In its 13 years around Saturn, Cassini has discovered a lot about its largest moon Titan. And while the mission might be over, exciting science is still being discovered. For example, researchers have observed a cloud of noxious gas forming in its atmosphere.
The cloud is located at an altitude of about 160 to 210 kilometers (100 to 130 miles) and extends between 75 and 85 degrees south of latitude. It is made of hydrogen cyanide and benzene rings, which have crystallized in a never-seen-before ice configuration.
The cloud is invisible to the human eye, and it was only discovered thanks to Cassini's Composite Infrared Spectrometer (CIRS). The cloud appeared in three observations taken between July and November 2015.
“This cloud represents a new chemical formula of ice in Titan’s atmosphere,” Carrie Anderson, a CIRS co-investigator at NASA’s Goddard Space Flight Center, said in a statement. “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.”
The CIRS detection confused researchers at first as they had not seen it before. To work out its origin, the team used an ice chamber in the lab to recreate the conditions high above the moon’s surface. The team tried different gas mixtures and different freezing scenarios, but it was only when they let hydrogen cyanide and benzene freeze together in well-ordered crystals that they obtained a close match to the observations.
The team had previously seen a similar ice formation in 2005 on the north pole of Titan, which was two years after the Winter solstice in the northern hemisphere. The cloud then was made of hydrogen cyanide and cyanoacetylene, one of the more complex organic molecules in the moon’s atmosphere.
When Cassini began studying Titan in 2004, it was summer in the southern hemisphere. Although each season lasts seven years, the spacecraft has been able to observe how the complex atmosphere of the moon has changed over the years, as well as the seasonal winds and cloud patterns.
“One of the advantages of Cassini was that we were able to flyby Titan again and again over the course of the thirteen-year mission to see changes over time,” said Anderson. “This is a big part of the value of a long-term mission.”
Cassini was a cooperative mission of NASA, the European Space Agency, and the Italian Space Agency. The craft was purposely destroyed on September 15 to avoid potential contamination of bacteria that might have hitched a ride.