NASA’s Cassini captures defiant clouds on Titan

NASA’s Cassini spacecraft has detected a massive cloud of hydrogen cyanide at Titan’s south pole which simply should not be there.

Titan's southern polar vortex
Titan’s southern polar vortex as seen by Cassini in 2012. Image credit: NASA/JPL-Caltech/ASI/University of Arizona/SSI/Leiden Observatory and SRON

Saturn’s largest moon, Titan, is engulfed in a dense atmosphere that is mainly comprised of nitrogen. Clouds of methane and ethane are interspersed in the atmosphere, and the liquid methane rains down onto the moon’s surface.

Titan boasts a dynamic atmosphere that changes along with the seasons. The Saturn system takes 29 years to orbit the Sun, so that each season on Titan lasts for around 7 years. In 2009, autumn encroached on Titan’s southern hemisphere, and in May 2012 Cassini spotted a swirling vortex at the south pole.

The cloud spans several hundred kilometres across, but its most unusual feature is its height in the atmosphere. Typically, clouds on Titan form at a height of around 45 kilometres above the surface, but this massive cloud is perched at a height of 300 kilometres.

“The cloud can be seen in images,” said Remco de Kok, lead author of the Nature paper discussing the unusual cloud. “When the cloud is seen on the horizon, you can determine the altitude directly.”

By training Cassini’s spectrometers on the cloud, the astronomers discovered that it was comprised of hydrogen cyanide. Normally present in small quantities as a gas in the atmosphere, the hydrogen cyanide in this cloud is made up of ice particles.

In order for hydrogen cyanide gas to condense to icy particles, the temperature needs to plummet to minus 148° Celsius, which is 100° colder than theoretical models predicted this region of the atmosphere to be.

Cassini can measure the temperature of the atmosphere at different altitudes, revealing that Titan’s southern hemisphere is indeed cold enough for hydrogen cyanide clouds to form, and that the temperature is steadily declining.

“We had actually measured the temperatures at this location above the South Pole only half an Earth year earlier and they were 50° Celsius warmer,” de Kok told Skymania.

The cool temperatures can be explained because hydrogen cyanide radiates strongly in the infrared, meaning that it can cool the surrounding air. As more accumulates, the air cools further, thus helping to lower the temperature. The abating autumn sunlight also helps to bring the temperatures down.

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By Amanda Doyle

I am an astrophysics postdoctoral research assistant at the University of Warwick. I obtained my PhD from Keele University in 2014 and my thesis title was "Spectral analyses of solar-like stars". My research involves refining stellar parameters with the aim of improving our understanding of both stars and planets. I completed my masters in astronomy at Swinburne University of Technology via the Swinburne Astronomy Online programme in 2010, and I obtained my degree in physics with astronomy from Dublin City University in 2008. When I'm not doing research, I like to write about all aspects of astronomy. I am a freelance science writer and I contribute to Astronomy Now, NASA's Astrobiology Magazine, BBC Sky at Night magazine, Skymania News, and Sen. I am also the editor of Popular Astronomy magazine.

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