Hemispheric colour differences on Saturn's moon Rhea are apparent in this false-colour image from NASA's Cassini spacecraft that shows the side of the moon that always faces the planet.
The left half of the visible disk of Rhea in this image faces in the direction of Rhea's orbital motion around Saturn, while the right side points in the trailing direction. It is not unusual for large icy Saturnian moons to exhibit hemispheric albedo 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 geographically coordinated changes can arise from numerous processes, such as the preferential infall or bombardment, on one side of Rhea, of coorbiting particles and meteoritic debris or a process called 'magnetic sweeping', that is, dragging and implantation into the icy surface of ions that are trapped in Saturn's magnetic field (which spins about Saturn faster than Rhea orbits it). The subtle reddish false-colour hues near Rhea's poles, which are relatively sheltered from bombardment by magnetic sweeping or more equatorial infalling meteoric debris, suggest that at least some of the colour differences are exogenic, or derived externally.
This view was captured during Cassini's flyby of Rhea on 2 March 2010. 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.
This view looks toward the Saturn-facing side of Rhea (1528 kilometres across). North is up. The images were taken with the Cassini spacecraft wide-angle camera. The view was acquired at a distance of approximately 35,000 kilometres from Rhea and at a Sun-Rhea-spacecraft, or phase, angle of 3 degrees. Image scale is 2 kilometres per pixel.