Because Tethys is made almost entirely of ice, the energy couldn't have come from radioactive decay of heavy elements in its core, he said.
Instead the heat must have been generated by tidal forces from Saturn's enormous gravity.
Similar tidal heating currently creates volcanoes on Jupiter's moon Io and most likely powers geyser-like ice jets on Saturn's moon Enceladus, Nimmo noted.
(Read "Saturn Moon's Ice Geysers Create 'Cosmic Graffiti'" [February 8, 2007].)
But it's difficult to calculate how the tides produced enough heat on Tethys unless the moon once had an underground ocean, he said.
An ocean would allow enough heating for the moon's crust to flex more strongly than it would if Tethys were solid all the way to its core.
Subsequent freezing would then have widened the rift as the water expanded into ice.
Testing this hypothesis might be difficult, Nimmo said, but it might be possible to find places where water once oozed onto the surface and then froze.
This is thought to have occurred on Jupiter's moon Europa, which is widely believed to still have a subsurface ocean.
If confirmed, Tethys would become the fifth moon in the outer solar system believed to have, or to once have had, an ocean.
"The more places we look, the more oceans we're finding," Nimmo said.
And where there once was water, life may have existed.
Depending on how much ammonia Tethys's ocean once contained, some liquid water might still exist below the moon's surface, Nimmo said.
"Ammonia acts as a good antifreeze," he noted. As the ocean freezes, much of the ammonia stays in the liquid water.
"It gets harder and harder to freeze the last little bit because it gets more and more concentrated in ammonia," Nimmo said.
Robert Pappalardo, a planetary scientist at NASA's Jet Propulsion Laboratory, called the find intriguing.
"This makes the exploration of icy satellites and their interiors even more important to understanding possible habitats for life in our solar system," Pappalardo said in an email.
The result has important implications for understanding how common life might be in the universe, he added.
Carolyn Porco, leader of the Saturn-orbiting Cassini spacecraft's imaging team, agreed.
Determining how common oceans and other possible precursors to life might be on icy moons "is one of the most gripping questions planetary scientists are asking today," she said in an email.
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