Showing that the ingredients for life in the universe may be distributed far more widely than previously thought, scientists have found traces of a key building block of biology in dust snatched from the tail of a comet.
Scientists at the Goddard Space Flight Center in Greenbelt, Md., have uncovered glycine, the simplest amino acid and a vital compound necessary for life, in a sample from the comet Wild 2. The sample was captured by NASA's Stardust spacecraft, which dropped it into the Utah desert in 2006.
"By detecting glycine, we now know that comets could have delivered amino acids to the early Earth, contributing to the ingredients that life originated from," said Jamie Elsila, a research scientist at Goddard and coauthor of a paper outlining the discovery in the journal Meteoritics and Planetary Science.
The idea that the ingredients for life were delivered to Earth from space, rather than developing out of Earth's original chemical soup, has been around for years. Amino acids previously have been discovered in meteorites. But this is the first time an amino acid has turned up in comet material.
"This is yet another piece of evidence that the ingredients for life are ubiquitous. These building blocks of life are everywhere," said Carl Pilcher, director of NASA's Astrobiology Institute, which helped fund the research. Pilcher said the discovery strengthens the argument that life in the universe may be common, rather than rare.
The Stardust spacecraft, managed jointly by the Jet Propulsion Laboratory in La Canada Flintridge and Lockheed Martin Space Systems in Denver, was launched in 1999 on a 2.9-billion-mile journey that made two loops around the sun before meeting up five years later with Wild 2, which orbits between Mars and Jupiter.
Flying as close as 147 miles to the hamburger-shaped comet, Stardust passed through its tail of dust and gas.
At its closest approach, the craft deployed a tennis-racket-shaped collector packed with a substance called aerogel, which harvested comet particles. The spacecraft then returned to Earth's orbit and jettisoned a capsule containing the sample. The capsule made what NASA called a "bulls-eye" landing in Utah on the morning of Jan. 15, 2006.
Jason Dworkin, a coauthor of the research paper, said glycine was first detected a few months after the sample landed. The next two years, he said, were spent verifying the result.
Don Brownlee, a University of Washington astronomer who served as chief scientist on the Stardust mission, called the work "a real tour de force technologically to make these measurements in such small samples."
Brownlee said the result is exciting because it represents a second, very large source of life-giving material. He estimated that there are as many as a trillion comets in and around the solar system, many of them in the chilly Kuiper Belt beyond Pluto, or in the Oort Cloud even farther out.
"There has been a huge question of where the prebiotic compounds came from on Earth," Brownlee said. "Did they come from space? Or were they made here? Or maybe they came from both places."
Just having the right materials is no guarantee that life will begin, of course, any more than leaving a hammer, nails and planks lying around will cause a barn to rise. Brownlee pointed out that many of the 30,000 or so meteorites that have been found on Earth bear traces of organic compounds, and there also is evidence that they were once warm and wet, all necessary conditions for life. Yet none of the meteorites has shown any evidence of life forms.
"They are all failed places where life could have arisen," Brownlee said.