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Re: Downgrading the Chicxulub Impact

Larry Febo wrote:
>Haven`t heard of any "downgrade", but just got finished reading Chatterjee`s
>book ..."The Rise of Birds", where he indicates an approximate 600 km
>"Shiva" crater that he believes was concurrent with the Chicxalube impact.
>He also indicates that  small fragments of  additional bolide strike (s) at
>the K-T boundary were found in a drill core from Deep Sea Drilling Platform
>576 in the western north pacific. All this indicating multiple strikes from
>numerous bolides, all concurrent, and all lying in  an apparently straight
>line path circling the globe, which he terms the "Alvarez Impact Belt"! So,
>it could have been very similar to the Shoemaker-Levy impact on Jupiter not
>so long ago.

Chatterjee is the only scientist I know who champions the 'Shiva' crater.
Other crater chains on Earth subjected to critical examination generally
have gone away because the time or impact correlations have not held up.
This is consistent with models that show a Shoemaker-9 chain type impact on
the Earth is quite unlikely. Nobody says it couldn't happen, just that it
isn't likely.

I also have not hear of any downgrading of Chicxulub, but keep in mind that
'crater size' is hard to measure when we're talking about a feature buried
under other deposits. At one point there was a transient hole in the ground
(and for a much briefer period, a hole in the water since the impact
evidently occurred offshore on the continental shelf). The simplest model
is a single simple crater wall around the hole, but some large lunar
craters are more complex. Remember, too, that the impact will affect a
larger zone surrounding the rim, for example shattering rock over a much
larger area. Functionally, the usual definition is assumed to be crater
wall, but some features (I believe the cenote ring is an example) are
outside the area thought to be the crater wall. There may also be
structures inside the crater wall, which could have smaller diameter.
(Somebody also could have been estimating diameter in _miles_ rather than
kilometers, a number which rounds to 100, so the 'downgrading' could be as
simple as a conversion mistake. -- Jeff Hecht