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NASA NTRS1982

Mass-radius relationships in icy satellites after Voyager

Using improved data for the masses and radii of the satellites of Jupiter and Saturn, models accounting for self-compression effects are presented for the interiors of Europa, Ganymede, Callisto, Rhea, and Titan. For the differentiated models, two different possible scenarios for heat transport are treated, as well as two different compositions for the silicate component. Undifferentiated models are also treated. In each case, the models of Ganymede, Callisto, and Titan show noticeable similarities. It is found that estimates of the ice-rock ratio are dependent upon the assumptions made about the heat transport mechanisms, the rock composition, and on the distribution of rock and ice in the satellite's interior.

Lupo, M. J.↗
NASA NTRS1983

Saturn's icy satellites - Thermal and structural models

Thermal history models which assume formation as homogeneous ice-silicate mixtures are constructed for the small, icy Saturnian satellites Mimas, Tethys, Dione, Rhea, and Iapetus, including the effects of radiogenic and accretional heating, conductive and subsolidus convective heat transfer, and lithosphgeric growth. Accretional heating is not likely to have melted the water ice in the interiors of these bodies, and solid state creep of the ice-dominated material precludes melting by radiogenic heating. The four largest satellites are sufficiently large and rich in heat-producing silicates to possess a solid state convection system beneath a rigid lithosphere, irrespective of initial conditions. The model thermal histories are qualitatively consistent with the present appearances of these satellites.

Ellsworth, K.↗
NASA NTRS1984

Voyager Feature-fixed Photometry of Iapetus

Iapetus is one of the most mysterious objects in the Solar System. As indicated by previous ground-based studies and dramatically confirmed by the Voyager images, the surface of this satellite has a strikingly bimodal distribution of albedo and color. While the underlying object seems to be a largely icy satellite similar to Rhea, the leading hemisphere of Iapetus is dominated by a very dark patch of hemispheric scale. The symmetry of this dark material with respect to the orbital motion of the satellite strongly suggests an external control, if not necessarily an external origin, for the dark material. Quantitative photometric techniques are used to map the photometric function of the surface of the material over Iapetus. The approach is to study clearly defined surface areas that appear on a number of Voyager images obtained under different spacecraft and lighting geometries.

Goguen, J.↗
NASA NTRS1983

The Voyager Mission - Encounters with Saturn

The trajectories of Voyager 1 and Voyager 2 through the Saturnian system were selected in such a way that the observations would be complementary. The Voyager 1 trajectory provided a close encounter with Titan, making possible atmospheric occultations, a search for an intrinsic Titanian magnetic field, and studies of the interaction of Titan with either the solar wind or the Saturnian magnetosphere. The trajectory was also designed to provide an optimum geometry for the transmission of S and X band radio waves from the spacecraft through the rings; in this way, the attenuation and scattering of 3.6- and 13-cm waves by the ring particles could be observed. Besides this, Voyager 1 provided radio and ultraviolet occultation studies of Saturn's atmosphere as well as close approaches to three of the moderate-sized icy satellites (Mimas, Dione, and Rhea) and to a number of minor satellites. The much later arrival of Voyager 2 and the Saturn flyby distance were chosen in such a way as to make possible a gravity-assisted continuation on to Uranus and Neptune.

Stone, E. C.↗
NASA NTRS1983

Implications of Voyager data for energetic ion erosion of the icy satellites of Saturn

The possible production of a neutral-particle torus by magnetosphere-particle sputtering of the icy satellites (Enceladus, Tethys, Dione, Rhea) of Saturn is discussed. Particle spectra from the Voyager low-energy-charged-particle experiment are used together with laboratory-derived erosion rates and velocity distributions for the ejected species. An extended torus region in the inner magnetosphere of Saturn is expected, with H2O escape fluxes in the vicinities of the satellites varying from about 2 to 7 x 10 to the 7th mol/sq cm sec for incident protons and from about 6 to 14 x 10 to the 9th mol/sq cm sec if the incident ions are all oxygen.

