Multi-ring basins

Callisto orbits Jupiter at a distance of 1.9 million kilometres its surface probably consists of silicate materials and water ice. There are only a few small craters (diameter less than a kilometre), but large so-called multi-ring basins are also present. In contrast to previous models, new determinations of the moon s magnetic field suggest the presence of an ocean under the moon s surface. It is unclear where the necessary energy comes from neither the sun s radiation nor tidal friction could explain this phenomenon. Ruiz (2001) suggests that the ice layers are much more closely packed and resistant to heat release than has previously been assumed. He considers it possible that the ice viscosities present can minimize heat radiation to outer space. This example shows the complex physical properties of water up to now, twelve different crystallographic structures and two non-crystalline amorphous forms are known Under the extreme conditions present in outer space, frozen water may well exist in modifications with as yet completely unknown properties. 

Wetherill G. W. (1981) Nature and origin of basin-forming projectiles. In Multi-ring Basins (eds. P. H. Schultz and R. B. Merrill). Pergamon, New York, pp. 1 — 18. 

Fig. 18.7 The Wilkes Land subgladal meteorite-impact crater was identified on the basis of gravity data obtained by the GRACE sateUite (Gravity Recovery and Climate Experiment). The location of the crater is indicated by a positive free-air gravity anomaly which occurs above a subglacial basin at 70°S and 120°E outlined by the solid white lines which delineate the rim of the crater. The dashed lines outline the hypothetical outer rings of this multi-ring basin. The coincidence of a positive gravity anomaly over a deep...

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