Callisto: A World in its Own Right

Callisto, once unknown and then disregarded after Voyager, has emerged in the post-Galileo era worthy of the same intense scientific scrutiny that is lavished upon her sisters, playing an essential role in our understanding of the evolution of icy moons, and in a larger sense, the grand tapestry of solar system history. Along with the discovery of Callisto’s conducting, probably fluid sub-surface layer, major Gulileo discoveries about Callisto include the complete absence of cryo-volcanic resurfacing, the relatively undifferentiated interior, and the presence of massive landform erosion from sublimation processes. Callisto’s landscape at decameter scales is unique among the Galilean satellites, and might be most akin to that of cometary nuclei. The process of sublimation degradation, previously underappreciated, is now recognized as a major surface modification process on Callisto. Its role in mass wasting and landslide initiation was elemental in creating the bizarre and astonishing scenery imaged by Galileo.