Jupiter’s giant moon Ganymede is an icy world, as are many satellites of the Outer Planets. But is also one of the few showing signs of some kind of tectonics. Its surface is made up of dark, cratered material, presumably an ancient mixture of rocky debris and ice, riven by swaths of lighter surface. The latter, which covers two-thirds, has little cratering and is a later feature of the moon’s surface. Somehow, Ganymede underwent a resurfacing, perhaps in a similar manner to neighbouring Europa – a simple ice ball – but not so all-consuming.
The event probably stemmed from the coming together of Jupiter’s largest moons into orbital resonance that generated sufficient gravitational energy to cause internal melting. Precisely how this achieved the intricacies of Ganymede’s surface is something of a mystery.
Images from Voyager and Galileo missions form stereoscopic pairs from which the moon’s topography can be derived with useful precision (Schenk, P.M. et al. 2001. Flooding of Ganymede’s’ bright terrains by low-viscosity water-ice lavas. Nature, v. 410, p.57-60). Using digital elevation data with high-resolution Galileo images, Schenk et al. have been able to subdivide the light swaths into three kinds of surface, reticulate, grooved and smooth at different elevations from highest to lowest. Large elevation differences of the order of 2 km are involved. That in itself is evidence that ice at the prevailing temperature behaves more like rock than glacial ice.
The greater surprise is that the lowest, smooth unit shows evidence of having formed by processes akin to volcanism, with calderas and features that engulf earlier structures. However, even the fine resolution of the latest images does not reveal “lava” flows. Some rifting mechanism seems to have encouraged emergence of water-ice “magma” to form the low smooth terrains. All very counter-intuitive for terrestrial volcanologists, because water “magma” must be more dense than the solidified flows forming from it, unlike silicate liquids or those rich in sulphur on Io. That makes the formation of high volcanoes impossible.
Presumably, the much higher grooved and reticulate terrains started in the same manner, as linear troughs, then to be deformed and thickened by “water tectonics”.