Ice cloud heralds fall at Titan's south pole

Apr 11, 2013 by Elizabeth Zubritsky
The change of seasons on Titan is creating new cloud patterns at Titan's south pole. Here, a combination of red, green and blue images taken by the wide-angle camera on NASA's Cassini spacecraft shows a vortex over the south pole in natural color. A more recent ice cloud, detectable only at infrared wavelengths, also has formed over this pole. Credit: NASA/JPL-Caltech/Space Science Institute/GSFC

(Phys.org) —An ice cloud taking shape over Titan's south pole is the latest sign that the change of seasons is setting off a cascade of radical changes in the atmosphere of Saturn's largest moon. Made from an unknown ice, this type of cloud 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 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, Md.

The southern ice cloud, which shows up in the far infrared part of the , is evidence that an important pattern of global air circulation on Titan has reversed direction. When Cassini first observed the , warm air from the 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.

Based on modeling, scientists had long predicted a reversal of this circulation once Titan's north pole began to warm and its began to cool. The official transition from winter to spring at Titan's north pole occurred in August 2009. But because each of the moon's seasons lasts about 7-1/2 Earth years, researchers still didn't know exactly when this reversal would happen or how long it would take.

The first signs of the reversal came in data acquired in early 2012, which was shortly after the start of southern fall on Titan, when Cassini images and visual and data revealed the presence of a high-altitude "haze hood" and a swirling at the south pole. Both features have long been associated with the cold . Later, Cassini scientists reported that infrared observations of Titan's winds and temperatures made by CIRS had provided definitive evidence of air sinking, rather than upwelling, at the south pole. By looking back through the data, the team narrowed down the change in circulation to within six months of the 2009 equinox.

Despite the new activity at the south pole, the southern ice cloud hadn't appeared yet. CIRS didn't detect it until about July 2012, a few months after the haze and vortex were spotted in the south, according to the study published in Astrophysical Journal Letters in December 2012.

"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.

At first blush, the southern ice cloud seems to be building rapidly. The northern ice cloud, on the other hand, was present when Cassini first arrived and has been slowly fading the entire time the spacecraft has been observing it.

So far, the identity of the ice in these clouds has eluded scientists, 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."

Explore further: NASA picks Boeing and SpaceX to ferry astronauts

Related Stories

The Titanian seasons turn, turn, turn

Jul 11, 2012

(Phys.org) -- Images from NASA's Cassini spacecraft show a concentration of high-altitude haze and a vortex materializing at the south pole of Saturn's moon Titan, signs that the seasons are turning on Saturn's ...

South polar region of Titan, Saturn's largest moon

Dec 29, 2011

(PhysOrg.com) -- This view from NASA's Cassini spacecraft looks toward the south polar region of Saturn's largest moon, Titan, and shows a depression within the moon's orange and blue haze layers near the ...

Saturn and its largest moon reflect their true colors

Aug 30, 2012

(Phys.org)—Posing for portraits for NASA's Cassini spacecraft, Saturn and its largest moon, Titan, show spectacular colors in a quartet of images being released today. One image captures the changing hues ...

The many moods of Titan

Feb 23, 2012

(PhysOrg.com) -- A set of recent papers, many of which draw on data from NASA's Cassini spacecraft, reveal new details in the emerging picture of how Saturn's moon Titan shifts with the seasons and even throughout ...

Cassini gazes at veiled Titan

Sep 23, 2010

(PhysOrg.com) -- NASA's Cassini spacecraft will swing high over Saturn's moon Titan on Friday, Sept. 24, taking a long, sustained look at the hazy moon. At closest approach, Cassini will fly within 8,175 kilometers ...

Surprise hidden in Titan's smog: Cirrus-like clouds

Feb 03, 2011

Every day is a bad-air day on Saturn's largest moon, Titan. Blanketed by haze far worse than any smog belched out in Los Angeles, Beijing or even Sherlock Holmes's London, the moon looks like a dirty orange ...

Recommended for you

Image: Rainbow aurora captured from space station

2 hours ago

Auroras occur when particle radiation from the Sun hits Earth's upper atmosphere, making it glow in a greenish blue light. ESA astronaut Alexander Gerst has one of our planet's best views of this phenomenon, ...

Experts: Mystery fireball was Russian satellite

5 hours ago

People from New Mexico to Montana saw the bright object break apart as it moved slowly northward across the night sky. Witnesses described it as three "rocks" with glowing red and orange streaks.

User comments : 1

Adjust slider to filter visible comments by rank

Display comments: newest first

Anda
not rated yet Apr 12, 2013
Always fascinating Titan...