[PPS01-P04] Ganymede's volume expansion in the intermediate stage : Clues to temperature change and phase change of ices.
Keywords:Icy satellites, Ganymede, Thermal evolution
We focus on the thermal evolution of the solid icy shell. Ganymede's ice shell can be divided into the upper elastic lithosphere and lower plastic asthenosphere due to the large viscosity contrast. In the lithosphere, thermal expansion coupled with thermal evolution could generate surface stress through entire evolution (Zuber et al., 1984). The asthenosphere is composed of high-density ices (ice II, ice III, ice V and ice VI) (Schubert et al., 1986). Phase change between high-density ice and low-density ice such as ice II to ice Ih could generate large amount of volume expansion. Detailed thermal profile and its temporal change, especially in the lithosphere where the temperature decreases drastically toward surface, are critical for the evaluation of origin of the volume expansion.
Here, we are going to show the results of numerical simulation for thermal evolution, and discuss about accumulated stresses of elastic lithosphere especially during intermediate stage of Ganymede. In our simulation of thermal evolution based on the mixing length theory, the largest contribution to the volume expansion is associated with phase change of Ice２ to IceIh which overcomes the volume decreases due to growth of high-density ices. Our results indicate that phase change of ices generates enough volume increase during the intermediate stage and therefore we consider it as a plausible origin of Grooves.