TALLAHASSEE, Fla.- An unusual meteorite that fell on a frozen lake inCanada five years ago has led a Florida State University geochemist toa breakthrough in understanding the origin of the chemical elementsthat make up our solar system.
Professor Munir Humayun of the National High Magnetic FieldLaboratory and the geological sciences department at FSU and AlanBrandon of NASA discovered an isotopic anomaly in the rare elementosmium in primitive meteorites. The anomalous osmium was derived fromsmall stars with a higher neutron density than that which formed oursolar system. The findings of the researchers, who also includedcolleagues from the University of Maryland and Bern University inSwitzerland, were recently published in the journal Science.
"Our new data enabled us to catch a glimpse of the differentstar types that contributed elements to the solar system, the parentalstars of our chemical matter," Humayun said. "It opens a treasure troveof prospects for exploring the formation of the elements."
For about 50 years, scientists have known that all the elementsbeyond iron in the periodic table were made in stars by up to threenuclear processes. Osmium is mainly formed by two of those processes,the so-called s-process in which neutrons are slowly added to nucleiover a period of perhaps thousands of years in aging, medium-size starsand the r-process that occurs in supernovae in which neutrons arepumped into nuclei at a rate of hundreds of neutrons in a few seconds.
The new data gathered by Humayun's team not only shows thedifferent star types that contribute elements to the solar system, italso will be used to test astrophysical models of production of thechemical elements at a more sophisticated level than previouslypossible, he said.
Humayun and colleagues studied samples from an extremelyfragile meteorite that fell on Tagish Lake on Jan. 18, 2000. Unlikeiron meteorites, primitive meteorites like this one are not preservedlong on the Earth's surface because they disintegrate and form mud whenexposed to water. This one was retrieved within 48 hours of its fall inthe dead of an Arctic winter.
Most meteorites have a uniform osmium isotopic distribution,but Humayun's team found that osmium extracted from the Tagish Lakemeteorite was deficient in s-process osmium. They are the first toreport an anomaly in the isotopic makeup of the element osmium frommeteorites.
Other researchers have found isotope anomalies in several otherelements in some primitive meteorites, but not in others. Because ofthe disparity, scientists believed that the ashes of stars thatpreceded the solar system must have been sprinkled in a non-uniform wayinto the solar nebula, the disk of gas and dust that formed the sun,planets and meteorites. Scientists had hypothesized that some of thedust could have been created by an active nearby star.
Humayun's findings challenge that explanation. He believes thatthe anomaly is an expression of presolar stardust that survived thehomogenization that affected nearly all other meteorites. Typically,stardust accretes to form meteorites and is then heated byradioactivity - a process that destroys the silicon carbide grains thatare the carriers of the anomaly. But in the case of the meteorites withosmium isotopic anomalies, the heat was not significant enough todestroy the silicon carbide.
"The previous interpretation of incomplete mixing of differentsources of dust at the scale of the solar nebula no longer seemstenable," he said. "We now interpret those anomalies as incompletedissolution of silicon carbide grains that carried traces ofmolybdenum, ruthenium and osmium. These anomalies reveal that the rawmaterials from which our solar system was built are preserved in a fewexceptional meteorites, from which we can now recover the prehistory ofour solar system."
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