Figure 36.27 Interior of Ganymede.

Voyager images were used to create a global view of Ganymede. The cutout reveals the four-layer interior structure, based on measurements of Ganymede's gravity field and theoretical analyses using Ganymede's known mass, size, and density. Ganymede's surface is rich in water ice, and Voyager and Galileo images show features that are evidence of geological and tectonic disruption of the surface in the past. These geological features reflect forces and processes deep within Ganymede's interior. Based on geochemical and geophysical models, scientists expected Ganymede's interior to consist of either an undifferentiated mixture of rock and ice or a differentiated structure with a large Moon-sized core of rock and possibly iron, overlain by a deep layer of warm, soft ice and capped by a thin, cold, rigid ice crust. Galileo's measurements of Ganymede's gravity field have basically confirmed the differentiated model and allowed scientists to estimate the size of these layers more accurately. In addition, the data strongly suggest a dense metallic core at the center of the rock core. This metallic core implies a greater degree of heating at some time in Ganymede's past than had been proposed before and may be the source of Ganymede's magnetic field, which was discovered by Galileo's space physics experiments. (NASA/JPL)

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