This is the first definitive detection of the chemical - used to make food-storage containers, car bumpers and other consumer products - on any moon or planet, other than Earth.
A small amount of propylene was identified in Titan's lower atmosphere by Cassini's Composite Infrared Spectrometer (CIRS), which measures the infrared light, or heat radiation, emitted from Saturn and its moons.
Propylene is the first molecule to be discovered on Titan using CIRS. By isolating the same signal at various altitudes within the lower atmosphere, researchers identified the chemical with a high degree of confidence.
"This chemical is all around us in everyday life, strung together in long chains to form a plastic called polypropylene," said Conor Nixon, a planetary scientist at NASA's Goddard Space Flight Center in Greenbelt, and lead author of the paper in Astrophysical Journal Letters.
The detection of the chemical fills in a mysterious gap in Titan observations that dates back to NASA's Voyager 1 spacecraft and the first-ever close flyby of this moon in 1980.
Voyager identified many of the gases in Titan's hazy brownish atmosphere as hydrocarbons, the chemicals that primarily make up petroleum and other fossil fuels on Earth.
On Titan, hydrocarbons form after sunlight breaks apart methane, the second-most plentiful gas in that atmosphere. The newly freed fragments can link up to form chains with two, three or more carbons.
The family of chemicals with two carbons includes the flammable gas ethane. Propane, a common fuel for portable stoves, belongs to the three-carbon family.
Voyager detected all members of the one- and two-carbon families in Titan's atmosphere. From the three-carbon family, the spacecraft found propane, the heaviest member, and propyne, one of the lightest members. But the middle chemicals, one of which is propylene, were missing.
As researchers continued to discover more chemicals in Titan's atmosphere using ground- and space-based instruments, propylene was one that remained elusive. It was finally found as a result of more detailed analysis of the CIRS data.
"This measurement was very difficult to make because propylene's weak signature is crowded by related chemicals with much stronger signals," said Michael Flasar, Goddard scientist and principal investigator for CIRS.
"This success boosts our confidence that we will find still more chemicals long hidden in Titan's atmosphere," he said.