Crescent Ganymede observed by Voyager 1 in March 1979 from a distance of 115,000 kilometers by the spacecraft's wide-angle camera. Ganymede's natural color is a little like milk chocolate, as shown in this approximately "natural" color view.

GANYMEDE

With a diameter of 5268 kilometers, Ganymede is the largest satellite in the solar system and larger than the planet Mercury. Ganymede and Callisto are similar in size and density. Yet compared to "boring" Callisto, Ganymede has a very complex geologic history. The different geologic histories of Ganymede and Callisto have been a continuing puzzle. Impact craters, volcanic deposits, and tectonic structures are all common on the surface of Ganymede. Its craters record the flux of impacting projectiles, and its volcanic and tectonic features record part of the thermal and compositonal evolution of the interior. Recent theoretical work suggests that the eccentricity of Ganymede's orbit was tidally pumped up in ancient times, probably accounting for the long complex geologic history of Ganymede. Unlike Europa and Io, Ganymede is not undergoing tidal heating today.

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Ganymede

The global geography of Ganymede is shown in this view obtained from 2.5 million kilometers by Voyager 2. The surface is roughly equally divided between relatively dark terrain, which is heavily cratered and relatively old, and bright terrain, which is younger and has been deformed tectonically. The large, dark, oval-shaped mass is Galileo Regio. Visible in north and south polar regions are diffuse relatively bright polar shrouds, probably consisting of water frosts (at top and bottom). While most of Ganymede is relatively brownish in color, possibly due to the contamination of the icy surface by meteoritic material, these polar frosts are more neutrally colored. Bright spots are fresh, young impact craters and their ejecta patterns.

RESURFACING AND TECTONICS


Bright and Grooved Terrain on Ganymede

These views of bright terrain are the highest resolution images obtained on Ganymede. Grooves are typically 5 to 10 kilometers across, and in some areas appear to be comprised of parallel blocks of crustal material separated by faults. They resemble similar structures seen in Canyonlands National Park, Utah. Grooves are common on bright terrain and are generally believed to be due to extension of the surface. The cause and the amount of this extension are poorly known, but will be investigated by Galileo.

Dark Terrain on Ganymede

Roughly half of the surface of Ganymede consists of dark terrain. High crater counts indicate this is the oldest terrain on Ganymede. High-resolution images reveal that the geology of this terrain is exceedingly complex. Long arcuate fractures (termed furrows) are probably related to ancient impacts. In some cases, bright material appears to have oozed up through the center of these fractures. Numerous smaller fractures also cross the terrain in many areas. Smooth dark patches and sinuous scarps are abundant and may be related to volcanic resurfacing or to erosional degradation. Also, the floors of some craters appear to be partially filled with possible volcanic deposits. Dark terrain is a primary target for Galileo observation in 1996 and 1997.

Volcanic Landforms on Icy Satellites

This mosaic illustrates the great variety of relief-forming volcanic landforms observed on the icy satellites, including Ganymede. Smooth plains units are common on the satellites and are not shown here. These image tiles are arranged horizontally according to satellite system (J: Jupiter, S: Saturn, U: Uranus, N: Neptune). Each image is shown to the same scale (scale bar is 50 kilometers). A companion image gives a key to the name of the satellite on which the feature appears and the type of feature. The thickness of these volcanic deposits range from a few hundred meters to 2-3 kilometers. Ammonia-water lavas may have extruded from the interiors of these moons and formed some of these features.

