Re: Impact Scenarios?

[Tom Holtz <tholtz@geochange.er.usgs.gov>]

>He cites such factors as the uneven distribution of mass within the earth
>can influence impacts and consequently hit the same spots on Earth. He
>points to craters from the time frame 50-100 million years ago and
>how they form a pattern (Chicxulub crater, Manson crater in Iowa, Avak
>in Alaska and three in Russia).

Given that Manson hit the Earth some 13.5 million years (finally checked
the absolute dates) before the K-T impacts, I think it is an amazing
stretch to associate these as gravitationally linked suites of asteroids.

>I also recently read (I _think_ it was an Aviation Week - wished I'd
>photocopied the article) about asteroids with companion satelites.
>These companions are either small chunks orbiting the larger piece
>and are broken off bits of the larger one or are gravitationally
>captured. At any rate, if one impacts, the other does too, and some
>crater sites (moon, etc.) can be explained as the result of a strike
>by such a pair.

There have been hypotheses that a single asteroid "calved" to produce
multiple impacts, and the recent discovery of asteroid "moons" suggests
that there are other mechanisms which could produce multiple associated
impacts.  However, one would expect that these collisions would be
temporally associated in the scale of days or months or years or centuries
(or minutes?), and not ten million years or more.

                                
Thomas R. Holtz, Jr.                                   
tholtz@geochange.er.usgs.gov
Vertebrate Paleontologist in Exile                  Phone:      703-648-5280
U.S. Geological Survey                                FAX:      703-648-5420
Branch of Paleontology & Stratigraphy
MS 970 National Center
Reston, VA  22092
U.S.A.