DIONE

one of the eight large, regular satellites of Saturn. It has a diameter of 1,120 kilometres (694 miles) and orbits Saturn in a prograde, nearly circular orbit at a mean distance of 377,400 km (233,990 mi). Like many objects in the Saturn system, Dione is involved in an orbital resonancei.e., its orbital period of about 66 hours is precisely twice that of Enceladus, another Saturn satellite. This phenomenon may be responsible for tidal heating in Enceladus. As a result of tidal friction, Dione's rotation is commensurate with its orbital motion, so that it always keeps the same side toward Saturn. The surface of Dione shows great brightness contrasts, with the trailing hemisphere generally darker than the leading one. On the average, however, Dione's reflectivity is very high, as would be expected for a surface composed of large amounts of water ice. The satellite's low density, which is 1.4 times that of water, is consistent with a bulk composition of approximately 60 percent ice and 40 percent rock. Apparently a substantial amount of ice melting and crustal resurfacing has occurred because the surface in general shows a low crater population compared with other Saturn satellites. Most craters are found on the bright, preceding hemisphere. The dark, trailing hemisphere is broken by many bright linear features that form a polygonal network. Some of these are associated with linear troughs and ridges. The bright features are thought to have been caused by the recondensation of volatile material that escaped from Dione's interior along linear fractures. The remarkable asymmetry of the satellite's surface is not understood, although there is evidence of a major impact near the centre of the linear network on the trailing side. in Greek mythology, a consort and, in one remote region, cult partner of Zeus, the king of the gods. Since the partner and wife of Zeus was normally the goddess Hera, it has been conjectured that Dione is an older figure than Hera. Dione was variously described. In the Iliad she is mentioned as the mother of the goddess Aphrodite by Zeus; in Hesiod's Theogony, however, she is simply identified as a daughter of Oceanus. Other writers have identified her as the mother of Dionysus.