http://www.space.com/scienceastronomy/titan-life-methane-
speculation-100607.html
Strange Discovery on Titan Leads to Speculation of Alien Life
By Charles Q. Choi
SPACE.com Contributor
posted: 07 June 2010
New findings have roused a great deal of hoopla over the
possibility of
life on Saturn's moon Titan, which some news reports have further
hyped up
as hints of extraterrestrials.
However, scientists also caution that aliens might have nothing to
do with
these findings.
All this excitement is rooted in analyses of chemical data returned by
NASA's Cassini spacecraft. One study suggested that hydrogen was
flowing
down through Titan's atmosphere and disappearing at the surface.
Astrobiologist Chris McKay at NASA Ames Research Center speculated
this
could be a tantalizing hint that hydrogen is getting consumed by life.
"It's the obvious gas for life to consume on Titan
[http://www.space.com/scienceastronomy/asphalt-lake-life-on-
titan-100505.html],
similar to the way we consume oxygen on Earth," McKay said.
Another study investigating hydrocarbons on Titan's surface found a
lack
of acetylene, a compound that could be consumed as food by life that
relies on liquid methane instead of liquid water to live.
"If these signs do turn out to be a sign of life
[http://www.livescience.com/strangenews/
070806_GM_life_universe.html], it
would be doubly exciting because it would represent a second form
of life
independent from water-based life on Earth," McKay said.
However, NASA scientists caution that aliens might not be involved
at all.
"Scientific conservatism suggests that a biological explanation
should be
the last choice after all non-biological explanations are
addressed," said
Mark Allen, principal investigator with the NASA Astrobiology
Institute
Titan team. "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."
"Both results are still preliminary," McKay told SPACE.com.
To date, methane-based life forms
[http://www.space.com/scienceastronomy/090625-am-titan-
chemistry.html] are
only speculative, with McKay proposing a set of conditions
necessary for
these kinds of organisms on Titan in 2005. Scientists have not yet
detected this form of life anywhere, although 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 minus 290 degrees Fahrenheit
(minus 179 degrees Celsius), any organisms would have to use a
substance
that is liquid as its medium for living processes. Water itself
cannot do,
because it is frozen solid on Titan's surface. The list of liquid
candidates is very short -- liquid methane and related molecules
such as
ethane. Previous studies have found Titan to have lakes of liquid
methane
[http://www.space.com/scienceastronomy/091221-titan-flash-lake.html].
Missing hydrogen?
The dearth of hydrogen Cassini detected is consistent with
conditions that
could produce methane-based life, but do not conclusively prove its
existence, cautioned researcher Darrell Strobel, a Cassini
interdisciplinary scientist based at Johns Hopkins University in
Baltimore, Md., who authored the paper on hydrogen appearing online
in the
journal Icarus.
Strobel looked at densities of hydrogen in different parts of the
atmosphere and the surface. Previous models from scientists 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's computer simulations suggest a hydrogen flow down to the
surface
at a rate of about 10,000 trillion trillion molecules per second.
"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. An unknown mineral could be acting as a
catalyst on Titan's surface to help convert hydrogen molecules and
acetylene back to methane.
Although Allen commended Strobel, he noted "a more sophisticated model
might be needed to look into what the flow of hydrogen is."
Consumed acetylene?
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 mapped hydrocarbons on Titan's surface, it detected no
acetylene on the surface, findings appearing online in the Journal of
Geophysical Research.
Instead of alien life on Titan
[http://www.space.com/scienceastronomy/090625-am-titan-
chemistry.html],
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.
In addition, Cassini detected an absence of water ice on the Titan
surface, but loads of benzene and another as-yet-unidentified
material,
which appears to be an organic compound. The researchers that a
film of
organic compounds are covering the water ice that makes up Titan's
bedrock. This layer of hydrocarbons is at least a few millimeters to
centimeters thick, but possibly much deeper in some places.
"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," said Cassini team scientist Roger Clark based at the U.S.
Geological Survey in Denver. "All that implies Titan is a dynamic
place
where organic chemistry is happening now."
Speculation 'Jumping the Gun'
All this speculation "is jumping the gun, in my opinion," Allen said.
"Typically in the search for the existence of life
[http://www.space.com/scienceastronomy/alien-life-needs-more-than-
water-100520.html],
one looks for the presence of evidence -- say, the methane seen in the
atmosphere of Mars, which can't be made by normal photochemical
processes," Allen added. "Here we're talking about absence of evidence
rather than presence of evidence -- missing hydrogen and acetylene
-- and
often times there are many non-life processes that can explain why
things
are missing."
