[meteorite-list] Mammoth Stew

From: lebofsky at lpl.arizona.edu <lebofsky_at_meteoritecentral.com>
Date: Mon, 17 Dec 2007 11:05:41 -0700 (MST)
Message-ID: <1524.>

Hi All:

I was wanting to find REAL numbers, but may have to rely on memory:

1. The Moon stops very little of what might hit the Earth. The cross
section of the Moon is pi x radius(Moon)**2. A sphere at the Moon's
distance is
4 x pi x radius(orbit)**2. Since the distance from the Earth to the Moon
is about 110 Moon diameters (220 Moon radii), the Moon on "stops"
1/(4 x220 x 220) of what might be heading toward the Earth, about
1/200,000. Not very many.

On the other hand, and this has to be from memory, the Earth's "cross
section" to impacts is about 10% greater than its true cross section
(thanks to its gravity; there is a similar effect for the Moon, but much

So, in reality, the Earth should have 10% more craters than the Moon per
unit surface area.

If you want to get fussy about shielding, it works both ways: while the
Moon shields the Earth, the Earth shields the Moon (much better).

I will continue to search for the actual cross section effect.


On Mon, December 17, 2007 10:34 am, E.P. Grondine wrote:
> Hi Sterling, list -
> Thanks for clarifying the impact crater situation,
> though I am still sceptical about the models of Moon impact rates and Earth
> impact rates.
> I know that the Moon absorbed some impactors that were
> headed for the Earth - at least it did so within human recorded memory, and
> if anyone wants proof of this, contact me off list and I'll you a copy of
> the Trempeauleau petroglyph.
> So at a minimum the Earth cross section needs to be
> reduced in modeling the combined Earth-Moon system. My guess is that this
> should reduce the impact rate/craters by about 10%, leaving say only 2.7
> million or so craters. (Imagine that, E.P. arguing for a lower Earth
> impact rate!)
> Could you give us the quick math for this? I can't do
> it myself anymore.
> I was also quite surprised by this cratering model you
> pointed to:
> http://www.news.uiuc.edu/scitips/02/1025craters.html
> given that the KT-fossil meteorite is carbonaceous chondrite, in other
> words a comet, and that the Sudbury impact appears to have been iron, as
> its remains are a source for our nickel steel. It seems likely to me that
> this Illinois team's computer model is off.
> Re: the apparent 31,000 BCE impact, my hope is that
> large irons may have survived in "defraction lenses" (is that the right
> term?) in the blast, irons large enough to survive later weathering.
> Trying to remember
> the find spot for the mammoth tusks, I seem to remember it was reported
> that they came from a shop in Calgary, further unknown.
> good hunting all, E.P. Grondine
> Man and Impact in the Americas
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Received on Mon 17 Dec 2007 01:05:41 PM PST

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