[meteorite-list] Comets As Toolkits For Jump-Starting Life (Stardust)

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 14 Dec 2006 13:28:14 -0800 (PST)
Message-ID: <200612142128.NAA09980_at_zagami.jpl.nasa.gov>

http://stardust.jpl.nasa.gov/news/news109.html

Comets As Toolkits For Jump-Starting Life
December 14, 2006

Just as kits of little plastic bricks can be used to make
everything from models of the space shuttle to the statue
of liberty, comets are looking more and more like one of
nature's toolkits for creating life. These chunks of ice
and dust wandering our solar system appear to be filled
with organic molecules that are the building blocks of
life.

The discovery of two kinds of nitrogen-rich organic
molecules in comet Wild 2 is the latest addition to the
set of bits and pieces useful to the origin of life that
has been found in comets.

These discoveries were made by members of the Stardust
Preliminary Examination Team, a group of scientists who
have been studying the samples returned from comet Wild 2
by NASA's Stardust spacecraft in January 2006.

"These results show that comets could have delivered
nitrogen rich organic compounds to the early Earth where
they would have been available for the origin of life,"
said Scott Sandford of NASA's Ames Research Center,
Moffett Field, Calif.

"This discovery shows that the menu of compounds available
for the origin of life was richer than had been previously
thought," said Jason Dworkin of NASA's Goddard Space Flight
Center, Greenbelt, Md.

"The two molecules we discovered in comet Wild 2,
methylamine and ethylamine, provide a source of fixed
nitrogen, a commodity which could have been rare on the
ancient Earth. Nitrogen fixation is the conversion of the
very stable nitrogen (N2) gas in our atmosphere to a
biologically usable form, like an amine or nitrate -- the
same compounds found in fertilizer. Enzymes that fix
nitrogen appear to be ancient, so finding a source of
fixed nitrogen would have been an early challenge for life
from the time of its origin. We determined that at least one
type of comet would have provided significant quantities of
stable, fixed nitrogen in the form of methylamine and
ethylamine," added Dworkin.

This is the first time these molecules have been detected in
comets. As the Stardust spacecraft sped through the comet's
tail at nearly 21,000 kilometers per hour (13,000 miles per
hour), a set of aerogel tiles mounted on a boom trapped dust
and gas from the comet. Often referred to as "frozen smoke,"
aerogel is the world's lowest density solid. Its low density
allows it to slow and capture comet dust particles without
vaporizing them.

Although the mission's goal was to return samples of comet
dust to Earth, the researchers looked for organic molecules
that were embedded in the aerogel itself, rather than trapped
in dust grains. "We found that the aerogel acted like a
sponge, absorbing organic gases from the comet nucleus," said
Daniel Glavin of NASA Goddard.

"And just like squeezing a sponge, we squeezed out all the
good stuff -- the water-soluble organics -- by boiling
samples of the aerogel in ultra-high purity water," added
Glavin. The team analyzed the aerogel water extract with a
liquid chromatograph mass spectrometer instrument to identify
the organic molecules.

Since Earth is crawling with life, the team had to rule out
contamination from our planet before it could say the
molecules likely came from the comet. Glavin and Dworkin
analyzed dozens of "pre-flight" aerogels that were not flown
on Stardust in order to understand the organic background
levels within the aerogel.

The team found high levels of both methylamine and ethylamine
in aerogel that was exposed to comet Wild 2. While they did
find small amounts of methylamine and trace levels of
ethylamine in the pre-flight aerogel, the total amount in
the unflown aerogel was over 100 times less. Also, the
relative amounts of the two molecules were very different
from that found in the comet-exposed aerogel. The different
total and relative amounts convinced the team that most of
the two chemicals in the Stardust sample came from the comet.

However, since Stardust was in space for seven years, the
team had to be sure that the two chemicals weren't simply
picked up while the spacecraft was cruising toward Wild 2.
Since the pressure in space is so low, the spacecraft can
release gas or volatile materials acquired during its
manufacture on Earth. This is called "outgassing," and it
could have contaminated the aerogel as well.

To reveal the levels of contamination from these two sources,
the Stardust team included a special piece of aerogel called
the "witness tile" on the spacecraft. It's a piece of aerogel
located behind a dust shield that protected the spacecraft
from high-speed collisions with comet particles. This
location kept the witness tile from being exposed to gas and
dust from the comet. But the witness aerogel was exposed to
everything else Stardust encountered, including the
manufacturing processes, shipping, the launch, spacecraft
outgassing, and Earth reentry.

"When we analyzed a sample of the witness tile, we did not
detect methylamine or ethylamine, so we don't think Stardust
was contaminated with these two chemicals on the way to
Wild 2," said Glavin.

NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages
the Stardust mission for NASA's Science Mission Directorate,
Washington. Dr. Peter Tsou of JPL is deputy principal
investigator and is a co-author on seven papers about the
mission's initial findings appearing in the Dec. 15 issue of
Science Express, the online edition of the journal Science.

For more information about Stardust studies and other
mission information, visit:

http://stardust.jpl.nasa.gov/

JPL Media Contact for Stardust:
DC Agle
818-393-9011
agle at jpl.nasa.gov
Received on Thu 14 Dec 2006 04:28:14 PM PST