Icy Satellites Of Saturn Research Articles

G-MODE CLASSIFICATION OF SPECTROSCOPIC DATA

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

A spectroscopic study of the surfaces of Saturn's large satellites: H 2O 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 / Δ λ ) 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 H 2O 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 H 2O 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 CO 2 (a few weight percent, depending on particle size and mixing), CH 4 (same), and NH 4OH (0.5 weight percent) in our globally averaged spectra of Rhea's leading hemisphere. New laboratory spectral data for NH 4OH are presented for the purpose of detection on icy bodies. These limits for CO 2, CH 4, and NH 4OH 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.

Atmospheres and Plasma Interactions at Saturn's Largest Inner Icy Satellites

We explore the possible existence of atmospheres on Saturn's icy satellites, Mimas, Enceladus, Tethys, Dione, and Rhea. Only the most massive satellite, Rhea, has the potential to hold a thin oxygen atmosphere/exosphere. The masses of the other satellites are too small to retain gravitationally atmospheres in the presence of large thermal escape rates. If the temperature of Rhea's atmosphere is close to its surface temperature, then a molecular oxygen column density of 6 × 1017 m-2 is possible and causes a discernable plasma interaction with Saturn's magnetosphere that can be probed by close flybys of the Cassini spacecraft.

Modeling global impact effects on middle-sized icy bodies: applications to Saturn's moons

Disrupted terrains that form as a consequence of giant impacts may help constrain the internal structures of planets, asteroids, comets and satellites. As shock waves and powerful seismic stress waves propagate through a body, they interact with the internal structure in ways that may leave a characteristic impression upon the surface. Variations in peak surface velocity and tensile stress, related to landform degradation and surface rupture, may be controlled by variations in core size, shape and density. Caloris Basin on Mercury and Imbrium Basin on the Moon have disturbed terrain at their antipodes, where focusing is most intense for an approximately symmetric spheroid. Although, the icy saturnian satellites Tethys, Mimas, and Rhea possess giant impact structures, it is not clear whether these structures have correlated disrupted terrains, antipodal or elsewhere. In anticipation of high-resolution imagery from Cassini, we investigate antipodal focusing during giant impacts using a 3D SPH impact model. We first investigate giant impacts into a fiducial 1000 km diameter icy satellite with a variety of core radii and compositions. We find that antipodal disruption depends more on core radius than on core density, suggesting that core geometry may express a surface signature in global impacts on partially differentiated targets. We model Tethys, Mimas, and Rhea according to their image-derived shapes (triaxial for Tethys and Mimas and spherical for Rhea), varying core radii and densities to give the proper bulk densities. Tethys shows greater antipodal values of peak surface velocity and peak surface tensile stress, indicating more surface damage, than either Mimas or Rhea. Results for antipodal and global fragmentation and terrain rupture are inconclusive, with the hydrocode itself producing global disruption at the limits of model resolution but with peak fracture stresses never exceeding the strength of laboratory ice.

Discovery of co 2 ice and leading–trailing spectral asymmetry on the uranian satellite ariel

New 0.8- to 2.4-μm spectral observations of the leading and trailing hemispheres of the uranian satellite Ariel were obtained at IRTF/SpeX during 2002 July 16 and 17 UT. The new spectra reveal contrasts between Ariel’s leading and trailing hemispheres, with the leading hemisphere presenting deeper H 2O ice absorption bands. The observed dichotomy is comparable to leading–trailing spectral asymmetries observed among jovian and saturnian icy satellites. More remarkably, the trailing hemisphere spectrum exhibits three narrow CO 2 ice absorption bands near 2 μm. This discovery of CO 2 ice on one hemisphere of Ariel is its first reported detection in the uranian system.

