One of the most surprising results from the Galileo spacecraft's images of Callisto was the close-up views of the moon's surface. The poor-resolution images taken by the Voyager spacecraft showed a landscape covered with craters -- everyone expected to see the same thing on smaller scales, if not more so; meteor size distribution should follow a power law, with many more small meteors than large ones. Instead, pictures like the one below (bottom, left) were taken, with many smooth, non-cratered regions. Even many small craters that were present seemed to be in the process of disintegrating!

But what could be erasing craters? Callisto has been geologically dead for over 3 billion years, and it has no atmosphere. Material isn't just falling downhill because the smooth regions aren't at the bottom of slopes. Some possible explanations of this strange data can be found here.

Different resolution images of Callisto's surface: