[meteorite-list] NASA-Funded Study Says Saturn's Moon Enceladus Rolled Over

[Ron Baalke]

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Carolina Martinez  818-354-9382
Jet Propulsion Laboratory, Pasadena, Calif.

Tim Stephens  831-459-2495
University of Santa Cruz, Calif.

Jim Scott  303-492-3114
University of Colorado, Boulder
				
NEWS RELEASE: 2006-080				May 31, 2006	
		   
NASA-Funded Study Says Saturn's Moon Enceladus Rolled Over
 
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.  

Enceladus recently grabbed scientists' attention when the Cassini 
spacecraft observed icy jets and plumes indicating active geysers 
spewing from the tiny moon's south polar region.

"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.

Nimmo and Pappalardo calculated the effects of a low-density blob 
beneath the surface of Enceladus and showed it could cause the 
moon to roll over by up to 30-degrees and put the blob at the 
pole. 
 
Pappalardo used an analogy to explain the Enceladus rollover. "A 
spinning bowling ball will tend to roll over to put its holes -- 
the axis with the least mass -- vertically along the spin axis.  
Similarly, Enceladus apparently rolled over to place the portion 
of the moon with the least mass along its vertical spin axis," he 
said.

The rising blob (called a "diapir") may be within either the icy 
shell or the underlying rocky core of Enceladus. In either case, 
as the material heats up it expands and becomes less dense, then 
rises toward the surface. This rising of warm, low-density 
material could also help explain the high heat and striking 
surface features, including the geysers and "tiger-stripe" region 
suggesting fault lines caused by tectonic stress. 

Internal heating of Enceladus probably results from its eccentric 
orbit around Saturn.  "Enceladus gets squeezed and stretched by 
tidal forces as it orbits Saturn, and that mechanical energy is 
transformed into heat energy in the moon's interior," added 
Nimmo. 
 
Future Cassini observations of Enceladus may support this model. 
 Meanwhile, scientists await the next Enceladus flyby in 2008 
for more clues. 
 
This research was supported by grants from NASA. The 
Cassini-Huygens mission is a cooperative project of NASA, the 
European Space Agency and the Italian Space Agency. JPL, a 
division of Caltech, manages the mission for NASA's Science 
Mission Directorate. The Cassini orbiter was designed, 
developed and assembled at JPL. 

For images and information about the Cassini mission, visit: 

http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov .

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