Ganymede's Surface Research Articles

The magnetic field and magnetosphere of Ganymede

Within Jupiter's magnetosphere, Ganymede's magnetic field creates a mini‐magnetosphere. We show that the magnetic field measured during Galileo‧s second pass by Ganymede, with closest approach at low altitude almost directly over the moon's polar cap, can be understood to a large measure in terms of the structure of a vacuum superposition model of a uniform field and a Ganymede‐centered dipole field. Departures from the simple model can be attributed principally to magnetopause currents. We show that the orientation of the observed magnetopause normal is qualitatively consistent with expectations from the vacuum superposition model. The magnetopause currents inferred from the inbound boundary crossing are closely related to expected values, and the magnetic structure of the boundary is similar to that observed at the magnetopause of Earth. We use the vacuum magnetic field model to infer the magnetic field near Ganymede's surface, and thereby predict the particle loss cones that should be present along the spacecraft trajectory. By mapping a fraction of the corotation electric field into the polar cap, we determine expected flow velocities near closest approach to Ganymede as a function of reconnection efficiency. We conclude by discussing prospects for measurements on Galileo's remaining passes by Ganymede.

Chemical schemes for surface modification of icy satellites: A road map

The icy Galilean satellites of Jupiter are subject to magnetospheric plasma ion bombardment, which induces chemical changes within the ice. The possible detection of CO2 on the surface of Ganymede by the Galileo spacecraft makes for a more complicated chemistry and increases the number of chemical compounds that may then be present. We outline chemical schemes for the irradiation of pure and mixed ices H2O/CO2 and suggest species which observers may detect on Europa, Ganymede, and Callisto, such as C3O2, H2CO3, H2O2, CO3, HO2, CO, H2CO, CH2CO, as well as K2O, KOH, and SO3, from plasma implantation. Column abundances of compounds in the ice are calculated using a specified energy input and G values (yield per 100 eV).

A Search for a Sodium Atmosphere around Ganymede

Long-slit high-resolution spectra have revealed that Ganymede has an atmospheric sodium column density, between a distance of 7800 and 15600 km from the satellite surface, of less than 1 × 10 8atoms cm −2, a factor of 13 times smaller than the sodium density in the atmosphere of Europa at the same distance. Assuming similar processes creating sodium on both Europa and Ganymede, we estimate that the source rate for sodium production on Ganymede should be within a factor of 2 of that on Europa. Two possibilities for the lack of detectable sodium around Ganymede are that Ganymede's surface is depleted in native sodium compared to Europa's or that Ganymede has a magnetic field capable of standing off impacting energetic particles.

Detection of Ozone on Ganymede

An absorption band at 260 nanometers on the trailing hemisphere of Ganymede, identified as the Hartley band of Ozone (O3), was measured with the Hubble Space Telescope. The column abundance of ozone, 4.5 x 10(16) per square centimeter, can be produced by ion impacts or by photochemical equilibrium with previously detected molecular oxygen (O2). An estimated number density ratio of [O3]/[O2] = 10(-4) to 10(-3) requires an atmospheric density orders of magnitude higher than upper limits from spacecraft occultation experiments. Apparently, this O2-O3 "atmosphere" is trapped in Ganymede's surface ice, an inference consistent with the shift and broadening of the band compared with the gas-phase O3 band.

Sputtering of ices in the outer solar system

Exploration of the outer solar system has led to studies in a new area of physics: electronically induced sputtering of low-temperature, condensed-gas solids (ices). Many of the icy bodies in the outer solar system were found to be bombarded by relatively intense fluxes of ions and electrons, causing both changes in their optical reflectance and ejection (sputtering) of molecules from their surfaces. The small cohesive energies of the condensed-gas solids afford relatively large sputtering rates from the electronic excitations produced in the solid by fast ions and electrons. Such sputtering produces an ambient gas about an icy body, often the source of the local plasma. This colloquium outlines the physics of the sputtering of ices and its relevance to several outer-solar-system phenomena: the sputter-produced plasma trapped in Saturn{close_quote}s magnetosphere; the O{sub 2} atmosphere on Europa; and optical absorption features such as SO{sub 2} in the surface of Europa and O{sub 2} and, possibly, O{sub 3} in the surface of Ganymede. {copyright} {ital 1996 The American Physical Society.}

Photometry and surface structure of the icy Galilean satellites

Observations of the icy Galilean satellites from both spacecraft and ground‐based telescopes have been fit to radiative transfer models describing the surfaces in terms of their roughness, compaction state, and single‐particle phase function and albedo. The macroscopically rough nature of Ganymede and Callisto is similar to the Moon's, while Europa is significantly smoother. The sharply peaked opposition surge on Europa may be due to an extremely uncompacted upper regolith. However, if the peak is attributed to coherent backscatter, its surface may be the most compacted of the Galilean satellites. The compaction state of Callisto is similar to that of the Moon, while Ganymede's surface is more compacted. Callisto and possibly Europa exhibit textural differences in their leading and trailing hemispheres. This dichotomy is due to enhanced erosion by meteorites on the leading side.

