This story originally ran on Aug. 18.
Have we seen the little green men, and are they us?
Scientists from NASA this week reported that they have detected the amino acid glycine in samples of the comet Wild 2 gathered by the spacecraft Stardust, adding ammunition to the theory that life on Earth may have extraterrestrial beginnings and that life may exist elsewhere in space.
The discovery was made by mass spectrometry and "supports the theory that some of life's ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts," Jamie Elsila, lead author of a study to be published on the research and an astrochemist at the NASA Goddard Space Flight Center, said in a statement.
She and her colleagues presented their findings earlier this week at the national meeting of the American Chemical Society in Washington.
In January 2004 Stardust passed through the dense gas and dust surrounding the icy nucleus of the comet Wild 2. As it did, it collected gas and dust samples in a collection grid filled with a special aerogel. The grid was then stowed in a capsule that detached from Stardust and parachuted to Earth two years later.
Since then, scientists worldwide have been analyzing the samples in order to learn more about comets and the formation of our solar system.
In preliminary tests, glycine was detected in both the sample of the aerogel and aluminum foil from the sides of the chambers that held the aerogel in the collection grid. In this stage of the research, liquid chromatography coupled with fluorescence detection and time-of-flight mass spectrometry was used, Elsila told ProteoMonitor in an e-mail.
"This was used to characterize the abundance and distribution of amines and amino acids in the Stardust-returned material," she said.
But though glycine was detected, it was unclear whether that was due to contamination from terrestrial sources. To determine whether the glycine had in fact come from outer space, Elsila and her fellow researchers performed gas chromatography coupled with mass spectrometry and isotope ratio mass spectrometry.
The most common form of carbon, carbon 12, has six protons and six neutrons in its nucleus. A glycine molecule from space, however, tends to have more of the heavier isotope carbon 13, which carries an extra neutron in its nucleus, than a glycine from Earth. Because the glycine from the Stardust samples contained carbon 13, "the carbon isotopic signature was extraterrestrial, proving that this glycine is cometary," Elsila said.
The discovery is far from definitive proof that life on Earth evolved from biological detritus from outer space, or that extraterrestrial life forms may actually exist, but "the discovery of glycine in a comet supports the idea that the fundamental building blocks of life are prevalent in space, and strengthens the argument that life in the universe may be common rather than rare," said Carl Pilcher, director of the NASA Astrobiology Institute, which co-funded the research.