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T. Denk, G. Neukum, T. Roatsch, K.-D. Matz, U. Wolf, R.J. Wagner, R. Jaumann (DLR)
Limb-fitting analyses of Voyager data show that the shape of Iapetus can be described by an ellipsoid with half-axes of 750 km x 715 km, somewhat larger than the IAU radius (718 km). However, note that Iapetus' shape is irregular rather than ellipsoidal, containing large depressions and bulges. Measured radii vary between ~700 km and ~780 km. An irregularly shaped, Iapetus-sized body is something quite unusual in the solar system. It suggests that Iapetus was already cold, brittle, and geologically dead before most craters were formed, presumably very early in the solar system's history.
Crater counts on the bright and dark side north-east of crater Marsilion revealed similar size-frequency distributions for both terrain types. Assuming Iapetus' bulk composition is water ice, it follows that the craters either formed on top of a very thick dark layer, or that the layer is a relatively young and thin structure. We prefer 'young and thin', because otherwise the edges of the dark terrain should show bright spots everywhere due to post-formation impacts (what is not the case), and because no bright impactor remnants have been detected on the dark side.
The dark-bright boundary's complex geometry suggests that mass-movement processes took place, and the dark layer should therefore have a macroscopic thickness. But a complex transition-zone geometry as well as a significant thickness of the dark layer argue against dark-side origin theories that are based on micrometeoroid-gardening and ballistic (re-)distribution scenarios, like the Phoebe dust theory. A suggested, alternative origin scenario is a recent catastrophic destruction of a former dark, outer Saturnian satellite by a dark interplanetary body, with parts of the debris being distributed over the leading side of Iapetus. Such a collision might have produced enough dark material at geologically recent times.
References: Denk et al., LPSC 2000, #1596 and #1660, and references therein.