Previous analyses into viscous relaxation of impact craters on Ganymede have predicted short characteristic relaxation times (of the order of 100 Myr or less for larger craters), in disagreement with the apparent great ages of Ganymede's terrains. By applying improved understanding of rheologic parameters and initial crater shapes to a viscoelastic model, we calculate upper limit (zero heat flow) relaxation times in excess of the age of the solar system, eliminating this discrepancy. These very long relaxation times are not due to elastic effects, but are simply due to the effective viscosity of water ice at the appropriate temperatures and stresses. ¿ 2000 American Geophysical Union