Why is there a lack of small craters on Callisto?
Compared with flamboyant Europa, Io and Ganymede, the outermost Galilean moon Callisto was always thought rather drab. In Voyager images it epitomized the traditional stereotype for icy satellites: an old, frozen, pockmarked mudball. But Galileo observations tell a different story. Callisto is covered with large impact scars, ranging from craters kilometers in diameter to the so-called palimpsest named Valhalla, some 1,500 kilometers across. The surface is believed to date back more than four billion years to the rain of meteoritic and cometary debris left after the formation of the planets and satellites. In this sense, Callisto is indeed old. Seen close-up, however, Callisto’s surface is blanketed by fine, dark debris. Small craters, which on most other bodies are produced in abundance, are largely absent. Surface features appear softened and eroded. Clearly, some young processes have been at work.
