Nov/Dec 2001 Table of Contents
X-ray image of a chondrite shows the distribution of silicate
chondrules and calcium-aluminum-rich inclusions. The letters
SO and CC refer to various chondrule textures. Courtesy of
Alexander Krot (University of Hawai'i).
are allowing us to glimpse the very beginnings of our solar system.
These primitive meteorites can contain thousands of seed-size droplets
known as chondrules. "Chondrules are among the oldest objects
in the solar system, dating back to the period when the Sun formed"
says Anders Meibom of Stanford University, coauthor of a new paper
that examines the chondrules in two chondrites to learn more about
how the planets in our solar system formed.
the standard view, chondrules were born in the disk of gas and dust
surrounding the young Sun, the same disk that gave rise to the planets.
Lightning zaps and shock waves briefly heated isolated regions of
the disk, partially melting the dust. The resulting silicate and
metal droplets cooled on a timescale of minutes to form the tiny
chondrules, which soon collided and stuck together with other minerals
to form chondrites.
two chondrites in this study, however, are different from the ones
that scientists usually study. "These meteorites are very rich
in metallic spherules and in an very unusual type of chondrules,
both of which might have formed by condensation," says Alexander
Krot of the Hawai'i Institute of Geophysics and Planetology, another
coauthor of the study. These objects could not have formed from
isolated lightning zaps or shock waves; they must have been heated
in a huge event that caused a region in the disk millions of kilometers
across to completely vaporize. Only in such a large-scale event
could the dust completely vaporize and the gas cool slowly enough
to condense the metallic spherules and form the unusual chondrules
that Meibom, Krot, and their collaborators observed.
with most front lines in science, however, planetary scientists
aren't quite sure what caused this massive heating. Star formation
expert Frank Shu of the University of California, Berkeley thinks
that Meibom and Krot's chondrules formed when the innermost regions
of the disk, heated by the Sun, could not cool quickly enough. In
his X-wind model that he developed in the 1990s, huge pockets of
vaporized material rose to the surface of the disk, like bubbles
in a boiling pot of water. When the material reached the surface,
the Sun's magnetic field lines grabbed it and flung it outward in
a diffuse wind toward cooler regions. Particles between a millimeter
and centimeter in size would eventually fall back onto the disk
at planetary distances. During this journey the chondrules solidified
Desch of the Carnegie Institution of Washington has a different
theory. Shu's model may still be correct, he says, but X-winds did
not form the chondrules. He believes that enormous shock waves had
enough energy not only to melt the disk, but also to completely
vaporize dust surrounding the young Sun. A shock wave of dense,
hot gas would hit a region of the disk and drag its material along.
The friction of this event vaporized the dust, and then the hot
gas of the wave kept temperatures warm enough so that the newly-formed
droplets did not cool immediately.
have been a lot of suggestions for how we get these shock waves,"
notes Desch. He theorizes that early in the formation of the solar
system, the disk surrounding the Sun was so massive that parts of
the disk collapsed under their own gravity, forming huge, over-dense
structures. Clumps of material 10 times denser than the surrounding
region formed about where Jupiter orbits today. Thin bars of dense
material extended from these clumps to the inner edge of the disk.
Material orbiting the Sun would run into these bars, quickly slow
down, and generate shock waves.
waves or X-winds, these two new meteorites provide scientists with
the data they need to test their models. As Meibom says, "We
now have actual rocks that provide hard numbers."
applied physics student RACHEL THESSIN
is an editorial intern at Sky & Telescope. She has previously
worked at the U.S. Naval Observatory in optical interferometry.