A Bit of Titan on Earth Helps in the Search for Life's
Origins
By Lori Stiles
May 17, 2004
While the Cassini spacecraft has been flying toward Saturn,
chemists on Earth have been making plastic pollution like that raining through
the atmosphere of Saturn's moon, Titan.
Scientists suspect that organic
solids have been falling from Titan's sky for billions of years and might be
compounds that set the stage for the next chemical step toward life. They
collaborate in University of Arizona laboratory experiments that will help
Cassini scientists interpret Titan data and plan a future mission that would
deploy an organic chemistry lab to Titan's surface.
Chemists in Mark A.
Smith's laboratory at the University of Arizona create compounds like those
condensing from Titan's sky by bombarding an analog of Titan's atmosphere with
electrons. This produces "tholins" � organic polymers (plastics) found in
Titan's upper nitrogen-methane atmosphere. Titan's tholins are created by
ultraviolet sunlight and electrons streaming out from Saturn's magnetic
field.
Tholins must dissolve to produce amino acids that are the basic
building blocks of life. But chemists know that tholins won't dissolve in
Titan's ethane/methane lakes or oceans.
However, they readily dissolve in
water or ammonia. And experiments done 20 years ago show that dissolving tholins
in liquid water produces amino acids. So given liquid water, there may be amino
acids brewing in Titan's version of primordial soup.
Oxygen is the other
essential for life on Earth. But there is almost no oxygen in Titan�s
atmosphere.
Last year, however, Caitlin Griffith, of UA�s Lunar and
Planetary Laboratory, discovered water ice on Titan�s surface. (See Titan
Reveals a Surface Dominated by Icy Bedrock.) UA planetary scientist Jonathan
Lunine and others theorize that when volcanoes erupt on Titan, some of this ice
could melt and flow across the landscape. Similar flows could result when comets
and asteroids slam into Titan.
Better still, Titan�s water may not immediately freeze because
it's probably laced with enough ammonia (antifreeze) to remain liquid for about
1,000 years, Smith and Lunine noted in a research paper published in last
November's issue of "Astrobiology."
So although Titan is extremely cold
-- about 94 degrees Kelvin (minus 180 degrees Celsius or minus 300 degrees
Fahrenheit) -- water may briefly flow across the surface, supplying oxygen and a
medium for chemistry, they conclude.
To further understand how all this
might work together, Smith's group is generating tholins in the lab, analyzing
their spectroscopic properties, and trying to understand their
chemistry.
�We�re trying to learn how the compounds will react with
molten water on Titan�s surface, what compounds they�ll make, and, therefore,
what we should really be looking for," Smith explained. "We�re not just looking
for atmospheric plastic sitting on the surface, but the result of time and
energy input over billions of years.
"We want to know what sorts of
molecules have evolved, and whether they've evolved along pathways that might
provide insights into how biological molecules developed on primordial Earth,�
he said.
�Some of what we�ve learned so far in our experiments is that
these materials are gross mixtures of incredibly complex molecules,� Smith
added. �Carl Sagan spent the last 10 years of his life studying these compounds
in experiments like ours. What we�ve found complements his work. We see the same
spectroscopic signatures."
But Smith's group also has found that there is
a component of these molecules that is very reactive and could easily, within a
reasonable time frame, react on the surface of Titan to yield oxygenated
compounds.
"And that�s what we�re just starting to unravel now,� Smith
said.
�Our work will get much more interesting this fall, in our
experiments at the Advanced Light Source of the Lawrence Berkeley Lab," he
added. "We�ll be using a synchrotron to create tholins photochemically, using
very energetic photons to break up this Titan gas by vacuum ultraviolet
radiation.�
Vacuum ultraviolet radiation hits nitrogen and methane
molecules in Titan's upper atmosphere and blasts them apart. Scientists don't
know if this produces the same kinds of polymers that are formed from an
electrical discharge.
�When you can crack nitrogen and methane molecules
with light, you might get polymers similar to those formed when an electrical
discharge cracks them apart," Smith said. "Or you may get different polymers.
The chemistry is quite complex, and we just don't know the answers to so many of
the simplest questions. But that's one of the reasons we'll conduct the
experiments at Berkeley.�
The work going on in Smith's lab is important to scientists on
NASA's Cassini Mission and possible follow-up missions to Saturn. The Cassini
orbiter was launched in 1997 and is to launch a probe into Titan's atmosphere in
December. This Huygens probe will float to Titan's surface next
January.
�Titan�s thick orange aerosol haze layer is basically a bunch of
organic plastics � polymers of carbon, hydrogen and nitrogen," said Smith, head
of UA's chemistry department. "The particulates eventually settle on Titan�s
surface, where they produce the organic feedstock for any organic chemistry
going on."
Cassini's Huygens probe will be the first instrument to
actually sample this aerosol. It will give scientists some rudimentary chemical
information on this material. But the probe won't tell them much about organic
chemistry at Titan's surface.
A follow-up mission to Titan that includes
a robotic organic chemistry laboratory will give scientists a much more detailed
look at the surface. The experiment is being designed by Lunine and Smith in
collaboration with researchers from Caltech and NASA's Jet Propulsion
Laboratory.
Lunine leads NASA�s Astrobiology Institute focus group on
Titan and is one of three interdisciplinary Cassini mission scientists for the
Huygens probe.
�We don�t really know how life formed on the Earth, or on
whatever planet it formed,� Lunine said. �There are no traces left of how it
happened on Earth, because all of Earth�s organic molecules have been processed
biochemically by now. Titan is our best chance to study organic chemistry in a
planetary environment that has remained lifeless over billions of
years.�
