Antarctica is nature's forensic freezer, preserving records of the past in layers of largely unsullied ice and snow that scientists have dug up to explore past geologic and atmospheric conditions. The southernmost continent has also proved an ideal hunting ground for meteorites, which stand out atop ice sheets and snowfields and tend to undergo little weathering and terrestrial contamination after arrival.
A new analysis in the May 7 issue of Science comes from a France-based team working at Antarctica's Concordia base that uncovered well-preserved meteorite samples from beneath the surface. The researchers dug up snow from decades past, which fell before the site had a potentially contaminating human presence. Therein they found so-called micrometeorites—tiny specks just a fraction of a millimeter across that nonetheless carry important clues to the birth and evolution of the solar system.
"Antarctica is really one of the best places to collect interplanetary dust," says lead study author Jean Duprat, a cosmochemist at the Nuclear Spectrometry and Mass Spectrometry Center, a joint lab of the French National Center for Scientific Research and the University of Paris-Sud. Surrounded by oceans and covered with ice and snow, Antarctica contains relatively little terrestrial dust to dilute the particles that arrive from space. In a sieved, melted sample collected in 2006, Duprat and his colleagues identified more particles from space than from Earth.
"Here we extracted particles from very, very clean snow layers that fell on Earth about 50 years ago," Duprat says. Among the 148 micrometeorites produced by melting three cubic meters of snow were two, loaded with organic material, that form the basis for the new research. Numerous studies have sought to explain how such carbon-based substances—including prebiotic compounds—arrived on Earth to sow the seeds of life, with research in recent years showing comets and asteroids to both be plausible delivery mechanisms.
"One of the main questions we are addressing with these particles is the birth of the solar system, the material out of which the planets are formed," he says. "If you want to do that, you really have to go for materials from small bodies like asteroids or comets"—primitive objects that have not undergone much melting in the intervening billions of years. The carbonaceous Antarctic micrometeorites appear to be of cometary origin, Duprat says.
Another question is whether the organics that eventually reached Earth are native to the solar system or were inherited from interstellar space and incorporated into the molecular cloud from which the sun and planets formed. Duprat says that the micrometeorites he and his colleagues turned up appear to fall into the native category, based on an analysis of minerals embedded in the particles. "These minerals, a lot of them, are crystalline phases that are typical of the protoplanetary disk and are not typical of interstellar space," Duprat says. "We don't see an interstellar heritage in the minerals."
Nevertheless, says cosmochemist Larry Nittler of the Carnegie Institution of Washington, D.C., who wrote a commentary on the research for Science, "it's not clear that these results shed any light on the origin of organic matter." The micrometeorites are extremely rich in deuterium, a heavy isotope of hydrogen, a trait that was once thought to be a sign of interstellar origin. Now the case is less clear, Nittler says, and some research indicates that deuterium excesses could be produced in the early solar nebula, which would jibe with the Duprat group's mineralogical analysis.
Whatever the ultimate provenance, the Antarctic meteorite particles make for good samples of organic material because they are relatively large, from an interplanetary dust perspective, and contain so much carbon. "Normally you would have something that is something like a few percent carbon, and here it's more than half in volume," Duprat says. As a result, cosmochemists can analyze the organic content in situ, without having to first extract it using chemical solvents.
With so few samples to draw on, Nittler says, it is important to add pieces to the puzzle as Duprat and his colleagues have done. "A lot of these studies are heavily biased to what we get," he says, adding that the new micrometeorites provide a different look at the chemistry of the early solar system. "It's almost like a new class of extraterrestrial material, because they're not like anything we've seen before."