A strange cloud that NASA's Cassini spacecraft spotted on Saturn's moon Titan two years ago appears to be made of icy hydrogen cyanide, a poison that on Earth has been used to kill everything from rats to whales.
The toxic cloud, described in the journal Nature, defies expectations for where and how clouds form on the Saturnian satellite and may force scientists to adjust their understanding of what goes on in the Titan's atmosphere.
The large spinning cloud appeared in Titan's southern hemisphere in May 2012, high in the atmosphere, about 200 miles above the surface. The cloud, which is still going strong, spins faster than the moon itself, taking nine hours to complete a rotation while Titan takes about 16 Earth days.
Using data from the Cassini spacecraft's visual and infrared mapping spectrometer, the researchers looked at the spectrum of light coming from the cloud and found the chemical signature of hydrogen cyanide ice. This was a surprise, because the spot where the cloud sits, about 190 miles above the surface, is not where hydrogen cyanide should be. At that height, it should be too warm for the toxic chemical to condense into ice; those kinds of clouds are thought to lie much lower in the atmosphere, around an altitude of 50 miles.
"The presence of a cloud at this location is therefore highly unexpected," the study authors wrote of the 2012 apparition.
In fact, just three months earlier, Cassini measured the temperature of the region to be minus-153.67 degrees Fahrenheit – 81 degrees too warm for hydrogen cyanide to condense, Caitlin Griffith of the University of Arizona, who was not involved in the study, wrote in a commentary on the paper.
But after studying the region's temperature profile with Cassini's composite infrared spectrometer, the researchers concluded that the area was probably cooling down fast enough to reach the frigid temperatures necessary to make these toxic clouds.
It's possible that because these molecules radiate very strongly at infrared wavelengths, they may end up cooling the surrounding air too fast for warming processes in Titan's atmosphere to keep up, the authors wrote.
"Our detection of HCN ice particles at these altitudes indicates that the polar atmosphere there is roughly 100K [180 degrees Fahrenheit] colder than predicted … contrary to expectations," the study authors wrote. "Hence, models of Titan's circulation require revision to understand the transitional behaviour of Titan's atmosphere around equinox."
Titan's solar "year" as it travels with Saturn around the sun lasts about 29 years, and each season is roughly seven years. The latest seasonal shift occurred in 2009, with the northern hemisphere moving from winter to spring and the southern hemisphere heading from summer to autumn. So this poisonous polar vortex, swirling around as Titan's southern half makes its way toward winter, sheds fresh light on the seasonal dynamics on this strange moon.
Understanding the dynamics of Titan's atmosphere could help researchers better understand those of primordial Earth, Griffith wrote. That's because some theories of a young Earth hold that the terrestrial atmosphere held a lot of methane, rather like that of Saturn's moon.