February 28, 2011 | 1
Asteroids and comets have long been suspected of helping nudge Earth toward life. The interplanetary interlopers have struck Earth since the beginning, delivering carbon, water, and sometimes organic compounds such as amino acids. By providing key chemicals or even pre-formed biological building blocks, as well as the heat and energy generated by a meteorite impact, space rocks may have played an important role in getting the biological ball rolling.
A new paper in the Proceedings of the National Academy of Sciences adds another piece of evidence to back the idea that biological precursors arrived aboard meteorites. In the study, published online February 28, Sandra Pizzarello of Arizona State University and her colleagues report that an Antarctic meteorite released copious ammonia when heated with water. Ammonia provides a ready source of nitrogen, which is a key ingredient in many biomolecules, including DNA and the amino acids.
Pizzarello and her co-authors subjected a sample of the meteorite, called Graves Nunataks (GRA) 95229, to temperatures of 300 degrees Celsius at high pressures in the presence of water to simulate hydrothermal conditions on the meteorite’s parent asteroid or on Earth. (The 129-gram meteorite of the carbonaceous chondrite variety was recovered in Antarctica in 1995 and was named for the ice field where it was found.) Under heat and pressure, GRA 95229 released almost nothing but ammonia, in amounts that constitute roughly 1 percent by mass of the type of meteoritic material examined. Its parent asteroid, the authors speculate, must have been rich in ammonia.
The formation of prebiotic compounds, the authors write, requires nitrogen in its reduced form—that is, bonded to hydrogen, as in ammonia (NH3). But if the early atmosphere of the planet was neutral rather than reducing, ammonia would be hard to come by without outside contributions. "The current geochemical evidence of a neutral early Earth atmosphere combined with the known photochemical destruction of ammonia has left prebiotic scenarios struggling to account for a constant provision of ammonia," the researchers report. "An abundant exogenous delivery of ammonia, therefore, might have been significant in aiding early Earth’s molecular evolution toward prebiotic syntheses."
Photo of meteorite GRA 95229: NASA