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A giant cloud that covers the south pole of Titan, Saturn's largest moon, has a surprisingly poisonous nature: It's made of cyanide gas.
The discovery suggests that the air above Titan's poles can get much cooler than previously thought, scientists said. The Egypt-sized cloud on Titan was first spotted by NASA's Cassini spacecraft in 2012, but only now has its cyanide composition been understood. [See more amazing photos of Titan]
The cloud is at a surprisingly high altitude, about 180 miles (300 kilometers) up. The presence of a cloud at such heights is unexpected because previous estimates of the temperatures at this altitude have been too high to allow cloud formation.
After analyzing years' worth of Cassini observations, the researchers determined that the cloud is made of "ice particles of hydrogen cyanide, or 'blauwzuur,' blue acid, as it's known in the Netherlands, which is highly toxic," according to lead study author Remco de Kok, a planetary scientist at Leiden University in the Netherlands.
Hydrogen cyanide is expected to condense in Titan's atmosphere at an altitude of about 50 miles (80 kilometers), and research has detected a nearly imperceptible tropical haze layer at this height matching this gas. However, Titan's atmosphere was thought to be too warm at higher altitudes for hydrogen cyanide to form.
This new finding reveals that the polar atmosphere above Titan's south pole must be extremely cold to allow hydrogen cyanide to condense, much colder than thought possible at those altitudes — minus 234 degrees Fahrenheit (minus 148 degrees Celsius), about 180 degrees F (100 degrees C) colder than once predicted.
The researchers said that dramatic cooling must be taking place inside the winter vortex of winds over Titan's south pole. "There may be gases present there that radiate a lot of energy away in the infrared, cooling the atmosphere there," de Kok said.
The Titan cyanide cloud study is detailed in the Oct. 2 edition of the journal Nature.