[meteorite-list] Re: Antarctic meteorite stats

From: Robert Verish <bolidechaser_at_meteoritecentral.com>
Date: Thu Apr 22 10:16:31 2004
Message-ID: <20030809090811.85520.qmail_at_web80510.mail.yahoo.com>

<ROBERT.D.MATSON_at_saic.com> wrote:
"With these factors in mind, has anyone attempted to
estimate the annual fall rate derived from the number
of Antarctic meteorite finds?"

Rob raises a very good question.

If the answer turns out to be, "Yes, it was attempted,
but various factors worked against obtaining a valid
number", then that leaves us with only one remaining
"dense collection area" from which stats can be used
to estimate the annual fall rate. What Rob is leading
us to conclude, is that the only remaining areas for
this type of study, are the dry lakes of the Mojave
Desert.

What this means is that Mojave Desert meteorite finds
should be the focus of a concerted terrestrial
age-dating effort. To this end, I am making each and
every one of my finds available to cosmogenic
radioisotope (age-dating) researchers.

In addition, pertinent field data is being recorded by
all of the meteorite-recovery searchers. Not just find
coordinates, but even if no finds were made, we are
recording the "man-hours" spent searching and the
exact area searched (in the form of GPS Tracklogs, as
per Rob's request). Every fragment is documented and
examined closely, because accurate pairings are
crucial.

It is quite remarkable the degree of response that is
given by all the volunteers, regardless of their
background, to these requests for various types of
field data. Although most of this field work is
conducted by volunteers on an individual basis
(usually no more than two-people per team), the real
team work comes about when the data is tabulated and
then reported to the m-recovery Group.

It helps to remind each other that there is no detail
too small, and that all of our efforts are useful
regardless of success at making a find. It's nice to
know that all this record keeping will eventually be
of some value to somebody, someday.

Bob V.

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[meteorite-list] Antarctic meteorite stats
Matson, Robert ROBERT.D.MATSON_at_saic.com
Fri, 8 Aug 2003 18:36:18 -0700

Next message: [meteorite-list] Antarctic meteorite
stats

Hi All,

On the question of the total mass of all Antarctic
meteorites, Al commented:

> This is and would be an important consideration.
> I have noticed that a lot of the Antarctic falls are

> sometimes very small. Total mass would shed an
> interesting correlation to non-Antarctic finds.
> ... Also the Antarctic falls are from hundreds and

> thousands of years ago. Perhaps as much as 800,000
> years ago, so there is a concentration of the falls
> on the ice sheets which may be distorting the
> numbers more.

This is part of the reason that despite the huge
number of statistical samples from Antarctica, it
would be very difficult to compute an accurate annual
meteorite fall rate from them. Among the many factors
you would have to consider:

1. Movement of the ice sheets over tens of thousands
of years. Where meteorites are found today is not
easily correlated to where they actually fell.
A square kilometer of a particular patch of ice today
may correspond to a quite different size and shape
for that surface in the past. You also have zones of
concentration, where large effective collection areas
have been compressed into small strips. Searching 1
km^2 of such a surface may be the equivalent of
searching 10, 100, or even a 1000 km^2.

2. Variable meteorite fall rate over the last
half-million or more years. The long lifetime of
meteorites in Antarctica means that any derived fall
rate will represent an average over that lifetime. It
is likely that the flux today is different from what
it was several hundred thousand years ago. I doubt
that
scientists have done terrestrial age dating on more
than a tiny fraction of Antarctic finds, so you have
both the uncertainty of the average age of all your
samples and the temporal variability in the flux rate.

3. Pairing uncertainty. Geographical location of the
finds doesn't help you much if the surface doesn't
stay put. ;-) Pairing of rare types can at least
give you a good estimate of the average number of
specimens per fall (somewhere in the range of 3-6), so
this ratio can simply be applied to the common types.

With these factors in mind, has anyone attempted to
estimate the annual fall rate derived from the number
of Antarctic meteorite finds?

Cheers,
Rob

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Received on Sat 09 Aug 2003 05:08:11 AM PDT