Spectral geometric albedos of the Galilean satellites from 0.24 to 0.34 micrometers: Observations with the international ultraviolet explorer

We present the results of an 8-year program of spectrophometry of the Galilean satellites of Jupiter that was undertaken using the International Ultraviolet Explorer (IUE) Spacecraft. The ultraviolet geometric albedos of all four satellites are low. This is consistent with the hypothesis that sulfurous materials escaping from the surface of Io are being distributed by magnetospheric processes to the surfaces of the other three objects. Although iron bearing silicates may also cause UV darkening, these materials also have spectral features in the visible region of the spectrum which are not found in the spectra of the Galilean satellites. For Io, we find that the ultraviolet geometric albedo is very low (P_uv ∼ 0.04). The trailing hemisphere has an albedo that is higher than that of the leading hemisphere. This is opposite of what is observed at visual wavelengths. The decrease of albedo shortward of 0.33 μm is consistent with groundbased observations (Nelson and Hapke, 1978) and the laboratory reflection spectrum of sulfur dioxide frost. The hemispheric albedo asymmetry is consistent with a variable distribution of the frost, it being present in greater abundance on Io's leading hemisphere. The strenght of this feature has not changed with respect to longitude over the8 years of this study. The phase coefficients and opposition surges at ultraviolet wavelenghts indicate that Io's surface regolith is very porous. Europa has the highest ultraviolet albedo of all the Galilean satellites (P_uv ∼ 0.2). This not inconsistent with the hypothesis of recent resurfacing. However, this albedo is not high enough to be consistent with a surface of pure water ice. We confirm a previously reported ultraviolet spectral asymmetry between Europa's leading and trailing hemispheres. The new data are consistent with the previous analyses which interpreted this as the spectral signature of sulfur ions from the Jovian magnetosphere which had been embedded preferentially on the trailing side of Europa's predominately water ice.surface. The opposition surge observed for Europa's trailing side is greater than that for the leading side. This implies that the trailing side is less compact than the leading side, perhaps due to gardening from the ion implantation process. Ganymede's ultraviolet albedo (P_uv ∼ 0.10) is lower than Europa's. Ganymede has an ultraviolet spectral asymmetry that is similar to Europa's for wavelenghts longer than 0.28 μm. However, at wavelengths shorter than 0.28 μm, the two objects have different opposite hemispherical spectral ratios, indicating that the same mechanism cannot be used to explain the ultraviolet spectral albedo of both objects. One possible explanation is that ozone is present in addition to sulfur embedded on Ganymede's surface. The ultraviolet albedo and opposite hemispherical spectral ratio of Calisto is spectrally flat, indicating that the surface is covered by a material that is spectrally absorbing in the ultraviolet but has no change in absorption at the ultraviolet wavelenghts. The orbital phase variation in the ultraviolet indicates that the absorber is assymmetrically distributed in longitude.     

Four-color photometry of the Galilean satellites

Photometry obtained in 1973 on the uvby system yields high-precision rotational light curves for Io, Europa, and Ganymede at a mean phase angle of ～6°. By combining our observations with photometry obtained by others over a broader range of phase angle, we alsi derive improved values for the phase coefficients and opposition surges of the four Galilean satellites. The values of V(1, 0) obtained by linear extrapolation to zero phase are accurate to ±0.03 magnitudes. We also derive the colors of the sun of the uvby system and use these to obtain albedos of the satellites in four colors.     

Early eclipses of the Galilean satellites

A brief summary of the development of the theory of motion of the Galilean satellites is presented. Over 7700 eclipse observations have been collected and reduced using the Ephemeris E-2. They are of great potential in improving the ephemerides of the satellites and can yield important information on the evolution of the Galilean system.Proceedings of the Conference on Analytical Methods and Ephemerides: Theory and Observations of the Moon and Planets. Facultés universitaires Notre Dame de la Paix, Namur, Belgium, 28–31 July, 1980Presently a recipient of the Humboldt Award of the Alexander von Humboldt Foundation at the Astronomisches Rechen-Institut in Heidelberg and on leave from the Jet Propulsion Laboratory     

