(meteorobs) Fwd: NEO News (10/13/00) Yukon meteorite

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Date: Fri, 13 Oct 2000 15:53:40 -0700
To: david.morrison@arc.nasa.gov
From: David Morrison <dmorrison@arc.nasa.gov>
Subject: NEO News (10/13/00) Yukon meteorite

NEO News (10/13/00) 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.nasa.gov.  For additional 
information, please see the website: http://impact.arc.nasa.gov.  If 
anyone wishes to copy or redistribute these notes fully or in part, 
please include this disclaimer. 

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