solar system other than Earth known to have liquid lakes, clouds and perhaps even rain. But instead of water, frigid Titan’s lakes and clouds are made of liquid hydrocarbons like ethane and methane.

While Earth’s northern hemisphere is moving into autumn, Titan’s northern hemisphere has been shifting from winter to spring for most of the six years Cassini has been there. A full year on Saturn — and therefore on all its moons — lasts 30 Earth years, and each season lasts about seven years.

Throughout the northern winter, Titan’s poles were shrouded in heavy clouds. But as the seasons changed, the clouds cleared, says planetary scientist Sebastian Rodriguez of the University of Paris Diderot.

“Over the past six years, we’ve found that clouds appear clustered in three distinct latitude regions of Titan: large clouds at the north pole, patchy cloud at the south pole and a narrow belt around 40 degrees south,” Rodriguez said in a press release. “However, we are now seeing evidence of a seasonal circulation turnover on Titan — the clouds at the south pole completely disappeared just before the equinox and the clouds in the north are thinning out.”

Rodriguez and colleagues presented the first long-term study of Titan’s weather that includes the spring equinox, which occurred in August 2009, at the European Planetary Science Congress in Rome on September 22.

The team used more than 2000 images from Cassini’s VIMS (Visual and Infrared Mapping Spectrometer) instrument to analyze the moon’s cloud patterns. They found that in the winter, north polar clouds of ethane form 18 to 30 miles up in the lower portion of Titan’s atmosphere by a constant influx of ethane and small particles from an upper layer. In the southern hemisphere, clouds are produced by air humid with methane upwelling from the surface.

Thanks to a mission extension, Cassini will continue to study Saturn and its moons until operating until May 2017, a few months past Saturn’s northern summer solstice. This will let scientists observe seasonal changes all the way through from mid-winter to mid-summer in the northern hemisphere.

“We have learned a lot about Titan’s climate since Cassini arrived at Saturn, but there is still a great deal to learn,” Rodriguez said. “With the new mission extension, we will have the opportunity to answer some of the key questions about the meteorology of this fascinating moon.”

Images: 1) Clouds in Titan’s atmosphere between July 2004 and April 2010. Black areas are cloud-free and yellow are fully covered. 2) Left: A flyby of Titan on May 12, 2008 shows a large cloud capping Titan’s north pole. Right: Another flyby on December 12, 2009 shows a band of clouds at 40 degrees south, but the north pole is cloud-free. Credit: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot

See Also:

• Cassini Skims Through Titan’s Upper Atmosphere
• Photo: Shining Lake Confirms Presence of Liquid on Titan
• Titan’s Strange Shape May Explain Polar Lakes
• The Lagoons of Titan: Oily Liquid Confirmed on Saturn Moon
• This Summer’s Sexiest Images From Saturn

Authors: Lisa Grossman

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