PASADENA, Calif., July 19 (UPI) -- U.S. scientists have determined Saturn's moon Iapetus is cryogenically frozen in the same state as it was more than 3 billion years ago.
Researchers at the U.S. National Aeronautics and Space Administration's Jet Propulsion Laboratory gained the latest information about Iapetus when the Cassini orbiter -- part of the Cassini-Huygens space probe -- flew by Iapetus in early 2005 and discovered the moon had a walnut shape, with a chain of mountains along its equator.
Scientists said they believe the moon's bulge and slow spin rate point to heating from long-extinct radioactive elements present when the solar system was born.
Dennis Matson, Cassini project scientist at JPL, said scientists calculate Iapetus originally rotated much faster, giving the moon an oblate shape. By the time the rotation slowed, the outer shell of the moon had frozen. Since the excess surface material was too rigid to go back smoothly into the moon, it produced equatorial mountains.
"This is the first direct evidence of the early spin history for a satellite in the outer solar system," said Matson.
Cassini's next close encounter with Iapetus will occur Sept. 10 at 620 miles.
The research appears online in the journal Icarus.
