This is the first definitive detection of the plastic ingredient on any moon
or planet, other than Earth.
Image Credit:
NASA's Goddard Space Flight
Center
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A small amount of propylene was identified in Titan's lower atmosphere by
Cassini's Composite Infrared Spectrometer (CIRS). This instrument measures the
infrared light, or heat radiation, emitted from Saturn and its moons in much the
same way our hands feel the warmth of a fire.
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. Details
are presented in a paper in the Sept. 30 edition of the Astrophysical Journal
Letters.
"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, Md., and lead
author of the paper. "That plastic container at the grocery store with the
recycling code 5 on the bottom -- that's polypropylene."
CIRS can identify a particular gas glowing in the lower layers of the
atmosphere from its unique thermal fingerprint. The challenge is to isolate this
one signature from the signals of all other gases around it.
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 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.
"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."
Cassini's mass spectrometer, a device that looks at the composition of
Titan's atmosphere, had hinted earlier that propylene might be present in the
upper atmosphere. However, 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. JPL manages the mission for NASA's
Science Mission Directorate in Washington. The CIRS team is based at
Goddard.
For more information about the Cassini mission, visit:
NASA
Guillermo Gonzalo Sánchez Achutegui
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