Denk, T., Neukum, G., Roatsch, Th., Giese, B., Wagner, R., Helfenstein, P., Burns, J.A., Turtle, E.P., Johnson, T.V., Porco, C.C. (2006). "Iapetus: Two Years of Observations by the Cassini ISS Camera." European Planetary Science Congress EPSC2006-A-00571.


Abstract

Since its arrival at the Saturn system in mid-2004, the Cassini orbiter has successfully performed seven observation campaigns of Saturn's strange satellite Iapetus at ranges of 1.4 million km or closer. Large parts of the surface have now been observed at 9 km/pxl resolution or better. The closest flyby was taking place on 31 Dec 2004, when Cassini passed the northern leading side of Iapetus at 124000 km altitude. The surface of Iapetus is heavily cratered on both the bright and the dark hemispheres, with an unusually high number of large basins of up to 800 km in size [1, 2, 3].
The irregular (non-spherical, rather ellipsoidal) shape of Iapetus [4] was confirmed by Cassini data [1, 5], and is considered as a remnant of the despinning process [6]. A major surprise was the discovery of a large, up to 20 km high ridge system on the leading side on 25 Dec 2004 [1, 2], which is located exactly at the equator, and which might also be explained by despinning early in Iapetus' history. An alternative hypothesis considers an ancient ring system around Iapetus as the cause for the ridge formation [7]. The ridge appears to be a geometric continuation of isolated bright mountains, which have been discovered in Voyager images on the anti-Saturn side [4], but remained unexplained at this time.
There is also progress in solving the centuries-old question of the formation of the unique dark/bright albedo dichotomy. ISS data have not shown any "hole" (fresh "punch-through" impact crater) at a resolution down to 750 m/pxl. RADAR data indicate that the dark layer is rather thin (at the order of decimeters, [8]), suggesting that it is either a young structure relative to major crater formation, or that an ongoing process is responsible. Earlier ideas for the albedo dichotomy formation included dust from retrograde outer satellites covering Iapetus' leading side [9, 10]. The advantage of this idea is that the exact alignment to the center of the leading side can be explained. However, the bright poles are a problem here. A recent idea from Spencer et al. [11] is based on data from the CIRS infrared spectrometer and suggests that a thermal re-distribution of water ice towards the poles is responsible for the albedo dichotomy. Here, the bright poles can be easily explained, but the leading/trailing side asymmetry requires different starting conditions for each hemisphere. Outer satellite dust was considered as a candidate.
This scenario is now supported by the ISS discovery of a color dichotomy in addition to the albedo dichotomy [12]. Different to the albedo dichotomy, the color dichotomy is quite precisely aligned to the leading side, including the bright poles. It allows to link the Spencer et al. [11] and Buratti et al. [10] hypotheses. The dust from outer satellites only causes a moderate darkening, but a significant reddening of the whole leading side. This process once triggered the thermal re-distribution of water ice to start on the leading side, but not on the trailing hemisphere, as suggested by Spencer et al. [11].
While the described mechanism appears straightforward, it is still questionable if the very complex albedo patterns at equatorial latitudes revealed by Cassini images [2] have also been produced entirely by these two rather simple processes. Imaging of the anti-Saturn side planned for the sole targeted flyby on 10 Sep 2007 will hopefully provide information that may help to ultimately solve the Iapetus enigma. References: [1] Porco et al. (2005), Science 307, 1237. [2] Denk et al. (2005), LPSC abstract 2268. [3] Giese et al. (2005), DPS abstract 47.08. [4] Denk et al. (2000), LPSC abstract 1596. [5] Thomas et al. (2006), LPSC abstract 1639. [6] Castillo et al. (2005), DPS abstract 39.04. [7] Ip (2006), GRL, in press. [8] Ostro et al. (2006), Icarus, in press. [9] Soter (1974), IAU Colloq. 28. [10] Buratti et al. (2002), Icarus 155, 375. [11] Spencer et al. (2005), LPSC abstract 2305. [12] Denk et al. (2006), EGU 06-A-08352.