Cassini watches as Titan's seasons turn

PASADENA, Calif., Oct. 21 (UPI) -- Cassini has a front seat for seasonal changes happening on Titan, Saturn's largest moon. Most recently, NASA's spacecraft captured images of a vortex forming in the upper atmosphere above Titan's southern hemisphere -- a sign winter is coming.

Cassini's instruments suggest the swirling, cyclone-like atmospheric pattern is enriched with trace gases. When the NASA probe first arrived at Saturn in 2004, similar seasonal weather patterns were observed in the northern hemisphere. Over the last decade, the moon's atmosphere has reversed its polar orientation.

"Cassini's long mission and frequent visits to Titan have allowed us to observe the pattern of seasonal changes on Titan, in exquisite detail, for the first time," Athena Coustenis, a researcher at Observatoire de Paris and a member of Cassini's Composite Infrared Spectrometer team, said in a news release. "We arrived at the northern mid-winter and have now had the opportunity to monitor Titan's atmospheric response through two full seasons."

It was summer in the southern hemisphere until late 2009-early 2010, when Saturn's orbit put the south in the shadows and soaked the north in sunlight. Researchers believe the reversal began in the wake of the 2009 equinox, as atmospheric gases in the north began to warm and colder gases began to cycle toward the south pole.

Over the last four years, temperatures in the southern hemisphere have dropped 40 degrees Celsius. Warming in the north hasn't been as dramatic. Temperatures in the summer hemisphere have risen gradually, just a six-degree increase since 2014.

Coustenis and her colleagues presented their latest observations of Titan's changing seasons at a conference in Pasadena, Calif., this week -- a joint affair, combining the 48th meeting of the American Astronomical Society Division for Planetary Sciences and 11th European Planetary Science Congress.