Posted by Emily Lakdawalla
04-12-2007 15:43 CST
Cassini nabbed a nice set of views of Epimetheus yesterday. Epimetheus is one of Saturn's smaller moons, orbiting between the main ring system and the larger round moons. Epimetheus is nearly co-orbital with Janus, which has four times Epimetheus' mass; the two dance past each other once every four years.This view is particularly nice because it's pretty low-phase, meaning that the Sun was close to being behind Cassini when the photo was taken. That is a good illumination geometry to reveal subtle color differences across the moon. For comparison, here's a view where Epimetheus was lit from a lower angle, highlighting its lumpy topography: Epimetheus' appearance is interesting. Closer to Saturn, small moons tend to be smooth-looking because they are covered in dust. Consider Pandora, which is just a little smaller than Epimetheus: The image of Epimetheus actually reminds me more of another moon entirely: Mars' moon Phobos. It has the same mix of craters, dust, and grooves. (It is, however, considerably smaller than Epimetheus.) I love pulling out these Phobos 2 images -- thanks again to Ted Stryk for resurrecting them. A reader recently pointed me to a paper presented at the 2006 Lunar and Planetary Science Conference on "New evidence on the origin of Phobos' parallel grooves," (PDF, 400k) arguing that Phobos' grooves are not -- as is popularly explained -- secondary craters from the enormous Stickney impact crater. Instead, the paper suggests that the grooves represent chains of secondary craters from impacts that happened on Mars. As evidence, the paper argues that most crater chains are on Phobos' leading hemisphere, and the geometry of the way they wrap around toward and then disappear in the trailing hemisphere is consistent with Phobos running in to streams of ejecta from Mars impacts. It seems likely that impacts happening within Saturn's cluttered ring system would spray ejecta all over the place, perhaps producing similar chains of impact-caused grooves on Epimetheus.
Epimetheus may look similar to Phobos, but it often looks dissimilar to its own self. Here's a nice montage of five images of Epimetheus, four from Cassini and one from Voyager.
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