Some time after the last equinox took place on Saturn’s largest moon, astronomers discovered a giant, toxic cloud swirling above Titan’s southern pole. The presence of frozen cyanide particles indicates that the atmosphere inside the winter polar vortex is much, much cooler than previously predicted -- about 100 degrees Celsius colder. The findings, published in Nature this week, requires us to rethink what we know about Titan’s atmosphere.
Like Earth, Titan experiences seasons. It orbits the sun every 29 years (along with Saturn) and each season lasts about seven years. The last time the seasons switched was in 2009: Winter gave way to spring in the northern hemisphere, and summer slid into autumn in the southern hemisphere. In May 2012 during the southern autumn, astronomers detected a large, swirling cloud above Titan’s south pole. It covers over one million square kilometers, and it has been seen at very high altitudes of 300 kilometers ever since. Researchers attributed this polar vortex to the shifting seasons, but previous estimates of the temperatures at this altitude have been too warm for clouds to form.
To see what gave rise to the mysterious cloud, a team led by Remco de Kok from Leiden University analyzed observations by NASA’s Cassini orbiter of the spectrum of sunlight reflected by Titan’s dense atmosphere. Turns out, the cloud is made up of hydrogen cyanide (HCN) ice particles. "The light coming from the polar vortex showed a remarkable difference with respect to other portions of Titan's atmosphere," de Kok says in a NASA release. "We could clearly see a signature of frozen HCN molecules."
As a gas, HCN is present in small amounts in the atmosphere, but finding these molecules in ice form was surprising: HCN condenses to form frozen particles only if the atmospheric temperature is as cold as minus 148 degrees Celsius. That’s about 100 degrees Celsius colder than what models of Titan's upper atmosphere predict, or about 50 degrees in a year.
"This is a very rapid change given Titan's long annual cycle and is much colder than previously thought possible,” de Kok says in a university statement. “It suggests that once the pole is in shadow, the upper atmosphere acts as a very efficient radiator of heat.” Cassini’s infrared observations confirm that Titan’s dark, southern hemisphere has been cooling rapidly. Atmospheric circulation has been drawing large masses of gas in that direction ever since the seasons changed in 2009. As HCN gas becomes more concentrated, its molecules shine brightly at infrared wavelengths -- cooling the surrounding air in the process.