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NASA NTRS1981

Voyager photometry of surface features on Ganymede and Callisto

Photometric properties of selected surface features on Ganymede and Callisto are studied, using Voyager images over phase angles from 10 to 124 deg, taken with a clear filter (effective wavelength of approximately 0.5 microns). Normal reflectances on Ganymede average 0.35 for the cratered terrain, and 0.44 for the grooved terrain; the ubiquitous cratered terrain on Callisto is 0.18. The photometric properties of these regions are described by a simple scattering function, where the function of the phase angle is qualitatively similar to that of the moon, i.e., concave upward. By contrast, bright craters on both satellites have functions of the phase angle which are concave downward. The scattering function is not Lambertian, and may be due to an admixture of a small amount of dark, opaque silicate grains with the frost deposits. The brightest craters on Callisto have reflectances which are 10% lower than the brightest craters on Ganymede, and both have similar scattering laws.

Squyres, S. W.↗

NASA NTRS1982

The tectonics of Ganymede

Evidence of tectonic activity has been found on the Jupiter moon Ganymede, which is the largest solar system icy object known. The transition of dense ice polymorphs in the deep interior of large icy bodies to either less dense polymorphs or liquid water at shallow depths, as internal differentiation and formation of a silicate core occurs, could increase Ganymede's surface area by as much as 7%. If the tectonic evolution of Ganymede is to be described by a progressive fragmentation of an initially uniform lithosphere in response to planetary expansion, the stress transmitted across different areas of bright terrain must differ significantly. Alternatively, lithospheric thickness variations or such other mechanisms as tidal distortion or convection stresses beneath the lithosphere may have been important.

Parmentier, E. M.↗

NASA NTRS1982

Tectonic deformation of Galileo Regio and limits to the planetary expansion of Ganymede

Galileo Regio is the largest and most prominent unit of dark, ancient, heavily cratered terrain on Ganymede. Its major tectonic feature, an arcuate system of rimmed furrows, formed very early, as nearly the entire cratering record post-dates it. Galileo Regio has undergone very little subsequent structural alteration, as opposed to the rest of the planet (i.e., replacement by grooved and smooth terrain). Its survival as an intact lithospheric unit during the era of grooved terrain formation constrains the concomitant expansion of Ganymede, if any, to be less than one percent in radius. This limit is derived from an analysis which considers Galileo Regio to be a thin, freely floating elastic spherical shell on an expanding planet. A comparison of tectonic features (both endogenic and impact produced) on Ganymede and Callisto suggests that the ultimate source that powered the creation of grooved terrain lies in the Ganymedean core.

Mckinnon, W. B.↗

NASA NTRS1982

Constraints on the expansion of Ganymede and the thickness of the lithosphere

Since grooved terrain has formed from extensional faulting of at least one half of the surface of Ganymede, grooves may be due to planetary expansion which can be constrained by determining the extension associated with groove formation. The geometry and kinematics of a fault or group of faults must be known in order to determine the extension. Although neither of these is known for grooves on Ganymede, geologically reasonable end members of the dip, displacement and extension of faults which bound the graben can be estimated in order to place valuable constraints on possible surface area and radius increases as well as on the thickness of the Ganymede lithosphere. Minimum lithosphere thickness is estimated to be 5 km in the Marius and 9 km in the Galileo regions, at the time of furrow formation, and 4 km at the time of groove formation.

Golombek, M. P.↗

NASA NTRS1982

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 micron. The 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 the analysis. The result indicates that the phase integrals (0.8 for Ganymede and 0.6 for Callisto) derived by S. W. Squyres and J. Veverka (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.

Squyres, S. W.↗

NASA NTRS1985

Polar Cap Formation on Ganymede

Since thermal migration is not an effective mechanism for water transport in the polar regions at the Galilean satellites, some other process must be responsible for the formation of Ganymede's polar caps. It is proposed that Ganymede's polar caps are the optical manifestation of a process that began with the distribution of an ice sheet over the surface of Ganymede. The combined processes of impact gardening and thermal migration led, in regions at latitudes less than 40 to 45 deg., to the burial of some fraction of this ice, the migration of some to the polar caps margins, and a depletion of free ice in the optical surface. At higher latitudes, no process was effective in removing ice from the optical surface, so the remanants of the sheet are visible today.

Pilcher, C. B.↗

NASA NTRS1985

Lineaments on Ganymede: New Evidence for Late Tectonic Activity

Families of lineaments in the Gilomesh and Ninki basins of Ganymede imply post impact tectonic activity. The grooves, previously considered the youngest tectonic features, are estimated to have formed between 3.8 and 3.1 Gyr ago. One rayed crater however, is probably less than 1 Gyr old, implying tectonic activity on Ganymede has extended nearly to the present. Like the grooves, the lineaments appear to be extensional. The parallel trends and nearly contiguous associations of the lineaments with the grooves imply that both are products of the same stress systems. The young inferred age of the lineaments implies that they (and presumably also the grooves) are not associated with stresses in the cooling of fresh deposits of bright terrain, but are probably associated with underlying convective stress patterns, the long implied duration of an extensional stress regime in Ganymede's lithosphere is consistent with the stress models of derived assuming a differentiated interior.

