New global colour maps of Saturn's five large innermost moons reveal complex patterns that provide fresh constraints on the dynamics of particles and grains within the Saturnian system.

The global maps cover Mimas, Enceladus, Tethys, Dione, and Rhea, and were presented at the AAS Division for Planetary Sciences meeting yesterday by Paul Schenk of Houston’s Lunar and Planetary Institute. The maps were compiled from images taken by Cassini over the first four years of the mission and have resolutions of between 400 and 750 metres, and cover the spectral range from infrared to ultraviolet.

Distinct patterns revealed by the mapping include colour asymmetries on four of the moons and equatorial banding on three, which confirms some of the markings observed by the Voyager spacecraft nearly 30 years ago. With the new data, the most striking patterns are revealed when dividing the infrared maps by the ultraviolet maps – the IR/UV ratio yields the "redness" of the surface, defined by enhanced absorption at shorter wavelengths. With the exception of Mimas the moons in this study all show strong IR/UV enhancements on their trailing hemispheres. A similar, although weaker reddening is also centred on the leading hemisphere, but the simultaneous reddening on both hemispheres is difficult to explain since most impact processes associated with surface weathering preferentially favour one hemisphere or the other.

Schenk and colleagues are investigating alternative hypotheses, such as the bombardment of Saturn's E ring grains on the leading hemisphere as the satellites overtake these grains in their orbit, or by radiation of charged particles. The latter theory favours both the trailing hemispheres, which are heavily bombarded by the cold plasma, or the leading hemispheres, which are preferentially impacted by high energy electrons.

A dark equatorial lens-shaped band was revealed in Voyager observations of Tethys, which is also very prominent in the new Cassini maps, but the real surprise was that a similar feature was also seen on Mimas, stretching – some 175 kilometres wide – across the leading hemisphere. Scientists suspect that the impact of high-energy electrons will produce similar features on the faces of Mimas and Tethys, but less so on Dione and Rhea.

A very narrow and straight band of discrete UV-bright features within two degrees of Rhea’s equator were resolved by Cassini as bluish marks that form a chain just a few kilometres wide across Rhea's leading hemisphere. This pattern has not been seen on any other icy satellite of Saturn, and in the absence of tectonic activity, and the features' proximity to the moon's equator, Schenk and colleagues suggest that the impact of debris from Rhea’s dusty ring system is to blame.

The science team will continue to work on these hypotheses and develop new models for the interaction of particles within the Saturnian system and how they modify the surfaces of these moons.

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