Isotopic Evidence for Extraterrestrial Non-Racemic Amino Acids in the Murchison Meteorite
"Isotopic Evidence for Extraterrestrial Non-Racemic Amino Acids in the Murchison Meteorite"
By: Michael H. Engel and Stephen A. Macko
Date: September 18, 1997
Source: Engel, Michael H. and Stephen A. Macko. "Isotopic Evidence for Extraterrestrial Non-Racemic Amino Acids in the Murchison Meteorite." Nature 389 (September 18, 1997): 265.
About the Author: Michael H. Engel is the Willard Miller Professor at the School of Geology and Geophysics at the University of Oklahoma in Norman, Oklahoma. His research focuses on the stable isotopes and steroisomer composition of terrestrial and extraterrestrial materials. Stephen A. Macko is a professor in the Department of Environmental Sciences, University of Virginia in Charlottesville, Virginia. His work involves using isotope analysis to study the cycling of organic matter in fossil and modern ecosystems.
The Murchison meteorite fell onto farmland near the town of Murchison, Victoria in Australia about 60 miles (97 kilometers) north of Melbourne on September 28, 1969. Witnesses near Murchison observed the fall of the meteorite and nearly 220 pounds (100 kilograms) of fragments were collected within a short period of time.
Terrestrial bacteria are quick to colonize surfaces and cracks in meteorites after they fall to Earth. Meteorites that are found long periods of time after they have fallen to Earth are almost certainly contaminated by terrestrial organisms. The rapid collection of the Murchison meteorite fragments increased chances that its composition was pristine. In addition, because the meteorite fell to Earth during the same period as NASA's lunar missions, laboratories around the world were prepared to perform analyses on extraterrestrial rocks. The Murchison meteorite garnered much attention in the scientific community for two major reasons: its organic chemical composition and the possibility that it contained fossilized bacteria.
The Murchison meteorite is a primitive meteorite containing materials that coalesced even before the Earth's solar system formed. It contains carbon-based molecules just like the molecules found in living organisms on Earth. In particular it contains a large number of amino acids, the molecules that make up proteins in living organisms. Many of these amino acids are found on Earth, but it contains more than fifty amino acids that are extremely rare. In addition, the chirality (structure) of the amino acids in the meteorite is closer to that of amino acids that are found in organisms on Earth, which are all left-handed, than the structure predicted for amino acids produced through chemical reactions in space, which are predicted to be equally left and right-handed.
In 1997 and again in 2000, scientists revealed data showing that the Murchison meteorite contained more than just organic molecules. Microscopic photographs from freshly broken regions of the meteorite contained microfossils that looked like bacteria. Although these claims generated much discussion among the scientific community, they were met with a great deal of skepticism. Many researchers believe that the microfossils resulted from contamination of terrestrial origin.
In 2001, researchers found that the Murchison meteorite contained polyols, which are organic sugar compounds similar to glucose. Glucose is considered fundamental to metabolism on Earth. Sugars also make up parts of cell membranes and the backbone of the DNA molecule.
In the period of time since it fell to Earth, the Murchison meteorite has become one of the most well-studied meteorites in the world. It has been closely scrutinized for the differences it reveals between organic molecules and microfossils found on Earth and those of purported extraterrestrial origin.
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Research into the origins of life on Earth predicts that life probably started around one billion years after the planet was formed. Several scientists, most notably Francis Crick, who helped discover the structure of DNA, have suggested that meteorites crashing into Earth during the early part of its formation may have seeded the planet with the types of molecules required for life.
Classic experiments by Stanley Miller and Harold Urey in the early 1950s at the University of Chicago showed that organic molecules present in the primordial Earth could, under the proper conditions, result in the chemical synthesis of amino acids. However, if meteorites like the Murchison meteorite were crashing into Earth and bringing with them complements of amino acids, this might have accelerated the process.
The theory that extraterrestrial forces influenced life on Earth is called panspermia. The idea has a variety of social and scientific implications, including the possibility that if life could have been seeded extra-terrestrially on Earth, such seeding could also have occurred on other planets.
One of the most debated issues around the composition of the Murchison meteorite concerns the chirality of its amino acids. Chirality refers to molecules that occur in forms that are mirror images of each other. A good example of chirality is a person's two hands. They have the same structure, but are mirror images of each other.
On Earth, all biological processes use one of the two chiral forms of amino acids, referred to as left-handed molecules or L-enantiomers. When chemists synthesize amino acids from molecules that have no chirality, the result is an equal number of right-handed and left-handed molecules, a mixture known as racemic. A meteorite formed in space is expected to have a racemic mixture of amino acids.
As stated by Engel and Macko, analyses of the chirality of the amino acids in the Muchison meteorite in 1997, as well as in 1971, 1982, and 1990, show that the composition of the meteorite is predominately left-handed. Such a finding supports the hypothesis that left-handed extraterrestrial molecules could have seeded life on Earth.
One of the major issues surrounding the findings from the Murchison meteorite involves questions of contamination. A variety of experiments were performed on the meteorite to address the question of contamination by terrestrial molecules. In one, several amino acids that are not found on Earth were studied. These were found to have a left-handed dominance just like the analysis of the studies of the full complement of amino acids. In addition, stable isotope analyses were performed on the isotopes of atoms found in the meteorite. These experiments showed that the isotopic composition of both the left-handed and right-handed molecules was identical. Although both of these studies suggest that contamination was unlikely, the question of contamination of the meteorite by terrestrial organisms is still heavily debated in the scientific community.
Engel, Michael H. and Stephen A. Macko. "Stable Isotope Analysis of Amino Acid Enantiomers in the Murchison Meteorite at Natural Abundance Levels." Proceedings of the SPIE—The International Society for Optical Engineering 3111 (1997): 82.
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MacDermott, Alexandra J. "Distinguishing the Chiral Signature of Life in the Solar System and Beyond." Proceedings of the SPIE—The International Society for Optical Engineering 3111 (1997): 272.
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