Changing Seasons On Titan
April 12, 2013

Ice Cloud Shows Titan’s Seasons Are Changing

April Flowers for - Your Universe Online

The latest sign that the change of seasons has started a cascade of radical changes in the atmosphere on Titan is an ice cloud taking shape over the south pole. This type of cloud, made from unknown ice, has long hung over the north pole. According to observations by the composite infrared spectrometer (CIRS) on NASA's Cassini spacecraft, the northern cloud is now fading.

"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 Donald E. Jennings, a CIRS Co-Investigator at NASA's Goddard Space Flight Center.

The southern ice cloud formation is evidence that an important pattern of global air circulation has reversed direction on Titan. The cloud shows up in the far infrared part of the light spectrum. Warm air from the southern hemisphere was rising high in the atmosphere and was transported to the cold north pole when the Cassini spacecraft first observed the circulation pattern. Once at the north pole, the air cooled and sank down to lower layers of the atmosphere to form ice clouds. Earth has a similar circulation pattern, called a Hadley cell, which carries warm, moist air from the tropics to the cooler middle latitudes.

Scientists have been expecting this reversal for a long time, based on modeling predictions. The official winter to spring transition at Titan's north pole began in August 2009. Each of the moon's seasons last approximately seven-and-a-half Earth years, making it nearly impossible for researchers to pinpoint when the reversal would happen, or how long it would take.

Data acquired in early 2012 showed the first signs of the reversal, which came shortly after the start of southern fall on Titan. Cassini images and visual and infrared mapping spectrometer data revealed the presence of a high-altitude "haze hood" and a swirling polar vortex at the south pole, two features long associated with the cold north pole. Cassini specialists later reported infrared CIRS observations of winds and temperatures provided evidence of air sinking at the south pole, rather than upwelling. A careful analysis of past data allowed the team to pinpoint the change in circulation to within six months of the 2009 equinox.

The ice cloud, however, was not detected by CIRS until about 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 Goddard.

The southern ice cloud seems to be building rapidly, while the northern cloud — which has been present since Cassini first arrived — has been slowly fading the entire time the spacecraft has been observing it.

The scientists have thus far been unable to identify the ice in the polar clouds. They have ruled out simple chemicals, such as methane, ethane and hydrogen cyanide, which are typically associated with Titan. It is possible that "species X," as some team members call the ice, is 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, 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's chemistry is still unknown. So it's important to learn as much as we can about this phenomenon, wherever we find it."

The findings of this study were published in the Astrophysical Journal Letters.