Upper Limits for Condensed O2on Saturn's Icy Satellites and Rings

New 4800–7200 Å spectra of Saturn's rings, and the icy saturnian satellites Enceladus, Tethys, Dione, Rhea, and Iapetus, do not show the weak 5773 and 6275Å absorption bands due to high-density condensed O2that are seen, with a maximum depth of 1.8%, on Ganymede's trailing side. The 5773Å band depth must be ≤0.6% (and very probably ≤0.3%) on the rings, Tethys, Dione, Rhea, and the trailing side of Iapetus, and ≤1.2% on the trailing side of Enceladus. The lack of detectable absorption does not rule out abundant O2molecules that are separated too well to produce these absorption bands. The observations show that the presence of O3, recently detected on Rhea and Dione (Nollet al.1997b,Nature388, 45–47), does not require the presence of detectable quantities of high-density O2. I discuss the implications of the new observations for models of O2formation on Ganymede.

Detection of the hydroxyl radical in the Saturn magnetosphere

THE magnetosphere in the vicinity of the orbits of Saturn's icy satellites consists of a low-density plasma, in which the electrons are an order of magnitude cooler than the accompanying heavy ions1. Most models2–12 neglect this fact, even though radiative cooling and diffusive loss rates are both too slow to account for the observed temperatures. Shemansky and Hall13 have recently proposed that the electrons could be cooled by the presence of a large abundance of neutral gas, derived mainly from the breakdown products of H2O (mainly O and OH) from the icy satellites. Hydrogen radicals have been reported in this region13, but these originate from the atmosphere of Saturn itself; no satellite-derived neutral species have been detected. Here we report the detection of neutral OH molecules near the orbit of Tethys, using the Faint Object Spectrograph on the Hubble Space Telescope. Our results suggest that neutral OH is one of the dominant species in Saturn's inner magnetosphere, implying a source rate for H2O twenty times greater than current theoretical estimates5,6. One possible explanation is that the micrometeorite erosion rates of the inner satellites are significantly higher than expected.

Convection and lithospheric strength in Dione, an icy satellite of Saturn

Finite element numerical simulations for the structure of convection cells within a self-gravitating, Boussinesq fluid sphere with variable viscosity are used to determine temperature distribution, stream functions, velocities, and intensities of the stresses within Dione, an icy satellite of Saturn. A non-Newtonian, i.e., stress-dependent, viscosity is considered, but the numerical simulation, according to U. R. Christensen (1984a, Geophys. J. R. Astron. Soc. 77, 343–384), is carried out knowing that the properties of power law convection can be closely simulated by Newtonian flow with a reduced value of the activation enthalpy. The initial temperature profile has been determined taking into account the amount of energy trapped as heat during the accumulation process and the other energy source is the radioactivity of the long-lived isotopes. The obtained results seem to show a long duration endogenic activity in agreement with the geological observations. The intensities of the stress are able to fracture the icy crust both in extensional and in compressional regimes. The thickness of the mechanical lithosphere evolves in a way determined by the complex interplay of the lithospheric temperature gradient and the strain rate.

Sputter‐produced plasma as a measure of satellite surface composition: The CASSINI mission

Measurements of the co‐orbiting neutral cloud or the plasma produced by sputtering of the icy Saturnian satellites can be used to determine the relative abundance of a minority surface species which would be difficult to determine from reflectance spectra. This is due to the fact that the sputter source rates, hence the plasma supply rates, are directly proportional to the bulk concentrations of mixed solids or clathrates, although the surface grains may be depleted in the most volatile species.

