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(meteorobs) Fwd: NEO News (10/13/00) Yukon meteorite
------- Forwarded Message
Date: Fri, 13 Oct 2000 15:53:40 -0700
To: david.morrison@arc.nasadot gov
From: David Morrison <dmorrison@arc.nasadot gov>
Subject: NEO News (10/13/00) Yukon meteorite
NEO News (10/13/00) Yukon meteorite
COMMENTS ON YUKON METEORITE
by Clark R. Chapman
12 October 2000
A report was published in today's Science Magazine of results on the
Tagish Lake meteorite, by Peter Brown and many co-authors, who have
participated in preliminary analysis of the visual and satellite
sightings of the bolide and laboratory analysis of some of the
meteorite fragments. The Tagish Lake meteorite is the most
significant "fall" of a carbonaceous meteorite since 1969, when the
famous meteorites Allende and Murchison fell in Mexico and Australia,
respectively. "Falls" are especially important for these fragile,
rare meteorites because the meteorites can be recovered for
laboratory analysis before they've been badly degraded by
contamination on the Earth's surface.
In the case of Tagish Lake, we are especially fortunate that the fall
occurred in a cold place (the Yukon) during winter and that a local
resident, Mr. Jim Brook, carefully collected some pieces (without
handling them with his bare hands) and kept them frozen.
Indeed, we are fortunate that the meteorites were found at all. The
most primitive kinds of carbonaceous chondrites are very fragile,
more like clods of dirt than hard rocks, and often burn up and
shatter in the Earth's atmosphere, or decompose on the Earth's
surface before they are found. Even in this case, only a few
kilograms of meteorite have been recovered from an object that was
the size of a room -- 5 meters across -- before it hit the Earth's
atmosphere. Finding even that small amount was assisted by
additional good fortune: much of meteoritic debris landed on a
frozen lake where it is much easier to find such small pieces
comparedwith fields and mountainous terrain.
The bolide was detected by optical sensors on a U.S. Defense
Department satellite, and is one of the biggest impacts detected by
these satellites during many years of operation. At 5 kilotons of
TNT equivalent explosive energy, it may have been the biggest event
detected over land. [Note from Morrison: the estimated energy of the
February 1994 bolide over the western Pacific was several times
greater, perhaps as high as 100 kilotons. Since it took place over
the ocean, no meteorites were recovered].
Measurement of the meteorite's composition makes it clear that it is
one of the most "primitive" meteorites ever studied. An assay of its
composition shows that it is very much like the Sun itself (except,
of course, for volatile, gaseous compounds) and it shows even less
alteration of its minerals -- by early heating -- than is typical for
these kinds of meteorites. Thus it is material closely
representative of the primordial "stuff" from which the planets were
made 4.5 Gyr ago.
Although carbonaceous meteorites are very rare among meteorites
collected on the Earth (because of their fragility, as described
above), they are very abundant in space. Indeed, most of the mass in
the asteroid belt is made up of carbonaceous asteroids -- very black,
low-density objects believed to be made of materials at least roughly
like Tagish Lake. The fact that these preliminary studies of Tagish
Lake show that it does not readily fit into the cubby-holes of the
couple-of-dozen other CI and CM meteorite falls tells us that we are
just beginning to scratch the surface of the mysteries of these
abundant remnants from planetary accretion that are orbiting in the
middle and outer parts of the asteroid belt.
+++++++++++++++++++++++++++++++++++++++++++
NEO News is an informal compilation of news and opinion dealing with
Near Earth Objects (NEOs) and their impacts with the Earth. These
opinions are the responsibility of the individual authors and do not
represent the positions of NASA, the International Astronomical
Union, or any other organization. If you wish to subscribe (or
unsubscribe) contact dmorrison@arc.nasadot gov. For additional
information, please see the website: http://impact.arc.nasadot gov. If
anyone wishes to copy or redistribute these notes fully or in part,
please include this disclaimer.
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