[meteorite-list] Mammoth Stew

From: Jason Utas <meteoritekid_at_meteoritecentral.com>
Date: Mon, 17 Dec 2007 10:24:35 -0800
Message-ID: <93aaac890712171024i29d34f25p9fbeb65253d525f4_at_mail.gmail.com>

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


On Dec 17, 2007 10:05 AM, <lebofsky at lpl.arizona.edu> wrote:
> 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
> less).
>
> 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.
>
> Larry
>
>
>
> 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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>
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Received on Mon 17 Dec 2007 01:24:35 PM PST