Crater palimpsests on Ganymede are circular features of high albedo apparently formed by impacts very early in the satellite's history. Some palimpsests have distinct outward-facing slopes at their outer margins, suggesting that they are distinct deposits of significant thickness. This view is supported by observations of superimposed craters that excavate through palimpsest deposits into underlying materials of contrasting albedo.

We suggest that crater palimpsests may result from volcanic extrusions triggered by large impact events early in Ganymede's history. Impact excavation that occurred to a sufficient depth would penetrate the satellite's thin, primordial lithosphere, allowing mobile, buoyant material from a vigorously convecting mantle to rise to the surface. Upon reaching the surface, the material would spread radially to produce a circular deposit of relatively high albedo. As the lithosphere cooled and thickened, palimpsest formation would have ceased, consistent with the observed concentration of palimpsest formation in the earliest part of Ganymede's recorded geologic history. Constraints on the nature of the extruded material may be derived from a simple model of emplacement. We use an analytic model for axisymmetric viscous flow to constrain the flow viscosity in terms of the aspect (height/radius) ratio &ggr; of the palimpsest. A measured value of &ggr;=0.02 would imply a viscosity marginally consistent with glacial ice viscosities. Smaller values of &ggr;, which are likely, would imply still lower viscosities, more appropriate to partially liquid flow. If such flows were described by a Bingham rheology, their yield strengths were of the order of 1 kPa. This is lower than is typical for even very mobile silicate lavas, but may be reasonable for a water/ice slush. Ice extrusion could have been as slow process taking thousands of years, while slush extension would have been essentially contemporaneous with the impact event. For the case of either solid ice and slush extrusion, our conclusions for the thermal structure of Ganymede at the time of palimpsest formation are similar: There was a thin (~10 km) conducting lithosphere underalin by a vigorously convecting mantle that was very close to the solidus temperature. ¿ American Geophysical Union 1990