|Researcher examines a Stardust aerogel tile under a stereo microscope.
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.
|Dr. Jason P. Dworkin
"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," said Dworkin.
"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.
|This aerogel array, which was mounted atop the Stardust spacecraft, was used to collect interstellar dust particles as well as dust from the tail of comet Wild 2.
"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 re-entry.
"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.