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SV: Giant Impact Near India -- Not Mexico -- May Have DoomedDinosaurs



Also a surprisingly large proportion of asteroids have satellites and
some asteroids are essentially just rockpiles which would certainly
break up inside the Roche limit (for example Itokawa that was imaged by
the Hayabusa probe), so you would actually expect a fairly large
proportion of multiple impacts. Rieskessel + Steinheim is a very
probable case and Chicxulub has often been linked with Boltysh in
Ukraine which is about the same age.
However since the separation between the impactors would be fairly small
one would expect the impacts to be on the same side of the Earth,
certainly not antipodal. A cometary impact series could theoretically be
globally distributed since the comet could have broken up well before
hitting Earth, like Shoemaker-Levy did. However such events must be
quite rare, and analyses of the ejecta from Chicxulub indicates that it
was very likely an asteroid. 
If there are more than two impacts you would expect them to fall more or
less on a great circle. It has been suggested that Red Wing, Manicougan,
Saint Martin, Rochechouart and Obolon may be linked, since they are all
the same age within the measuring error (late Triassic) and fall on a
great circle with Triassic continental position.
Unfortunately there are other mechanisms that are expected to cause
temporal clustering of impacts. A star moving through the Oort cloud
would probably increase the number of comets many orders of magnitude
for a few million years, and a major collision event in the asteroid
belt (which are known to have happened) would have similar effect for
asteroids/meteorites.   
All this said I am personally very skeptical about the Shiva crater
(which incidentally was almost antipodal to Chicxulub).

Tommy Tyrberg

-----Ursprungligt meddelande-----
Från: owner-DINOSAUR@usc.edu [mailto:owner-DINOSAUR@usc.edu] För Graydon
Skickat: den 16 oktober 2009 00:07
Till: Mark Wildman
Kopia: paleovouga@gmail.com; dinosaur@usc.edu
Ämne: Re: Giant Impact Near India -- Not Mexico -- May Have
DoomedDinosaurs

On Thu, Oct 15, 2009 at 10:54:57PM +0100, Mark Wildman  scripsit:
> Who would have thought it? Two giant asteroids hitting the planet at
the
> same time. What are the odds? 

Not that bad; asteroids do break up due to gravitational influence prior
to impact.

> And still more life forms were extinguished at the end of the Permian
> than at the end of the Cretaceous. Hmmm - the cynic in me regarding
> asteroid extinctions is getting a little more cynical every day......

Last I heard, the chronological correlation with the extinction and
bloody great hole in the Yucatan was pretty tight.  There's also the
point that asteroid impacts that happen to occur around the time there's
already major flood vulcanism going on seem to do more ecological
damage and produce extensive mass extinctions.

-- Graydon