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The Story
of a Meteorite
Timothy J. McCoy
Contained in every good meteorite, there’s
a clue to the story of the solar system’s formation and evolution. And
sometimes, even coating the meteorite’s outer surface as lacquer or streaks
of paint, there’s the story of the meteorite’s fall to Earth.
One of my favorite stories is that of the Nakhla meteorite that fell
as a shower of 40 stones near Alexandria, Egypt, in 1911. The story goes
that one of the stones hit and killed a dog. The 1,300-million-year-old
meteorite was plunged into fame a second time years later when it was shown
to be one of only 13 known martian meteorites.
As classifiers of meteorites at the Smithsonian Institution’s National
Museum of Natural History in Washington, I and my colleagues collect these
stories, both scientific and human. Most of the meteorites arrive at the
museum through the U.S. mail. More than 7,500 of the 9,250 distinct meteorites
cataloged in the collection come from the efforts of the U.S. Antarctic
Research Program, a joint effort of the Smithsonian, NASA and the National
Science Foundation. Some the museum buys from meteorite dealers. Some it
acquires through trades with other museums. And still others come from
people who are watching the sky and the ground for something unusual. Blame
it on sensationalist news, but people are looking up at the sky waiting
for something to hit them. The Smithsonian receives about 100 calls a year
from people who think they’ve found meteorites. About two of those calls
yield true meteorites. Most of the samples turn out to be industrial slags,
iron concretions or pieces of basalt.
Every so often, the museum gets a surprise.
Scene of the crime: The home of Frances and Arthur
Pegg, where the Burnwell
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Burnwell, Ken. Sept. 4, 1990. 3:45 p.m. EDT: Frances Pegg was carrying groceries into the kitchen of her Burnwell home along the Kentucky side of the Tug Fork River. Arthur Pegg was working nearby when a sound like the chopping of a helicopter ripped toward the house, changing pitch as it approached. A bright object blazed from the south-southwest. It was a meteorite, and it struck the old house, passing through the roof, ceiling and porch floor. The impact sounded like a gunshot and threw splintered wood as far as 20 feet. Arthur Pegg originally thought that a wrench or a part from a passing plane had penetrated the house, and the next day he removed every object from an enclosed area under the front porch. Amidst glass from an old aquarium and destroyed cans of paint and lacquer, he found the meteorite. Scratches and some lacquer still cover the rock |
| For seven years, the meteorite remained a protected conversation piece in the Pegg home, and the Peggs appeared on television and were quoted in the Daily News of Williamson, W.V., about their famous meteorite. In January 1997, the Peggs called the Smithsonian, offering to sell the meteorite. At first, the Smithsonian curators were skeptical, as they must be about the many calls they receive; but the Peggs’ call was not a false alarm. A week later, I found the package in my mailbox: it contained small pieces of the suspected meteorite, along with photographs and a newspaper clipping. We could see that the pieces were definitely part of the real thing. I set out for Kentucky to acquire the Burnwell meteorite, which became part of the national meteorite collection in January 1997. It has proven to be a unique scientific specimen in the study of asteroids. | 
The Burnwell meteorite: Its exterior is almost completely covered by a shiny, black fusion crus5t formed when the rock entered the atmosphere. Its interior, exposed on one end, is gray. Smithsonian Museum of Natural History |
The oddities of Burnwell
Researchers are trying to learn about the total range of materials that
were in the solar nebula. That’s why a strange meteorite like Burnwell,
which doesn’t completely fit into any of the established meteorite classifications,
is a useful
surprise.
Chondrites are the most primitive meteorites known and are composed of particles (including chondrules, from which they take their name) from the solar nebula. They are divided into a range of classifications based on oxygen isotopic compositions, metal abundances, bulk compositions, chondrule sizes and mineral compositions (specifically, the amount of iron oxide in the silicates olivine and pyroxene, and the amount of cobalt in the iron-nickel metal).
Chondrites are subdivided into the discontinuous groups H, L and LL. H chondrites have abundant metal, while LL chondrites contain very little metal. Harold Urey and Harmon Craig of the University of Chicago first recognized the differences between H and L chondrites in 1953. LL chondrites were recognized as a distinct group in the early 1960s. Scientists eventually recognized that each group originates from a different asteroid. Researchers have long postulated that additional groups might exist, including a group hypothetically termed “HH” even richer in metallic iron than H chondrites.
Colleagues Sara Russell, Eugene Jarosewich, Richard Ash and I have shown
that the Burnwell meteorite is the first
to have all the properties of the postulated “HH” chondrite group.
The hallmark of Burnwell is its reduced nature. While the differences between
H, L and LL chondrites reflect both differences in oxidation state and
bulk composition, it is generally true that H chondrites tend to be more
reduced and L and LL chondrites more oxidized. Burnwell is an extreme in
this trend. It is rich in iron-nickel metal and its silicates are relatively
poor in iron oxide (olivine has a fayalite concentration of 15.8 compared
to 17–20 in H chondrites). Compared to that in H chondrites, the metal
in Burnwell is also poor in the element cobalt.
Burnwell also displays an anomalous oxygen isotopic composition. While we typically think of Earth as diverse in its oxygen isotopic composition, all of its rocks and water (both liquid and ice) are related through mass fractionation. In contrast, meteorite oxygen isotopic compositions are not related to Earth through mass fractionation, but probably reflect heterogeneity of the solar nebula before Earth’s formation. LL chondrites plot furthest from Earth values and H chondrites closest. Burnwell plots even closer to Earth values than H chondrites. But while it is tempting to think that Burnwell might represent the primordial material from which Earth formed, the differences in oxygen isotopic composition between Earth and Burnwell are significant. In fact, there is no compelling reason to believe that the material from which Earth formed will ever be found as a distinct meteorite.
