Callisto
Callisto is the least active geologically of the Galilean satellites. Basically a dead world, it bears the scars of innumerable meteoric impacts, with virtually no sign of major internal activity. Callisto is a world of craters, and to understand it we must explore the role that cratering plays in molding planetary surfaces.
The space between the planets is filled with debris, ranging from the larger asteroids, hundreds of kilometers in size, down to microscopic grains of dust. Inevitably, each planet collides with some of these fragments. The smaller particles do little damage; in the case of a planet with an atmosphere, like Earth, they burn up as meteors before reaching the surface, whereas on an airless planet, they erode the surface by sandblasting the exposed rock. The larger impacts are another matter, and the craters they produce can be the dominant features on the surface of a planet.
Voyagers 1 and 2 photographed most of the surface of Callisto at resolutions of a few kilometers or better. Shown here is a preliminary shaded relief map. Additional measurements will improve the accuracy of the coordinate system. [260-672]
Callisto is a world of craters, as is well shown in this Voyager 2 photomosaic taken from a distance of 400 000 kilometers. Craters about 100 kilometers in diameter cover the surface uniformly. Many have bright rims, perhaps composed of exposed water-ice. There are very few craters larger than 150 kilometers in diameter, however, indicating that the scars of very large impacts do not survive on the surface of Callisto. [P-21746B/W]
The concentric rings surrounding Valhalla are perhaps the most distinctive geological feature on Callisto. This Voyager 1 close-up shows a segment of the ridged terrain. The presence of superposed impact craters shows that the rings formed early in Callisto’s history; however, the density of craters is less here than on other parts of the satellite, where the surface is older. [P-22194]
We who live on Earth tend not to realize the importance of cratering, for the simple reason that our planet has very few craters, and these are frequently of volcanic rather than meteoric origin. Why are we so favored? Is there an invisible shield to protect us from the cosmic shooting gallery? Clearly not; the Earth has experienced just as many cratering impacts as has the Moon or other planets. The difference is not that craters are formed less often, but that the great geological activity of Earth—erosion, volcanism, mountain building, continental drift, etc.—erases craters as fast as they are formed. On the average, a 10-kilometer-wide crater is formed on Earth about once every million years, but all those older than a few million years have been eroded away, filled in, or crushed beyond recognition by crustal motion.
If a planet lacks great internal geologic forces, large craters can survive almost indefinitely. Such is the case for the Moon. Most of the volcanism and other activity on the Moon ceased 3½ billion years ago, as the dating of lunar samples obtained by the Apollo astronauts showed. Since that time, the lunar surface has been passively accumulating impact scars. The longer any particular surface area has been exposed, the more densely packed are the craters. Thus crater density is the first thing a planetary geologist looks for in photos of a new world. Craters are the touchstone of this field, revealing the degree of internal activity and allowing the determination of the relative ages of different surface units.
On Callisto the density of craters is very high. In some places they are packed as closely as one can imagine, particularly for craters several tens of kilometers in diameter. Although no one knows the exact rate of formation of impact craters on the Jovian satellites, geologists on the Voyager Imaging Team estimate that it would require several billion years to accumulate the number of craters found on Callisto. They therefore conclude that Callisto has been geologically inactive almost since the time of its formation.
Although superficially similar to the heavily cratered surfaces of the Moon and Mercury, Callisto is far from identical to these rocky worlds. One of the most obvious differences is a lack of craters larger than about 150 kilometers on Callisto, together with a tendency for large craters to have much shallower depths. Apparently the ice-rock composition of Callisto alters the ability of the crust to retain large craters. Geologists speculate that the ice flows over many millions of years, filling in crater floors and gradually obliterating the largest craters. There is also a conspicuous absence of mountains on Callisto, again suggestive of a weak, icy crust.
The IRIS instrument measured the temperature of spots on the surface of Callisto as each Voyager sped past. The measurements shown here were all made at equatorial latitudes (between -10° and 25°). Shown are very low predawn temperatures (-190° C) followed by an increase to a noon-time maximum of about -120° C, and then a drop again as the Sun sets. [260-735]
The most prominent features in the Voyager pictures are the ghost remnants of what must have been immense impact basins. The largest of these, the “bullseye” of the Voyager 1 images, has been named Valhalla, for the home of the Norse gods. These ghost basins have lost nearly all their vertical relief. What remains is a central, light-colored zone (probably the location of the original crater), surrounded by numerous concentric rings of subdued, bright ridges. Such features had never been seen before on any planet, and they appear to be the characteristic geologic feature of an ice-rock planet.
Little is known about the composition of Callisto’s surface, the material from which sunlight is reflected. It appears to be primarily dark rock or soil, but it lacks diagnostic spectral features, except for one infrared band due to water molecules bound in the soil. The many lighter spots and arcs that outline craters in the high-resolution pictures may be regions in which the ice is showing through, but these cover only a very small fraction of the exposed surface. (It should be noted that, although Callisto is the darkest of the Galilean satellites, the term “dark” is relative, for even Callisto is brighter than Earth’s moon.)
The daytime surface temperature of Callisto, observed both from the ground and by Voyager, is about -118° C. The Voyager infrared interferometer spectrometer also determined the minimum temperature, reached just before dawn, of -193° C. No atmosphere is expected at these cold temperatures, and none was seen.
Analysis of Voyager images provided an improved diameter for Callisto of 4840 kilometers, yielding an average density of 1.8 grams per cubic centimeter. As noted previously, it is this low density that leads to the conclusion that ice or water is an important component of the interior of Callisto. The ice has never been detected directly, but the peculiar nature of the craters seen by Voyager adds strong circumstantial support to this conclusion.
Callisto, with its heavy cratering, is the most familiar-looking of the Galilean satellites; if all of them had turned out to be as geologically dead as Callisto, planetary geologists would certainly have been disappointed. However, each satellite, progressing in toward Jupiter, presents increasing evidence of internal activity.