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Friday, March 9, 2001

By Craig T. Kojima, Star-Bulletin
Associate researcher Alexander Krot and Dr. Klaus Keil,
rear, show an electron microscope close-up of a slice
of meteorite on a lab monitor.

Oddball meteorites
hold clues to stars’ birth

By Helen Altonn

University of Hawaii researchers have raised new questions about the solar system's creation with analysis of two unusual meteorites.

Most meteorites found on Earth were believed to be fragments of asteroids formed when the solar system was created about 4.56 billion years ago.

But the UH study, reported recently in the journal Science, takes a different twist.

The UH researchers, led by Alexander Krot, Hawaii Institute of Geophysics and Planetology associate researcher, examined two rare meteorite specimens.

HH 237, about the size of a grapefruit, was recovered from northern Africa. QUE 94411, a walnut-size sample, was discovered in an Antarctica mountain range. Both were loaded with pristine iron and nickel.

Krot's collaborators included institute Director Klaus Keil; Anders Meibom, former institute scientist now at Stanford University; Sara S. Russell and Timothy E. Jeffries of the Natural History Museum in London; and Conel M. Alexander of the Carnegie Institute of Washington's Department of Terrestrial Magnetism.

Krot participated in the expedition that found the primitive meteorite in Antarctica in 1991, and the UH team and colleagues discovered its preserved 4.56 billion-year-old metal grains. They were the first solids found in the solar system.

That discovery was reported in Science last May. The group's latest findings were published in the March 2 issue.

Discussing them in interviews, Krot and Keil explained that primitive meteorites, or chondrites, contain thousands of tiny particles known as chondrules.

As the oldest rocks in the solar system, chondrites tell of the solar system's origin, Keil said. "They contain these millimeter-sized spheres, chondrules, named after the Greek word for grains."

Most meteorites were thought to have formed in the asteroid belt region between the Jupiter and Mars orbits, about 140 million miles from Earth, the scientists said.

It's believed chondrules started as dust balls that were melted by a heating mechanism that briefly raised the asteroid belt's temperatures -- then just below 700 degrees -- to about 3,000 degrees, Keil said.

"Imagine a rain of little dust balls, and some heating mechanism. We're not clear what that was," he said, noting ideas that it could have been a burst of lightning or shock waves.

"They cooled relatively quickly and accreted to form parent bodies of meteorites that we have today -- the asteroids," Keil said.

By Craig T. Kojima, Star-Bulletin
A researcher shows a carrier holding
five different samples of meteorites.

However, chondrules in the two meteorites discovered in Antarctica and Africa did not form by that process, he said.

Instead of forming in the asteroid belt, Krot said, "We think they formed in a place close to the sun. It tells us there was significant mixing of materials in the solar system when the sun formed."

Chemical analysis of the two meteorites suggests the metal grains formed at very high temperatures in the solar nebula, the scientists said. The chondrules condensed from very hot gas into liquid droplets that were quickly blown away from the sun to cooler regions, they said.

The team's findings support a theory proposed in 1996 by astronomer Frank Shu of the University of California based on images from the Hubble Space Telescope.

Hubble allowed scientists for the first time to observe the birth of new stars elsewhere in the Milky Way. They found most young stars are created from enormous disks of whirling gas and dust.

As the disk contracts, it rotates faster and faster, funneling tons of interstellar dust toward the center, where temperatures reach 3,000 degrees or more -- hot enough to melt metal and vaporize most solids.

The rotating disk also produces enormous gas jets capable of launching debris far into space at speeds of hundreds of miles per second.

Based on these Hubble images, Shu proposed that chondrules in Earth's solar system were created near the hot central disk of a newly emerging sun -- not in the relatively cool asteroid belt hundreds of millions of miles away.

Shu suggested that dust particles were melted by the sun, then launched into space by powerful gas jets and solar wind -- some landing in the asteroid belt and others ending up as materials forming Earth, Mars and other planets.

The UH study provided the first evidence to support Shu's theory of chondrule and star formation.

The next goal will be to date all the components of the meteorites, Krot said. "Basically, using this technique, we can date components from 1 to 2 million years after the formation of our sun."

Aside from exploring the first few million years of the solar system, Keil said the most exciting thing in the development is that the scientists have bridged a gap between cosmochemistry -- the study of extraterrestrial materials -- and astrophysics.

"We now can ... provide some real hard evidence for astrophysical theories."

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