Scientists spot massive methane rainstorm over Titan

Titan is bigger than Mercury. It's the only moon with a dense atmosphere – 98 percent nitrogen. And it's bitterly cold. The average temperature at the surface is -179 degrees Celsius (-290 degrees Fahrenheit). On Earth, water is a major sculpting agent. On Titan, water is frozen hard as a rock; the sculpting agent is liquid methane and ethane, which rains out of clouds composed of these hydrocarbons.

The moon's atmosphere is thought to resemble that of Earth early in its history. This has raised the question of whether Titan is a spot in the solar system where chemical reactions that form the foundation for organic life are taking place. Some researchers suggest that simple forms of microbial life could exist beneath Titan's crust.

Cloudbursts of liquid methane

Clouds are important tools for studying a planet's atmosphere, but on any given year, clouds cover roughly 1 percent of Titan's disk versus 65 percent for Earth.

After noteworthy sightings of clouds on Titan in 1995 and again in 2004, a team of astronomers from the University of Hawaii, the Lowell Observatory, and the California Institute of Technology mounted a two-year program to track the moon's clouds. They used NASA's Infrared Telescope Facility on Hawaii's Mauna Kea volcano.

In April 2008, a large cloud system appeared, the team reports.

For lead investigator and University of Arizona planetary scientist Emily Schaller, "it was Titan's little present to me." Her newly minted PhD from Caltech involved the study of Titan's clouds, but for most of the time she was gathering data for it, "nothing much happened," she recalls. Then, the day she handed in her work, the storm appeared.

To the researchers' surprise, the vast storm system appeared in the tropics where it is rarely cloudy – its trigger unknown. In addition, it apparently spawned cloud formation near the south pole – during a season when the south polar region should be cloud-free.

Cloudbursts of liquid methane are not the only forces shaping Titan's surface. At last week's IAU meeting, scientists described volcanic activity, with slush-like mixtures of water ice and ammonia – a natural antifreeze – replacing scorching lava.

In addition, researchers presented models for mountain-building on Titan where mountain ranges resulted from the surface getting squeezed as it cooled and contracted. The surface also hosts the Titan equivalent of wind-drive sand dunes, as well as hydrocarbon lakes.

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