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Mercury, Nov/Dec 2001 Table of Contents

This 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).

by Rachel Thessin

Chondrites 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.

In 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.

The 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.

As 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 as chondrules.

Steve 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.

"There 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.

Shock 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."

Caltech applied physics student RACHEL THESSIN (rthessin@caltech.edu) is an editorial intern at Sky & Telescope. She has previously worked at the U.S. Naval Observatory in optical interferometry.