experiments: (1) the torus beyond ～5.7R_J was found to be at least 1.5 to 2 times denser in 1981 than at the time of the Voyager 1 measurements in 1979, (2) the outer torus SII ion temperatures were approximately two times cooler than those measured by Voyager 1, and (3) in 1981, the outer torus OII mixing ratios were lower than were suggested by the Voyager 1 experiments. The 1981 ground-based OII/SII intensity ratios are found to be consistent with a radial peak near 6.0R_J in the ratio of oxygen to sulfur. At its maximum this ratio is ～2, and it falls to ～1 within ～0.5R_J inside and outside of this radius. Viewing geometry variations were found to be inadequate to account for the longitudinal variations observed by Morgan (1984). Intrinsic longitudinal intensity changes of about a factor of 2 are required to match the 1981 observations. Convective motions were found to adequately explain the observed optical east-west intensity asymmetry, but problems in interpreting the [OII] doublet line ratios still remain. It is suggested that systematic errors are present in the measurements of the [OII] line ratios.