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Nico Schmedemann, Freie Universitaet Berlin (Germany)
Tilmann Denk, Freie Universitaet Berlin (Germany)
Roland Wagner, DLR (Germany)
Gerhard Neukum, Freie Universitaet Berlin (Germany)
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The examination of the geologic history of the Saturnian Satellites is a major goal of the Cassini imaging experiment (ISS). Crater counting for the determination of model ages is a powerful tool to understand stratigraphic relationships between different terrain units. The shapes of the measured crater-size frequency distributions follow very closely the distribution of Earth's moon (after correction for the different impact conditions; Neukum et al. 2006), justifying its usage here for model age determinations.
- Iapetus: Following the models of Castillo-Rogez et al. (2007) and Neukum et al. (2006), an age of 4.4 Gyr is expected for the oldest parts of Iapetus's surface. Based on these models, we measured different ages at neighboring morphologic units. A small part of the equatorial ridge near 96°W longitude and an "average" dark terrain sample north of the ridge shows densely createred surfaces which represent the most ancient areas (∼4.4 Gyr). The central areas of two basins on the leading side at 34°N/80°W and 20°N/20°W, as well as another basin on the trailing side at 40°S/260°W are featuring slightly younger ages of about 4.3 Gyr. The "landslide" crater (diameter ∼120 km; 6°N/36°W) and the landslide itself are sparsely cratered with a model age of 4.1 Gyr. These might be among the youngest areas on Iapetus.
The examination of the most recent data (targeted flyby on Sep 10, 2007, Denk et al. 2008) with spatial resolutions down to 10 m/pxl revealed a set of relatively bright and therefore presumably young craters on the dark side. From this data set we estimated the timescale and development of the darkening process. It was approximated by a power function, which shows a steep drop at the youngest ages. The crater brightness drops to 1/3 of the initial value after ∼10 kyr and to 1/5 after ∼4 Myr. The transition to the complete darkening is hard to define but is on the order of about 1 Gyr. In addition we'll present data of other landslides and average areas on the bright trailing side of Iapetus. Furthermore, data of the major units of other Saturnian Satellites like Mimas and Enceladus will be presented for comparison with Iapetus. Mimas is characterized by heavily cratered plains, which indicates a high surface age similar to Iapetus. Enceladus, on the contrary, contains areas of very young age, but also terrains which have been formed billions of years ago.
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