When the Cassini-Huygens space probe landed on the surface of Saturn's largest moon, Titan, last year, it found no evidence of the ethane ocean long thought to cover the satellite's surface.

Instead, scientists observed dune structures that could be dust-like combinations of ethane and smog particles, according to a new study in the current issue of Nature.

Titan's dense atmosphere is composed mostly of nitrogen with a small amount of methane. This methane is broken up by the Sun's ultraviolet light to produce a dense orange-brown smog that hides the satellite's surface.

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Scientists thought that ethane, one of the products of these reactions with the Sun's rays, was abundant enough to have condensed and rained down to form a kilometer-deep ocean across the entire surface of the satellite.

But observations of the surface suggest that it is instead covered by dunes, which Donald Hunten of the University of Arizona thinks could be made of a combination of ethane and smog particles.

Titan's ethane can't condense into liquid rain because "the smog particles grab the ethane before it has a chance to form drops," Hunten said.

The resulting particles deposit on the moon's surface and pile up to form dunes that might be as deep as several kilometers, Hunten says.

The particles would be more like dust than sand, though, so Hunten has dubbed them "smust" (a combination of "smog" and "dust").

Hunten based his proposal for this mechanism on the observed behavior of ethane in Jupiter's atmosphere, where at certain levels, it condenses onto smog particles.

"Basically, I used Jupiter as the laboratory to show that the ethane is sticking to the particles," Hunten told Space.com.

"I think there's a very deep deposit of them on the ground [of Titan], but we can't confirm that with observations," he added.

To confirm Hunten's theory, laboratory experiments would have to be conducted to show that ethane does indeed condense onto the smog particles.

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