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NEMESIS - Part 4 of 4

WEISSMAN P.R. (1990) Are Periodic Bombardments Real? (Sky & Telescope,
March 1990, 266-270) - The Death Star, pp. 267-269:

Another argument against periodicity comes from examining the list of 16
"periodic" craters used by Alvarez and Muller. For eight of these,
geologists have attempted to identify the type of impactor from trace
elements left in once-molten rocks at the craters' bottoms. In six cases
the incoming bodies appear to have been differentiated, that is, at
least partially molten, long ago. Almost certainly, these objects were
not comets, which have likely never been heated enough to melt even
their ices much less the rock-hydrocarbon mixes in them. The other two
craters could have been formed by comets or "primitive"
(undifferentiated) asteroids.
Thus, many of the craters in the Alvarez-Muller list appear to have been
formed by the impacts of asteroids. Unfortunately no one has come up
with a way of causing periodic minor-planet showers every 26 million
years. Worse, the "primitive"-impactor crater in Lappajarvi, Finland, is
77 million years old. That places it right between the late Cenomanian
extinction about 95 million years ago and the K-T event at 65 million
years. Even Alvarez and Muller admitted that this had to be a random
comet or asteroid impact.
In 1985 another hypothesis emerged to explain periodicity. Whitmire and
colleague John Matese (University of Southwest Louisiana) proposed that
comet showers are caused by a 10th planet, or Planet X, that circulates
beyond Pluto in a hypothetical ring of comets beyond the orbit of
Neptune, now referred to as the Kuiper belt (after Gerard P. Kuiper, who
first suggested its existence in 1951). They suggested that the body's
orbit is highly inclined and that gravitational perturbations from the
other planets cause it to precess at just the proper rate to bring it
into the flattened disk of comets every 26 million years or so. At such
times a shower would result.
An immediate problem with this idea is that showers would extend over a
fairly long time, smearing out the periodicity. This behavior is
expected because the proposed body would start perturbing comets as its
orbit approached and left the ecliptic plane, not just while it was
exactly within it. Moreover, the planet's gravitational tug would drive
the comets into inclined orbits, further smearing the periodicity.
Finally, it is unlikely that the number of bodies sent into
Earth-crossing orbits would be great enough to cause very many impacts
on our planet.


Best wishes,

Bernd

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