Croft, S. K.↗

NASA NTRS1985

Polar frost formation on Ganymede

Voyager photographs have shown the presence of polar frost on Ganymede, a satellite of Jupiter. A number of models have been proposed for the formation of this feature. The models are based on the transport of material from the equatorial to the polar regions. The present paper is concerned with a model regarding the origin and appearance of the Ganymede caps which does not depend on such a transport. The model is based on observations of the surficial changes produced by ion bombardment. It is pointed out that experiments on ion and electron bombardment of water ice at low temperatures have shown that these particles sputter significant quantities of water molecules. In addition, they also change the visual characteristics of the surface significantly. Ion bombardment competing with thermal reprocessing may be sufficient to explain the latitudinal differences observed on Ganymede.

Johnson, R. E.↗

NASA NTRS1986

Early thermal profiles and lithospheric strength of Ganymede from extensional tectonic features

The early thermal profiles and the lithospheric stability and strength of Ganymede are quantitatively determined on the basis of brittle lithosphere thickness estimates derived from the width and spacing of extensional tectonic features, together with lithospheric strength envelopes for ice. Plots of the brittle and ductile yield stress vs. depth for the icy lithosphere of Ganymede exhibit a linear increase in brittle strength with depth to a maximum at the brittle-ductile transition that is followed by an exponential decrease in ductile yield stress with depth. The results obtained imply that the thermal gradient and lithospheric strength have varied laterally by factor as great as 5, and that Ganymede underwent cooling in a highly inhomogeneous fashion with lateral thermal anomalies. The present analysis furnishes reasons for the stability of large cratered terrain remnants.

Golombek, Matthew P.↗

NASA NTRS1988

Dome craters on Ganymede

Voyager observations of Ganymede show broad, high-albedo, topographic domes situated within the central pits of some impact craters, referred to in this study as 'dome craters'. Of 56 dome craters identified on Ganymede, all but two can be placed into one of two classes, based on the ratio of dome diameter to crater rim diameter. Two new hypotheses for the origin of the domes involving diapirism as an agent of dome formation are offered. Implicit in both hypotheses are possible regional heat flux variations. Under these scenarios, 'relaxation' of crater relief may not be homogeneous in space and/or time, and crater morphology may not be a consistent indicator of crater age. Plutonic intrusions within the upper lithospheres of Ganymede (and Callisto) may have played a far more important role in heat transport on these satellites than previously noted.

Moore, Jeffrey M.↗

NASA NTRS1990

Formation of crater palimpsests on Ganymede

A model is presented for the mechanism of the formation of crater palimpsests (circular features of very little relief) on Ganymede, that resolves some of the inconsistencies present in previous hypotheses of the palimpsest formation. It is suggested that palimpsest formation may be a result of cratering during a period of vigorous convection within the satellite early in the Ganymede's history. The morphology of palimpsests is reviewed and an updated compilation of palimpsests distribution and dimensions is obtained on the basis of final Voyager data. Calculations are performed that show that palimpsest formation is expected when large impacts take place in an icy lithosphere that is underlain by a zone of warm buoyant vigorously convecting material. The implications of the model for the thermal history of Ganymede is discussed.

Thomas, Paul J.↗

NASA NTRS1991

Tidal origin of the Laplace resonance and the resurfacing of Ganymede

In the present scenario for the Laplace resonance's tidal origin, the Laplace relation for the three inner Galilean satellites Io, Europa, and Ganymede may have been established within a previous three-body resonance which excited Ganymede's orbital eccentricity to a high value. The tidal heating enhancement associated with that high-eccentricity period furnishes a plausible explanation for the Ganymede surface, which appears to be geologically younger than that of Callisto.

Malhotra, Renu↗

NASA NTRS1995

Estimates of Comet Fragment Masses from Impact Crater Chains on Callisto and Ganymede

Chains of impact craters, or catenae, have been identified in Voyager images of Callisto and Ganymede. Although these resemble in some respects secondary crater chains, the source craters and basins for the catenae cannot be identified. The best explanation is a phenomenon similar to that displayed by former comet Shoemaker-Levy 9; tidal (or other) breakup close to Jupiter followed by gradual orbital separation of the fragments and collision with a Galilean satellite on the outbound leg of the trajectory. Because the trajectories must pass close to Jupiter, this constrains the impact geometry (velocity and impact angle) of the individual fragments. For the dominant classes of impactors, short period Jupiter-family comets and asteroids, velocities at Callisto and Ganymede are dominated by Jovian gravity and a satellite's orbital motion, and are insensitive to the pre-fragmentation heliocentric velocity; velocities are insensitive to satellite gravity for all impactor classes. Complex crater shapes on Callisto and Ganymede are determined from Voyager images and Schmidt-Holsapple scaling is used to back out individual fragment masses. We find that comet fragment radii are generally less than about 500 m (for ice densities) but can be larger. These estimates can be compared with those for the Shoemaker-Levy 9 impactors.

