There is a giant, toxic cloud circling Titan, Saturn's largest moon. The cloud was spotted recently in data revealed by the ESA's Cassini mission.

The cloud is on Titan's south side. It seems to have formed after the atmosphere of the planet cooled down.

Remco de Kok, the lead author of the research paper detailing what we know about the cloud, said that this giant cloud might mean that Titan's southern half is cooling much more quickly than scientists thought would happen.

The cloud is composed of hydrogen cyanide, a poisonous gas. The cloud is very large, four times larger than the United Kingdom. It was spotted by the Cassini mission as early as May 2012.

Titan is the only moon in the Solar System with a nitrogen-centric atmosphere. Titan experiences different seasons as it orbits the Sun. However, since Titan takes 29 years to make its journey around the Sun, its seasons are much longer than the ones we experience on Earth. The scientists think that this cloud might be a sign of the coming winter on Titan. Because Titan's seasons are so long, the moon has a long winter coming that might make even the Starks of Winterfell shiver - winter lasts seven years on Titan.

"The cloud was first seen in images from Cassini's cameras taken in 2012. It started off quite small but soon grew to cover the entire south polar region. This was totally unexpected and set us puzzling over what the cloud could be made of. Unfortunately, while the images showed that the cloud was very high up, at over 250km above the surface, they did not allow us to figure out what the cloud was actually made of or why it was there," said Dr. Nick Teanby, one of the co-authors of the paper.

de Kok said that the team did not expect hydrogen cyanide to be able to form so high up in Titan's atmosphere, because it would have to cool at an extremely fast rate to allow that to happen. The formation of the hydrogen cyanide suggests that the atmosphere cooled more than 50 degrees in the past year, reaching negative 150 degrees Celsius.

"This is a very rapid change given Titan's long annual cycle and is much colder than previously thought possible. It suggests that once the pole is in shadow the upper atmosphere acts as a very efficient radiator of heat, perhaps due to the high abundance of exotic hydrocarbon and nitrogen based compounds, which emit strongly in the infrared," said de Kok.