Cassini data explains Titan's cold winters

The winter skies of Titan, Saturn’s largest moon, are unseasonably cold, and a recent study using Cassini data suggests that the world’s unique atmosphere could be responsible.

The discovery could help planetary reasearchers understand some of the strange atmospheres that exist around exoplanets.

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Titan is the only known moon within the Solar System to host a significant atmosphere
Credit: NASA/JPL/Space Science Institute

 

On the rocky planets of our Solar System that have atmospheres, the high-altitude air over the polar region is usually warm during winter, no matter how cold it may be on the ground.

This is due to the sinking air compressing and heating the atmosphere.

But when NASA’s Cassini spacecraft studied Saturn's moon Titan, the only moon in the Solar System known to have a significant atmosphere, it revealed that the polar vortex is instead extremely cold.

 


Read more about the Cassini mission from BBC Sky at Night Magazine:


 

“For Earth, Venus and Mars, the main atmospheric cooling mechanism is infrared radiation emitted by the trace gas carbon dioxide,” says Nick Teanby from the University of Bristol, who led the study.

“Because carbon dioxide has a long atmospheric lifetime it is well mixed at all atmospheric levels and is hardly affected by atmospheric circulation.

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

Where carbon dioxide insulates the planet, these more complex chemicals allow the heat out, causing extremely cold atmospheric temperatures.

 


A view of Titan passing in front of Saturn and its rings, captured by the Cassini spacecraft.
Credit: NASA/JPL-Caltech

 

The cold spot over Titan was discovered by the Cassini spacecraft, which watched the planet for nearly half of its 29.5 Earth-year long year.

At the start of the moon’s winter in 2009, the spot did initially start to warm, but the temperature dropped in 2012, reaching temperatures as cold as 120K (-150ºC).

Only in the last two years has the hot-spot returned.

The finding could help researchers interpret moons and worlds far beyond our Solar System.

“This effect is so far unique in the Solar System and is only possible because of Titan’s exotic atmospheric chemistry,” Teanby concludes. “But a similar effect could also be occurring in many exoplanet atmospheres having implications for cloud formation and atmospheric dynamics.”


 

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