A paper that describes the first storm observed in the tropical latitudes of Saturn's moon Titan will be published in the journal Nature on August 13. The rain from large clouds such as these is actually liquid methane and may be responsible for forming the channels and other features near the equator observed by the Huygens probe in 2005.

The huge storm, observed with the NASA Infrared Telescope Facility and the Gemini North telescope on Mauna Kea, Hawaii, covered over almost 1.2 million square miles (three million square kilometers, about the size of India). While the diameter of Earth is 7,926 miles (12,756 km), Titan's is 3,193 miles (5,150 km), just slightly larger than Mercury, the smallest planet in our solar system.

Titan is the only moon in the solar system with a thick atmosphere, and like Earth, it has a weather cycle, including clouds and rain. However, on Titan, the substance that forms clouds and rains down on the surface is not water but methane (natural gas).

It is so cold on Titan (-288 F, -178 C) that methane is a liquid, and there are "boulders" made of frozen water rather than rock.

Clouds on Titan are generally much smaller and occur much more infrequently than on Earth, which led scientists to wonder how the rivers and channels seen by the Cassini spacecraft and the Huygens probe were formed. "After three years of observing Titan and finding little to no cloud activity, Titan suddenly put on quite a show," quipped Schaller.

Unlike the large channels on Mars, which were probably carved millions or billions of years ago by liquid water, Titan's carved surface features, like those on Earth, are still being formed today. Observations over the next several years by telescopes on Earth and by instruments aboard Cassini will continue to reveal more clues about the meteorology and geology of this world, which has many similarities to our own.

The Cassini-Huygens mission was launched in 1997 and arrived at Saturn in July 2004. Cassini completed its initial four-year mission to explore the Saturn system in June 2008.

It is now working on an extended mission, seeking answers to new questions raised during its first years at Saturn. The Huygens probe separated from the Cassini orbiter on December 25, 2004, and landed on Titan, Saturn's largest moon, on January 14, 2005. It continued to transmit information for over an hour despite a hard landing and Titan's high atmospheric pressure.

The lead author, Dr. Emily Schaller, wrote the paper while working as a Hubble Fellow at the University of Hawaii, Institute for Astronomy. The other authors of the paper, "Storms in the tropics of Titan," are H. G. Roe (Lowell Observatory), and T. Schneider and M. E. Brown (both California Institute of Technology).