Carbonaceous chondrite meteorites contain vital clues to the evolution of carbon compounds in our solar system preceding the origin of life.
Adapted from an Arizona State University press release
Scientists have conducted an organic analysis of the Tagish Lake meteorite, a rare, carbon-rich meteorite classified as a carbonaceous chondrite. The meteorite fell on a frozen Canadian lake in January 2000, and is the most pristine carbonaceous chondrite specimen ever studied.
The analysis suggests there can be a different outcome for the evolution of organic chemicals in space than from what has been observed in other carbonaceous meteorites. This difference could be due to the possibility that the Tagish Lake meteorite contains carbon molecules that may have accumulated during the formation and development of the solar system.
Carbonaceous chondrite meteorites contain vital clues to the evolution of carbon compounds in our solar system. For example, the Murchison meteorite, a carbonaceous chondrite found in Australia in 1969, contains numerous amino acids and a variety of other organic compounds that are the building blocks for life. Many scientists now believe that such meteorites could have seeded the Earth with the ingredients necessary for life to arise.
A team headed by chemist Sandra Pizzarello, a research scientist at Arizona State University, conducted the organic analysis of the Tagish Lake meteorite. 4.5 grams were taken from the sealed interior of the meteorite, and while the organic compounds found in the meteorite have some similarities to other known carbonaceous chondrites, there are also clear differences - most notably the near-absence of amino acids.
"The chemistry here is different from that we have seen in any other meteorite," says Pizzarello. "It's simple when compared with Murchison, and probably represents a separate line of chemical evolution. However, it still includes compounds that are identical to biomolecules."
In an article published in the August 24 issue of the online journal Science Express, the team notes that the chemistry of the Tagish Lake meteorite appears to have preserved organics that accumulated or developed in the early history of the Solar System. This includes molecular bubbles of carbon (fullerenes or "buckyballs") containing the noble gases helium and argon in a ratio similar to the gas and dust cloud that formed the planets. Thus, the meteorite perhaps reflects an early stage of evolution of complex carbon compounds in space.
The Science paper notes that many of the organic compounds found in the Tagish Lake sample have also been found in other meteorites, but that the distribution of compounds is different, particularly for the amino acids and carboxylic acids.
"We found some compounds identical to some in Murchison that show the same - interstellar connection - in their abundance of deuterium (heavy hydrogen), while some others differ from Murchison in amounts and variety," says Pizzarello, meaning that for some groups of organic molecules, only the simplest species were found in Tagish Lake, as opposed to a broader distribution of species found in Murchison. "Overall, Tagish Lake represents a simpler, more unaltered stage than we have seen before."
Other members of the research team include Yongsong Huang from the Department of Geological Sciences at Brown University; Luann Becker from the Institute for Crustal Studies at the University of California Santa Barbara; Robert J. Poreda from the Department of Earth and Environmental Sciences, University of Rochester; George Cooper from the NASA Ames Research Center; and Ronald A. Nieman and Michael Williams, both also from ASU.
"Some people have been disappointed that we found virtually no amino acids, but scientifically this is very exciting," Pizzarello said. "This meteorite shows the complexity of the history of organic compounds in space - it seems to have had a distinct evolution."
Pizzarello notes that while the meteorites like Tagish Lake may lack amino acids, they still could have contributed the molecular precursors of biomolecules that are necessary for the origin of life.
Louis Allamandola, astrochemist with the NASA Ames Research Center and NAI member, says the absence of amino acids and other simple organic molecules could mean the meteorite was exposed to high heat or energy levels during its travels through space.
"The main signature of the meteorite is that of sooty, cross polymerized aromatic material -- probably much like the black soot from a diesel engine or sooty candle flame," says Allamandola. "From an organic chemical point of view, this is the kind of material you get when you heat any organic material above about 400 degrees Celsius (752 degrees Fahrenheit). So, all this means is that this particular rock had a different history from that of the Murchison meteorite. It had a rougher go on its transit from deep space to Earth."
Carbonaceous chondrite meteorites generally show little evidence of being shaped by high temperatures. Even entry into our atmosphere does not heat their interiors to any great degree, as their porous texture tends to bleed away heat. But Allamandola says it wouldn't come as a surprise if the meteorite had been heated or energetically processed before entering the Earth's atmosphere - the vast reaches of space contain many different levels of radiation, temperatures, densities, and environments.
"There are at least two ways to look at it," says Allamandola. "Either carbon goes through some sort of process to form amino acids, and this meteorite therefore represents an early window on the evolution of carbonaceous chondrites before the amino acids develop. Or, the rock might have been so energetically processed that the amino acids were destroyed."
"Basically," Allamandola says, "this draws attention to the fact that only very few meteorites have been studied in any detail, and that we really don't know what most of them contain. It's a tough game."