Observations of Mars, Uranus, Neptune, Io, Europa, Ganymede, and Callisto at a wavelength of 2.66 mm

Brightness temperature observations at a wavelength of 2.66 mm of Uranus, Neptune, and the Galilean Satellites calibrated against Mars are presented. A detailed procedure is reported for using an accurate thermal model of Mars as a calibrator which yields brightness temperatures for Uranus and Neptune of 122 ± 5 K and 114 ± 7 K in excellent agreement with published “absolutely calibrated” measurements of these bodies. The results for the satellites are: Io, 101 ± 9 K, Europa, 102 ± 10 K, Ganymede, 78 ± 6 and 83 ± 5 K, and Callisto, 105 ± 6 K. The measurements for Io, Europa, and Callisto are consistent with thermal models of the lunar type but Ganymede is strongly anomalous with its 2.7-mm brightness temperature remaining very low and equal to those at centimeter wavelengths. It appears that for Ganymede alone, the millimeter surface emissivity is very low as it is at centimeter wavelengths for all the satellites. Apparently, the near surface of Ganymede is different from that of the other satellites in some unknown way to cause this phenomenon, possibly being more scattering at short wavelengths.

Power Law Rheology of Ice and the Relaxation Style and Retention of Craters on Ganymede

Numerical models of viscous relaxation of craters in ice, using rheological laws determined from laboratory data, previously indicated excessively short relaxation times (te, the time required for crater depth to become 1/e of its initial value) for craters on the icy satellites. A typical value of te was ∼104 years for a 100‐km‐diameter crater. These time scales are totally incompatible with the large regions of heavily cratered terrain on the icy satellites. We discuss the application of a numerical unite element model incorporating the most recent rheological data for ice at temperatures and pressures appropriate to the near‐surface regions of Ganymede. The model permits a full representation of a temperature and stress dependent (power law) viscosity for ice. For temperature gradients in reasonable agreement with those obtained from thermal and structural models of Ganymede we obtain relaxation times te ≳ 10 years for craters with diameters less than 100 km. For all craters with diameters greater than 10 km, the strong dependence of viscosity on stress was found to significantly shorten the relaxation time. An attempt was also made to determine whether Newtonian or non‐Newtonian rheological laws dominated relaxation for craters of various sizes. Newtonian rheologies appear to dominate crater relaxation only for situations where the effective temperature is several tens of degrees warmer than the surface is observed to be (perhaps because of the effect of an insulating regolith) and where temperature gradients are small. In this case, viscous relaxation of craters with diameters smaller than ≈25 km is expected to be dominated by Newtonian flow laws, provided that the currently accepted parameters (gross extrapolations from near‐melting temperature measurements) are valid for the low temperatures of the icy satellites. On the other hand, larger diameter craters undergo (at least initially) viscous relaxation determined by a strongly non‐Newtonian rheological law and have geologically short relaxation times (te ∼107 years, for a 300‐km crater), although these times may be extended by silicate suspensions and by dispersion hardening. This presumably accounts for the absence of such craters, in unmodified form, on Ganymede. Finally, the large stress exponent (n = 4.7) of the current rheological data for ice indicates a strong role for non‐Newtonian flow in determining the topography of viscously relaxing craterforms of this size. In principle, a threshold crater diameter (and hence stress) for the onset of non‐Newtonian viscous behavior could be obtained from Galileo images, allowing estimates to be made of near‐surface temperatures and thermal gradients in Ganymede's past. In practice, however, any morphological variability is almost certainly too subtle to be detected from imagery. The calculated relaxation times from this work are still much shorter than those indicated by the topographic relief of Ganymede's surface. This remains an unresolved problem. Possible ways in which the observations can be resolved with current laboratory data on ice deformation are discussed.

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.

The tectonics of Ganymede

The surface of Ganymede, the largest icy jovian satellite, is composed of two distinct terrain types showing abundant evidence of global-scale tectonics. There is geological evidence consistent with the formation of younger bright terrain by the emplacement of relatively clean ice into rift zones developed in the more silicate-rich dark terrain.

Grooved terrain on Ganymede

The icy crust of Ganymede comprises bright and dark areas. Investigation of Voyager 1 and 2 images has shown that bright terrain is grooved and separates dark polygons of cratered terrain. The grooved terrain contains alternating ridges and grooves in straight and curvilinear sets, which are locally interrupted by smooth patches and swaths. Cratered terrain, where 'it occurs in small wedges and slivers, has a pervasive grain of narrowly spaced furrows, and thus is transitional to grooved terrain. An analysis of the morphology of terrain types, and of superposition and cross-cutting relations, suggests that grooved terrain grew at the expense of cratered terrain, that tracts of cratered terrain were converted into grooved terrain in situ, and that vertical tectonism and shear movements dominated in the restructuring of Ganymede's surface. It is postulated that during a period in the planet's history when the lithosphere was thin, upwelling convection currents caused incipient rifting accompanied by intensive normal faulting; where rifting went to completion, crustal segments separated, locally spread apart, and sheared past one another. In places subduction and compression may have occured, but the evidence is inconclusive. Thus, the grooved terrain on Ganymede may record an early phase of ice-plate tectonics that caused rifting and drifting of the icy lithosphere, but, unlike silicate plate tectonics on Earth, may have resulted in only minor vertical turnover.

Ganymede: A Bit of Everything

Ganymede: A Bit of Everything

Surface compositions of the satellites of Saturn from infrared photometry

JHKL photometry (1.2-3.5 microns) of Rhea, Dione, Tethys, and the high-albedo side of Iapetus reveals broadband spectral signatures essentially identical to those of the water-ice-covered surfaces of Ganymede and the rings of Saturn, although significantly different from either the surface of Europa or laboratory measurements of pure water ice. The dark side of Iapetus has more neutral infrared colors, indicative of a nonicy surface.