The observations were made in December 2001 using the W. M. Keck II 10-meter and the Gemini North 8-meter telescopes atop Hawaii's Mauna Kea volcano. Both telescopes are outfitted with adaptive optics that provide unprecedented detail of features not seen even by the Voyager spacecraft during its flyby of Saturn and Titan.

Analyses of the observations are being published by the Caltech team in the Dec. 19 issue of Nature and by the UC Berkeley and NASA Ames team in the Dec. 20 issue of the Astrophysical Journal.

Titan is Saturn's largest moon, larger than the planet Mercury, and is the only moon in the solar system with a thick atmosphere. Like Earth's atmosphere, Titan's is mostly nitrogen. Unlike Earth, Titan is inhospitable to life because of a lack of atmospheric oxygen and because of its extremely cold surface temperature - minus 183 degrees Celsius (-297 degrees Fahrenheit). Along with nitrogen, Titan's atmosphere contains a significant amount of methane.

Earlier spectroscopic observations had hinted at the existence of clouds on Titan, but gave no clue as to their location. These early data were hotly debated, since Voyager spacecraft measurements of Titan appeared to show a calm and cloud-free atmosphere. Furthermore, previous images of Titan had failed to reveal clouds, finding only unchanging surface markings and very gradual seasonal changes in the haziness of the atmosphere.

Improvements in the resolution and sensitivity achievable with ground-based telescopes led to the present discovery. The observations used adaptive optics, in which a flexible mirror rapidly compensates for the distortions caused by turbulence in Earth's atmosphere.

These distortions cause the well known twinkling of the stars. Using adaptive optics, details as small as 300 kilometers (186 miles) across can be distinguished at the enormous distance of Titan, 1.3 billion kilometers (820 million miles), equivalent to reading an automobile license plate from 100 kilometers (63 miles) away.

The images presented by the two teams clearly show bright clouds near Titan's south pole.

"We see the intensity of the clouds varying over as little as a few hours," said post-doctoral fellow Henry Roe, lead author for the UC Berkeley group. "The clouds are constantly changing, although some persist for as long as a few days."

Titan experiences seasons much like the Earth, though its year is 30 times longer due to Saturn's distant orbit from the sun. Titan is currently in the midst of southern summer, and its south pole has been in continuous sunlight for more than six Earth years. The researchers believe that this fact may explain the location of these large clouds.

"These clouds appear to be similar to summer thunderstorms on Earth, but formed of methane rather than water. This is the first time we have found such a close analogy to the Earth's atmospheric water cycle in the solar system," says Antonin Bouchez, one of the Caltech researchers.

In addition to the clouds above Titan's south pole, the Keck images, like previous data, reveal the bright continent-sized feature that may be a large icy highland on Titan's surface, surrounded by linked dark regions which are possibly ethane seas or tar-covered lowlands.

"These are the most spectacular images of Titan's surface which we've seen to date," says Michael Brown, associate professor of planetary astronomy and lead author of the Caltech paper. "They are so detailed that we can almost begin to speculate about Titan's geology, if only we knew for certain what the bright and dark regions represented."

In 2004, Titan will be visited by NASA's Cassini spacecraft, which will look for clouds on Titan during its multi-year mission around Saturn.

"Changes in the spatial distribution of these clouds over the next Titan season will help pin down their detailed formation process," says Imke de Pater, professor of astronomy at UC Berkeley. The Cassini mission includes a probe named Huygens that will descend by parachute into Titan's atmosphere and land on the surface near the edge of the bright continent.

The team conducting the Gemini observations consists of Roe and de Pater from UC Berkeley, Bruce A. Macintosh of Lawrence Livermore National Laboratory and Christopher P. McKay of the NASA Ames Research Center. The team reporting results from the Keck telescope consists of Brown and Bouchez of Caltech and Caitlin A. Griffith of the University of Arizona.

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