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Rare meteorites rekindle controversy over birth of the solar system
A new meteorite study is rekindling a scientific debate over the creation of our solar system.
The study, published in the March 2 issue of the journal Science, is based on the microscopic analysis of two rare meteorites recently discovered in Antarctica and Africa.
Most meteorites found on Earth are believed to be fragments of asteroids -- ancient rocks that formed during the creation of the solar system about 4.56 billion years ago. Thousands of asteroids still orbit the Sun in the asteroid belt between Mars and Jupiter, about 140 million miles from Earth.
"Asteroids and meteorites are solids that never got incorporated into the planets. These objects have survived, unchanged, for 4.56 billion years," says physicist Anders Meibom, a postdoctoral fellow in the Stanford Department of Geological and Environmental Sciences who co-authored the Science study.
Chondrites and chondrules
Using electron microscopy and other laboratory techniques, Meibom and his colleagues conducted a detailed chemical analysis of two chondrites - primitive meteorites made up of thousands of tiny round particles called chondrules.
"Chondrules are among the oldest objects in the solar system, dating back to the birth of the Sun," says Meibom, "so when we look at chondrules, we're actually looking at the very first steps toward the creation of our solar system."
Meibom points out that most chondrules are made of silicates and metals that can only be produced at very high temperatures. Exactly how chondrules formed in the early solar system is a hotly debated topic among scientists.
"The conventional view," notes Meibom, "is that chondrules started out as dust balls in the asteroid belt region some 4.56 billion years ago. Today, the asteroid belt is ultra-cold, but at that time, the temperature was just below 700 degrees Fahrenheit. The dust balls melted after they were zapped by quick bursts of lightning or shock waves, which briefly raised temperatures to about 3000 degrees F."
According to this theory, as the melted particles cooled, they turned into millimeter-size chondrules, which eventually clumped together to form larger chondrites.
But in 1996, astronomer Frank Shu of the University of California proposed a different theory based in part on dramatic images from the Hubble Space Telescope, which for the first time allowed astronomers to witness the actual birth of new stars elsewhere in the Milky Way.
The Hubble revealed that 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 3000 degrees F or more - hot enough to melt metal and vaporize most solids.
The rotating disk also produces enormous jets of gas capable of launching debris far into space at speeds of hundreds of miles per second.
Using the Hubble images as a guide, Shu proposed that chondrules in our solar system were created near the hot central disk of the newly emerging Sun not in the relatively cool asteroid belt hundreds of millions of miles away.
According to Shu, dust particles were melted by the Sun, then launched into space by powerful jets of gas and solar wind. While in flight, the molten particles solidified into spherical chondrules, some of which landed in the asteroid belt a few days later. Others ended up as the raw materials that formed the Earth, Mars and the rest of the planets in our solar system.
According to Meibom, the March 2 chondrite study in Science magazine gives Shu's version of chondrule creation a tremendous boost.
"Our findings demonstrate that Frank Shu's ideas are not just some fantasy," he notes. "We now have actual rocks that provide hard numbers, which fit very nicely into the general framework of Shu's theory."
Meibom and his colleagues based their study on two rare meteorite specimens - HH 237, a grapefruit-size chondrite recovered from the Hammadah al Hamra region of north Africa; and QUE 94411, a walnut-size sample collected from the Queen Alexander mountain range in Antarctica.
"Most chondrites are only seven to 10 percent metal by volume, but these two specimens are about 70 percent iron and nickel," says Meibom.
Microscopic analysis revealed that these iron-nickel compounds formed by condensation from hot gas when the temperature was around 2500 degrees F.
"Because HH 237 and QUE 94411 contain pristine samples of condensed iron and nickel, we were able to determine that these metal grains formed on a time scale of a few days. Furthermore, the newly created metal grains must have been transported out of their hot formation region very quickly.
"Shu's model provides those kinds of temperatures and time scales, and the jets certainly provide a way to kick the grains out to much colder regions of the solar nebula," adds Meibom.
"The scenario we are suggesting is that of big blobs of hot gas rising up through the disk almost like bubbles in boiling spaghetti sauce. As the gas bubbles rose and cooled, silicate and metal grains began to condense out of the gas. When these grains got close enough to the surface of the disk, they became trapped in the powerful jet streams. Days later, the particles arrived in the asteroid belt, where the relatively cold temperatures preserved them from destruction."
These chondrites allow us to look at the very frontier of the solar system, concludes Meibom.
"For the first time, we're really building a bridge between what we observe in the meteorites and what astrophysicists like Shu are telling us."
Frank Shu agrees.
"In these two very special meteorites we finally have direct evidence that certain portions of rock had to move from some place very hot to some place very cold in a very short period of time," comments Shu. "This is a very important study."
Meibom's other collaborators in the Science study are Alexander N. Krot and Klaus Keil of the University of Hawaii; Sara S. Russsell and Timothy E. Jeffries of the Natural History Museum in London; and Conel M. O'D. Alexander of the Carnegie Institution of Washington's Department of Terrestrial Magnetism.
By Mark Shwartz