August 23, 2010 | 14
A new analysis of a meteorite shows that an inclusion within the carbonaceous stone is older than any known material in the solar system. The finding pushes back the estimated age of the solar system to 4.568 billion years, older than previous estimates by up to 1.9 million years.
A piece of the meteorite, known as Northwest Africa 2364, was purchased in 2004 in Morocco and is now part of a collection at Northern Arizona University in Flagstaff, Ariz. About 150 miles south of Flagstaff, two researchers at the Arizona State University Center for Meteorite Studies in Tempe, Audrey Bouvier and Meenakshi Wadhwa, dated an especially primitive piece of the meteorite known as a calcium-aluminum-rich inclusion, or CAI. Their findings appeared online August 22 in Nature Geoscience.
Bouvier and Wadhwa measured the meteorite’s ratio of two lead isotopes, whose relative proportions change on geologic timescales. Each variety of lead is a decay product of uranium, but the two parent uranium isotopes have very different half-lives: uranium 238 decays to lead 206 with a half-life of about 4.5 billion years, whereas uranium 235 decays to lead 207 with a half-life of about 700 million years. So the relative abundances of lead 206 and 207 can be used—along with some calibration for initial uranium abundances—to determine the age of ancient objects.
The balance of lead isotopes in the Northwest Africa 2364 meteorite point to an age of 4,568,200,000 years, plus or minus a few hundred thousand years. Taking the measurement uncertainties into account, that is between 300,000 and 1.9 million years more ancient than the oldest CAIs found in other meteorites. The lead-derived age of Northwest Africa 2364, Bouvier and Wadhwa write, is "the oldest absolute age yet obtained for any Solar System material and is, therefore, the best estimate for the time of formation of the Solar System."
Artist’s impression of a protoplanetary disk: ESO/L. Calçada