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Emily LakdawallaDecember 4, 2007


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.


NASA / JPL / SSI / Ian Regan

Cassini captured this low-phase view of Epimetheus on December 3, 2007. At 144 x 108 x 98 kilometers in size, Epimetheus is the 12th largest moon of Saturn, co-orbital with the larger Janus, outside the rings but inside the orbit of Saturn's bigger icy moons. It appears to be coated in dust, but some crater rims are relatively sharp. Near the top of this image appear some grooves reminiscent of those on Phobos.
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:

Cassini captured this view on March 30, 2005, from a distance of only 74,600 kilometers. The view is actually a false-color composite of ultraviolet, green, and infrared images. Color variations in the image are most likely a result of "photometric effects," changes in the reflectivity of the surface of Epimetheus with the angle of the incoming sunlight. The large circular crater in the lower center is Hilairea, which has a diameter of about 33 kilometers (21 miles). To the lower left is the crater Pollux, with a reddish interior. Source
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:


Saturn's moon Pandora as seen by Cassini on its September 5, 2005 flyby. Resolution in the full-size image is approximately 300 meters per pixel. Source
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.)

Russian Academy of Sciences / Ted Stryk

This view of Mars' moon Phobos (which is about 25 kilometers in diameter) was captured by the Phobos 2 mission on February 28, 1989. Phobos 2 took a total of 13 color sets of images of Phobos before contact was lost on March 27, 1989.
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.

Views of Epimetheus from Cassini and Voyager

NASA / JPL / SSI / Calvin Hamilton / Gordan Ugarkovic / Exploitcorporations

Views of Epimetheus from Cassini and Voyager
Lumpy Epimetheus looks different with each spacecraft flyby. Four of the five images are from Cassini; the center image is from Voyager 1 (the black stripe across the moon is the shadow of the F ring). In the bottom row, colored dots show features that correlate between adjacent images.

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Emily Lakdawalla

Solar System Specialist for The Planetary Society
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