Scientists using data from NASA's Cassini mission to Saturn have found evidence of a toxic hybrid ice in a wispy cloud high above the south pole of ringed planet's largest moon, Titan.
The finding is a new demonstration of the complex chemistry occurring in Titan's atmosphere - in this case, cloud formation in the giant moon's stratosphere - and part of a collection of processes that ultimately helps deliver a smorgasbord of organic molecules to Titan's surface.
Invisible to the human eye, the cloud was detected at infrared wavelengths by the Composite Infrared Spectrometer (CIRS) on the Cassini spacecraft.
Located at an altitude of about 160 to 210 kilometres, the cloud is far above the methane rain clouds of Titan's troposphere, or lowest region of the atmosphere.
The new cloud covers a large area near the south pole, from about 75 to 85 degrees south latitude.
Laboratory experiments were used to find a chemical mixture that matched the cloud's spectral signature - the chemical fingerprint measured by the CIRS instrument.
The experiments determined that the exotic ice in the cloud is a combination of the simple organic molecule hydrogen cyanide together with the large ring-shaped chemical benzene.
The two chemicals appear to have condensed at the same time to form ice particles, rather than one being layered on top of the other.
"This cloud represents a new chemical formula of ice in Titan's atmosphere," said Carrie Anderson of NASA's Goddard Space Flight Center in the US.
"What's interesting is that this noxious ice is made of two molecules that condensed together out of a rich mixture of gases at the south pole," said Anderson.
In Titan's stratosphere, a global circulation pattern sends a current of warm gases from the hemisphere where it is summer to the winter pole.
This circulation reverses direction when the seasons change, leading to a buildup of clouds at whichever pole is experiencing winter.
Researchers use CIRS to sort through the complex set of infrared fingerprints from many molecules in Titan's atmosphere.
The new cloud, which the researchers call the high- altitude south polar cloud, has a distinctive and very strong chemical signature that showed up in three sets of Titan observations taken from July to November 2015.
Since Titan's seasons last seven Earth years, it was late fall at the south pole the whole time.
The spectral signatures of the ices did not match those of any individual chemical, so the team began laboratory experiments to simultaneously condense mixtures of gases.
The best result was achieved by introducing both hydrogen cyanide and benzene into the chamber and allowing them to condense at the same time.
Additional studies will be needed to determine the structure of the co-condensed ice particles. The researchers expect them to be lumpy and disorderly, rather than well- defined crystals.
The Cassini spacecraft ended its Saturn mission on September 15 by crashing into the planet.
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