When astronomer Radwan Tajeddine and colleagues decided to measure the wobble of Saturn's small moon Mimas they didn't expect to find anything interesting.
With its heavily cratered, geologically dead surface, Mimas was considered to be scientifically boring.
But appearances can be deceiving. Using data from NASA's Cassini spacecraft, new research shows something strange inside Mimas that is causing the moon to sway as it orbits around Saturn.
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Computer models point to two possibilities. First is that Mimas, which is about 250 miles in diameter, has an oblong or football-shaped core, a clue that the moon may have formed inside Saturn's ice rings. The second option is that Mimas has a global ocean located 16 miles to 19 miles beneath its icy crust.
Both explanations have problems, however. Telltale clues of an oddly-shaped core should be visible on the moon's surface and so far scientists have found none.
Likewise, the moon's surface shows no signs of heating, though some energy would be needed to melt ice and sustain a liquid ocean.
One theory is that Mimas' eccentric orbit could be responsible, as gravitational tugging by Saturn is stronger when the moon orbits closer to the planet and weaker when it is farther away.
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"This phenomenon will create friction inside the moon, which will induce heating that will melt the ice and form an ocean. The ocean will sustain as long as the orbit is eccentric," Tajeddine, of Cornell University, said.
Soon after Mimas formed, heat released by the natural decay of radioactive materials could have started the ice melt. Later, chemical reactions between silicate rock and water could have added to the pool.
"Together, all those energy sources might provide enough heat to produce a thin global subsurface ocean. Once a thin global ocean has formed, heat produced by tidal heating will be significantly larger, and likely be able to sustain a global ocean," planetary scientist Attilio Rivoldini, with the Royal Observatory of Belgium, wrote in an email to Discovery News.
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The amount of tidal heating depends on Mimas' orbit, which can change over time, Rivoldini added.
Follow-up measurements of Mimas' surface temperatures could show whether there is any heat in the moon. If Mimas does have an ocean, it would join a growing list of solar system planets and moons that could be friendly to life.
The research appears in this week's Science.