Dione

In terms of size, Dione is the near-twin of Tethys. A bulk density of 1.43 g/cm3 and an overall albedo of about 50 per cent indicated a predominantly icy body with an exposed icy surface. Telescopic studies noted a 0.6-magnitude variation around its orbit.43 The Voyager imagery revealed why: there is dark mottling with an albedo of about 20 per cent on the trailing hemisphere, with a pattern of bright streaks with 70 per cent albedo superimposed upon it making a striking contrast.44,45

A physiographic study identified several associated terrain types.46 Predominant is a rugged 'highland' terrain with many craters up to 100 kilometres in diameter. In general, these craters are shallower than on Tethys. Most of the larger craters have terraced walls and central peaks. Scarps up to 100 kilometres in length run over this terrain. In addition to a less rugged cratered terrain, defined as cratered plains, there are also smooth plains. There are some troughs on the smooth plains, and although these are typically less than 100 kilometres in length, a few exceed 500 kilometres, and in most cases there is a 'pit' at either end. In fact, there may be a global network of fractures. The largest craters - about 200 kilometres across - are on the trailing hemisphere; however, Dione has no crater to match Tethys's Odysseus. Evidently, early resurfacing erased the record of the post-accretional bombardment and the extant cratering derives from the later population of impactors. Dione's relatively high density indicates that it contains a significant proportion of rock, so radiogenic heating would have kept its interior warm. It has been suggested that after the formation of the brittle lithosphere, internal heat prompted fluid ammonia-water ice to erupt from fractures to form the plains.47 Indeed, a series of ridges that rise only a few hundred metres but extend up to 100 kilometres in length may be the fronts of low-viscosity flows. The fact that both Enceladus and Dione, whose orbits are in mutual resonance, have undergone sufficient internal melting to prompt surface flows is evidence for tidal stresses being the main heating agent. In the middle of the radiating pattern of broad wispy streaks on the trailing hemisphere there is an elliptical feature (named Amata) several hundred kilometres in diameter with a dark central patch which might mark an impact, but the streaks are not rays of bright ejecta splashed out from it. In higher resolution imagery, narrow linear troughs and ridges are evident emerging from some of the bright streaks, projecting around onto the leading hemisphere.48 The fact that the streaks seem to be associated with lineaments hints they might be pyroclastic deposits of a clathrate that was warmed at shallow depth by a pocket of radioactive elements and vented under pressure through fissures.49 However, volcanic models are problematic due to an absence of expected visible evidence, such as overwhelmed or flooded craters. Nevertheless, Dione has undergone significant endogenic activity. As for the dark mottling, the 'middle' icy moons orbit in the most intense region of the rapidly rotating magnetosphere, and their trailing hemispheres are irradiated as they synchronously

The largest crater in this Voyager mosaic of part of the Saturn-facing hemisphere of Dione is Aeneas, and the prominent curvilinear feature near the terminator is Latium Chasma. The brighter feature towards the south pole is Palatine Chasma. Notice the extremities of the wispy streaks radiating over the limb from the trailing hemisphere.

rotate. Perhaps the dark material is a modification of the surficial material by this plasma sputtering out ions and inducing chemical reactions.50

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