Unique atmospheric chemistry explains cold vortex on Saturn's moon Titan

New analysis suggests unique atmospheric chemistry explains why the polar vortex on Saturn's moon Titan isn't behaving as expected.

Titan is Saturn's largest moon, and is the only moon in the solar system with a sizable atmosphere. Recent observations suggest that atmosphere hasn't been behaving as predicted by models.

During the winter, air in the upper atmosphere tends to warm as cold air sinks, compresses and becomes heated. However, recent readings suggest the moon's polar vortex is oddly cold.

Before its fiery demise in Saturn's atmosphere on September 15, the Cassini spacecraft obtained a long series of observations of Titan's polar atmosphere covering nearly half of Titan's 29.5 year solar year - as measured by the time Saturn and Cassini take to circle the Sun - using the Composite Infrared Spectrometer (CIRS) instrument.

Through the years, NASA's Cassini spacecraft -- which dove into Saturn's atmosphere for a final time earlier this year -- executed a number of Titan flybys, measuring temperature and collecting samples from the polar atmosphere above the winter hemisphere.

The probe's data confirmed a hot spot began to form in 2009 -- as expected. But readings showed a cold spot began to form in 2012. Temperatures dipped to 120 Kelvin in 2015, before the hot spot returned in 2016 and 2017.

Researchers at the University of Bristol hypothesize that the unique chemical composition of Titan's atmosphere accounts for the unexpected variability.

"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," Nick Teanby, planetary scientist at Bristol, said in a news release.

"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."

The hydrocarbons found in Titan's atmosphere aren't evenly distributed. Thus, perturbations in the polar vortex can quickly alter the makeup of the different atmospheric layers.

Researchers suggest the sinking of cold air can leave high concentrations of trace gas chemicals in the upper atmosphere, causing rapid cooling.

Their analysis -- published this week in the journal Nature Communications -- explains the presence of hydrogen cyanide ice clouds above Titan's winter pole, as photographed by Cassini's camera in 2014.

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

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