Methone: 3-kilometer diameter Smooth Egg Moon of Saturn - Why is this moon shaped like a smooth egg? The robotic Cassini spacecraft completed the first flyby ever of Saturn's small moon Methone in May and discovered that the moon has no obvious craters. Craters, usually caused by impacts, have been seen on every moon, asteroid, and comet nucleus ever imaged in detail -- until now.
Dione (A moon of Saturn) This view highlights tectonic faults and craters on Dione, an icy world that has undoubtedly experienced geologic activity since its formation. Image credit: NASA/JPL/Space Science Institute Source link: http://www.nasa.gov/images/content/627613main_pia07691-full_full.jpg