Saturn’s
giant moon Titan hides within 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, cloaking it in
soot and, with a brutally cold surface temperature of around minus -270
F, forming lakes of liquid methane and ethane.
However
the most important raw ingredient in this chemical factory—methane gas,
a molecule made up of one carbon atom joined to four hydrogen
atoms—should not last for 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 as two papers
appearing in the April 20 issue of the Astrophysical Journal.
These
papers used data from 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.
This false-color image shows a thin, detached haze layer (purple line) that appears to float above Titan’s main atmospheric haze. The Cassini spacecraft narrow-angle camera took the image on July 3, 2004. Credit: NASA/JPL/Space Science Institute |
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 NASA 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 of these 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,” adds 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 paper by Kathleen Mandt of the Southwest Research Institute, San
Antonio, Texas, 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.
This false-color image shows evidence for lakes of liquid hydrocarbons, probably methane or ethane, on Titan’s surface. The lakes are represented as dark areas, but are not what the human eye would see, because radar was used to penetrate the thick haze obscuring Titan’s surface. The Cassini radar instrument acquired this image on July 22, 2006. The image is centered near 80 degrees north, 35 degrees west and is about 140 km (84 miles) across. The strip of radar imagery is foreshortened to simulate an oblique view of the highest latitude region, seen from a point to its west. Smallest details in this image are about 500 m (1,640 feet) across. Credit: NASA/JPL/USGS |
“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 has 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.
Source: NASA Goddard Space Flight Center