A close approach to Saturn’s moon Rhea by the Cassini spacecraft has revealed details of its icy surface that suggest major fault scarps cutting across the trailing hemisphere of the moon. These scarps gave bright radar reflections at a distance that mission scientists had thought might indicate cryovolcanism. The flyby and 3D images obtained show instead that significant tectonic activity has produced lines of cliffs and uplifted blocks cutting through the densely cratered plains that make up most of Rhea’s surface. This information strengthens the link between Rhea and Dione, suggesting that they are very similar in origin as well as being neighbours around Saturn.
Cassini data also show a difference between the leading and trailing hemispheres as the moon orbits Saturn. These are highlighted with false colour in the image, which combines ultraviolet, green and infrared information. The poles appear reddish in this image, while the trailing hemisphere appears bluer than the leading hemisphere. The variations may indicate compositional changes, or a change in the size distribution of grains at the surface. The poles may receive a different meteorite flux of different patterns of embedded ions, for example. And an orbiting moon is likely to have different patterns of meteoritic debris. The differences can also arise from “magnetic sweeping”, when ions that are trapped in Saturn’s magnetic field drag over and implant themselves in Rhea’s icy surface.