What is Consuming Hydrogen and Acetylene on Titan?

MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
PASADENA, CALIF. 91109 TELEPHONE 818-354-5011
http://www.jpl.nasa.gov

Jia-Rui Cook 818-354-0850
Jet Propulsion Laboratory, Pasadena, Calif.
jia-rui.c.cook@jpl.nasa.gov

Cathy Weselby 650-604-2791
NASA Ames Research Center, Moffett Field, Calif.
cathy.weselby@nasa.gov

NEWS RELEASE: 2010-190 June 3, 2010
WHAT IS CONSUMING HYDROGEN AND ACETYLENE ON TITAN?

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2010-190&cid=release_2010-190

PASADENA, Calif. – Two new papers based on data from NASA's Cassini spacecraft
scrutinize the complex chemical activity on the surface of Saturn's moon Titan. While
non-biological chemistry offers one possible explanation, some scientists believe these
chemical signatures bolster the argument for a primitive, exotic form of life or precursor to
life on Titan's surface. According to one theory put forth by astrobiologists, the
signatures fulfill two important conditions necessary for a hypothesized "methane-based
life."

One key finding comes from a paper online now in the journal Icarus that shows
hydrogen molecules flowing down through Titan's atmosphere and disappearing at the
surface. Another paper online now in the Journal of Geophysical Research maps
hydrocarbons on the Titan surface and finds a lack of acetylene.

This lack of acetylene is important because that chemical would likely be the best energy
source for a methane-based life on Titan, said Chris McKay, an astrobiologist at NASA
Ames Research Center, Moffett Field, Calif., who proposed a set of conditions necessary
for this kind of methane-based life on Titan in 2005. One interpretation of the acetylene
data is that the hydrocarbon is being consumed as food. But McKay said the flow of
hydrogen is even more critical because all of their proposed mechanisms involved the
consumption of hydrogen.

"We suggested hydrogen consumption because it's the obvious gas for life to consume on
Titan, similar to the way we consume oxygen on Earth," McKay said. "If these signs do
turn out to be a sign of life, it would be doubly exciting because it would represent a
second form of life independent from water-based life on Earth."

To date, methane-based life forms are only hypothetical. Scientists have not yet detected
this form of life anywhere, though there are liquid-water-based microbes on Earth that
thrive on methane or produce it as a waste product. On Titan, where temperatures are
around 90 Kelvin (minus 290 degrees Fahrenheit), a methane-based organism would have
to use a substance that is liquid as its medium for living processes, but not water itself.
Water is frozen solid on Titan's surface and much too cold to support life as we know it.

The list of liquid candidates is very short: liquid methane and related molecules like
ethane. While liquid water is widely regarded as necessary for life, there has been
extensive speculation published in the scientific literature that this is not a strict
requirement.

The new hydrogen findings are consistent with conditions that could produce an exotic,
methane-based life form, but do not definitively prove its existence, said Darrell Strobel,
a Cassini interdisciplinary scientist based at Johns Hopkins University in Baltimore, Md.,
who authored the paper on hydrogen.

Strobel, who studies the upper atmospheres of Saturn and Titan, analyzed data from
Cassini's composite infrared spectrometer and ion and neutral mass spectrometer in his
new paper. The paper describes densities of hydrogen in different parts of the atmosphere
and the surface. Previous models had predicted that hydrogen molecules, a byproduct of
ultraviolet sunlight breaking apart acetylene and methane molecules in the upper
atmosphere, should be distributed fairly evenly throughout the atmospheric layers.

Strobel found a disparity in the hydrogen densities that lead to a flow down to the
surface at a rate of about 10,000 trillion trillion hydrogen molecules per second. This is
about the same rate at which the molecules escape out of the upper atmosphere.

"It's as if you have a hose and you're squirting hydrogen onto the ground, but it's
disappearing," Strobel said. "I didn't expect this result, because molecular hydrogen is
extremely chemically inert in the atmosphere, very light and buoyant. It should 'float' to
the top of the atmosphere and escape."

Strobel said it is not likely that hydrogen is being stored in a cave or underground space
on Titan. The Titan surface is also so cold that a chemical process that involved a catalyst
would be needed to convert hydrogen molecules and acetylene back to methane, even
though overall there would be a net release of energy. The energy barrier could be
overcome if there were an unknown mineral acting as the catalyst on Titan's surface.

The hydrocarbon mapping research, led by Roger Clark, a Cassini team scientist based at
the U.S. Geological Survey in Denver, examines data from Cassini's visual and infrared
mapping spectrometer. Scientists had expected the sun's interactions with chemicals in
the atmosphere to produce acetylene that falls down to coat the Titan surface. But Cassini
detected no acetylene on the surface.

In addition Cassini's spectrometer detected an absence of water ice on the Titan surface,
but loads of benzene and another material, which appears to be an organic compound that
scientists have not yet been able to identify. The findings lead scientists to believe that
the organic compounds are shellacking over the water ice that makes up Titan's bedrock
with a film of hydrocarbons at least a few millimeters to centimeters thick, but possibly
much deeper in some places. The ice remains covered up even as liquid methane and
ethane flow all over Titan's surface and fill up lakes and seas much as liquid water does
on Earth.

"Titan's atmospheric chemistry is cranking out organic compounds that rain down on the
surface so fast that even as streams of liquid methane and ethane at the surface wash the
organics off, the ice gets quickly covered again," Clark said. "All that implies Titan is a
dynamic place where organic chemistry is happening now."

The absence of detectable acetylene on the Titan surface can very well have a non-
biological explanation, said Mark Allen, principal investigator with the NASA
Astrobiology Institute Titan team. Allen is based at NASA's Jet Propulsion Laboratory in
Pasadena, Calif. Allen said one possibility is that sunlight or cosmic rays are transforming
the acetylene in icy aerosols in the atmosphere into more complex molecules that would
fall to the ground with no acetylene signature.

"Scientific conservatism suggests that a biological explanation should be the last choice
after all non-biological explanations are addressed," Allen said. "We have a lot of work to
do to rule out possible non-biological explanations. It is more likely that a chemical
process, without biology, can explain these results – for example, reactions involving
mineral catalysts."

"These new results are surprising and exciting," said Linda Spilker, Cassini project
scientist at JPL. "Cassini has many more flybys of Titan that might help us sort out just
what is happening at the surface."

The Cassini-Huygens mission is a cooperative project of NASA, the European Space
Agency and the Italian Space Agency. JPL, a division of the California Institute of
Technology, manages the mission for NASA's Science Mission Directorate, Washington,
D.C. The Cassini orbiter was designed, developed and assembled at JPL.

For more information about the Cassini-Huygens mission visit http://www.nasa.gov/cassini and http://saturn.jpl.nasa.gov.

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