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Re: extinction5



"David Marjanovic" <david.marjanovic@gmx.at> writes:

> Considering the shape of the crater, it must have been something 
> _plus_ the
> rotational velocity of the Earth.

Yes, but compared to the relative collision velocity of the Earth with
its impactor, the contribution from the rotation of the Earth was tiny.

The shape of the Chicxulub Crater suggests that the impactor came in from
the southeast at a *moderately* low inclination.  The Chicxulub area was
at a low latitude during the Maastrichtian.  The Earth rotates with an
angular velocity of 0.36 km/sec at the equator.  That means that rotation
would have added a *maximum* of, very roughly, 0.36 km/sec to the
collision velocity.  The actual velocity contribution from rotation was
probably much less.  A maximum added contribution of 0.36 km/sec implies
that the impactor "skimmed" the Earth's surface and that the impactor
came in from due-east.  Neither of these two conditions appear to have
been the case at Chicxulub.

Wandering further off-topic:
It is interesting to note (and folks can critique my conclusions
themselves by grabbing that unused globe out of their closet) that,
assuming that the object was a typical earth-crossing asteroid with
typical earth-crossing orbital elements, then the relative velocity of
the impact varies greatly depending on the time of day.  For instance,
the lowest relative impact velocity would have occurred at noon or at
midnight.  The highest relative impact velocity would have occurred at
dawn or at dusk.  (To visualize this, it helps to use a little vector
algebra).  These observations are totally independent of the specific
(and unknown) orbital elements of the impactor.  What is assumed is that
the object orbited in the ecliptic plane, that the object orbited in the
same direction as the Earth, and that the object entered the atmosphere
from the SE at a low inclination.

However, if you assume that the K-T impactor was a comet, then none of
the above assumptions work, because comets don't necessary orbit in the
ecliptic plane and they don't necessarily orbit in the same direction as
the planets.

Another off-topic musing:
>From experimental results, the late Gene Shoemaker (along with others)
found that a VERY low angle of entry is necessary in order to produce an
asymmetrical crater.  Even entry angles of 50 degrees from horizontal
produced symmetrical craters.  Due to the slight asymmetrical shape of
the Chicxulub crater, the inclination of entry may have been smaller than
is now commonly believed.  It is possible that the K-T impact is an
example of an "Almost Near Miss".

<pb>
--








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