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Cloudy with a chance of…

Experts at NASA have sighted thin, wispy clouds of ice particles similar to Earth’s cirrus clouds on one of Saturn’s moons, Titan.

The findings were made using the Composite Infrared Spectrometer (CIRS) on NASA’s Cassini spacecraft.

The clouds are perfectly white, and their presence is the latest clue to the workings of Titan’s intriguing atmosphere and its one-way “cycle” that delivers hydrocarbons and other organic compounds to the ground as precipitation.

“This is the first time we have been able to get details about these clouds. Previously, we had a lot of information about the gases in Titan’s atmosphere but not much about the [high-altitude] clouds,” said Robert Samuelson at NASA’s Goddard Space Flight Center in Greenbelt, Md.

Compared to earlier-found methane and ethane clouds, these are much thinner and located higher in the atmosphere.

“They are very tenuous and very easy to miss,” said Carrie Anderson, lead author.

Even before Voyager 1 reached Titan, scientists knew the moon was wrapped in a thick atmosphere that probably contained hydrocarbons. It also picked up indications that Titan’s stratosphere, the second-lowest layer of its atmosphere, harboured ices made from some exotic organic compounds,” Samuelson said.

Pinpointing the altitudes where such gases turn into ices is painstaking work. Anderson and Samuelson are using the CIRS (pronounced “sears”) instrument on Cassini to find them.

In addition to spotting the clouds, the researchers gathered enough information to measure the sizes of the ice particles.

“If Titan has any water on the surface, it would be solid as a rock,” said Goddard’s Michael Flasar, the Principal Investigator for CIRS.

The cloud-forming temperatures occur in the “cold, cold depths of Titan’s stratosphere,” said Anderson.

Researchers think that the compounds get moved downward by a constant stream of gas flowing from the pole in the warmer hemisphere to the pole in the colder hemisphere. There, the gas sinks. This circulation pattern steals so much gas from the warmer hemisphere that researchers can measure the imbalance. The influx of all this gas gives the colder hemisphere more clouds.

“We are starting to find out how similar Titan’s clouds are to Earth’s. How do they compare? How do they not compare?” said Anderson.

“The big question is: will the vortex go out with a bang or whimper? On Earth, it goes out with a bang. It’s very dramatic. But on Titan, maybe the vortex just gradually fizzles out like the smile of the Chesire cat,” said Flasar.

The study appears in Icarus.