UC Berkeley press release


UC Berkeley theorists propose explanation for puzzling composition of meteorites

by Robert Sanders

Berkeley -- A new and detailed picture of how the Sun and planets formed from a swirling mass of gas and dust also explains the puzzling composition of the most common meteorites, the chondrites, which date from the formation of the solar system some 4.5 billion years ago.

Ever since the first meteorite was cracked open over a hundred years ago astronomers and meteoriticists have puzzled over the paradoxical mix of minerals that had been heated to high temperatures and others that obviously had formed at cold temperatures.

Based on a detailed theoretical model of how clouds of gas and dust condense into stars and planets, UC Berkeley astronomer Frank Shu and his colleagues Typhoon Lee and Hsien Shang propose that stars like the Sun recycle some of the dust falling into the star, throwing it out from the center in a fiery spray that seeds the colder matter with small "chondrules" or beads of melted rock. [Click here for graphic.]

Chondrules eventually coalesced with the remaining cold matter in the planetary disk to form asteroids, which are thought to have aggregated into the planets. Asteroids today are found primarily in a belt between Mars and Jupiter, serving as the source of meteors that frequently cross paths with the Earth.

The theory predicts that chondrules of only a certain size, ranging from a millimeter to about a centimeter in diameter, would fall back into the disk, in agreement with the size of chondrules found in the most common types of meteorites, the ordinary chondrites and the carbonaceous chondrites. The radioactive elements in these primitive chondrules have been used to date the origin of the solar system 4.56 billion years ago.

The theory has broader implications, since the formation of large asteroids and planets may not have been possible without these droplets of melted star dust in the early planetary nebula. Plus it explains why the Earth and many asteroids are deficient in certain elements.

"This is the first theory to explain all the disparate features of chondrites, such as the narrow range of sizes of chondrules and why their composition is different from that of the Sun," Shu says. "And because the planets formed from chondritic asteroids, it also explains the composition of the planets."

The scientists also predict that comets should contain similar inclusions from the early years of the solar system.

Shu will discuss their theory of the formation of chondrites in a talk June 11 during a meeting of the American Astronomical Society, which takes place June 8-12 in Winston-Salem, N.C. Shu, a professor of astronomy at the University of California at Berkeley, is past president of the American Astronomical Society and a prominent theorist in the formation of solar systems and galaxies.

Co-author Shang is a graduate student in astronomy at UC Berkeley, while Lee is a research fellow at the Institute of Earth Science of the Academia Sinica in Taipei, Taiwan.

The picture of the early solar system painted by the theory is one of a swirling disk of gas and dust with a rapidly rotating proto-Sun at the center. The spinning star drags its strong magnetic fields around with it like an eggbeater, stirring up the inner regions of the disk and throwing gas and dust about like tiny bits of batter.

The gas is channeled by the magnetic fields into jets perpendicular to the plane of the disk, like those seen today shooting from the center of new planetary systems.

In contrast, the dust -- about one percent of the entire matter in the cloud -- finds itself swept out of the shade of the disk into the powerful hot glare of the star.

Dust closest to the star would be melted by the heat, condensing into dense globules called CAIs, or calcium-aluminum-rich inclusions, which today are found as pinkish, centimeter-sized spheres in chondrites. Geologists today estimate these CAIs were melted for perhaps a day to several days before cooling, consistent with the time such "dustballs" would be within heating distance of the Sun before being swept further out by the powerful stellar wind.

Geologists estimate that the other common meteorite inclusion, blue-gray spheroids called chondrules, were melted for a mere hour. To date no one has been able to adequately explain such a short timescale, or why all chondrules are on the order of a millimeter or two in diameter while all CAIs are on the order of a centimeter in diameter.

Shu and his colleagues, together with Alfred Glassgold of New York University, propose that intense X-ray flares caused the quick melting of dust grains into chondrules. Flares on forming stars are a million times more powerful than flares on our Sun today, and have been observed to last about an hour, Shu says. They could easily melt dust grains kicked out of the planetary disk.

Both chondrules and CAIs would then be blown and sorted by the stellar wind, the smaller ones being swept out of the solar system, the larger ones quickly dropping back into the disk, and only the medium-sized grains raining down on the disk in the area where planets eventually formed.

Unheated dustballs rich in organic molecules would eventually aggregate with the chondrules and CAIs in the disk to form asteroids, Shu argues.

Previous theories have tried to explain how dust grains could be selectively melted within the current asteroid belt, about 2 1/2 times the distance of the Earth from the Sun, where it is too cold to melt anything, let alone a rock, he says. Collisions, lightning and shock waves have all been invoked, but no theory has been able to explain the rock textures of chondrules, the frequency and sizes of chondrules and the elemental peculiarities of chondrites.

"We have one simple, clean explanation for all the observed phenomena," Shu says. "With the new theory, the clues contained in the meteoritic record no longer conflict with the astronomical evidence concerning the nebular disks that we find around other young sunlike stars."

One peculiarity, for example, is that relative to the composition of the Sun the Earth is deficient both in volatile compounds rich in carbon, nitrogen, and oxygen, and in hard-to-melt compounds rich in elements such as calcium and aluminum.

Their model shows that the volatile compounds in dustballs would have been easily vaporized and blown away by the solar wind. The hard-to-melt compounds would have remained as dustballs and also been easily blown away. The result is that ordinary chondrites and terrestrial planets ended up depleted of both types of compounds.

The planets, in fact, reflect the composition of chondrules because chondrules comprise 80 percent of chondrites from which they were formed. The other 20 percent of chondrites is a black, carbon-rich matrix closer in composition to the original solar nebula.

"The results are important because chondritic meteorites retain within them the best clues of the physical conditions that prevailed in the inner solar nebula when the first steps were taken in the transformation of dust grains to terrestrial planets," Shu says.

Shu notes too that if none of the dust in the solar nebula had been heated to form dense chondrules and then sprayed into the planetary disk, the fluffy dustballs alone might not have been dense enough to stimulate a gravitational collapse into a planet.

Some evidence already supports Shu's prediction that comets contain material similar to that in asteroids, even though comets come from a much more distant zone beyond Pluto. Astronomers observing comet Hale-Bopp detected crystalline compounds, presumably once melted, as well as volatile compounds, for example.

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