[meteorite-list] Deciphering Denshal Dog Data (delete if dull)

From: David Weir <dgweir_at_meteoritecentral.com>
Date: Thu Apr 22 09:52:12 2004
Message-ID: <3D587614.BAD1C9DC_at_earthlink.net>

Hello List,

Even though my skeptical nature leads me to scrutinize the impact dog
event, I remain open-minded to any new evidence supporting either side.
Despite the absence of eyewitnesses and newspaper articles, I am of the
mind that the probability of such a dog impact in Denshal can be further
assessed through the scientific method, using data and theoretical
applications currently available.

While looking through the literature for any helpful data, I found a
"non-peer-reviewed" paper, published by Eugster et al. in LPSC 33
(2002), in which they describe research on "The Pre-Atmospheric Size Of
Martian Meteorites". The upper limit of the radii of Martian ejecta
translates to masses of 150-270 kg - too high to be a limiting factor
when considering a Nakhla strewn field that may be extended into
Denshal. However, in a diagram that compares the minimum pre-atmospheric
weights of several Martian meteorites - including Nakhla, Zagami,
Shergotty, QUE 94201, Chassigny, Los Angeles, and SaU 005 - it is Nakhla
that has the lowest (the smallest size). Therefore, one might reasonably
expect Nakhla to also be at the low end of weights of all Martian
meteorite falls, especially if a pattern is evident. The falls include
the following four, in order from smallest to largest minimum calculated
pre-atmospheric size, with the respective fall weights in parentheses:
Nakhla (10 kg), Zagami (18 kg), Shergotty (5 kg), and Chassigny (4 kg).

For those Martian meteorites that are finds, the two with the largest
minimum pre-atmospheric masses are SaU 005 (1.3 kg, but with all paired
masses included, 10.6 kg) and then Los Angeles (0.7 kg), either of which
may or may not be representive of their cumulative fall weights. In
addition, having a minimum pre-atmospheric size similar to that of
Chassigny is the Antarctic QUE 94201 (0.012 kg), which likely doesn't
represent it's total fall weight. Though not included in this study, two
other Martians with large recovered weights can be mentioned for
comparison - EET 79001 (7.9 kg) and the DaG 476 strewn field (6.3 kg).

While I don't see a pattern, I would not "expect" the Nakhla fall to be
much bigger than what is presently accepted. To my speculation, a
greatly extended strewn field for Nakhla, with the usual pattern of
larger masses falling further down range (into Denshal and the dog),
would significantly increase the fall weight of Nakhla. The presently
acknowledged weight seems to fit amongst the others just fine,
especially considering Nakhla was ascribed the lowest minimum
pre-atmospheric weight.

While this is admittedly only a tiny stab, I think there might be other
data out there, which taken together, could establish a preponderance of
evidence and tip the scale one way or the other.

For instance, in The Shergotty Consortium, published in Geochimica vol.
50, 1986, there are "peer-reviewed" papers concerned with the
pre-atmospheric and final sizes of certain shergottites. Following a
determination of the CRE age from known profiles, cosmic ray track
densities in specific samples were used to calculate their shielding
depth and ablation characteristics on the pre-atmospheric meteoroid.
This information was then used to calculate the size of the
pre-atmospheric mass. From this resolved meteoroid size, the production
rate of cosmogenic nuclides at different depths was used to better
constrain the CRE age. For Shergotty, a pre-atmospheric size of ~12 cm
was calculated. This is equal to a mass of 26 kg, of which only 5 kg was
recovered, inferring an ablation rate of 80%. Ablation rates of 50-80%
were determined for other shergottites.

This type of study could be done for Nakhla. Each piece of Nakhla
studied would have cosmic ray track densities that were consistent with
a specific shielding geometry, which should be consistent with the
pre-atmospheric size as calculated from production rates of cosmogenic
and radiogenic nuclides. An examination of a representative sampling of
Nakhla fragments should be able to constrain its size and ablation
characteristics, and perhaps determine if any significant anomalies in
its fall weight are evident. If not, then that would be evidence tipping
the scale in favor of a limited strewn field, ruling out a dog impact.

David
Received on Mon 12 Aug 2002 10:59:32 PM PDT