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News feature: 2012-115 April 24, 2012
Cassini Investigates Titan's Chemical Factory
The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2012-116&cid=release_2012-116
Saturn's giant moon Titan hides behind a thick, smoggy atmosphere that's well known to scientists as
one of the most complex chemical environments in the solar system. It's a productive "factory"
cranking out hydrocarbons that rain down on Titan's icy surface and cloak it in soot. With a brutally
cold surface temperature of around minus 270 degrees Fahrenheit (minus 170 degrees Celsius), the
hydrocarbons form lakes of liquid methane and ethane.
However, the most important raw ingredient in Titan's chemical factory is methane gas. Methane is a
molecule made up of one carbon atom joined to four hydrogen atoms. It should not last long because
it's being continuously destroyed by sunlight and converted to more complex molecules and particles.
New research from NASA-funded scientists attempts to estimate how long this factory has been
operating. The results are presented in two papers appearing in the April 20 issue of the Astrophysical
Journal.
These papers used data collected by two instruments onboard NASA's Cassini spacecraft in orbit
around Saturn and one instrument on the European Space Agency's Huygens probe that landed on
Titan's surface in January 2005. All three instruments were built at NASA's Goddard Space Flight
Center in Greenbelt, Md. A paper led by Conor Nixon of the University of Maryland, College Park
uses infrared signatures (spectra) of methane from Cassini's composite infrared spectrometer to
estimate how much "heavy" methane containing rare isotopes is present in Titan's atmosphere.
Isotopes are versions of an element with different weights, or masses. For example, carbon 13 is a
heavier (and rare) version of the most common type of carbon, called carbon 12. Occasionally, a
carbon-13 atom replaces a carbon-12 atom in a methane molecule. Because methane made with
carbon 12 is slightly lighter, the chemical reactions that convert it to more complex hydrocarbons
happen a bit faster. This means carbon-12 methane gets used up at a slightly faster rate than heavy
carbon-13 methane, so the concentration of heavy methane in Titan's atmosphere increases slowly.
By modeling how the concentration of heavy methane changes over time, the scientists predicted how
long Titan's chemical factory has been running.
"Under our baseline model assumptions, the methane age is capped at 1.6 billion years, or about a
third the age of Titan itself," said Nixon, who is stationed at Goddard. "However, if methane is also
allowed to escape from the top of the atmosphere, as some previous work has suggested, the age must
be much shorter -- perhaps only 10 million years -- to be compatible with observations."
Both scenarios assume that methane entered the atmosphere in one burst of outgassing, probably from
the restructuring of Titan's interior as heavier materials sank towards the center and lighter ones rose
toward the surface.
"However, if the methane has been continuously replenished from a source, then its isotopes would
always appear 'fresh' and we can't restrict the age in our model," said Nixon. Possible sources include
methane clathrates, basically a methane molecule inside a "cage" or lattice of ice molecules. Methane
clathrates are found in the frigid depths of Earth's oceans, and some scientists think there could be an
ocean of liquid water mixed with ammonia (acting as antifreeze) beneath Titan's water-ice crust. If
this is so, methane might be released from its clathrate cages during the eruptions of proposed
'cryovolcanoes' of water-ammonia slurry, or more simply could slowly seep out through fractures in
the crust.
The second Titan paper by Kathleen Mandt of the Southwest Research Institute, San Antonio, and
colleagues also models the time-evolution of methane. In this work, the concentration of the heavy
methane is determined from measurements by Cassini's ion and neutral mass spectrometer, which
counts molecules in the atmosphere of different masses (weights). Measurements made by the
Huygens gas chromatograph mass spectrometer, which also counts molecules of different masses,
were used to constrain the impact of escape on the heavy methane in the atmosphere.
"We compute that, even if methane has been replenished from the interior over time to match or
exceed the amounts fed into the atmospheric chemical factory, the process must have been running
for a maximum of one billion years," said Mandt. "If the process had started any earlier, we would
see a build-up of methane in the lakes on the surface and in the atmosphere beyond what is observed
today."
Together these papers add important new perspectives and constraints on the history of Titan's
methane atmosphere, confirming that it must have formed long after Titan itself. Previous work
considering the evolution of Titan's interior predicted the last major methane eruption occurred 350
million to 1.35 billion years ago, while crater counting has put the age of the current surface at 200
million to one billion years. (Crater counting works on the principle that an older surface has more
craters, just as the longer you're in a paintball game, the more hits you'll get.)
The present work for the first time estimates the methane age from the atmosphere itself, at less than
one billion years, considering both papers.
This research was supported by the NASA Cassini Mission and the NASA Cassini Data Analysis
Program grant NNX09AK55G. The Cassini-Huygens mission is a cooperative project of NASA, the
European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of
the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission
Directorate, Washington.
For related images to this story, please visit:
http://www.nasa.gov/mission_pages/cassini/whycassini/factory20120420.html
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