VLA observations of the Galilean satellites

Jupiter's Galilean satellites I–IV, Io, Europa, Ganymede, and Callisto have been observed with the VLA at 2 and 6 cm. The Jovian system was about 4.46 AU from the Earth at the time the observations were taken. The flux densities for satellites I–IV at 2 cm are 15 ± 2, 5.6 ± 1.2, 22.3 ± 2.0, and 26.0 ± 2.5 mJy, respectively, which corresponds to disk brightness temperatures of 92 ± 13, 47 ± 10, 67 ± 6, and 92 ± 9°K, respectively. At 6 cm flux densities of 1.10 ± 0.2, 0.55 ± 0.12, 2.0 ± 0.2, and 3.15 ± 0.2 mJy were found, corresponding to temperatures of 65 ± 11, 44 ± 10, 55 ± 6, and 105 ± 7°K, respectively. The radio brightness temperatures are lower than the infrared, the latter generally being consistent with the temperature derived from equilibrium with absorbed insolation. The radio temperature are qualitatively consistent with the equilibrium temperature for fast rotating bodies considering the high radio reflectivity (low emissivity) as determined from radar measurements by S. J. Ostro (1982). In Satellites of Jupiter (D. Morrison, Ed.). Univ. of Arizona Press, Tucson).     

Internal structures of the Galilean satellites

New models for the interiors of Io, Ganymede, and Callisto are proposed. The model of Io consists of a thin, high-rigidity outer layer separated from a solid interior by a thin, molten or partially molten shell. The modulus of rigidity of the outer layer must be at least 100 times larger than that of the underlying partially molten shell. These layers have thicknesses of order 100 km or less. The near-surface partially molten layer was most likely produced early in Io's history as a consequence of accretional heating; enhanced tidal heating in the outer rigid layer has kept the underlying region partially molten to the present day. The model of Ganymede consists of an ice outer layer, a shell of undifferentiated, primordial ice-silicate mixture, and a rock core. Accretional heating is responsible for melting the ice in the outer layers of Ganymede's initially homogeneous ice-silicate interior. Most of the rock in this outer layer accumulates in a shell on top of Ganymede's early cold and rigid central region; the water in the outer layer quickly refreezes. Heating of the undifferentiated region by the decay of radioactive elements in the silicate fraction would gradually warm it and reduce its viscosity. The rock layer would become gravitationally unstable and sink through the undifferentiated materials to form a rock core. Callisto's heavily cratered surface strongly suggests that relatively little, if any, ice-rock differentiation has occured in its interior.     

Mid-eclipse brightening of the Galilean satellites

Brinkmann (1973) has suggested that the Galilean satellites might briefly manifest a brightening at mid-eclipse due to a concentration of light refracted into the geometric umbra of Jupiter by the atmosphere around the terminator. Results obtained using two different models of the Jovian atmosphere indicate that such a brightening is unlikely even for Callisto due to the probable aerosol concentration in the Jovian atmosphere at the relevant altitudes.     

UBV photometry of the Galilean satellites

UBV observations of the Galilean satellites made at Lowell Observatory and Cerro Tololo Inter-American Observatory during 1973 and 1974 are reported. The dependence of brightness on solar phase angle for various faces of each satellite is determined. Significant differences in this dependence are found between different faces of the same satellite, between satellites, and between the present results and those of previous investigators. Rotational light and color-index curves are presented for the satellites and compared with earlier work. An apparent secular brightening of all four satellites between 1973 and 1974 is discussed.     

Eclipse spectrophotometry of the Galilean satellites

Narrowband spectrophotometry of satellite eclipses is presented for each of the Galilean satellites. Comparing the partially eclipsed full-phase satellite disk to the uneclopsed disk can reveal colorimetric inhomogeneities on the surface. The trailing half-disk of Ganymede is slightly blue compared to the leading half-disk. The trailing and leading half-disks of Callisto are similar to each other, as are the northern and southern half-disks. The northern half-disk shows evidence of blue and red portions. The trailing half-disk of Europa is redder than the leading half-disk.     

Galilean satellites and the Galileo space mission

The Galileo spacecraft arrived at Jupiter in December 1995 to start its two-year mission of exploring the Jovian system, The spacecraft will complete eleven orbits around Jupiter and have ten more close encounters with the outer three Galilean satellites, after the initial close approach to lo on December 7, 1995, Since the lo encounter occurred closer to lo than originally designed, the spacecraft energy change was greater than nominally planned and resulted in an initial spacecraft orbital period about 7 days less than that designed in the nominal tour, A 100-km change in the Io-encounter distance results in an 8-day change in initial period of the spacecraft. Hence the first Ganymede encounter was moved forward one week, and the aim points for the first two Ganymede encounters were altered, but all other encounters would occur on their nominal dates and at the nominal altitudes, This was accomplished without expending spacecraft fuel and resulted in the first Ganymede flyby occurring on June 27, 1996 rather than the nominally scheduled July 4.Earth- and spacecraft-based data were employed in developing ephemerides in support of the Galileo space mission. An analysis of CCD astrometric observations from 1992–1994, of photographic observations from 1967–1993, of mutual event astrometric data from 1973–1991, of Jovian eclipse timing data from 1652-1983, of Doppler data from 1987–1991, and of optical navigation data from the Voyager spacecraft encounter in 1979, produced the satellite ephemerides for the Galileo space mission.     

