[Illustration: 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]] [Illustration: 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.