[meteorite-list] Mammoth Stew, Overcooked

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Mon, 17 Dec 2007 18:51:24 -0600
Message-ID: <027301c84110$1d93b240$b842e146_at_ATARIENGINE>

Hi, Everybody,

    In calculating the "geometric" cross section of the
Earth, you have to increase its "static" radius by the
effective thickness of the atmosphere. The question
is how much to increase it. I'd say 60 to 80 km. On the
other hand, if you miss the Moon... you miss the Moon.

    Another consideration is that we are modeling these
cross sections "equally" in three dimensions (as every
physics course teaches us), as if impactors came equally
from all directions. But of course, most impactors hang
out in the ecliptic plane or at least within the 10 degree
neighborhood. So, in some ways, it makes as much sense
to model the situation in two dimensions as three (shades
of Flatland!). This would change the "shielding" situation.

    As far as the 3,000,000 craters goes, ignoring all the
gravitational "focusing" entirely and just crudely calculating
by geometric cross sections alone lowers the figure to
2,350,000. A focusing effect of ~10% raises it to about
2,600,000 craters on Earth, and a focusing effect of ~20%
raises it to about 2,800,000 craters made on Earth. I say,
no matter how you look at it -- that's a heck of a lot of
craters!

    IF (big if) Mars was subject to the same impactor flux
as Earth, it would have accumulated 850,000 craters during
its lifetime. Since Mars has 260,000 craters remaining, it is
clear that Mars' surface is largely ancient and unaltered, that
"erosion" in the terrestrial sense does not operate there. Unlike
the Earth where one crater in 18,000 survives, on Mars one
out three craters survives for the life of the planet. Caveat: It
is possible that Mars was subjected to an even higher flux of
impactors, of course, since Mars does live in what I call
a "bad" neighborhood for rocks...

    Craters on Venus? Oh, go away! There are 900, and the
smallest is 2 kilometers, but only craters of 35 km and larger
fit any sane distribution statistics. The surface preserves only
the last 500 million years OR LESS. It is geologically active,
but we don't know how much or how fast. The craters could
be only a few million years old or half a billion. Venus is a big
shrug.

    Impactor flux at Mercury? "New crater size-shape data
were compiled for 221 fresh lunar craters and 152 youthful
mercurian craters. Terraces and central peaks develop initially
in fresh craters on the Moon in the 0-10 km diameter interval.
Above a diameter of 65 km all craters are terraced and have
central peaks. Swirl floor texture is most common in craters
in the size range 20-30 km, but it occurs less frequently as
terraces become a dominant feature of crater interiors. For
the Moon there is a correlation between crater shape and
geomorphic terrain type. For example, craters on the maria
are more complex in terms of central peak and terrace detail
at any given crater diameter than are craters in the highlands.
These crater data suggest that there are significant differences
in substrate and/or target properties between maria and highlands.
Size-shape profiles for Mercury show that central peak and
terrace onset is in the 10-20 km diameter interval; all craters are
terraced at 65 km, and all have central peaks at 45 km. The crater
data for Mercury show no clear cut terrain correlation. Comparison
of lunar and mercurian data indicates that both central peaks and
terraces are more abundant in craters in the diameter range 5-75 km
on Mercury. Differences in crater shape between Mercury and
the Moon may be due to differences in planetary gravitational
acceleration (gMercury=2.3gMoon). Also differences between
Mercury and the Moon in target and substrate and in modal impact
velocity may contribute to affect crater shape."
http://www.springerlink.com/content/w0785mv1q8q045tw/
    Bottom line? 2/3rd of the craters of the Earth-Moon system,
based on current solar system flux, or as the authors put it, the
flux at Mercury cannot differ from the lunar flux by more than
+/- 50%.

    The Winnah and Still Champeen? The Earth, The Most
Cratered Planet in the Solar System! (Callisto is the Most
Cratered Body.) Like a boxer, it's how many times you get
hit... not how many scars you have.


Sterling K. Webb
--------------------------------------------------------------------
----- Original Message -----
From: <lebofsky at lpl.arizona.edu>
To: "E.P. Grondine" <epgrondine at yahoo.com>
Cc: <meteorite-list at meteoritecentral.com>
Sent: Monday, December 17, 2007 12:05 PM
Subject: Re: [meteorite-list] Mammoth Stew

#1

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


#2

Hi Again:

I found it!

It (the impact cross section) is (I should have realized) incoming
velocity dependent. For objects going at 50 km/s the cross sectional area
of the Earth is increased by 5%. However, for something approaching at 25
km/s, this increases to 20%!

For something going really slowly relative to the Earth (catching up), the
impact cross section can be more than double the actual cross section of
the Earth

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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Received on Mon 17 Dec 2007 07:51:24 PM PST