Dione's relatively high density suggests that it has a rocky core, making up about one third of its total mass, surrounded by water ice. In composition, albedo, and terrain, it is similar to Rhea. Both moons have markedly different leading and trailing hemispheres. The surface includes heavily-cratered terrain, moderately cratered plains, lightly cratered plains, and bright, wispy material. The heavily-cratered regions feature craters of all diameters up to and exceeding 100 km (60 miles), whereas the plains areas tend to have craters no larger than about 30 km across.
Contrary to what might be expected of a satellite in gravitational lock, it is the trailing hemisphere of Dione on which the majority of intense cratering exists. This suggests that, during the early period of heavy bombardment in the solar system, Dione was tidally locked to Saturn in the opposite direction to which it now spins. Subsequent major collisions with any objects large enough to cause craters more than about 35 km (21 miles) in diameter, of which there are many on Dione, would have been capable of altering the rate and direction of the moon's rotation. The likelihood is that Dione has been tidally locked in its current state for several billion years judging by the average albedo of the leading and trailing hemispheres. The albedo decreases from the leading to the trailing hemisphere since the former has suffered a higher rate of micrometeorite impacts. Streaks of bright material which interlace a dark portion of the surface are of uncertain origin but may be fractures associated with faults through which water has upwelled.
Related entries Dione photo gallery
Related category PLANETS AND MOONS
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