Although the researchers now know much about Burnwell and its relation to other meteorites, they remain uncertain about its origin. Did it originate from a fourth, distinct ordinary chondrite asteroid? Or, does Burnwell simply represent another piece of an H chondrite asteroid, one that is more heterogeneous than studies have showed they could be?
One way to find out is to study a number of similar but poorly characterized meteorites from the extensive U.S. Antarctic meteorite collection, housed at NASA’s Johnson Space Center in Houston, Texas, and at the Smithsonian. If the H chondrites and Burnwell-like meteorites do not share a continuous range of properties, then the two groups may have originated from different asteroids. Also, if the cosmic-ray exposure ages—the time meteorites sat exposed in space—are similar, then both H chondrites and meteorites like Burnwell may have been knocked off the same asteroid during a large impact event.
From Earth to space
The national meteorite collection contains all the pieces of the puzzle,
but we really can’t put them together. The missing pieces of the puzzle
come from earth-bound studies of asteroids, and from space missions. For
example, during the International Conference on Asteroids, Comets and Meteors
held at Cornell University in July, Schelte J. Bus, who recently earned
his doctorate from the Massachusetts Institute of Technology, described
how CCD (charge-coupled device) spectroscopy is helping asteroid researchers
define the compositions, and thus the evolutions, of asteroids in the Main
Asteroid Belt (see “Asteroid Family Trees,” Geotimes, September 1999).
| Research on Burnwell may help scientists correlate ground-based observations
with data from space probes such as the NASA Near-Earth Asteroid Rendezvous
(NEAR) Mission. Ground-based observers have found that as some asteroids
rotate, the spectra of light reflected from their surfaces change. This
spectral heterogeneity may imply that melting and differentiation could
have produced regions within these asteroids of drastically different mineralogies
and chemical compositions. Alternatively, an asteroid’s underlying bedrock
could be globally homogeneous and the spectral variance may reflect exposure
to radiation and impacts in space—the so-called “space weathering” effect.
The heterogeneity could also represent the diversity of materials incorporated
into an asteroid during its accretion—materials unchanged by melting or
space weathering. If Burnwell and H chondrites come from a single asteroid,
this fact would greatly expand the understanding of how heterogeneous a
single, unmodified asteroid might be.
NEAR’s Feb. 14, 2000, visit to its target asteroid, 433 Eros, will provide a unique opportunity to test these theories. NEAR was launched in February 1996. In June 1997, NEAR returned digital “photographs” of the asteroid Mathilde, only the third asteroid visited by a spacecraft. |
A mosaic of the asteroid 951 Gaspra compiled from two images taken by Galileo from 5,300 kilometers, about 10 minutes before closest approach on Oct. 29, 1991. The sun is shining from the right. Studies of asteroids and classifications of meteorites, give researchers data for understanding how asteroids formed and what their formations say about the early solar system. NASA Jet Propulsion Laboratory |
When NEAR arrives at Eros, it will go into orbit around the kidney-shaped asteroid, which measures about 40 kilometers in length and 14 kilometers in diameter at the center. NEAR carries a suite of instruments, including a near-infrared spectrometer, a multi-spectral imager, an X-ray/gamma-ray spectrometer and a laser rangefinder. For one year, the spacecraft will collect data about the shape, size, density, topography, mineralogy and chemistry of Eros. At times, the spacecraft will orbit only 15 kilometers above the asteroid’s surface. NEAR’s suite of instruments provides complementary pieces of information, all of which are necessary to unravel the asteroid’s history.
By knowing both the bulk composition and mineralogy of 433 Eros at a scale down to a few meters, the NEAR science team should be able to determine if 433 Eros is unmelted or has experienced igneous differentiation or space weathering. A relatively homogeneous composition might point to a primitive asteroid that is largely unchanged since its formation at the birth of the solar system.
Burnwell is particularly important in this scenario, because researchers
don’t know exactly how much heterogeneity
to expect in an undifferentiated asteroid. An asteroid that melted
might contain a variety of rock types, including basalts, ultramafic rocks
and metal-rich rocks produced by partial melting of the asteroid and melt
migration to different regions. An asteroid modified by space weathering
might display a relatively homogeneous chemical composition. However, it
might also exhibit differences in the spectra of reflected light measured
by the near-infrared spectrometer, a symptom of different grain sizes in
the fragmented rock, glass production by small-scale impact melting and
reduction of iron oxide to iron by the solar wind.
Burnwell promises to add to our knowledge of the processes that formed and continue to shape asteroids.
Timothy J. McCoy
McCoy is the associate curator of the national meteorite collection
at the Smithsonian’s National Museum of Natural History. He also worked
with meteorites while serving a post-doctoral fellowship from 1994 to 1996
at NASA’s Johnson Space Center in Houston, Texas. McCoy earned his doctorate
in 1994 from the University of Hawaii, Manoa.
The Gold Rush in North Carolina
Dennis J. LaPoint
In 1970, Dr. H. G. Jones, then director of the
North Carolina Department of Archives and History, walked over the site
of the country’s first documented gold discovery and became obsessed with
its preservation. The Kelly family (of tire fame) bought the central North
Carolina property in the 1890s, and, by coincidence, in 1970 the Kelly
heirs were considering an offer from a Charlotte developer to buy the property
as the exclusive “Gold Mine Estates.” Thanks to Jones’ personable nature,
the Kelly family instead donated the mine site to the state and sold the
remainder of the land to North Carolina for a modest amount. The Reed Gold
Mine is now a state historic site. This year, which also marked the 175th
anniversary of the North Carolina Geological Survey, the state celebrated
the 200th anniversary of the discovery of gold at the Reed property and
of the beginning of America’s first gold rush.
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