Is Callisto's surface really made of clay? The main information that scientists have to work with is the infra-red spectrum of radiation from the surface. Water ice is present, but not nearly as much as on Europa, or even Ganymede. Some estimates of the non-ice material show that it may be 55-75% by weight -- clearly it's not a simple skating rink. So what is the non-ice component? People have roughly matched Callisto's spectrum to a mixture of ice, hydrated minerals, and other minerals such as serpentine and magnetite [T.L. Rousch et al, Icarus 86, 355 (1990), W.M.Calvin and R.E. Clark, Icarus 89, 305 (1991)]. More recent work has hinted that the spectrum of the surface seems to mimic NH₄-bearing clays! [W.M.Calvin and R.E. Clark, Icarus 104, 1599 (1993)]. This theory is currently still speculative, but this fictional story assumes that at least some of the Callistan surface has clay properties.