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comet at K/T boundary

Josh Smith asks me about the evidence that it was a comet, not
an asteroid, at the Cretaceous-Tertiary boundary.  Much of my book
(CArlisle: Dinosaurs. diamonds, and things from outer space: the
great extinction - Stanfurd Univ Press, June 1995) is devoted to
just that, so if you want the evidence in detail try reading that.
It's not too technical, but meant for the general scientifically
literate reader.

Briefly, the evidence concerns the chemistry of carbon in the 
bnoundary clay and in meteorites.  CArbonaceous chondrtic 
meteorites show clear signs of formation beyond the  asteroid belt,
in the Oort Cloud, and seem to derive from secondary asteroids  
that have  come from the Oort cloud in the first place and then
been trapped by resonance effects (chefly by Jupiter's graviational
field) in the asteroid belt.  In that belt they have heated enough
by the sun to lose all their volatile components that normally
give a comet a tail wh en it  appraches the  sun.  Susch dead
comets behave like asteroids, but have a quite different
chemistry.  The carbon isotope ratio is that of interstellar
dust, not that of the inner solar system.  They also contain
many amino acids not found in the inner solar system.  

In the K/T boundary clay there is whole  suite of non-terrstrial
aminoacids corresponding with those of carbonaceous meteorites,
and nanometre-sized diamonds, also otherwise known only from
these same comet fragments.  And the carbon isotope ratios
are all those of interstellar dust.

That, in short, is the nature of the evidence that a comet
was involved at the K/T.  But read my book.


>From: David Brez Carlisle
bk090@Freenet Carleton.CA