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Saturn's moon Enceladus
This Voyager 2 mosaic of Enceladus was made from images taken through the clear, violet and green filters on August 25, 1981, from a distance of 119,000 kilometres (74,000 miles). It shows the water-ice-covered surface of Enceladus, one of Saturn's icy moons. Enceladus' diameter of just 500 km would fit across the state of Arizona. Yet despite its small size, Enceladus exhibits one of the most interesting surfaces of all the icy satellites. Its surface reflects about 90% of the incident sunlight (about like fresh-fallen snow), placing it among the most reflective objects in the Solar System. Several geologic terrains have superposed crater densities that span a factor of at least 500, thereby indicating huge differences in the ages of these terrains. It is possible that the high reflectivity of Enceladus' surface results from continuous deposition of icy particles from Saturn's E-ring, which in fact may originate from icy volcanoes on Enceladus' surface.
Some regions of Enceladus show impact craters up to 35 kilometres (22 miles) in diameter, whereas other areas are smooth and uncratered. Linear sets of grooves tens of kilometres long traverse the surface and are probably faults resulting from deformation of the crust. The uncratered regions are geologically young and suggest that Enceladus has experienced a period of relatively recent internal melting. The rims of several craters near the lower centre of the picture have been flooded by the smooth terrain. Features as small as 2 kilometres (1.2 miles) are visible in this highest-resolution view of Enceladus. Some terrains are dominated by sinuous mountain ridges from 1 to 2 km high (3300 to 6600 feet). Some terrains appear to have formed by separation of icy plates along cracks, and other terrains are exceedingly smooth at the resolution of this image. The implication carried by Enceladus' surface is that this tiny ice ball has been geologically active and perhaps partially liquid in its interior for much of its history. The heat engine that powers geologic activity here is thought to be elastic deformation caused by tides induced by Enceladus' orbital motion around Saturn and the motion of another moon, Dione.