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Ice cloud heralds a change of season at Titan's south pole

Jenny Winder, News Writer
Apr 14, 2013, 7:00 UTC

Sen—An ice cloud forming over Titan’s south pole is the latest sign that seasonal changes are causing radical alterations in the atmosphere of Saturn's largest moon.

This type of cloud, made from an unknown ice, has long hung over Titan's north pole, where it is now fading, according to observations made by the Composite Infrared Spectrometer (CIRS) on NASA's Cassini spacecraft.

"We associate this particular kind of ice cloud with winter weather on Titan, and this is the first time we have detected it anywhere but the north pole," said the study's lead author, Donald E. Jennings, a CIRS Co-Investigator at NASA's Goddard Space Flight Center in Greenbelt, Maryland.

The southern ice cloud shows up in the far infrared and is evidence that an important pattern of global air circulation on Titan has reversed direction. When Cassini first observed the circulation pattern, warm air from the southern hemisphere was rising high in the atmosphere and got transported to the cold north pole. There, the air cooled and sank down to lower layers of the atmosphere, where it formed ice clouds. A similar pattern, called a Hadley cell, carries warm, moist air from Earth's tropics to the cooler middle latitudes.

Scientists predicted a reversal of this circulation once Titan's north pole began to warm and its south pole began to cool. The official transition from winter to spring at Titan's north pole occurred in August 2009, but researchers didn't know when this reversal would happen or how long it would take.

The first signs of reversal came in early 2012, shortly after the start of southern autumn on Titan, when Cassini data revealed the presence of a high-altitude "haze hood" and a swirling polar vortex at the south pole. Later, infrared observations of Titan's winds and temperatures made by CIRS provided definitive evidence of air sinking, rather than upwelling, at the south pole. Looking back through the data, the team narrowed down the circulation change to within six months of the 2009 equinox. CIRS didn't detect the southern ice cloud until July 2012, a few months after the haze and vortex were spotted in the south.

"This lag makes sense, because first the new circulation pattern has to bring loads and loads of gases to the south pole. Then the air has to sink. The ices have to condense. And the pole has to be under enough shadow to protect the vapors that condense to form those ices," said Carrie Anderson, a CIRS team member and Cassini participating scientist at NASA Goddard.

So far, the identity of the ice in these clouds is unknown, though they have ruled out simple chemicals, such as methane, ethane and hydrogen cyanide, that are typically associated with Titan. One possibility is that species X, as some team members call the ice, could be a mixture of organic compounds.

"What's happening at Titan's poles has some analogy to Earth and to our ozone holes," said the CIRS Principal Investigator, NASA Goddard's F. Michael Flasar. "And on Earth, the ices in the high polar clouds aren't just window dressing: they play a role in releasing the chlorine that destroys ozone. How this affects Titan chemistry is still unknown. So it's important to learn as much as we can about this phenomenon, wherever we find it."

The Cassini-Huygens mission, which is a cooperative project of NASA, the European Space Agency and the Italian Space Agency, launched in October 1997 and arrived in Saturn's orbit on June 1, 2004.

The Huygens probe landed on the surface of Titan in January 2005, providing valuable data about the nature of Saturn's largest moon.

NASA's Jet Propulsion Laboratory (JPL) based in Pasadena, California, a division of the California Institute of Technology, manages the overall mission for NASA.