An amino acid has been found on a comet for the first time, a new analysis of samples from NASA''s Stardust mission reveals. The discovery confirms that some of the building blocks of life were delivered to the early Earth from space.
Amino acids are crucial to life because they form the basis of proteins, the molecules that run cells. The acids form when organic, carbon-containing compounds and water are zapped with a source of energy, such as photons – a process that can take place on Earth or in space.
Previously, researchers have found amino acids in space rocks that fell to Earth as meteorites, and tentative evidence for the compounds has been detected in interstellar space. Now, an amino acid called glycine has been definitively traced to an icy comet for the first time.
"It''s not necessarily surprising, but it''s very satisfying to find it there because it hasn''t been observed before," says Jamie Elsila of NASA''s Goddard Space Flight Center, lead author of the new study. "It''s been looked for [on comets] spectroscopically with telescopes but the content seems so low you can''t see it that way."
Comets and asteroids are thought to have bombarded the Earth early in its history, and the new discovery suggests they carried amino acids with them.
"We are interested in understanding what was on the early Earth when life got started," Elsila told New Scientist. "We don''t know how life got started ... but this adds to our knowledge of the ingredient pool."
Jonathan Lunine of the University of Arizona agrees. "Life had to get started with raw materials," he told New Scientist. "This provides another source [of those materials]."
Tiny sample size
The samples in Elsila''s study came from four squares of aluminium foil, each about 1 centimetre across, that sat next to a lightweight sponge-like "aerogel" that was designed to capture dust from the comet''s atmosphere, or coma.
The researchers reported finding several amino acids, as well as nitrogen-containing organic compounds called amines, on the foil in 2008. But it was not clear whether the discoveries originated in the comet or whether they were simply contamination from Earth.
The researchers spent two years trying to find out – a painstaking task since there was so little of the comet dust to study. In fact, there was not enough material to trace the source of any compound except for glycine, the simplest amino acid.
With only about 100 billionths of a gram of glycine to study, the researchers were able to measure the relative abundance of its carbon isotopes. It contained more carbon-13 than that found in glycine that forms on Earth, proving that Stardust''s glycine originated in space.
"It''s a great piece of laboratory work," says Lunine. "It''s probably something that couldn''t have been done remotely with a robotic instrument – it points to the value of returning samples."
Elsila says she would like to see samples returned not just from a comet''s coma but from its main body, or nucleus. "There might be more complex mixtures [of amino acids] and higher levels of them in a comet nucleus," she told New Scientist.
Europe''s Rosetta spacecraft should help shed light on the issue. The first mission designed to orbit and land on a comet''s nucleus, it will reach the Comet 67P/Churyumov-Gerasimenko in 2014 after a 10-year journey from Earth.
Journal reference: Meteoritics & Planetary Science (forthcoming)