This Cassini image of Enceladus is featured on the Astronomy Picture of the Day site. Geologists are still trying to understand what they see. They work on some basic principles: cratered surfaces are older, smooth surfaces are new and indicate resurfacing, ridges and grooves show inner processes massaging the outer crust. But there’s more to it than that. Scientists want to know how and why certain structures form. They want to know the history behind it. They certainly want to know what’s inside this moon, not quite big enough to reach from San Francisco to Los Angeles.