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NEMESIS - Part 3 of 4

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

If the companion was likely to escape in 600 million years, which is
only 13 percent of the age of the solar system, then it must have been
captured by the Sun only recently. Everyone agreed that the capture of a
random passing star by the solar system was very unlikely. That left
just one other alternative, namely, that Nemesis had formed much closer
to the Sun and had slowly evolved outward due to stellar perturbations,
just as comets in the Oort cloud tend to do. If so, its period must have
been shorter in the past, making comet showers and cratering episodes
more frequent.
This alternative could also be tested. Geologists estimate the rate of
terrestrial impacts by counting craters and using radioisotope
techniques to date the rocks in which they are found. (In general,
terrestrial crater counts are valid only for the past 600 million years.
Geologic processes and weathering tend to destroy evidence of all but
the largest older impacts.) The result is that Earth suffers about 11
impacts every 1 million years that lead to craters 10 km or more in
diameter, with roughly 70 percent of the impacts in the oceans. This
correlates well with the rates expected from the calculated number of
asteroids and comets in Earth-crossing orbits. Asteroids provide about
two-thirds of the projectiles while the rest are comets, mostly
long-period ones from the distant Oort cloud.
The Apollo missions and their returned samples made it possible to
estimate cratering rates for the Moon as well. Because there is no
weathering on our satellite and geologic processes are much less active
there, the lunar record extends back over 3 billion years. Surprisingly,
the rate found for the Moon is about one-half that for Earth (per unit
area), though the error bars on the two estimates do overlap. This
raises the possibility that the solar system's cratering rate was
actually lower in the past.
One can estimate the expected number of craters on the Earth and Moon if
a companion star has been sending in showers of comets every 26 million
years over the history of the solar system. The result (including the
random influx of observed comets and asteroids) is 4.5 times as many
craters as are actually found on Earth and over eight times as many as
on the Moon!
There is a way out of this dilemma, but it doesn't make matters any
easier for either periodic-bombardment hypothesis. If we are
experiencing the tail end of a small comet shower right now, then we
could be overestimating the number of objects in the Oort cloud and thus
in a shower. This would result in fewer craters on the Earth and Moon
but would also lower the chance that a single impact during a shower
would be large enough to cause a global extinction.
Until now we have concentrated on the companion-star hypothesis, but
cratering rates argue just as effectively against showers caused by
encounters with giant molecular clouds (GMC's). An even stronger
argument came from Patrick Thaddeus and Gary Chanan (then both at
NASA-GISS). They pointed out that GMC's are not located exclusively in
the galactic plane but typically are spread above and below it by 150 to
250 light-years. Interestingly, this is about the same distance that the
solar system oscillates above and below the plane. Consequently, the Sun
is only about 50 percent less likely to encounter a GMC at the end
points of its motion (where it moves more slowly, enhancing the cloud's
gravitational effect) than it is at plane crossings. Thus a periodic
signature signal would not be expected.

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