Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2005 February 15
See Explanation.  Clicking on the picture will download
 the highest resolution version available.

Saturn's Moon Rhea from Cassini
Credit: Cassini Imaging Team, SSI, JPL, ESA, NASA

Explanation: Each moon of Saturn seems to come with its own mystery. Rhea, Saturn's second largest moon behind Titan, shows unusual wisps, visible above as light colored streaks. Higher resolution images of similar wisps on Dione indicate that they might be made of long braided fractures. Rhea is composed mostly of water ice, but likely has a small rocky core. Rhea's rotation and orbit are locked together, just like Earth's Moon, so that one side always faces Saturn. A consequence of this is that one side always leads the other. Rhea's leading surface is much more heavily cratered than the trailing surface, pictured above. The above image in natural color was taken last month by the Cassini robot spacecraft in orbit around Saturn.

Tomorrow's picture: sun morph

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