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 Dr Jamie Elsila of Nasa’s Goddard Space Flight Centre (Goddard) in Greenbelt, Maryland. “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.”
Elsila is the lead author of a paper on this research accepted for publication in the journal Meteoritics and Planetary Science. The research was presented during the meeting of the American Chemical Society in Washington, DC, on 16 August.
“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 Dr Carl Pilcher, Director of the Nasa Astrobiology Institute, which co-funded the research.
Proteins are the workhorse molecules of life, used in everything from structures like hair to enzymes, the catalysts that speed up or regulate chemical reactions. Just as the 26 letters of the alphabet are arranged in limitless combinations to make words, life uses 20 different amino acids in a huge variety of arrangements to build millions of different proteins.
Stardust passed through dense gas and dust surrounding the icy nucleus of Wild 2 (pronounced “Vilt-2”) on 2 January 2004. As the spacecraft flew through this material, a special collection grid filled with aerogel – a novel sponge-like material that’s more than 99 per cent empty space – gently captured samples of the comet’s gas and dust. The grid was stowed in a capsule which detached from the spacecraft and parachuted to Earth on 15 January 2006. Since then, scientists around the world have been busy analysing the samples to learn the secrets of comet formation and our solar system’s history.
“We actually analysed aluminium foil from the sides of tiny chambers that hold the aerogel in the collection grid,” said Elsila. “As gas molecules passed through the aerogel, some stuck to the foil. We spent two years testing and developing our equipment to make it accurate and sensitive enough to analyse such incredibly tiny samples.”
Earlier, preliminary analysis in the Goddard labs detected glycine in both the foil and a sample of the aerogel. However, since glycine is used by terrestrial life, at first the team was unable to rule out contamination from sources on Earth. “It was possible that the glycine we found originated from handling or manufacture of the Stardust spacecraft itself,” said Elsila. The new research used isotopic analysis of the foil to rule out that possibility.
Isotopes are versions of an element with different weights or masses; for example, the most common carbon atom, Carbon 12, has six protons and six neutrons in its centre (nucleus). However, the Carbon 13 isotope is heavier because it has an extra neutron in its nucleus. A glycine molecule from space will tend to have more of the heavier Carbon 13 atoms in it than glycine that’s from Earth. That is what the team found. “We discovered that the Stardust-returned glycine has an extraterrestrial carbon isotope signature, indicating that it originated on the comet,” said Elsila.
“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,” adds Dr Daniel Glavin of Nasa Goddard.
“The discovery of amino acids in the returned comet sample is very exciting and profound,” said Stardust Principal Investigator Professor Donald E Brownlee of the University of Washington, Seattle, Washington. “It is also a remarkable triumph that highlights the advancing capabilities of laboratory studies of primitive extraterrestrial materials.”