chondrites_010302 Asteroid bits that fall to Earth may have been born in a hot flash with the Sun, not in the
strip between Mars and Jupiter where most of the ancient space rocks currently reside, a new study says.The finding lends support to a theory that says the solar system formed from the
gravitational collapse of a gas and dust disk that created the Sun in a burst of high-impact sculpture while also winging off space objects across vast distances on a blast of energy called the X-wind.The clue that cracked the case open came in the form of two strange
meteorites recently discovered in Antarctica and Africa and bearing unusual concentrations of certain elements. Most meteorites found on Earth are believed to be fragments of asteroids -- ancient rocks that formed during the creation of the solar system some 4.5 billion years ago. Thousands of asteroids still orbit the Sun in the "Main Belt" between Mars and Jupiter about 140 million miles (225 million kilometers) from Earth.
Alexander N. Krot of the University of Hawaii and Anders Meibom, now a postdoctoral fellow at Stanford University, used electron microscopy and other techniques to conduct a detailed chemical analysis of two
chondrites -- primitive meteorites made up of thousands of tiny round particles called chondrules.In contrast with most chondrites, they found both rocks lack sulfur and have a high concentration of nickel and iron.
That rare rocky recipe suggests the chondrites condensed out from a consistently superhot environment -- about 2,500 degrees Fahrenheit (1,370 degrees Celsius) -- such as a spinning protoplanetary disk giving birth to the Sun, rather than the ultracold Asteroid Belt that may only occasionally receive bolts of heat energy.
"As opposed to flash-heating models to create chondrules in the Asteroid Belt, we are looking at a much larger-scale thermal event in the inner solar system to form these objects," Meibom said.
"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."
The study was published in the March 2 issue of the journal Science.
Tightening the Asteroid Belt
Most chondrules are made of chemistry that can only be produced at very high temperatures. The question is how you get there.
The conventional view is that chondrules started out as
dust balls in the Asteroid Belt 4.56 billion years ago when its temperature was just below 700 degrees F (370 degrees C) -- quite hot for a place that is now deathly frigid, but not hot enough by itself to make chondrules. To do that, the dust balls were presumably zapped by quick bursts of lightning or shock waves that briefly raised temperatures to about 3,000 degrees F (1,650 degrees C).As the melted particles cooled, the thinking goes, they turned into millimeter-size chondrules, which eventually clumped together to form larger chondrites.
Now Meibom and Krot's rocks suggest another process for how chondrules form. "We are just saying this is an alternative and it's consistent with astronomical modeling of large-scale, very hot processes in the
solar system," Meibom said.Enter X-wind
The astronomical modeling Meibom refers to was made public by University of California astronomer Frank Shu in 1996, who proposed an engine for star formation based in part on images from the Hubble Space Telescope showing the creation of new stars from enormous disks of whirling gas and dust.
As a disk contracts, it rotates faster and faster, funneling tons of interstellar dust toward the center, where temperatures reach 3,000 degrees F (1,650 degrees C) 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 (kilometers) 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 rather than in the Asteroid Belt.
According to Shu's theory, dust particles were melted by the Sun, then launched into space by powerful jets of gas and solar wind called the X-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 Earth, Mars and the rest of the planets in our solar system.
"Shu's model provides those kind 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," said Meibom.
Multiple birthplaces?
While Shu's model suggests that these asteroid bits formed only in the
inner solar system, Meibom says chondrules could have formed simultaneously in the nebula close to the Sun and in the Asteroid Belt.As to whether all chondrules form this way, meteorite researcher Matthew Genge of the Natural History Museum in London says the chemical mixes in primitive meteorites vary subtly, suggesting they formed in separate regions before being incorporated into parent asteroids.
"The results of Krot and coworkers are very interesting, and for these two meteorites are consistent with the X-wind model," Genge said. "These meteorites are very different from most, and these 'odd ball' stones might be the exception to the rule [rather] than the key to the formation of asteroids."
Krot and Meibom's other collaborators in the Science study are Klaus Keil of the University of Hawaii; Sara S. Russell 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.
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