Relaxation of impact basins on icy satellites

The extent and style of viscous relaxation of the largest crater basins on the icy satellites of Saturn can be used as a probe of the viscosity of the deep interiors of these bodies. In particular, some constraints on the size of a silicate core and the magnitude of the internal viscosity gradient can be determined if the relaxation process is sufficiently well understood. We discuss the application of a Unite element model incorporating spherical symmetry to the general problem of relaxation of large craters on spherical bodies. We examine the influence of core size, geothermal gradient, and rheologic law on relaxation flow and consider the implications for the internal structures of Mimas and Tethys. Our model results show that relaxation of crater topography on a spherical satellite is qualitatively and quantitatively different from relaxation on a planar surface once the crater diameter becomes a significant fraction of the satellite radius. For small‐to moderate‐sized basins, relaxation flow is bidirectional, with flow both from rim to bowl and rim to surrounds. This is the same style of flow observed for craters on planar surfaces. For large enough basins, however, the spherical geometry of the problem leads to flow that is unidirectional, toward the crater bowl throughout the satellite. The transition from bidirectional to unidirectional flow appears to be very sensitive to the basin diameter. The critical basin diameter for transition increases with core size, thermal gradient, rim height, and rheologie law stress exponent. Unidirectional and bidirectional flow produce fundamentally different stress fields in the regions surrounding the relaxing crater. Relaxation velocities for large craters on spherical bodies can be appreciably more rapid than those for equivalent craters on planar surfaces, again because of the geometric effects of relaxation on a sphere. The difference in degree of relaxation between the large basins Odysseus on Tethys (which has undergone significant modification) and Herschel on Mimas (which appears essentially unmodified) can be explained by different thermal histories for Tethys and Mimas, by the effects of stress‐dependent rheology, or by a combination of the two. The influence of a core (at sizes consistent with the observed densities of the satellites) on relaxation time scales is unimportant. If the Ithaca Chasma graben system on Tethys is the result of stresses generated by relaxation of the Odysseus basin, then Tethys's core must be small enough that it allows unidirectional relaxation flow for a basin of Odysseus's size. If this assumption it correct, we estimate an upper limit of roughly 20% of Tethys's radius for the radius of the satellite's core.

Enceladus: Implications of its unusual photometric properties

We have investigated the photometric properties of Enceladus up to phase angles of 43°, using Voyager 1 and 2 images. Among the satellite's unusual photometric characteristics are its very high geometric albedo (1.0 ± 0.1 near 0.47 μ), which extends down to at least 0.34 μ, and the uniformity of albedos, colors, and scattering properties over the geologically varied surface. Although some albedo variations of up to 15% occur in low resolution Voyager 1 images, the albedos of the four major geological units imaged in the Voyager 2 near-encounter sequence differ by 1–2% or less, even though the ages of these units probably differ by at least a factor of 10 (3.8 billion years to a few hundred million years). The lack of correlation of spectrophotometric properties with terrain type suggests that the optical characteristics of Enceladus are determined by a recently deposited ubiquitous surface layer, possibly originating in Saturn's E-ring. The high geometric albedo implies that the surface layer is remarkably free of opaque material and is much more backscattering than is common for natural or laboratory frost layers on Earth. The unique phase and photometric functions of Enceladus can be explained by the high degree of multiple scattering in its surface. The existing observations suggest that the textural characteristics of Enceladus' regolith are similar to those of other icy Saturnian satellites.

A simple two-layer model for the Uranus satellites

A two-layer model of a satellite interior with a rocky core with a density 3–3.4 g cm-3 and with a H2O mantle with a density 0.94–1.2 g cm-3 is applied for the icy satellites. The case of Mimas is discussed separately. A comparison of the results with these obtained for more complicated models as applied for Jupiter and Saturn icy satellites has been carried out. This comparison shows that the two-layer model offers a reasonable approximation and, therefore, it can be applied for the satellites of Uranus. We obtained the dimensionless core radii 0.55–0.74, 0.45–0.68, 0.59–0.67, 0.55–0.65, and dimensionless core masses 0.42–0.72, 0.26–0.63, 0.47–0.61, 0.41–0.57, for Ariel, Umbriel, Titania, and Oberon, respectively.

