[meteorite-list] Mammoth Stew

From: E.P. Grondine <epgrondine_at_meteoritecentral.com>
Date: Mon, 17 Dec 2007 17:40:18 -0800 (PST)
Message-ID: <370426.11900.qm_at_web36914.mail.mud.yahoo.com>

Hi Jason, all -

Yes, you're making sense: we don't know the rate of
impacts for the last 50,000 years.

For that matter, we don't know what the current rate
of impact is. And therein lies the problem that I had
been working on for some 7 years before my stroke,
recovering accounts of comet and asteroid impact from
historical and proto-historical records.

I have to agree with you that many asteroids must have
accreted with larger bodies long ago. But another
thing we don't know is the timing of asteroid
production, nor do we have really good models for the
timing of their accretion.

What we do know is that the impact rate is variable,
and it is suspected by many to be periodic based on
ELEs (Extinction Level Events). It is also suspected
by many that comets play a larger part in the current
(for the last couple hundred million years or so)
cratering process than asteroids, and the late Eugene
Shoemaker thought that we are entering a period of
higher risk from comet impact. The data from the
Americas for the last 12,000 years would seem to
support his opinion.

One of the wonderful things in life is that we have
samples of both asteroids and comets, called
meteorites, and the people on this list find them and
trade them and study them intensely. Right now there
is the very exciting possibility of large irons
hammers coming on the market. It looks like the Nakla
dog is going to get some great competition.

good hunting all,
E.P. Grondine
Man and Impact in the Americas

PS - I wish the Hupe's the best success with their
most recent offerings. What a magnificent selection of
lunar and martian meteorites! and once again, the best
of luck to everyone out there.

Jason wrote:
Hola All,
But the main problem is that impact rates have not
been constant since the formation of a solid lunar
crust a number of billions of years ago, and as such,
this declining rate biases the results put forth.

Simply put, we're talking about craters having formed
in the past ~50k years, as that's the time period that
we're discussing, because before this, impact rates
were different (greater).

Older craters on earth erode, to the point of being
unrecognizable, another reason for us to use the ~50k
age range, as simple wind and rain will take care of
even the largest craters given only a few hundred
million years (never mind the ~2 billion year old
pristine lunar surface), to the point of making them
inconspicuous, at best.

Using such absolute numbers as the total number of
lunar impact craters is simply biased towards a period
of time two billion years ago, and unless one knows
the approximate age of all lunar craters,there's just
no point of using it as a comparison for the number of
impacts that was occurring ~50k years ago, as we
simply don't know what the rate was.

...Am I making sense?
Regards,
Jason


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Received on Mon 17 Dec 2007 08:40:18 PM PST