Scientists Think They Know Why Titan's Polar Vortex Is So Cold

The ice cloud associated with the polar vortex. NASA/JPL-Caltech/Space Science Institute

An international team of researchers might have finally worked out why Titan’s winter polar vortex behaves unlike any other in the Solar System. The team thinks it’s all due to Titan’s peculiar atmospheric composition.

In winter, polar regions are typically cold. As the air begins to sink, it becomes compressed and heated, so higher altitude layers of the atmosphere are usually warmer. But Titan, which is Saturn's largest moon, actually develops a colder upper atmosphere. As reported in Nature Communications, the phenomenon has not been observed anywhere else in the Solar System.

"For the Earth, Venus, and Mars, the main atmospheric cooling mechanism is infrared radiation emitted by the trace gas CO2 and because CO2 has a long atmospheric lifetime it is well mixed at all atmospheric levels and is hardly affected by atmospheric circulation," lead author Dr Nick Teanby, from the University of Bristol, UK, said in a statement.

"However, on Titan, exotic photochemical reactions in the atmosphere produce hydrocarbons such as ethane and acetylene, and nitriles including hydrogen cyanide and cyanoacetylene, which provide the bulk of the cooling."

A single year on Titan is equivalent to about 29.5 Earth years and the Cassini mission, which ended a few months ago, was able to study the moon for over half of that time. August 11, 2009, marked the start of winter for Titan's South Pole, and the researchers witnessed the formation of an expected hot spot in the atmosphere. By 2012, the hot spot was gone, and a colder region had appeared with a temperature of -153°C (-243°F). This polar vortex persisted up until the last couple of years.

Titan is the only moon in the Solar System to have a significant atmosphere and from their discovery of the polar vortex, the researchers have tried to understand what novel mechanism has been at play. Some of the chemicals in the atmosphere are produced at a really high altitude, and even modest circulation could lead to a region rich in these molecules. And it turns out that these chemically richer regions matched with the cooler ones.

"This effect is so far unique in the solar system and is only possible because of Titan's exotic atmospheric chemistry. A similar effect could also be occurring in many exoplanet atmospheres having implications for cloud formation and atmospheric dynamics," Teanby added. 

The study also provides an explanation for the mysterious hydrogen cyanide ice clouds seen over the pole in 2014.

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