Hemispheric colour differences on Saturn’s moon Rhea are apparent in this false-colour view from Nasa’s Cassini spacecraft. This image shows the side of the moon that always faces the planet.
In this image, the left half of the visible disc of Rhea faces in the direction of Rhea’s orbital motion around Saturn, while the right side faces the trailing direction. It is not unusual for large icy Saturnian satellites to exhibit hemispheric albedo (reflectivity) and colour differences. These differences are likely related to systematic regional changes in surface composition or the sizes and mechanical structure of grains making up the icy soil. Such large-scale variations can arise from numerous processes, such as meteoritic debris preferentially hitting one side of Rhea.
The differences can also arise from “magnetic sweeping”, a process that happens when ions that are trapped in Saturn’s magnetic field drag over and implant themselves in Rhea’s icy surface. The slightly reddish false-colour hues near Rhea’s poles identify subtle composition changes that might be caused by differences in the surface exposure to meteoric debris falling into the moon or implantation of ions. These differences could vary by latitude. They suggest that at least some of the colour differences are exogenic, or derived externally.
This view was captured during Cassini’s 2 March 2010 flyby of Rhea. To create the false-colour view, ultraviolet, green and infrared images were combined into a single picture that isolates and maps regional colour differences. This “colour map” was then superimposed over a clear-filter image that preserves the relative brightness across the body.
Image credit: Nasa/JPL/SSI