Lanzerotti, L. J.↗
NASA NTRS1983

Corotation anisotropies in Saturn's magnetosphere

Ion data from the Voyager 1 and 2 low-energy charged particle (LECP) experiment are fit to a second-order harmonic expansion to determine anisotropies within the Saturnian magnetosphere. Anisotropies from the low-energy channels (28-130 keV) are often consistent with those expected from corotation (generally with a small radial component on the dayside) but are at times distinctly different from corotation. On the dayside, near the noon meridian, first-order anisotropies often depart significantly from the corotation direction. On the nightside, amplitudes are consistent with full corotation but begin to drop below corotation values at a dipole L of approximately 27 Saturn radii (corresponding to a velocity of approximately 265 km/s). Second-order anisotropies are significant, even dominant at times, on the dayside where a relatively broad range of pitch angles is sampled by the instrument. In all cases there is a departure of approximately 180 deg from the first-order anisotropies expected due to corotation beginning outside the Rhea L shell and continuing through at least the orbit of Dione. This is the region where the cold and hot plasma tori have been observed. The anisotropies expected from rigid corotation of the observed flux distributions are computed, and it is concluded that parts of the Saturnian magnetosphere are not rigidly corotating. Calculations based on the combined effects of corotation plus solar wind generated convective electric fields appear to provide insights into the anisotropies along the dawn meridian. However, on the dayside near noon, turbulence and/or time variations seem necessary to explain the departure from simple convection models.

Carbary, J. F.↗
NASA NTRS1984

Voyager disk resolved photometry of the Saturnian satellites

The scattering properties of the icy satellites of Saturn are studied. Scans of intensity along the photometric equator near opposition are analyzed, and disk-resolved measurements of intensity are fitted to simple photometric functions. Histograms of the albedo distribution across the satellite disks are constructed, and the geometric albedos are determined as a function of wavelength. It is found that, at low phase angles, Rhea and Dione closely follow lunar behavior with almost no limb darkening. Mimas, Tethys, and especially Enceladus show significant limb darkening at low phase angles, which suggest multiple scattering is important for their surfaces. No satellite's surface can be described by Lambert's law. The darker satellites are redder, and the darker areas of each satellite are redder. A longitudinal dark stripe which represents a 15 percent decrease in albedo is situated near the center of the trailing side of Tethys. Mimas and Enceladus both have remarkably flat spectral reflectances into the ultraviolet.

Buratti, B. J.↗
NASA NTRS1985

Voyager Cartography

The Jovian and Saturnian satellites are being mapped at several scales from Voyager 1 and 2 data. The maps include specially formatted color mosaics, controlled photomosaics, and airbrush maps. More than 500 Voyager images of the Jovian and Saturnian satellites were radiometrically processed in preparation for cartographic processing. Of these images, 235 were geometrically transformed to map projections for base mosaic compilations. Special techniques for producing hybrid photomosaic/airbrush maps of Callisto are under investigation. The techniques involve making controlled computer mosaics of all available images with highest resolution images superimposed on lowest resolution images. The mosaics are then improved by airbrushing: seams and artifacts are removed, and image details enhanced that had been lost by saturation in some images. A controlled mosaic of the northern hemisphere of Rhea is complete, as is all processing for a similar mosaic of the equatorial region. Current plans and status of the various series are shown in a table.

Batson, R. M.↗
NASA NTRS1985

The Control Networks of the Satellites of Jupiter and Saturn

Geodetic control networks are being computed photogrammetrically for the large satellites of Jupiter and many of the satellites of Saturn using pictures from the Voyager 1 and 2 encounters. Control points have been identified on the satellites and their coordinates computed by single-block analytical triangulations. Contents of data sets for Io, Europa, Ganymede, Callisto, and Rhea are summarized.

Davies, M. E.↗
NASA NTRS1984

Production of ammonia-depleted surface layers on the Saturnian satellites by ion sputtering

On the basis of laboratory-derived data on the ion erosion of water and ammonia ices, it is suggested that the surfaces of the Saturnian satellites Enceladus, Tethys, Dione, and Rhea could have become enriched with water ice over a reasonable geological time period owing to the preferential erosion of the ammonia by magnetosphere ion bombardment. This mechanism serves as an alternative explanation to other processes invoked for the absence of ammonia spectral features in these satellites.

Lanzerotti, L. J.↗
NASA NTRS1985

Near equality of ion phase space densities at earth, Jupiter, and Saturn

Energetic-ion phase-space density profiles are strikingly similar in the inner magnetospheres of earth, Jupiter, and Saturn for ions of first adiabatic invariant near 100 MeV/G and small mirror latitudes. Losses occur inside L approximately equal to 7 for Jupiter and Saturn and inside L approximately equal to 5 at earth. At these L values there exist steep plasma-density gradients at all three planets, associated with the Io plasma torus at Jupiter, the Rhea-Dione-Tethys torus at Saturn, and the plasmasphere at earth. Measurements of ion flux-tube contents at Jupiter and Saturn by the low-energy charged-particle experiment show that these are similar (for O ions at L = 5-9) to those at earth (for protons at L = 2-6). Furthermore, the thermal-ion flux-tube contents from Voyager plasma-science data at Jupiter and Saturn are also very nearly equal, and again similar to those at earth, differing by less than a factor of 3 at the respective L values. The near equality of energetic and thermal ion flux-tube contents at earth, Jupiter, and Saturn suggests the possibility of strong physical analogies in the interaction between plasma and energetic particles at the plasma tori/plasma sheets of Jupiter and Saturn and the plasmasphere of earth.

