Stratigraphic Relationships among the Upper Layers of the Outer Galilean Satellites, Inferred from the Investigation of their Ray Systems

Abstract

Callisto and Ganymede are the outermost Jovian icy satellites: they exhibit similar low density and inferred internal structure, but their surface appearance and their albedo are markedly different. Callisto is very dark and uniformly cratered, whereas Ganymede is splitted in dark cratered terrains and relatively bright grooved terrains. This distinction is confirmed by rayed craters distribution and occurrence: Ganymede has many remarkable ray systems, Callisto exhibits mostly haloed craters. The different scenario between the two satellites is believed to be related to the stratigraphy and the thickness of the upper layers of their crust. The study of Ganymede’s rayed craters distribution on grooved and cratered terrains reveals further dissimilarities. Dark rayed craters are present almost exclusively on cratered terrains and only in the small size range; bright rayed craters, on the other hand, occur in all size ranges and on both grooved and cratered regions. Supposing that a projectile excavates material from a depth of one tenth of the final crater size, we suggest the presence of a continuous layer, 4 ÷ 5 km thick, composed of a mixture of ice and rock un der Ganymede’s dark cratered areas. An analogous layer could be present on Callisto, but it should be ubiquitous and sufficiently thick to avoid the formation of prominent bright rayed craters.