Cratering rates on the Galilean satellites

We exploit recent theoretical advances toward the origin and orbital evolution of comets and asteroids to obtain revised estimates for cratering rates in the jovian system. We find that most, probably more than 90%, of the craters on the Galilean satellites are caused by the impact of Jupiter-family comets (JFCs). These are comets with short periods, in generally low-inclination orbits, whose dynamics are dominated by Jupiter. Nearly isotropic comets (long period and Halley-type) contribute at the 1-10% level. Trojan asteroids might also be important at the 1-10% level; if they are important, they would be especially important for smaller craters. Main belt asteroids are currently unimportant, as each 20-km crater made on Ganymede implies the disruption of a 200-km diameter parental asteroid, a destruction rate far beyond the resources of today's asteroid belt. Twenty-kilometer diameter craters are made by kilometer-size impactors; such events occur on a Galilean satellite about once in a million years. The paucity of 20-km craters on Europa indicates that its surface is of order 10 Ma. Lightly cratered surfaces on Ganymede are nominally of order 0.5-1.0 Ga. The uncertainty in these estimates is about a factor of five. Callisto is old, probably more than 4 Ga. It is too heavily cratered to be accounted for by the current flux of JFCs. The lack of pronounced apex-antapex asymmetries on Ganymede may be compatible with crater equilibrium, but it is more easily understood as evidence for nonsynchronous rotation of an icy carapace.     

Radar observations of the icy Galilean satellites

We report 12.6-cm-wavelength radar observations of Europa, Ganymede, and Callisto made at the Arecibo Observatory in November 1977 and February 1979. When combined with previous observations, our results establish firmly the distinguishing radar properties of these satellites: (i) high geometric albedos, α; (ii) circular polarization ratios, μ_C, which anomalously exceed unity; (iii) linear polarization ratios, μ_L, which are approximately 0.5; and (iv) diffuse scattering which varies as cosⁿθ, where θ is angle of incidence and 1 ? n ? 2. We tabulate weighted-mean values of α, μ_C, μ_L, and n derived from observations between 1975 and 1979. The values of μ_C for Ganymede and Europa are nearly identical and significantly larger than that for Callisto. The values of n for Ganymede and Callisto are nearly identical and significantly smaller than that for Europa. Although significant albedo and/or polarization features are common in the radar spectra, the fractional rms fluctuation in disk-integrated properties is only ～10%. No time variation in the radar properties has been evident during 1976–1979.     

Surface color variations of the Galilean satellites

Color variations of the four Galilean satellites have been monitored during the summer of 1971 with the McDonald Observatory area-scanning photometer. All were found to vary with orbital phase, with the exception of Europa in B-V. The curves suggest complex variations in surface make-up with some large features, possibly the result of interaction with the Jovian environment.     

Problems on the Galilean satellites of Jupiter

The purpose of this paper is to present a critical review of some problems concerning the dynamics of Jupiter's Galilean system of satellites. Theory, ephemeris and observation are considered.Two theories were proposed by Ferraz-Mello and by Sagnier. The main characteristics of these theories are that the frequencies are allowed to be kept fixed for all times from the earlier stages, and so to have a purely trigonometric solution.For a completely satisfactory work we need many more observations than actually exist. Two kinds of observations seem to be the best suitable: long-focus photographic plates and photometric records of mutual events.The most recent photographic observations are discussed in order to state guidelines for future work. The problem of the precision of Sampson's tables is discussed on the grounds of the recent observations.Paper presented at IAU Colloquium, No. 28, Ithaca, N.Y., August, 1974.     

Strength of the lithosphere of the Galilean satellites

Several approaches have been used to estimate the ice shell thickness on Callisto, Ganymede, and Europa. Here we develop a method for placing a strict lower bound on the thickness of the strong part of the shell (lithosphere) using measurements of topography. The minimal assumptions are that the strength of faults in the brittle lithosphere is controlled by lithostatic pressure according to Byerlee's law and the shell has relatively uniform density and thickness. Under these conditions, the topography of the ice provides a direct measure of the bending moment in the lithosphere. This topographic bending moment must be less than the saturation bending moment of the yield strength envelope derived from Byerlee's law. The model predicts that the topographic amplitude spectrum decreases as the square of the topographic wavelength. This explains why Europa is rugged at shorter wavelengths (∼10 km) but extremely smooth, and perhaps conforming to an equipotential surface, at longer wavelengths (>100 km). Previously compiled data on impact crater depth and diameter [Schenk, P.M., 2002. Nature 417, 419-421] on Europa show good agreement with the spectral decrease predicted by the model and require a lithosphere thicker than 2.5 km. A more realistic model, including a ductile lower lithosphere, requires a thickness greater than 3.5 km. Future measurements of topography in the 10-100 km wavelength band will provide tight constraints on lithospheric strength.     

