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UC Berkeley Theorists Propose Explanation For Puzzling Composition of Chondrites

University of California Berkeley

NEWS RELEASE: 6/11/97

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.