Ganymede
The largest of the Galilean satellites (5270 kilometers in diameter), Ganymede was expected to be similar to Callisto in many ways. Both have low densities (for Ganymede, 1.9 grams per cubic centimeter), indicating a bulk composition of about half rocky materials and half water. In addition, their diameters differ by only eight percent, and both are far enough from Jupiter to escape the severe pounding Io receives from magnetospheric charged particles. Thus it was with great interest that Voyager scientists looked at the differences that emerged between these two satellites.
Shaded relief map of Ganymede. [260-673]
The surface of Ganymede as revealed by the Voyager cameras is one of great diversity, indicating differing periods of geologic activity. At one extreme there are numerous dark areas that resemble the surface of Callisto in both albedo (reflectivity) and crater density. The largest of these, Regio Galileo, stretches from the equator to latitude +45° and is 4000 kilometers across, nearly as large as the continental United States. This ancient terrain even preserves the remnants of a Callisto-type impact basin in the form of a system of parallel, curving, subdued ridges about 10 kilometers wide, 100 meters high, and spaced about 50 kilometers apart. The central part of this ghost basin is missing, however; it was presumably destroyed by subsequent geologic activity.
Other regions of the surface of Ganymede are clearly the product of intense internal geologic activity. Generally, these regions are of higher albedo and consist of many straight parallel lines of mountains and valleys. The Voyager geologists call these the grooved terrain because of their appearance from a great distance. Typically, these mountain ridges are 10-15 kilometers across and about 1000 meters high, similar in scale to some sections of the Appalachian Mountains in the Eastern United States. No higher relief exists, presumably for the same reason it is absent on Callisto. In many places the grooved terrain forms between areas of the older, darker surface, giving the appearance of mountains extruded between separating plates of ancient crust. In other areas the relationships are much more complex, with curved systems of grooves and ridges overlying each other, displaying intricate crosscutting relationships. Apparently Ganymede has experienced a series of mountain-building events.
The grooved terrains show a substantial range in age, as indicated by the crater densities. The oldest have nearly the same density as the ancient, dark plains, suggesting that formation of the grooved terrain began early, perhaps 4 billion years ago. The youngest grooved terrain has only about one-tenth as many craters, but this is still as many as are seen in the 3.5-billion-year-old lunar plains. The Voyager geologists believe that even in these areas geologic activity ceased billions of years ago.
The hemisphere of Ganymede that faces away from the Sun displays a great variety of terrain. In this Voyager 2 mosaic, photographed at a range of 300 000 kilometers, the ancient dark area of Regio Galileo lies at the upper left. Below it, the lighter grooved terrain forms bands of varying width, separating older surface units. On the right edge, a prominent crater ray system is probably caused by water-ice splashed out in a relatively recent impact. [260-671]
The sinuous nature of some of the narrower Ganymede groove systems can be seen in this oblique view, obtained on March 5 by Voyager 1. The area shown is about the size of California, with features visible as small as 5 kilometers across. The ridges appear to be the result of deformation of the crust of Ganymede. [P-21235]
Other types of surfaces are seen on Ganymede. Some regions are lightly cratered and smooth, with no indication of mountain building. In one place, there is a rough mountainous area that looks more like the jumbled lunar mountains than the long ridges and valleys of the rest of Ganymede. Many of the larger craters are distinguished by brilliant white halos and rays that suggest that impacts may have splashed large quantities of water or ice over the surface.
Many of the geologic features seen on Ganymede appear to have been caused by breaking, faulting, or spreading of the crust. In a few cases, there even seem to be indications of transverse, or sideways, motion along faults. This evidence is extremely exciting to geologists, since similar crustal motion on Earth is associated with the drift of continental plates, drawn by convection currents deep in the mantle. Such activity has never been seen before on another planet.
Astronomers on Earth had known since 1971 that about half the surface of Ganymede was covered with exposed water ice and about half with darker rock. An examination of the albedo variations in the Voyager pictures suggests that the ice is exposed near large craters and, to a lesser extent, in the grooved terrain, but no direct measurements were made by Voyager of the composition of different parts of the surface.
The presence of ice on the surface suggested to many astronomers that Ganymede might have a very tenuous atmosphere of water vapor or oxygen, which might be released by the breakdown of water vapor by sunlight. During the Voyager 1 flyby, a sensitive test for an atmosphere was made by the ultraviolet instrument from observations of the star Kappa Centauri as it was occulted by Ganymede. No dimming of the starlight was seen, yielding an upper limit for the surface pressure of the gases oxygen, water vapor, or carbon dioxide of 10⁻¹¹ bar, or one hundred-billionth the atmospheric pressure at Earth.
The differences between the geologic histories of Ganymede and Callisto are surprisingly large. No one knows the reason. Perhaps only a small increase in internal temperature is necessary to initiate geologic activity in an icy planet, and for some reason Ganymede crossed this threshold for a part of its history, whereas Callisto did not.
The complex patterns of the grooved terrain on Ganymede are apparent in high-resolution images. This picture, taken by Voyager 1 on March 5, has a resolution of about 3 kilometers and shows a region about the size of the state of Pennsylvania. The mountain ridges are spaced about 10 to 15 kilometers apart and rise about 1000 meters, similar to many of the mountains of Pennsylvania. The transections of different mountain systems indicate that they formed at different times. A degraded crater near the left center of the picture is crossed by ridges, indicating that it predated the period of crustal deformation and mountain building. [P-21279]
Ray systems of exposed water-ice are visible in this high-resolution mosaic of Ganymede, obtained by Voyager 2 on July 9 at a range of about 100 000 kilometers. The rough mountainous terrain at lower right is the outer portion of a large fresh impact basin that postdates most of the other terrain. At the bottom, portions of grooved terrain transect other portions, indicating an age sequence. The dark patches of heavily cratered terrain (right center) are probably ancient icy material formed prior to the grooved terrain. The large rayed crater at upper center is about 150 kilometers in diameter. [P-21770B/W]
The many variants of smooth and grooved terrain on Ganymede suggest a complex geologic history for this satellite. Four high-resolution views by Voyager 2 are grouped together. [260-678A]
A band of low mountain ridges has apparently been cut and offset by a fault.
Multiple sets of mountain ridges transect at nearly right angles. Some impact craters were formed before, and some after, the grooved terrain.
Many short parallel ridges butt into each other, making a crazy-quilt pattern.
In the center of this frame is an unusual smooth area, perhaps the result of flooding of the surface by material that filled in the grooves.