CRATERS

Crater Types on Ganymede

This mosaic illustrates the great variety of unusual impact crater landforms observed on Ganymede. (Similar features are observed on Callisto as well). These image tiles are arranged according to crater size and age. Each image has been scaled so that the crater rims all appear to be the same size, so that the relative dimensions of associated internal and external structures and deposits can be compared. Scale bars are shown at the bottom of each image and give a sense of the true dimensions of each crater (each scale bar is 30 kilometers long). A companion image gives a key to the name for each crater type. Except for the two craters shown along the left-hand edge, all the crater types shown here are unique to Ganymede and Callisto. This probably reflects the icy composition of the crusts of these bodies. With increasing age (downward on the mosaic), the structures within craters become less pronounced and more difficult to distinguish. This may reflect increased heat flow in ancient times, leaving the near surface layers too weak to support deep impact craters. These ancient craters promptly collapsed, leaving relatively flat scars. The unusual bright circular features seen in the centers of larger craters (central dome craters) and surrounding larger ancient craters (palimpsests) may be relatively bright material uplifted from several kilometers below the surface.

Central Dome Crater Isis in 3-D

This stereo view dramatically reveals the morphology and structure of typical large craters on Ganymede. The narrow rimwall, lack of extensive terraces, and shallow topography are evident; these features contrast sharply with those of lunar craters of similar size. Also visible is a large, irregular, caldera-like feature that may be volcanic in origin (right of Isis).

Memphis Facula

Palimpsests are very ancient, relatively bright, circular impact scars very different in appearance from normal craters (visible throughout this scene). Memphis Facula (diameter 340 kilometers) is typical. Very little topography is apparent due to viscous relaxation (creep) of the outer layers of a planet, probably during the impact itself. The bright material is only a few hundred meters thick (except at the center) and may be material excavated from the interior of Ganymede. Such structures hold important clues to the early thermal history and composition of Ganymede. A relatively young impact basin, Nidaba, is visible at right. This ~300-kilometer-wide basin is younger and has more topography than Memphis.

Gilgamesh

At a diameter of 580 kilometers, Gilgamesh is the largest preserved impact basin on Ganymede. This mosaic shows the major components of the basin, together with an overview of the entire structure (lower right). The rim of the basin is shown at upper left, and the ~300 kilometers wide smooth central depression is shown at upper right. A portion of the ejecta blanket and secondary crater chains blasted out from the basin are shown at lower left. These are approximately 500 kilometers from the basin center.

Crater Chains

Three unusual crater chains have been identified on Ganymede. These were not formed by large impact basin forming events, but appear to be the impact scars of tidally disrupted comets that struck Ganymede. Similar crater chains are found on Callisto. These features serve to record the chacteristics of comets and support the "rubble-pile" model for comet nuclei, in which nuclei are formed of many small, loosely bound fragments.

Dark Ray Crater Kittu

Dark ray craters are rare and occur mostly on Ganymede. Kittu is ~30 kilometers across. Dark ray craters may form by the concentration of meteoritic material in ejecta patterns of craters that form on the trailing hemisphere of Ganymede (Ganymede rotates synchronously and one hemisphere always faces in the direction of orbital motion). Mapping the composition of these dark deposits may tell us what types of asteroids and comets pass through the Jupiter system.

All images by Paul M. Schenk, Lunar and Planetary Institute, Houston, TX.

©Lunar and Planetary Institute, 1997

Supplemental Reading Materials

Beatty J. K., O'Leary B., and Chaikin A., eds. (1990) The New Solar System. Sky Publishing Corporation, Cambridge, Massachusetts; Cambridge University Press, New York. 326 pp.

Moons and Rings (1991) Voyage Through the Universe series. Time-Life Books, Alexandria, Virginia. 144 pp.

Rothery D. (1992) Satellites of the Outer Planets. Clarendon Press, Oxford. 208 pp.

McKinnon W. and Parmentier E. (1986) Ganymede and Callisto. In Satellites (J. Burns and M. Matthews, eds.), p. 437, University of Arizona Press, Tucson.

Morrison D., ed. (1982) Satellites of Jupiter. University of Arizona Press, Tucson. 972 pp.

Schenk P. (1995) Volcanic constructs on Ganymede and Enceladus. Journal of Geophysical Research, 100, 19009-19022.

Schenk P. (1993) Central pit and dome craters: Exposing the interiors of Ganymede and Callisto. Journal of Geophysical Research, 98, 7475.