Fog Seen on Saturn Moon Titan--A First

Earth-like fog shrouds chilly lakes on the south pole of Saturn's moon Titan, scientists say. (See a Titan picture.)

On Earth, fog typically forms when moisture-rich air cools rapidly, which reduces the air's ability to hold water vapor.

"But on Titan, you can't do that," said study leader Mike Brown, an astronomer at the California Institute of Technology.

"It's already very cold there and it's hard to cool [the air] down any further. So the only way to make fog is to have the air be in contact with liquid on the ground."

(Related: "Huge Space Lake Confirmed on Saturn's Moon Titan.")

The team thinks liquid evaporating from Titan's lakes makes the surrounding air increasingly humid. When moist air then brushes the cool surfaces of the lakes, the otherworldly fog is born.

Fog on Titan confirms that at least some of the moon's lakes are filled with liquid methane and not the chemical ethane, which can't evaporate in Titan's cold temperatures.

"For a long time we've been pretty sure that there are liquid lakes on Titan," Brown said. "But if the lakes were all just ethane, it would be a very inactive place."

Instead, the presence of fog confirms that Titan has an active "methane-cycle" similar to Earth's water cycle, he said.

Foggy Patches

The team detected the fog in data from NASA's Cassini spacecraft, which has been exploring the Saturnian system since 2004.

Brown and colleagues first looked at the data using a digital filter they developed that reveals details only on the moon's surface.

In the filtered images, the fog appears during the moon's late southern summer as bright, reddish-white patches hovering above the surface of lakes.

Those patches are not visible in images filtered to show only higher altitudes of the same region, thus ruling out the patches as clouds, which have been seen before on the large moon.

As the fog dissipates, Brown suspects, methane vapor rises higher in Titan's atmosphere to form the clouds, which to the naked human eye would look identical to clouds on Earth.

Previous studies have suggested that those clouds, in turn, pelt the surface with cold methane rain—maybe even enough to occasionally create new lakes on the Saturn moon.