Gigantic Ice Cloud Spreads Across Titan

A south polar vortex (small, bright “button”) has been forming, seen in this 2013 image. NASA/JPL-Caltech/Space Science Institute

Winter is coming on Titan’s south pole, according to the latest analysis from Cassini. The probe has been observing the formation of a large frozen cloud in Titan’s stratosphere.  

The cloud was detected by Cassini’s Composite Infrared Spectrometer (CIRS), which was able to look through the different layers of the atmosphere and discover the huge perturbation, according to preliminary findings. The cloud is 240 kilometers (150 miles) across, and composed mostly of hydrocarbons, nitrogen compounds called nitriles, and hydrogen cyanide

The cloud peaks at an altitude of 200 kilometers (124 miles). It has a very low density, which is reportedly akin to Earth’s fog but likely flat on top. This is not the first polar cloud observed around the south pole; another smaller one was observed in 2012 reaching a higher altitude. 

When Cassini arrived in 2004, the south pole was mid-summer and ice clouds were seen in the opposite pole. But as the seasons changed, the global atmospheric circulation changed, moving gas from the north to the south.

Titan’s seasons last for 7.5 years. The seasons depend on Saturn’s orbit inclinations; only one pole is illuminated at a time. This disparity in sunlight generates movements in the atmosphere and sends warmer gas from the summer pole towards the other, where it cools down and sinks into a cloud. 

All the information that was extrapolated from the cloud image (its size, altitude and composition) are used by scientists to understand what the onset of winter is like on Titan. Based on the 2012 cloud observations, the temperature in the south pole must get down to at least -150°C (-230°F). The new observations will help refine this estimate. 

“The opportunity to see the early stages of winter on Titan is very exciting,” said Robert Samuelson from NASA's Goddard Space Flight Center, who worked on this research, in a statement. “Everything we are finding at the south pole tells us that the onset of southern winter is much more severe than the late stages of Titan’s northern winter.”

Cassini will continue to periodically study the southern hemisphere of Titan until 2017, when its mission will end.

Image in text: Cassini’s camera spotted this impressive cloud hovering in a 2012 close-up of Titan’s south pole, by NASA/JPL-Caltech/Space Science Institute

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