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bolides, impacts, KT boundary



Jeffrey Martz <martz@holly.ColoState.EDU> recently wrote:


>       Its important to keep in mind that the earth has almost certainly 
> been creamed by as many meteors and comets as the moon, and that the 
> impact craters have simply been eroded or subducted or whatever.  This 
> means not only that species CAN survive impacts, but that impacts are 
> frequent enough (geologically speaking) to be considered as possible 
> causes of extinction. (This brings up a point I believe Gene Shoemacher 
> made.  The majority of the objects that cross the earth's orbits are comets, 
> making the most likely object to strike the earth a comet.  Wouldn't this 
> imply that most of the craters on the moon were caused by comets?)

Yet another astrophysics impact thread... A few interesting points
worth making.  The lunar crater record is not quite comparable to the
terrestrial one because many lunar impact features date to times so
early that the population of impactors was still the planetesimals
(think of today's rare impacts as the last stage of planet
formation). Objects continue to orbit the sun until they hit
something, so the remaining asteroids (minor planets, planetoids, take
your pick) are the ones that haven't hit anything else yet (or did but
it was so small that the combined fragments are still loose). This
process has modified the original population of asteroids much more
strongly than comets, which spend very little time in the inner solar
system, may be only recently introduced to such orbits by
perturbations, and, if they start in the Oort cloud, have high
inclinations with correspondingly smaller cross-sections for crossing
planetary orbits.

The recent Earth-approaching statistics bear out Gene Shoemaker's
point (hey, I can't argue with him about impacts), but the
predominance of comets over more solid projectiles isn't huge. Another
factor weighs in in favor of comets - higher impact kinetic energy per
unit mass because they can hit us faster than most minor
planets. Minor planets are mostly in pretty circular orbits so they
will almost match velocity with the Earth, while comets are travelling
close to the solar escape velocity at our distance and can be going in
any direction (even the worst-case retrograde one where our velocities
add). Surely I wasn't the only one watching in bemusement as you could
in fact see Hyakutake's motion against the stars last week. OTOH, we
don't know of any comets that have been seen with really big nuclei
(Chicxulub level things), though they could of course be rare ones
anyway.

As someone else (whose message I erased) pointed out, the standard
astronomy textbook definition of a bolide has long been a meteor seen
to explode in the atmosphere. There has been some confusion, since
foreball usually meant just an exceptionally bright meteor but has
also been used for a particular phase of the impact explosion. I'd
like to stick with the term impact hypothesis for this interpretation
of mass extinctions to avoid further confusion.

Finally, most of you will likely be pleased to know that I just
complained in a review for a textbook publisher when some authors
sounded a little too sure that it was an impact that ended the
Cretaceous...

Bill Keel
Astronomy, University of Alabama