Saturn Moon Titan May Have Underground Ocean
Richard A. Lovett
for National Geographic News
|March 20, 2008|
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.
Scientists involved in the study have calculated that this could be done by winds generated from seasonal changes as Titan and Saturn slowly circle the sun.
With no underground ocean, the winds would have to push against the inertia of the entire moon, which is about 3,200 miles (5,150 kilometers) in diameter.
"You would have to have really killer winds to have any effect," Stiles said.
But if there is an ocean 60 to 180 miles (100 to 300 kilometers) below the surface, it would be easy for such winds to push the crust around in the observed manner, Stiles said.
It's an exciting prospect, said the study's lead author, Ralph Lorenz, a planetary scientist at Johns Hopkins University in Baltimore, Maryland.
That's because the ocean is most likely comprised of water and ammonia, while the planet's surface contains large quantities of hydrocarbons and other organic molecules. (Related: "Saturn Moon Has Lakes, "Water" Cycle Like Earth's, Scientists Say" [January 5, 2007].)
"Mixing organics and water is a very appealing astrobiological recipe," Lorenz said by email.
Stiles added: "It gives us one more place to look for life."
This is a "very important finding," said Christophe Sotin, a geophysicist at NASA's Jet Propulsion Laboratory who was not involved in the study.
But it needs to be confirmed, he said, by watching what happens to Titan's rotation as the winds shift in upcoming years.
If the crust's drift doesn't slow down and reverse course as the wind pattern changes, something else is causing the deviation from the previously expected rotation.
But if the ocean does exist, it would raise to four the number of large moons of Saturn and Jupiter thought to have such subterranean seas.
"Large reservoirs of water, a condition for life to form and develop, [would thus be] a common feature in the solar system," a team led by Sotin wrote in a commentary article in tomorrow's issue of Science.
But even without that carrot, the prospect of an ocean simply adds to Titan's mystique.
"Titan is very Earthlike compared to other bodies," Stiles said. "We see lakes—not water, but liquid. We see dunes, like in the Sahara desert. We see mountains and channels that look like river channels. It's one of the few other bodies that has weather."
(Related: "Titan Forecast: Cold, Drizzly Mornings" [October 11, 2007].)
"There aren't too many other bodies in the solar system where you see all these things going on," he added. "This deep ocean just adds to it."
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