NASA-funded Study Says Saturn's Moon Enceladus Rolled Over (JPL)

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NEWS - News Releases - 2006
NASA-funded Study Says Saturn's Moon Enceladus Rolled Over
May 31, 2006
(Source: Jet Propulsion Laboratory / University of Santa Cruz, Calif. / University of Colorado, Boulder)

Saturn's moon Enceladus -- an active, icy world with an unusually warm south pole -- may have performed an unusual trick for a planetary body. New research shows Enceladus rolled over, literally, explaining why the moon's hottest spot is at the south pole.
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"The mystery we set out to explain was how the hot spot could end up at the pole if it didn't start there," said Francis Nimmo, assistant professor of Earth sciences, University of California, Santa Cruz.

The researchers propose the reorientation of the moon was driven by warm, low-density material rising to the surface from within Enceladus. A similar process may have happened on Uranus' moon Miranda, they said. Their findings are in this week's journal Nature.

"It's astounding that Cassini found a region of current geological activity on an icy moon that we would expect to be frigidly cold, especially down at this moon's equivalent of Antarctica," said Robert Pappalardo, co-author and planetary scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. "We think the moon rolled over to put a deeply seated warm, active area there." Pappalardo worked on the study while at the University of Colorado.

Rotating bodies, including planets and moons, are stable if more of their mass is close to the equator. "Any redistribution of mass within the object can cause instability with respect to the axis of rotation. A reorientation will tend to position excess mass at the equator and areas of low density at the poles," Nimmo said. This is precisely what happened to Enceladus.
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more: http://saturn.jpl.nasa.gov/news/press-release-details.cfm?newsID=662