18 March 2011

Cassini tracks equatorial methane rainstorms on Titan

by Kate Melville

Showers on Saturn's largest moon, Titan, have brought methane rains to its equatorial deserts, as revealed in images captured by NASA's Cassini spacecraft. The observations, reported in Science, are the first of rain falling on Titan's surface at low latitudes. The enormous arrow-shaped storm was captured by Cassini's cameras as it soaked an area the size of Arizona and New Mexico combined.

The new observations show the weather systems of Titan's thick atmosphere are affected by the changing seasons. The Saturn system experienced equinox (when the sun lies directly over a planet's equator and seasons change) in August 2009. Years of Cassini observations suggest Titan's global atmospheric circulation pattern responds to the changes in solar illumination, influenced by the atmosphere and the surface. "It's amazing to be watching such familiar activity as rainstorms and seasonal changes in weather patterns on a distant, icy satellite," said Elizabeth Turtle, the report's lead author.

Clouds on Titan are formed of methane as part of an Earth-like cycle that uses methane instead of water, explained Turtle. On Titan, methane fills lakes on the surface, saturates clouds in the atmosphere, and falls as rain. Though there is evidence that liquids have flowed on the surface at Titan's equator in the past, liquid hydrocarbons, such as methane and ethane, had only been observed on the surface at polar latitudes. The vast expanses of dunes that dominate Titan's equatorial regions require a predominantly arid climate.

Scientists suspected that clouds might appear at Titan's equatorial latitudes as spring in the northern hemisphere progressed. But they were not sure if dry channels previously observed were cut by seasonal rains or remained from an earlier, wetter climate. Then, an arrow-shaped storm appeared in the equatorial regions on Sept. 27, 2010 (the equivalent of early April in Titan's year) and a broad band of clouds appeared the next month.

The new observations suggest that recent weather on Titan is similar to that over Earth's tropics. In tropical regions, Earth receives its most direct sunlight, creating a band of rising motion and rain clouds that encircle the planet.

"These outbreaks may be the Titan equivalent of what creates Earth's tropical rainforest climates, even though the delayed reaction to the change of seasons and the apparently sudden shift is more reminiscent of Earth's behavior over the tropical oceans than over tropical land areas," said NASA's Tony Del Genio, a member of the Cassini imaging team.

On Earth, the tropical bands of rain clouds shift slightly with the seasons but are present within the tropics year-round. On Titan, such extensive bands of clouds may only be prevalent in the tropics near the equinoxes and move to much higher latitudes as the planet approaches the solstices. Cassini's imaging team intends to watch whether Titan evolves in this fashion as the seasons progress from spring toward northern summer.

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Titan Moist, Say Cassini Boffins

Source: University of Arizona