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

[Ron Baalke]

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