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Excerpt from nbcnews.com
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’s Ames Research Center speculated
that this could be a tantalizing hint that hydrogen is getting consumed
by life.
“It’s the obvious gas for life to consume on Titan, 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,
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.”
McKay told Space.com that “both results are still preliminary.”
To date, methane-based life forms
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
(-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.
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. Strobel wrote the paper on hydrogen appearing online in
the journal Icarus.
Strobel looked at densities of hydrogen in different parts of the
atmosphere and at 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
Titan’s surface. But when Cassini mapped hydrocarbons on Titan’s
surface, it detected no acetylene on the surface, according to findings
appearing online in the Journal of Geophysical Research.
Instead of alien life on Titan,
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 Titan’s
surface, but loads of benzene and another as-yet-unidentified material,
which appears to be an organic compound. The researchers said that a
film of organic compounds is 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 Roger Clark, a Cassini team scientist based at the
U.S. Geological Survey in Denver. “All that implies Titan is a dynamic
place where organic chemistry is happening now.”
All this speculation “is jumping the gun, in my opinion,” Allen said.
“Typically in the search for the existence of life,
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
oftentimes there are many non-life processes that can explain why things
are missing.”
These findings are “still a long way from evidence of life,” McKay said. “But it could be interesting.”
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Strange find on Titan sparks chatter about life
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