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i- m dissociation of water ice exposed on the surface. Hydrogen atoms are knocked off the surface by bombardment from charged particles. This process is called 'sputtering'. Because the hydrogen is lighter than oxygen, it leaks to space, leaving the oxygen behind. This type of atmosphere is very, very thin [431].

There is no sign of current cryovolcanism on Ganymede's surface [432]. In fact, the observations by the Galileo spacecraft provide little evidence for cryovolcanism on Ganymede; yet, Galileo did make a more detailed investigation of a suspicious region first seen by Voyager. Galileo confirmed the appearance of a feature in Sippar Sulcus, indicating a series of fluid eruptions creating a flow that appears to have eroded into the icy surface creating its own 'caldera' [433] (Fig. 6.36). So, while a few isolated cryovolcanic flow features have been discovered, there is little evidence for widespread volcanic landforms on Ganymede [434].

Ganymede is strongly differentiated. Previous modeling supports the hypothesis that Ganymede has an outer shell composed of various ice solid phases and a massive metallic core in which a magnetic field might be generated (Fig. 6.37). As a result of the Galileo spacecraft mission, the magnetic field has been confirmed and Ganymede is the first satellite found to have its own magnetosphere [435]. Not only does Ganymede posses its own intrinsic magnetic field (not an induced one), it also has its own magnetosphere, one that deflects Jupiter's magnetosphere! The core size of Ganymede ranges from 25 to 33% of its surface radius. The thickness of the silicate mantle above the core varies between 900 and 1,100 km [436]. Spectroscopic data suggests that ice is the major component on Ganymede's surface. Its mean density suggests that Ganymede's interior consists of approximately 60% silicate rock plus metal and 40% volatile ices by mass.

Its moment of inertia factor is the smallest measured value for any solid body in the Solar System and indicates a strong concentration of mass towards its center

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