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.     

Optical polarimetry of the Galilean satellites of Jupiter

New measurements of the amount of polarization of the Galilean satellites are given and, within the context of other data, are interpreted as follows. The polarization of Europa is consistent with a water-frost surface. Io has a surface of partly absorbing crystals thought to result from evaporates released from the mantle and damaged by radiation. Ganymede has alternating water-frost areas and darker terrain, possibly of a silicaceous nature. Callisto is explained as having a mantle of ice containing embedded blocks of rocks, which occurred when recent evaporation left the blocks piled at the surface in a chaotic manner. This event occurred after the vicinity of Jupiter had been cleared of small orbiting objects able to impact Callisto. Meteorites which continue to enter within the sphere of influence of Jupiter can collide with Callisto only on its leading hemisphere, which is thereby comminuted by impacts. The surface of the trailing hemisphere is not regolithic.     

Infrared albedos and rotation curves of the Galilean satellites

Infrared (1.5–5 μm) albedos and rotation curves of the Galilean satellites have been obtained. The data suggest that the rotational variation in the infrared is less than ±10% for all four satellites. While no conclusion about rotational variation could be reached for Io, the 1.57 μm data for the outer three satellites marginally suggest phase correlation with the visual variation. The geometric albedos obtained are in general agreement with earlier results. For Io, the absorption feature near 1.5 μm found by Pilcher et al. (1972) is confirmed, thus contradicting the flat spectrum measured by Fink et al. (1973). Io and Ganymede were observed in the 1.57 μm bandpass as they reappeared from eclipse. The curve for Io shows a slight (<10%) overshoot similar to those sometimes reported for visual measurements. This result is based on a single reappearance, and is extremely tentative.     

Thermal migration of water on the Galilean satellites

We have modeled the thermal migration of water on the Galilean satellites under the assumption of ballistic molecular trajectories. We find that water migrating owing to solar radiation on an ice-covered satellite will build up in the temperate latitudes, in general not reaching the poles. As much as 50 m of ice may have been lost by this process from the equatorial regions of Europa over the age of the solar system. The disappearance of patches of ice—for instance, the bright rays surrounding some impact craters—from the equatorial regions of Ganymede and Callisto may approach a value (the irreversible evaporation rate) three orders of magnitude larger than the net equatorial loss rate for ice-covered Europa. The presence of water ice pole caps on Ganymede extending to the latitudes at which thermal migration becomes important suggests that some process distributed an extensive, thin covering of water on the satellite, and that the equatorial regions were subsequently cleared by the thermal process.     

Equipotential surface and normal gravity of the Galilean satellites

In this article, expanded equations of normal gravity on the equipotential surface are proposed for a natural satellite whose orbital plane is close to its equatorial plane. Tidal effects on the normal gravity are also discussed. The authors apply these to the Galilean satellites. Calculations suggest that the tides raised by Jupiter weakly affect the Galilean satellites. The radial displacements of the gravity due to the tides are in the range between 10⁻³ and 10⁻⁵ m s⁻², which are similar to the latitudinal and longitudinal displacements. The variations along the latitude circle are larger than those along the longitude circle. We conclude that the tidal effects on most of the Galilean satellites are larger than those on the Moon.     

Diameters of the Galilean satellites from pioneer data

Pioneers 10 and 11 have transmitted seven images of the Galilean satellites with surface resolutions on the order of several hundred kilometers. Because the point-spread function is well determined, it has been possible to measure the radius of each of the four satellites to a precision of typically ±30 km. The method used fits a semicircle to the illuminated limb by varying the center coordinates and radius until the best-fit criteria are satisfied. Careful attention is given to locating the true edge position within the blurred image. The radius determinations and corresponding densities for the satellites are: Io (1840 ± 30, 3.41 ± 0.19), Europa (1552 ± 20, 3.06 ± 0.15), Ganymede (2650 ± 25, 1.90 ± 0.06), and Callisto (2420 ± 20, 1.81 ± 0.05), where the units are in kilometers and grams per cubic centimeters, respectively. Since three images of Callisto were received, it has been possible to substantially decrease the uncertainties of the radius and density.