Saturn’s largest moon Titan fascinates planetary scientists because in many ways it resembles the Earth. It has a dense atmosphere, lakes, rivers and coastal features though it rains petrochemicals rather than water.
Now another similarity has been discovered by NASA’s Cassini spacecraft. This intriguing moon’s atmosphere also contains an ingredient we use on our own world to make plastic storage containers for the fridge.
Despite our light-hearted headline, we’re not suggesting that they hold Tupperware parties on Titan. But this is the first detection ever of propylene, also used for car bumpers, on any other place other than Earth.
A small amount of the stuff was picked out in Titan’s lower atmosphere by Cassini’s Composite Infrared Spectrometer, which measures the infrared light, or heat, radiating from Saturn and its moons.
Propylene is the first molecule to be discovered on Titan using CIRS and researchers are highly confident about their find, having isolated the same signal at various altitudes in the lower atmosphere. The experiment can identify a particular glowing gas from its unique thermal fingerprint. The challenge is to isolate this one signature from those of other gases around it.
Planetary scientist Conor Nixon, of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, is lead author of a paper about the discovery in the Astrophysical Journal. He said: “This chemical is all around us in everyday life, strung together in long chains to form a plastic called polypropylene. That plastic container at the grocery store with the recycling code 5 on the bottom – that’s polypropylene.”
The detection of propylene fills in a mysterious gap in Titan observations that dates back to the first-ever close flyby of this moon in 1980 by NASA’s Voyager 1 spacecraft. 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.
A NASA video about the discovery. Credit: Goddard Space Flight Center
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 and 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.
Michael Flasar, Goddard scientist and principal investigator for CIRS, said: “This measurement was very difficult to make because propylene’s weak signature is crowded by related chemicals with much stronger signals. This success boosts our confidence that we will find still more chemicals long hidden in Titan’s atmosphere.”
Cassini’s mass spectrometer, a device that looks at the composition of Titan’s atmosphere, had previously hinted that propylene might be present in the upper atmosphere but a positive identification had not been made.
“I am always excited when scientists discover a molecule that has never been observed before in an atmosphere,” said Scott Edgington, Cassini’s deputy project scientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif. “This new piece of the puzzle will provide an additional test of how well we understand the chemical zoo that makes up Titan’s atmosphere.”
The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. One of the mission’s first major successes was to send ESA’s Huygens probe to a soft landing on the surface of Titan.