Saturn’s third-largest moon, 900-mile-wide Iapetus, is girded at the equator with a unique ridge of mountains (photo on the left and in detail at right) that reach a height of 6 miles, taller than Mount Everest. “We think it’s basically a remnant of a time when Iapetus spun more quickly and was mushy and warm,” Porco says. 

Iapetus then cooled while still spinning rapidly, researchers believe, freezing its equatorial bulge in place. Cassini also resolved a longstanding mystery about why one half of the moon’s surface is 10 times as bright as the other: The leading hemisphere of the moon picks up dark debris that is warmed by the sun, while brighter ices condense on the colder, trailing hemisphere.