This Cassini image posted on their site yesterday shows a natural light contrast between one of Saturn’s major moons and one of the shepherd moons of the major ring system.
Dione, above, shows those streaks that intrigued Voyager scientists in the early 80’s. Cassini confirmed they were not deposits as such, but high cliffs exposing brighter interior material.
Janus, of irregular shape, is at the bottom of the image.
Early last week, this press release discussed the dark material prevalent on some of Saturn’s moons. Dione was singled out in one recent paper by Roger Clark of the USGS:
“We see the same spectral signature on all the moons that have coatings of dark material.” His team found the dark material (on Dione) to be extremely fine-grained, making up only a very thin layer on the moon’s trailing side. Its distribution and composition, as measured by the Cassini visual and infrared mapping spectrometer, indicate that the dark material is not native to Dione. And scientists see many of the same signatures there that appear on the moons Phoebe, Iapetus, Hyperion and Epimetheus, and also in Saturn’s F-ring.
While it seems there is some kind of mechanism moving dark grains from one moon to another, I think it’s important to realize the geological processes here may be complex and multivalent. Other moons show patterns of icy brightness. Saturn’s five large inner moons (Mimas, Enceladus, Tethys, Dione and Rhea) all have significantly brighter surfaces (even counting the dark patches) than the small moons nearby. They are even brighter than the bright areas of Iapetus.
It is possible that some geological ejections from moons, especially smaller ones, are strong enough to escape their weak gravity and deposit elsewhere. Other activity, especially on the larger moons may have insufficient energy to leave the satellite. Enceladus shows both examples: the icy geysers contributing to the E-ring and landing particles in the rings, and a smooth surface over much of its globe that indicates a resurfacing over old craters.
We might have to wait for surface probes to decipher an accurate geology of these moons, to determine how and when they got resurfaced in bright and dark, and solve the riddle of the sharing of material in the Saturn system.