Mass movement and landform degradation on the icy Galilean satellites: Results of the Galileo nominal mission

The Galileo mission has revealed remarkable evidence of mass movement and l andform degradation on the icy Galilean satellites of Jupiter. Weakening of surface materials coupled with mass movement reduces the topographic relie f of landforms by moving surface materials down-slope. Throughout the Galil eo orbiter nominal mission we have studied all known farms of mass movement and landform degradation of the icy galilean satellites, of which Callisto , by far, displays the most degraded surface, Callisto exhibits discrete ma ss movements that are larger and apparently more common than seen elsewhere . Most degradation on Ganymede appears consistent with sliding or slumping, impact erosion, and regolith evolution, Sliding or slumping is also observ ed at very small (100 m) scale on Europa. Sputter ablation, while probably playing some role in the evolution of Ganymede's and Callisto's debris laye rs, appears to be less important than other processes. Sputter ablation mig ht play a significant role on Europa only if that satellite's surface is si gnificantly older than 10(8) years, far older than crater statistics indica te. Impact erosion and regolith formation on Europa are probably minimal, a s implied by the low density of small craters there. Impact erosion and reg olith formation may be important on the dark terrains of Ganymede, though s ome surfaces on this satellite may be modified by sublimation-degradation. While impact erosion and regolith formation are expected to operate with th e same vigor on Callisto as on Ganpmede, most of the areas examined at high resolution on Callisto have an appearance that implies that some additiona l process is at work, most likely sublimation-driven landform modification and mass wasting. The extent of surface degradation ascribed to sublimation on the outer two Galilean satellites implies that an ice more volatile tha n H2O is probably involved. (C) 1999 Academic Press.