Wagner, R.J., Neukum, G., Giese, B., Roatsch, T., and Denk, T. (2007). "Geology and Geomorphology of Rhea: a First Look at the High-Resolution Cassini ISS Images from the Targeted Flyby on Aug. 30, 2007." Eos Trans. AGU, 88(52) Fall Meet. Suppl., Abstract P12B-06.

Background: Rhea, 1528 km in diameter, is Saturn's second-largest satellite. Its low average density of 1.233 gcm-3 and gravitational data imply that Rhea is more or less an undifferentiated, homogeneous body composed preferentially of water ice [1][2]. Rhea's major geologic units are vast expanses of densely cratered plains, featuring craters in a wide range of degradation stages and impact basins up to several 100 km in diameter [3][4][5]. The Cassini spacecraft has been in orbit about Saturn since July 1, 2004, and the onboard ISS cameras have taken images of Rhea during several non-targeted flybys [6]. During one targeted flyby on Aug. 30, 2007 (orbit 049RH), the cameras acquired more than 270 images of the anti-saturnian hemisphere at resolutions of up to 35 m/pxl. Results: A regional mapping image sequence with an average resolution of 300 m/pxl shows the densely cratered plains between about 90 to 250° W. These data can be combined with images from the same region taken in earlier flybys (e.g. 018RH) to derive stereo information. Large, old, heavily degraded impact features can be mapped in these data and are used to derive the size-frequency distribution of large impact structures. One remarkable high-resolution target was a bright, 48-km large bright, fresh ray crater. Stereo data reveal the topography of this crater and its surroundings in high detail. The new images show that the frequency of small craters on the ray crater floor and in the continuous ejecta blanket is very low, inferring that this crater is stratigraphically very young. Secondary chains and bright rays extend several hundreds of kilometers outward. One intriguing feature in this crater is the occurrence of small clusters, resembling small secondaries, within the crater floor and in the continuous ejecta, but restricted to the eastern part of the crater. Since there is no crater younger than the ray crater at close range as a possible source, two explanations remain: (1) the small clusters were created by material ejected at a steep angle which re-impacted a part of the crater floor and continuous ejecta [7]. (2) The crater clusters may originate from material disintegrated from the major projectile which impacted upon the formation of the larger primary crater.

References: [1] Thomas P. C. et al., LPSC 37th, abstr. No. 1639, 2006. [2] Anderson J. D. & G. Schubert, GRL 34, L02202, 2007. [3] Moore J. M. et al., JGR 90 (suppl.), C785-C795, 1985. [4] Moore J. M. et al., Icarus 171, 421-443, 2004. [5] Wagner R. J. et al., LPSC 38th, abstr. No. 1958, 2007. [6] Porco C. C. et al., SSR 115, 363-497, 2004. [7] Greeley R. et al., Satellites of Jupiter, (D. Morrison, ed.) UofA Press Tucson, Az., 340-378, 1982.