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

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

Correction to “Implications of Voyager data for energetic ion erosion of the icy satellites of Saturn”

Correction to “Implications of Voyager data for energetic ion erosion of the icy satellites of Saturn”

Saturn's icy satellites: Thermal and structural models

Thermal histories of the small icy Saturnian satellites Mimas, Tethys, Dione, Rhea, and Iapetus are constructed by assuming that they formed as homogeneous ice-silicate mixtures. The models include effects of radiogenic and accretional heating, conductive and subsolidus convective heat transfer, and lithospheric growth. Accretional heating is unlikely to have melted the water ice in the interiors of these bodies and solid state creep of the predominately ice material precludes melting by radiogenic heating. Mimas is so small that its thermal evolution is essentially purely conductive; at present it is a cold, nearly isothermal body. Any subsolidus convection or thermal activity in Mimas would have been confined to a brief period in its early history and would have been due to a warm formation. The four largest satellites are big enough and contain sufficient heat-producing silicates that solid state convection beneath a rigid lithosphere is inevitable independent of initial conditions. Dione and Rhea have convective interiors for most of their thermal histories, while Tethys and Iapetus have mainly conductive thermal histories with early periods of convective 0activity. The thermal histories of the five satellites for the last 4 by are independent of initial conditions; at present they have cold, conductive interiors. The model thermal histories are qualitatively consistent with the appearances of these satellites: Mimas has an ancient heavily cratered surface, Tethys and probably Iapetus have both heavily cratered and more lightly cratered areas, and Dione and Rhea have extensively modified surfaces. Because of their similar sizes and densities, Mimas and Enceladus are expected to have similar surfaces and thermal histories, but instead Enceladus has the most modified surface of all the small icy Saturnian satellites. Our results suggest a heat source for Enceladus, in addition to radiogenic and accretional heating; tidal dissipation is a possibility. Because the water ice in these bodies does not melt, resurfacing must be accomplished by the melting of a low-melting-temperature minor component such as ammonia hydrate.

Diameters and albedos of satellites of Uranus

The five known satellites of Uranus are too small and distant to permit direct measurement of their diameters. Consequently, products of the masses of the satellites, and estimates of their bulk densities and surface albedos have been used in discussions of their probable dimensions in the absence of measurements. The presence of water ice on the surfaces of Ariel, Umbriel, Titania and Oberon has been established spectrophotometrically1–4 and the brightnesses of the satellites have been measured photoelectrically5. Determination of the masses of the uranian satellites depends on observations of perturbations of the satellite orbits and results chiefly in upper limits to the products of the masses of various pairs of satellites6,7. The new diameter measurements reported here were made using the photometric/radiometric technique of diameter determination. This widely used technique has enabled the measurement of the diameters and albedos of approximately 250 asteroids and many planetary satellites. A recent recalibration of the technique using independent measurements of the diameters of three Solar System objects results in absolute accuracies of better than 5%. Our new albedo measurements show that Umbriel, Titania and Oberon are similar in albedo to J4 Callisto while Ariel is very similar in albedo to S7 Hyperion; the diameters of all four satellites are similar to those of the large, icy saturnian satellites S5 Rhea and S8 Iapetus.

A possible resurfacing mechanism for icy satellites

Photographs from Voyager Missions reveal a great variety of surface features for the icy satellites of Saturn1. Mimas is covered with many old craters. Enceladus has a rather smooth surface with few and rather young craters. Some parts of Dione's surface have an important crater density, whereas other parts are relatively smooth. Thus some of Saturn's icy satellites seem to have undergone important surface modifications during their history while others did not. I suggest here that the irreversible phase transition between amorphous and cubic ice and similar phase transitions may provide a low energy resurfacing mechanism for these icy satellites.

Crater densities and geological histories of Rhea, Dione, Mimas and Tethys

Crater density determinations for parts of the surfaces of several of Saturn's icy satellites including Rhea, Dione, Mimas, Tethys and 1980S3 reveal significant variations. These data, combined with observations of surface morphology, indicate that each satellite has undergone extensive post heavy bombardment geological evolution.