Titan's Swirling Polar Cloud is Cold and Toxic

©ESA

Polar Clouds on Titan

The international Cassini mission has revealed that a giant, toxic cloud is hovering over the south pole of Saturn's largest moon, Titan, after the atmosphere has cooled in a dramatic fashion.

Scientists analysing data from the mission found that this giant polar vortex contains frozen particles of the toxic compound hydrogen cyanide.

"The discovery suggests that the atmosphere of Titan's southern hemisphere is cooling much faster than we expected," says Remco de Kok of Leiden Observatory and SRON Netherlands Institute for Space Research, lead author of the study published in the journal Nature.

Unlike any other moon in the Solar System, Titan is shrouded by a dense atmosphere dominated by nitrogen, with small amounts of methane and other trace gases. Almost 10 times further from the Sun than Earth, Titan is very cold, allowing methane and other hydrocarbons to rain onto its surface to form rivers and lakes.

Like Earth, Titan experiences seasons as it makes its 29-year orbit around the Sun along with Saturn. Each of the four seasons lasts about seven Earth years and the most recent seasonal switch occurred in 2009, when summer transitioned to autumn in the southern hemisphere.

In May 2012, images from Cassini revealed a huge swirling cloud, several hundred kilometres across, taking shape at the south pole.

Titan's changing seasons (larger image)

This polar vortex appears to be an effect of the change of season, with large amounts of air being heated by sunlight during the northern spring and flowing towards the southern hemisphere.

A puzzling detail about this swirling cloud is its altitude, some 300 km above Titan's surface, where scientists thought it was too warm for clouds to form.

"We really didn't expect to see such a massive cloud so high in the atmosphere," says Dr de Kok.
Keen to understand what could give rise to this mysterious cloud, the scientists turned to the rich data from Cassini. After careful scrutiny, they found an important clue in the spectrum of sunlight reflected by Titan's atmosphere.

A spectrum splits the light from a celestial body into its constituent colours, revealing signatures of the elements and molecules that are present. The Visual and Infrared Mapping Spectrometer on Cassini takes spectra at many different points on Titan, mapping the distribution of the chemical compounds in its atmosphere and on its surface.

"The light coming from the polar vortex showed a remarkable difference with respect to other portions of Titan's atmosphere," says Dr de Kok. "We could clearly see a signature of frozen hydrogen cyanide molecules - HCN."

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