As spring continues to unfold on Saturn（土星） , April showers on the planet's largest moon, Titan, have brought methane（甲烷） rain to its equatorial（赤道的） deserts, as revealed in images captured by NASA's Cassini spacecraft. This is the first time scientists have obtained current evidence of rain soaking Titan's surface at low latitudes. The observations are released today in the journal Science.

"Titan continues to surprise and amaze us," said Alfred McEwen, a planetary scientist at the UA'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," said Elizabeth Turtle, a Cassini imaging team associate at the Johns Hopkins University Applied Physics Lab in Laurel, Md., and lead author of today's publication. "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（昼夜平分点） , 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（饱和烃） clouds in the atmosphere, and falls as rain.