Meteorite 'could have carried nitrogen to Earth'



By Neil Bowdler for BBC News


28 February 2011


A meteorite found in Antarctica could lend weight to the argument that life on Earth was aided by an extraterrestrial body, scientists claim. Chemical analysis of the meteorite shows it to be rich in the gas ammonia, which contains the element nitrogen - found in the amino and nucleic acids which form the basis of life. Analysis of other meteorites has revealed organic compounds which the authors of the new paper believe are too complex to have played a role.


Details of the study by researchers at Arizona State University and the University of California, Santa Cruz, are published in the journal Proceedings of the National Academy of Sciences. The new study is based on analysis of just under 4g of powder extracted from a meteorite called Grave Nunataks 95229 (named after its place of discovery in Antarctica). The meteorite was found in 1995 and belongs to the "Renazzo" family of "carbonaceous chondrite" meteorites, a group of meteorites that retain much of their original composition and have not been melted by their parent body. They can contain high proportions of water and organic compounds. The powder sample was shown to contain abundant amounts of ammonia as well as hydrocarbons (including the amino acids glycine and alanine). Analysis of the isotopes of nitrogen and hydrogen found within the sample suggest the material originated from a "cold cosmic" environment, and were not the result of Earthly contaminants.


Professor Sandra Pizzarello, who led the research, says the study "shows that there are asteroids out there that when fragmented and become meteorites, could have showered the Earth with an attractive mix of components, including a large amount of ammonia". She claims the meteorite provides better evidence of the possible "prebiotic" role played by meteorites than the "Murchison" group of carbonaceous chondrites. The professor says the Murchison meteorites represent "too much of a good thing" and contain hydrocarbon molecules which you would expect to find at the end rather than the start of the life story. She believes the composition of these compounds are too complex and too random in their molecular distribution to have played a role.


The theory that our planet may have been seeded by a comet or asteroid arises partly from the belief the formative Earth might not have been able to provide the full inventory of simple molecules needed for the "prebiotic" processes which led to primitive life. The suggestion is that the Asteroid Belt, between Mars and Jupiter, away from the heat and pressure of the forming planets, could have been a better place for such processes. Collisions between asteroids within the belt produce meteoroids which shoot off around the Solar System and which can carry materials to the Earth. Dr Caroline Smith, a meteorite expert at London's Natural History Museum agrees the important element in the new study is the nitrogen, even though she would like to see similar results repeated in other meteorites. "One of the problems with early biology on the early Earth is you need abundant nitrogen for all these prebiological processes to happen - and of course nitrogen is in ammonia.


"A lot of the evidence shows that ammonia was not present in much abundance in the early Earth, so where did it come from?" What specifically caused life to begin on Earth remains a mystery. Professor Pizzarello hypothesises material from a meteor may have interacted with environments on Earth such as volcanoes or tidal pools, but says all remains a matter of guess work. "You find these extraterrestrial materials (in meteorites) which have what you need," she says, "but on the how and when, in which environments and by what means - really, we don't know. You can only say that yes, it seems that the extraterrestrial environments could have had the good stuff."


[I think it’s reasonable to suggest that meteor impacts have had a hand in shaping life on Earth – beyond the one 65 million years ago that killed the dinosaurs and gave us our big break. We have been impacted by millions of rocks since the Great Bombardment and I wouldn’t be in the least bit surprised if they had added vital material to our biosphere. How, or if, they were involved in the beginnings of life is still unclear. Maybe one day we’ll find definitive evidence to support the idea.]