A NASA probe that ferried material from a comet to Earth appears to have brought back an amino acid from that encounter, bolstering a theory that life's precursors may have arrived on our planet from outer space.

The Stardust spacecraft, launched in 1999, passed through Comet Wild 2 in 2004, soaking up particulates and gas with a unique, lightweight capture material known as aerogel. The sample-return portion of the spacecraft dropped safely to Earth during a flyby of our planet two years later.

Early analysis of the samples revealed the presence of biological building blocks such as amino acids, but terrestrial contamination remained a possible explanation for their occurrence.

In research presented Sunday at the fall meeting of the American Chemical Society, NASA researchers announced that the simple amino acid glycine in Stardust's sample is enriched with carbon 13, a heavy isotope of the element that is relatively rare on Earth but more prevalent in space. (Isotopes of chemical elements have different numbers of neutrons in the nucleus and hence different masses—carbon 12 has six protons and six neutrons, whereas carbon 13 has six protons and seven neutrons.)

"We discovered that the Stardust-returned glycine has an extraterrestrial carbon isotope signature, indicating that it originated on the comet," Jamie Elsila, one of the NASA researchers, said in a statement. Elsila said that Wild 2's glycine is the first amino acid to be found in a comet.

Amino acids are critical to life and biochemistry; glycine is one of the so-called standard amino acids used to synthesize proteins.

As for Stardust, it is heading to a new cometary investigation under a secondary mission. In 2011 the probe will fly by Comet Tempel 1, which NASA blasted with part of a spacecraft called Deep Impact in 2005 to explore its composition. The cometary probe is now known as Stardust-NExT (New Exploration of Tempel) after its new target.

Photo of Wild 2 from the Stardust Navigation Camera: NASA/JPL-Caltech