for National Geographic News
Saturn's moon Titan may have an underground ocean on which its crust slides like a giant, floating icecap, pushed and pulled by climate-driven winds.
The discovery, to be announced in tomorrow's issue of the journal Science, was made when scientists tried to match corresponding features on radar maps created during two-and-a-half years of flybys by the Cassini spacecraft.
Each of the flybys had mapped a swath of the moon's surface; on 19 occasions, the craft crossed over the same areas twice.
But on the revisits, easily recognizable geologic features weren't at the expected coordinates, with some of them out of place by about 20 miles (30 kilometers).
Obviously, hills and valleys weren't swapping places, said Bryan Stiles, a radar engineer at NASA's Jet Propulsion Laboratory in Pasadena, California.
Rather, the maps didn't align because Titan's surface wasn't rotating at quite the expected rate.
That was a surprise, because scientists had thought that Saturn's enormous gravity would keep the same side of Titan pointed directly at the planet, just as one side of Earth's moon always faces us.
One possible explanation was that Titan's rotation had been altered by a large, recent asteroid impact. But such impacts occur infrequently, Stiles said.
A more likely prospect, he said, is that Titan's crust is moving on top of something more slippery than rock or ice.
"There has to be some effect causing the spin to be different from what we had thought. And the most obvious cause that we can come up with is this deep ocean."
Even with a low-viscosity layer like an ocean underlying Titan's crust, something has to be pushing the surface around.
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