It was assumed that when Voyager 1’s infrared instrument observed a high-altitude ice cloud on Titan made of dicyanoacetylene (C_{4}N_{2}) in an atmosphere with less than 1 percent of the vapor needed for the cloud to condense, that the Voyager instrument was not sensitive enough to detect it. But when Cassini’s composite infrared spectrometer (CIRS) had the same result, Carrie Anderson, a CIRS co-investigator at NASA’s Goddard Space Flight Center, and her colleagues turned to solid-state chemistry to explain the cloud’s formation.
Instead of the cloud forming by condensation, researchers think the C4N2 ice forms because of reactions taking place on other kinds of ice particles − solid crystals rather than vapor. The process is similar to the formation of nacreous clouds: chlorine-bearing chemical pollutants stick to crystals of water ice, which facilitates chemical reactions that release ozone-destroying chlorine molecules.