[meteorite-list] Brenham article - Geotimes

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Jan 26 13:22:23 2006
Message-ID: <200601261820.k0QIKj216128_at_zagami.jpl.nasa.gov>

>
>
> Hello all,
>
> Here's one of the better articles about Steve and
> Phil's new Brenham find. It's in this month's
> "Geotimes" magazine and online at:
> http://www.geotimes.org/current/NN_Brenhammeteor.html
>


Mammoth meteorite unearthed
Kathryn Hansen
Geotimes
January 2006

In October, Steve Arnold was scanning a field in Kansas with his
modified metal detector when he heard a familiar tone, indicating the
presence of a metallic object belowground. The sound could have
indicated just another wagon wheel or piece of lost farm equipment, but
Arnold instead pulled from a depth of 2 meters a rare meteorite - the
largest of its kind discovered anywhere in the world.

Steve Arnold, meteorite hunter and broker, poses with the 650-kilogram
meteorite that he found in a Kansas farm field. Image courtesy of Qynn
Arnold.

The 650-kilogram (1,430-pound) rock is one of numerous debris chunks
that scattered when a meteor crashed near what is now Haviland, Kan.,
sometime between 1,000 and 2,000 years ago. This particular piece of the
"Brenham" meteorite contains an iron-nickel alloy embedded with olivine
crystals that scientists classify as a "pallasite." But it was not the
composition that proved unique (most Brenham specimens are pallasites);
rather it was the rock's size combined with how it entered the
atmosphere that made it a rare find.

The specific locations of such meteorites in the Brenham fall might
provide new insight to the dynamics of meteorite impacts. For Arnold,
such information could possibly help him find additional specimens.
Based in Kingston, Ark., he works as a broker for collectors, matching
up prospective buyers with specimens on the market - some of which he
finds during meteorite hunting expeditions.

After browsing documentation of the specific locations of previous
Brenham findings, which Arnold calls "a treasure map," he says that he
thought that more rocks remained undiscovered. But 16 square kilometers
(4,000 acres) was too large an area to search, so he narrowed his focus
to a smaller area that he thought records indicated had not been
thoroughly searched with modern metal-detecting equipment. "If he was
partly right, we could recover some meteorites and make a fair profit,"
says Phil Mani, a geologist turned oil and gas attorney who financed the
search and recovery efforts.

Arnold and Mani gathered equipment, drove to Kansas, started hunting and
"two weeks later, we run into this big thing," Arnold says, which was
larger than either of them had hoped for. Also surprising was the
meteorite's shape. Most meteorites enter the atmosphere tumbling, which
effectively rounds the rock. Instead, this rock was cone-shaped with a
jagged backside, indicating it had a stable, or "oriented," position
upon entry. "When you find oriented meteorites, they tell you something
different than nonoriented meteorites," Mani says.

Impacts typically distribute meteorites over a "strewn field" shaped
like an ellipse, the exact parameters of which depend on physical
factors such as the object's angle and speed as it falls, and possibly
the shape of the rock. While documenting a different meteor fall in Park
Forrest, Ill., Arnold and colleagues found that oriented stones fall in
a pattern different from non-oriented stones. He says that in that case,
oriented meteorites tended to stay closer to the axis of the ellipse,
possibly because they had a more stable flight.

Arthur Ehlmann, a geologist at Texas Christian University in Fort Worth,
who studies meteorites, says that it makes sense that oriented pieces
should fall straight down the center, while irregular shapes would "veer
off to the side." Arnold and Mani hope to see if this pattern also holds
true for the Brenham strewn field.

A legal contract with the land-owners for exclusive meteorite-hunting
rights will allow Arnold and Mani to document the size, shape and
location (via GPS) of each Brenham meteorite that they find. But just as
important, they say, are areas that do not turn up meteorites because
they help to define the strewn field perimeter. Defining the patterns
and borders of the strewn field will help in "maximizing recovery of
Brenham meteorites," Mani says.

Selling the recently discovered meteorite for profit does not concern
Ehlmann, who says that other than helping to define the strewn field,
the large piece is unlikely to turn up any new scientific results. Most
scientific information has already been gleaned from previous specimens
recovered from the same impact, which scattered about 3 tons of material
over Kansas farmland. Other specimens, for example, have been sufficient
for scientists to arrive at the leading hypothesis that a pallasite
forms when an early planet's cooling molten core forces it to mix with
olivine in its mantle.

Denton Ebel, curator of the Arthur Ross Hall of Meteorites at the
American Museum of Natural History in New York City, says that
purchasing the meteorite as a whole would be beyond the budget of the
museum, but if the meteorite were cut into sections, he would be
interested in procuring a piece. "After all," he says, "it's not like a
dinosaur skeleton where if you take the head off, it changes it." Arnold
and Mani say that they prefer to sell the meteorite to a party that
plans to keep it in one piece.
Received on Thu 26 Jan 2006 01:20:44 PM PST