Discovery By Stardust Probe In Wild 2 Comet Suggests Life On Earth Began In Space

A fundamental building block of biology has been discovered in wisps of stardust from the tail of a comet, offering fresh evidence that life on Earth could have begun from matter that arrived on our planet from space.

New analysis of tiny particles captured by the Stardust probe has revealed the presence of traces of an amino acid called glycine, a basic component of proteins without which life as we know it could not exist.

The discovery, by NASA scientists, supports a theory that the raw material from which life began first formed in space, and was carried to Earth by comets that crashed into the planet.

It also means that other planets are likely to have been seeded with amino acids by comets in similar fashion, suggesting that extraterrestrial life may have evolved elsewhere in the Universe and could be common.

“Glycine is an amino acid used by living organisms to make proteins, and this is the first time an amino acid has been found in a comet,” said Jamie Elsila, of NASA’s Goddard Space Flight Center in Maryland, who led the research.

“Our discovery supports the theory that some of life’s ingredients formed in space and were delivered to Earth long ago by meteorite and comet impacts.”

Carl Pilcher, director of the NASA Astrobiology Institute, said: “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.”

The discovery is the latest to come from NASA’s unmanned Stardust mission, which flew through the cloud of dust and debris trailing the Wild 2 comet on January 2, 2004.

Millions of tiny particles from the comet’s tail were captured by a grid filled with aerogel, a super-light, sponge-like material sometimes nicknamed “frozen smoke” because 99 per cent of its volume is empty space.

A capsule containing the collection grid detached from the spacecraft soon after its close encounter with Wild 2 and returned to Earth, where it parachuted to the surface on January 15, 2006.

Scientists have since been examining the contents of the capsule for clues about the early solar system.

All forms of life on Earth rely on proteins, which drive chemical reactions in their cells and form many of the structural elements around which organisms are built. This huge variety of proteins are all made up of chains of 20 amino acids, much as 26 letters can spell all the words in the English language.

The origin of amino acids, and hence of life on Earth, has long been debated among scientists, with some favoring the view that they emerged in the primordial soup of the planet’s youth, and others proposing that they formed in space and were carried here on comets and meteorites.

The discovery of glycine in the Stardust samples points towards an extraterrestrial origin for at least one of the 20 amino acids.

Dr. Elsila’s team first identified traces of glycine last year, in particles removed both from the aerogel and aluminum foil around it.

As glycine is also present on Earth, however, the scientists had to confirm that it originated from space and not from contamination. “It was possible that the glycine we found originated from handling or manufacture of the Stardust spacecraft itself,” said Dr. Elsila.

The team therefore used a method known as isotopic analysis to examine the chemical composition of the glycine they found. Many elements occur in different isotopes, or versions, which have different masses. Carbon, for example, occurs in isotopes called carbon-12, carbon-13 and carbon-14, which differ in the number of protons and neutrons in the nucleus.

The balance of carbon-12 and carbon-13 isotopes can determine its origin. Molecules formed in outer space have proportionately more carbon-13, and this is the signature found in the Stardust glycine samples.

“We discovered that the Stardust-returned glycine has an extraterrestrial carbon isotope signature, indicating that it originated on the comet,” said Dr. Elsisa.

Daniel Glavin, another member of the study team, said: “Based on the foil and aerogel results it is highly probable that the entire comet-exposed side of the Stardust sample collection grid is coated with glycine that formed in space.”

Details of the discovery were presented at the American Chemical Society meeting in Washington, D.C., and a paper on the findings will be published in the journal Meteoritics and Planetary Science.

Professor Donald Brownlee, of the University of Washington in Seattle, who is principal investigator of the Stardust mission, said: “The discovery of amino acids in the returned comet sample is very exciting and profound. It is also a remarkable triumph that highlights the advancing capabilities of laboratory studies of primitive extraterrestrial materials.”

The Stardust probe itself is continuing to explore the solar system’s comets, and will fly past the comet Tempel 1 in 2011. It will get a close look at a scar gouged out of the comet by another spacecraft, Deep Impact, in 2005.