New Study Explains Unexpected Cooling of Titan’s Polar Atmosphere

A study published in the journal Nature Communications has shown that a recently reported unexpected polar vortex behavior on Saturn’s hazy moon Titan is due to its exotic atmospheric chemistry.

Titan’s south polar vortex stands out against the orange and blue haze layers that are characteristic of the hazy moon’s atmosphere. This view looks towards the trailing hemisphere of Titan (3,200 miles, or 5,150 km across). North on Titan is up and rotated 17 degrees to the left. Images taken using red, green and blue spectral filters were combined to create this natural-color view. The image was taken with the Cassini spacecraft narrow-angle camera on July 30, 2013. The view was acquired at a distance of approximately 895,000 miles (1.441 million km) from Titan. Image credit: NASA / JPL-Caltech / Space Science Institute.

The polar atmosphere of Titan recently experienced a rapid cooling, contrary to all model predictions and differing from the behavior of all other terrestrial planets in the Solar System.

Usually, the high altitude polar atmosphere in a planet’s winter hemisphere is warm because of sinking air being compressed and heated — similar to what happens in a bicycle pump.

Puzzlingly, Titan’s polar vortex seems to be extremely cold instead.

Before its final plunge into Saturn’s atmosphere on September 15, 2017, NASA’s Cassini orbiter obtained a long series of observations of Titan’s polar atmosphere using Cassini’s Composite Infrared Spectrometer (CIRS) instrument.

The CIRS observations showed that while the excepted polar hot spot did begin to develop at the start of winter in 2009, this soon developed into a cold spot in 2012, with temperatures as low as minus 243 degrees Fahrenheit (minus 153 degrees Celsius) being observed until late 2015. Only in the most recent 2016 and 2017 observations has the expected hot-spot returned.

“The Cassini spacecraft orbited Saturn from July 1, 2004, to September 15, 2017, and in total made 127 Titan flybys, providing coverage of nearly half of Titan’s 29.5-year orbit around the Sun,” said study lead author Dr. Nick Teanby, a researcher at the University of Bristol, UK, and colleagues.

“Saturn and Titan have an obliquity of 26.7 degrees, which gives rise to pronounced seasonal effects. Cassini’s observations cover early northern winter in 2004 to northern summer solstice in 2017, with northern spring equinox occurring on August 11, 2009.”

“For the first time this unique time series allows us to observe the detailed formation and evolution of Titan’s winter polar vortices.”

“For the Earth, Venus, and Mars, the main atmospheric cooling mechanism is infrared radiation emitted by the trace carbon dioxide and, because this gas has a long atmospheric lifetime, it is well mixed at all atmospheric levels and is hardly affected by atmospheric circulation,” Dr. Teanby said.

“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.”

“These gases are produced high in the atmosphere, so have a steep vertical gradient, meaning that their abundances can be significantly modified by even modest vertical atmospheric circulations.”

“Therefore, winter polar subsidence led to massive enrichments of these radiatively active gases over the southern winter pole.”

Dr. Teanby and co-authors used the temperature and gas abundances measured with Cassini, coupled with a numerical radiative balance model of heating and cool rates, to show that trace gas enrichment was large enough to cause significant cooling and extremely cold atmospheric temperatures.

This explains earlier observations of strange hydrogen cyanide ice clouds that were observed over the pole in 2014 with Cassini’s cameras.

“This effect is so far unique in the Solar System and is only possible because of Titan’s exotic atmospheric chemistry,” Dr. 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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Nicholas A. Teanby et al. 2017. The formation and evolution of Titan’s winter polar vortex. Nature Communications 8, article number: 1586; doi: 10.1038/s41467-017-01839-z