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Re: astroblemes

From: pjanke@maroon.tc.umn.edu
 > [Nemesis hypothesis reference deleted]
 > Just because the Nemesis model has been properly discredited does
 > mean that periodicity is not to be found in impacts.

But the periodicity has *also* been shown to be based on
invalid analysis.  As far as I know (last I heard) there
*is* no real evidence of periodicity.

 > The periodic nature of impacts is now being viewed by some as
 related to the
 > passage of our solar system through the galactic plane,

Before they do *that* they had better demonstrate that
there *is* a periodicity to explain.
 > Now let's examine the 30 million year interval spanning the K/T:
 > Crater       diameter      time of impact
 > --------------------------------------------
 > Manson          35km            74 mya
 > Chicxulub     >200km            65 mya
 > Kamensk         25km            65 mya
 > Ust-kara        25km            57+-9 mya
 > Kara            60km            57+-9 mya
 > Logancha        20km            50+-20 mya
 > We know of at least 5 impacts in this 30 million year interval that
 > are greater than *any* in the last 30 million years. I cite this not
 > as evidence of periodicity, but rather to rebut the statistical
 > generalization

Very interesting.

The time span here is *far* greater than the period covered by
the K-T extinctions.  (In fact I think the earlier impacts
predate the *Turonian* minor extinction event).

 > My personal theory is that the largest impacts do have a periodic
 > which is related to the geometry of our galaxy itself.
 > To illustrate this theory, lets look at the 3 biggest(by far)
 > events.
 > Period                     time            crater        diameter
 > Cretaceous/Tertiary        65 mya         Chicxulub     >200km
 > Permain/Triassic          250 mya         S. Atlantic   >300km*
 > Ordovician/Silurian       439 mya         no known crater(yet!)

Hmm, I have not heard of a well-validated crater at or about
the P-Tr boundary.   What references do you have to substantiate
this citation?
 > The 185 million year period is approximately the time required for
 > solar system to make one lap around the galactic nucleus.

If you thought that the orbit of Nemesis would be unstable,
you really ought to see how unstable the Sun's orbit around
the galaxy is!

I doubt any real long term periodicity would be noticable.

swf@elsegundoca.ncr.com         sarima@netcom.com

The peace of God be with you.