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Re: impacts are cool!

Thomas R. Holtz, Jr. wrote:
> At 10:50 PM 10/29/97 -0500, Michael Teuton wrote:
> >The amount of iridium favors a comet strike.
> In point of fact, the amount of iridium favors an *asteroid* strike, not a
> cometary one.  Whereas the bulk of a comet's mass is frozen gasses,
> asteroids are mostly metal and silicate.
> Comets got into the picture due to the (still very questionable) periodic
> extinction theory, which is not necessarily supported by the data.
Well, I don't do any of this work, but I do follow it and have talked
with some of the people involved.  Here are their arguments and some
references that support a comet strike.  The typical velocity of an
asteroid is 15 km/s with a typical density of 2.5 g/cm^3.  The typical
velocity of a short-period comet is 30 km/s with a typical density of 1
g/cm^3. An asteroid with a diameter of 9 - 17 km or a comet of a
diameter 10 - 17 km would produce the energy of the impact at Chicxulub. 

Asteroids would produce excess iridium deposited worldwide in the range
of 1.5x10^12-8.7x10^12g.  Comets would produce 2.4x10^11-5.5x10^11g.
Excess iridium at the K/T boundary is estimated to be 3.6x10^11g.  While
this may appear to be a small difference it apparently is not and falls
within the comet range. Also, asteroids do not contain amino acids.  So,
the evidence favors a comet strike more than an asteroid one according
to the geologists.

Energy vs. impactor diameter : The relationships given in Toon et al.,
1997. Environmental pertubations caused by the impact of asteroids and
comets, Reviews of Geophysics, 35, 41-78.

Iridium abundance in CI Carbonaceous chondrites and comets : Geiss, J.,
1988. Modern Astronomy, 1, 1.
Estimated excess iridium at K/T boundary: quoted in Yabushita, S. and
Allen, A., 1997. 

Did an impact alone kill the dinosaurs?, Astronomy and Geophysics, 38,
15-19. Derived from the figures of Kyte and Wasson, 1986, Science,    
232, 1225. 
       Michael Teuton

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