Denk, T., Neukum, G., Roatsch, T., Wagner, R., McEwen, A., Turtle, E. P., Johnson, T., Helfenstein, P., Porco, C. C., et al. (2004). "First Cassini ISS Observations of Iapetus." American Astronomical Society, DPS meeting #36, #4.08.

Shortly before and after Saturn arrival in July, the Cassini Imaging Subsystem (ISS) observed Iapetus at spatial resolutions of up to 15 km/pxl. These are coarser pixel scales than Voyager 2 images over some regions of the moon, but with higher signal-to-noise ratios. We report new results from the July data: 1. Craters are now clearly visible and measurable in the dark hemisphere (while Voyager data provided only superficial indications). 2. Wide parts of the southern hemisphere were observed for the very first time. 3. The Saturn-facing hemisphere, previously seen only by Voyager 1, was observed at improved spatial resolution. 4. At least three huge, up to 500 km-sized impact basins are seen in the Cassini images. Two prominent rings can be seen in at least one of these structures on the Saturn-facing hemisphere. Previously, the largest-known impact structures on Iapetus were about 150 km diameter. The presence of large impact basins implies that the crust of Iapetus dates back to an early period of heavy bombardment. The crater-diameter to satellite-diameter ratio of the largest impact structure is 1/3, and is comparable to similar structures on Tethys (Odysseus crater, 0.4) and Mimas (Herschel crater, 0.3). 5. All of the large structures are located in the dark terrain on the leading hemisphere, except for one near 270 deg West. -- We will also report additional results from the October flyby ( 3x closer than in July). On Jan 1 2005 Cassini will approach Iapetus within 55300 km and return high-resolution (330 m/pxl) images that should provide new constraints on origin of the dark material.