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 > > > > > > _________________________________________________________________________ > ___________ > Looking for last minute shopping deals? > Find them fast with Yahoo! Search. > http://tools.search.yahoo.com/newsearch/category.php?category=shopping > ______________________________________________ > http://www.meteoritecentral.com > Meteorite-list mailing list > Meteorite-list@meteoritecentral.com > http://six.pairlist.net/mailman/listinfo/meteorite-list > > ______________________________________________ http://www.meteoritecentral.com Meteorite-list mailing list Meteorite-list@meteoritecentral.com http://six.pairlist.net/mailman/listinfo/meteorite-list