The geology of Saturn's satellite Dione observed by Cassini's ISS camera

Over two decades ago, Voyager images of Saturn's satellite Dione (1124 km diameter) showed cratered terrains and plains with varied crater densities and ages, tectonic features such as linear or arcuate troughs (chasmata), and enigmatic ''wispy streaks'' (lineae) [Plescia, 1983; Moore, 1984]. Since the Cassini spacecraft entered Saturn orbit in July 2004, the ISS camera has imaged Dione's surface with resolutions as high as 430 m/pxl. The areas imaged by ISS generally show uniformly, heavily cratered plains units whose crater size distribution at the smallest measurable scale is close to an equilibrium distribution and infers a very high surface age, probably on the order of 4 Gyr [Neukum et al., LPSC 2005]. Areas much less densely cratered are associated with stratigraphically younger, relatively large craters and their continuous ejecta blankets. Wispy streaks seen in Voyager images were once believed to represent possible surface deposits of erupted cryovolcanic materials. However, to date, Cassini ISS has found no evidence for cryovolcanism. Instead, high-resolution Cassini images show that the wispy streaks are caused by light scattered from exposed scarps within a global-scale network of crudely parallel faults. Crosscutting relationships between these lineae imply that episodic tectonic activity has occured. Globally, their trends are either sinusoidal (along meridians), or follow small circles with varied radii. In the latter case, some streaks were previously thought to represent impact features (e.g. Amata); one feature, previously interpreted as a young ray crater (Cassandra), is actually a system of radial fractures and scarps. A double-ring (so far unnamed) basin seen in Voyager data at low resolution near the south pole [Stooke, LPSC 2002] exhibits a lower superimposed crater frequency on its floor and ejecta, implying a stratigraphically younger age than the cratered plains.