McKinnon, William B.↗

NASA NTRS2000

Trapped Energetic Electrons in the Magnetosphere of Ganymede

On May 7, 1997, the Galileo orbiter flew through the magnetosphere of Ganymede and crossed flux tubes connected at both ends to the satellite. Energetic electrons, observed during this encounter by means of the Energetic Particle Detector on board Galileo, showed double loss cones and "butterfly" type pitch angle distributions, as has been noted in past publications. In addition, as the spacecraft flew toward Ganymede, both the shape and magnitude of the spectrum changed. The intensities decreased, with the greatest depletion observed at the lowest energies, and the monotonic slope characteristic of the Jovian environment was replaced by a rollover of the spectrum at the low-energy end. The spectra lead us to infer a strongly energy-dependent injection efficiency into the trapping region. As on previous encounters, the pitch angle distributions confirmed the position of the magnetopause as indicated by the magnetometer measurements, but the spectra remained Jovian until the trapping region was reached. Various physical mechanisms capable of generating the observed spectra and pitch angle distributions, including downstream reconnection insertion followed by magnetic gradient drift and absorption of the lowest-energy electrons by Ganymede and injection from Jovian flux tubes upstream are assessed.

Eviatar, Aharon↗

NASA NTRS2003

Manifestations of Strike-Slip Faulting on Ganymede

Voyager images of Ganymede suggested that strike-slip faulting may have taken place [1, 2], but the role of this process in shaping grooved terrain was uncertain. In Galileo high-resolution images of Ganymede's surface, we recognize three signature features of strike-slip faulting: (1) en echelon structures, (2) strike-slip duplexes, and (3) offset preexisting features. We have undertaken a study to recognize and map these features, and identify any morphological progressions of strike-slip features. This will allow a better understanding of the structural history of Ganymede, and the formation and evolution of grooved terrain.

DeRemer, Lindsay C.↗

NASA NTRS2005

Numerical Modeling of Extensional Necking Instabilities: Application to Ganymede's Grooved Terrain

Ganymede s pervasive 5-10 km-wavelength grooves have been suggested to result from a necking instability during an epoch of lithospheric extension, but to date few quantitative studies of groove formation have been performed. We present two-dimensional numerical models of necking instabilities under conditions that are appropriate to Ganymede at the time of groove formation. Preliminary simulations indicate that extensional necking instabilities can occur under a range of conditions, many of which may be relevant to Ganymede. The form of the surface topography produced by these instabilities varies as a function of the strain rate, amount of extension, initial topographic perturbation, and rheological parameters.

Bland, M. T.↗

NASA NTRS2008

Material Units, Structures/Landforms, and Stratigraphy for the Global Geologic Map of Ganymede (1:15M)

In the coming year a global geological map of Ganymede will be completed that represents the most recent understanding of the satellite on the basis of Galileo mission results. This contribution builds on important previous accomplishments in the study of Ganymede utilizing Voyager data and incorporates the many new discoveries that were brought about by examination of Galileo data. Material units have been defined, structural landforms have been identified, and an approximate stratigraphy has been determined utilizing a global mosaic of the surface with a nominal resolution of 1 km/pixel assembled by the USGS. This mosaic incorporates the best available Voyager and Galileo regional coverage and high resolution imagery (100-200 m/pixel) of characteristic features and terrain types obtained by the Galileo spacecraft. This map has given us a more complete understanding of: 1) the major geological processes operating on Ganymede, 2) the characteristics of the geological units making up its surface, 3) the stratigraphic relationships of geological units and structures, and 4) the geological history inferred from these relationships. A summary of these efforts is provided here.

Patterson, G. Wesley↗

NASA NTRS2022

Chapter 2.6: Physical Chemistry and Thermal Evolution of Ices at Ganymede

Ganymede’s surface is composed mostly of water ice and other icy materials in addition to minor non-ice components. The formation and evolution of Ganymede’s landforms highly depend on the nature of the icy materials as they present various thermal and rheological behaviors. This chapter reviews the currently known thermodynamic parameters of the ice phases and hydrates reported on Ganymede, which seem to affect the evolution of the surface, using mainly results from the Voyager and Galileo missions

C Ahrens↗