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As spring continues to unfold on Saturn, showers on the planet’s largest moon, Titan, have brought methane rain to its equatorial deserts, as revealed in images captured by NASA’s Cassini spacecraft. The observations are reported in the journal Science.

“Titan continues to surprise and amaze us,” says Alfred McEwen, a planetary scientist at the University of Arizona’s Lunar and Planetary Lab and a co-author on the paper. “After years of dry weather in the tropics, an area the size of Arizona and New Mexico combined was darkened by methane rain over a period of just a few weeks.”

Extensive rain from large cloud systems, spotted by Cassini’s cameras in late 2010, has apparently darkened the surface of the moon. The best explanation is these areas remained wet after methane rainstorms.

The new findings, combined with earlier results reported in Geophysical Research Letters last month, show the weather systems of Titan’s thick atmosphere and the changes wrought on the moon’s surface are affected by the changing seasons.

“It’s amazing to be watching such familiar activity as rainstorms and seasonal changes in weather patterns on a distant, icy satellite,” says Elizabeth Turtle, a Cassini imaging team associate at the Johns Hopkins University Applied Physics Lab and lead author of the report in Science.

“These observations are helping us to understand how Titan works as a system, as well as similar processes on our own planet.”

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, as detailed in the Geophysical Research Letters paper.

Cassini found the surface temperature responds more rapidly to sunlight changes than does the thick atmosphere. The changing circulation pattern produced clouds in Titan’s equatorial region.

Clouds on Titan are formed of methane as part of an Earth-like cycle that uses methane instead of water. 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 in lakes at polar latitudes. The vast expanses of dunes that dominate Titan’s equatorial regions require a predominantly arid climate.

Titanic Deluge: The storm created large effects in the form of dark – likely wet – areas on the surface of the moon. After it dissipated, Cassini observed significant changes on Titan’s surface at the southern boundary of the dune field named Belet. Those changes covered an area of roughly Arizona and Utah. Scientists interpret the changes seen in these images (blue outlines) to be evidence of methane rain wetting the surface. (Credit: NASA/JPL/SSI)