Denk, T., Neukum, G., Roatsch, T., Giese, B., Wagner, R., Turtle, E. P., Helfenstein, P., Burns, J. A., Thomas, P. C., Veverka, J., Johnson, T. V., Porco, C. C. (2004). "Cassini ISS Iapetus Observations 2004-2006." American Astronomical Society, DPS meeting #38, #69.


Abstract
Since its arrival at Saturn in mid-2004, the Cassini spacecraft has performed eight observation campaigns of Iapetus at spatial resolutions up to 740 m/pxl. The whole surface, except for parts of the north pole and southern trailing side, has now been imaged at resolutions better than 10 km/pxl. As expected, impact craters of all sizes are the dominating landform. However, the surface of this 1500-km-sized moon shows many very unusual properties. Most intriguing is the extreme brightness dichotomy documented 334 years ago by J.D. Cassini. Another strange feature is the unique ridge that runs along Iapetus' equator over at least 2000 km and that was first recognized in Cassini ISS data from 25 Dec 2004. Another notable property of Iapetus is the relatively large number of big basins on the surface, with the largest showing a diameter of ~800 km. While Iapetus' basic shape appears to be a rotational ellipsoid, extreme local topography is probably a remnant of very ancient impact craters. The equatorial transition zones on the sub-Saturn and anti-Saturn sides both show very complex albedo patterns. At these locations, the dark terrain extends ~700 km into the trailing side. Additionally, dark material located at the trailing side shows a different color compared to leading-side dark material, with the latter being redder. Within the dark terrain near the equator, no bright spots (i.e., recent craters)have been identified. Between ~30A° and ~55A° latitude, poleward-facing crater walls appear bright. Above ~55A° latitude, the dark terrain ends, but equator-facing walls show dark material at even higher latitudes. Bright terrain on the leading side is generally slightly darker and redder than on the trailing side. This means that Iapetus also hosts a hemispheric color dichotomy in addition to the obvious albedo dichotomy. A targeted flyby is planned for Sep 2007.