Dehydration of primordial hydrous rock in Ganymede: formation of the conductive core, the grooved terrain, and the contrasting interior from Callisto

The small value of the moment of inertia factor and the strong intrinsic magnetic field observed for Ganymede are consistent with a highly differentiated interior with a conductive dense core. The Gany-mede's surface is commonly divided into two units, termed dark terrain and bright terrain. The latter, or grooved terrain, has been interpreted as grabens resulting from lithospheric extension. According to geological estimates, 3-4% increase in the satellite radius may be required for their formation. Hence Ganymede has likely undergone significant tempera-ture rise inside allowing the separation of a conduc-tive core and global expansion during its history. But its mechanisms still remain an open question. This study numerically investigates the possible influ-ence of hydrated rock on the thermal history of Ganymede. Here we assumes that Ganymede had an initial structure with a relatively thin H2O ice mantle and a low temperature primordial core made of the mixture of hydrous rock and Fe-sulfide similar to hydrated primitive meteorites. The primordial core is heated by the decay of long-lived radioactive nu-clides. The rise of core temperature is kept slow after the occurrence of effective thermal convection in the core having low viscosity (~1E20 Pas) of hydrous rock. However, once the temperature reaches the dehydration point (~900 K) then the highly viscous, anhydrous region begins to grow associated with the release of H2O to the mantle. The core temperature thereby becomes to increase faster with accelerating the further dehydration of primitive matter. The core temperature subsequently exceeds the eutectic point of the Fe-bearing sulfide and oxide so that the formation of a conductive dense core could occur by their gravitational segregation. Such thermal runaway occurs when the dry rock mass fraction including sulfide is larger than 48 wt%, which is consistent with the geodetic constraints. Dehydration of serpentine has increase in total vol-ume of ~10 %. This implies that the dehydration of primordial core in Ganymede may result in the in-crease in satellite radius about 1~2%, near the geo-logical estimates. The dehydration occurs at 1 to 2 Gyr after the satellite formation, giving an explana-tion for the cratering age of grooved terrain. The thermal history is also dependent on the satellite mass. When the dry rock mass fraction is between 48-54%, the model satellite with the Ganymede mass fully differentiates whereas that with the Callisto mass does not heat up sufficiently to melt the sulfide component or dehydrate the primordial core because of the efficient heat loss for smaller body. These agreements with observations support that the thermal runaway of primordial hydrated core is a favourable process triggering the formation of a dense conductive core and the grooved terrain of Ganymede, contrasting with Callisto having been geologically inert and partially differentiated.