Cheng, A. F.↗
NASA NTRS1985

Lunar and Planetary Science Conference, 15th, Houston, TX, March 12-16, 1984, Proceedings. Part 2

Subjects of lunar petrology are discussed, taking into account Apollo 14 aluminous mare basalts and their possible relationship to KREEP, the petrology and geochemistry of clasts from consortium breccia, the depths of the mare basalt source region, the origin of olivine at Copernicus, a transient heating event in the history of a highlands troctolite from Apollo 12 soil, and the composition and evolution of the lunar crust in the Descartes highlands. Other topics explored are related to early earth and magmatic processes, differentiated meteorites, chondritic meteorites, other planets and remote sensing, and cratering. Attention is given to the gravity field of Venus at constant altitude and comparison with earth, a spectral analog of Martian soil, dark halo craters and the thickness of grooved terrain on Ganymede, the geomorphology of Rhea, a Monte Carlo model of lunar megaregolith development, the scaling of complex craters, crustal radiogenic heat production and the selective survival of ancient continental crust, and the formation of an impact-generated H2O atmosphere and its implications for the early thermal history of the earth.

Ryder, G.↗
NASA NTRS1985

Application of a radiative transfer model to bright icy satellites

A radiative transfer model, derived largely from the work of Hapke (1981) and Goguen (1981), 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 function, 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.

Buratti, B. J.↗
NASA NTRS1984

Satellites of Saturn - Geological perspective

The Voyager encounters have added the satellites of Saturn to those planetary bodies for which geological studies are possible. These satellites are surprisingly heterogeneous, in spite of their apparently common composition of more than half water ice. All are more or less heavily cratered, but most also show clear evidence of substantial endogenic modification and resurfacing during the first few hundred million years of their existence. In the case of Enceladus, this internal activity has been most dramatic and may have persisted into the past billion years of solar-system history. Iapetus also remains a major mystery with its still unexplained hemispheric dichotomy, for which both external and internal causes have been suggested. The current knowledge of the satellites is reviewed from a geologic perspective, using primarily Voyager imaging data. Emphasis is placed on the six larger bodies (except Titan): Rhea, Iapetus, Dione, Tethys, Enceladus, and Mimas.

Morrison, D.↗
NASA NTRS1985

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

Spectrophotometry is presented in the 2.7-4.1 micrometer spectral region for icy satellites of Saturn (Tethys, Dione, Rhea, Iapetus and Hyperion) and Jupiter (Europa, Ganymede and Callisto). The 3.6-micrometer 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 at 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.

Lebofsky, L. A.↗
NASA NTRS1985

Resurfacing Saturn's satellites - Models of partial differentiation and expansion

A number of possible internal sources for the extensional tectonic features on the icy moons of Saturn are discussed, including the freezing of an initially molten body, the expansion of a warming icy moon which never melts, and the refreezing of an ammonia-water solution inside these moons. Computer calculations indicate that the most reasonable cause of rifting on Dione and Rhea is the refreezing of an ammonia-water eutectic melt inside these moons aproximately two-billion years after their formation.

Consolmagno, G. J.↗
NASA NTRS1986

Corotation of the Kronian magnetosphere

Observations of the radial variation of azimuthal plasma velocity in the inner part of the magnetosphere of Saturn show deviations from corotation occur as far in as L = 4. Major deviations from rigid corotation occur near the orbits of Rhea and Dione. Voyager-derived neutral and plasma density and temperature vs. altitude profiles in the upper atmosphere to verify that the atmospheric torque needed to drive corotation through neutral-ion coupling is available are used. It is found that the observed azimuthal velocities and calculated Pedersen conductance are consistent with one another, but that the conductance is much less than previous values estimated for Saturn and Jupiter. The lower than anticipated conductivity is attributed to the large heliocentric distance, to the absence of an active satellite (such as Jovian Io) and to a possible role of ring material in depleting the ionosphere.

Eviatar, A.↗
NASA NTRS1989

Gravity field of the Saturnian system from Pioneer and Voyager tracking data

Doppler-tracking data and star-satellite imaging from the Voyager 1 and 2 spacecraft are used along with Pioneer 11 Doppler tracking data to study the gravity field of the Saturnian system. The present analysis has yielded improved values for the masses of Rhea, Titan, and Iapetus, and Saturn. The results are consistent with the findings of Null et al. (1981) and Nicholson and Porco (1988) for the Saturn zonal harmonic coefficients J2, J4, and J6. The ratio of the mass of the sun to the mass of the Saturnian system is found to be 3497.898 + or - 0.018

Campbell, J. K.↗