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From: Dave-Platt%LADC@CISL-SERVICE-MULTICS.ARPA (Dave Platt)
Newsgroups: net.space
Subject: Timing of comet showers
Message-ID: <8601102258.AA02103@s1-b.arpa>
Date: Fri, 10-Jan-86 17:32:00 EST
Article-I.D.: s1-b.8601102258.AA02103
Posted: Fri Jan 10 17:32:00 1986
Date-Received: Sat, 11-Jan-86 07:45:50 EST
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I'm no expert on the subject, but the discussions I've heard have
pointed out that it doesn't actually take all that high a comet
density to cause extinctions on Earth... given that the effect
of even one 5-mile-diameter solid comet nucleus hitting Earth
would be rather atrocious.  An impact that would leave a crater of
moderate size on Luna would probably have a far severer effect here
on Earth... most of the effects which might lead to a mass extinction
on Earth would not apply on Luna. Consider: firestorms, dust diffusing
through the atmosphere, feedback effects resulting from decreased
insolation -> greater snowfall -> higher albedo -> colder weather,
possible changes in seawater chemistry, triggering of volcanic activity
with massive releases of sulphate and acid rain, etc.

When the island of Tambora (I think it was) in Indonesia blew up back
in the late 1800's (?), it triggered a "year without a summer" across
much of the Northern hemisphere... snow in June, massive crop failures
and famine, and so forth.  Tambora's erruption was a good deal larger
than Mt. St. Helens or Krakatao, but it was probably much smaller than
the ruckus raised by (for example) the massive erruptions that occurred
in what is now Yellowstone (ash carried all the way out into the Atlantic).
I suspect that the impact of a 5-mile-diameter comet would have been at
least as impressive... especially if it happened to strike in an area
with a substantial body of magma at a shallow depth (on a mid-ocean
ridge, for example).

Also, Earth is a substantially larger target than Luna... both because
of its larger diameter and because its gravity well is deeper.  Although
it's true that Luna's surface has been mapped quite well by telescope
and orbiting satellite, very few of the craters have actually been
physically sampled in a way that permits an accurate on-the-spot
measurement of their age.  The currently-popular theories do indicate
that the majority of the cratering on Luna occurred billions of years
ago... but it's quite conceivable that the large number of old craters
conceal a smaller, but still very significant number of younger ones.

I saw a mention somewhere within the past couple of years that
there is historical evidence that people actually saw the impact
of a good-sized object on Luna.  I believe that an Arab writer
(historian, natural philosopher, or something of the sort) wrote that
he, and quite a few other people, saw a very bright light spring out
on the surface of the Moon, in the dark area between the "horns" of
the new Moon;  it faded to a dimmer glow over a period of several
minutes.  If I recall correctly, some scientists believe that this
was probably the impact of an asteroid: a member of the class known
as Apollo objects, which have orbits that cross Earth's;  they even
suggested one specific crater as being the probable point of
impact (somewhere near Tycho, I think???).

There's been another interesting speculation recently, concerning
Apollo objects of the sort that may have hit Luna.  Quite a few of
these object have been detected... a pretty good-sized one passed
within the bounds of Luna's orbit within the past five years or so,
and something not previously plotted cut a swath through the outer
layers of Earth's atmosphere during the '70s, and plowed its way
back out again (a VERY impressive fireball and trail, according to
those who saw it... photograph appeared in Astronomy magazine during
'84 or '85, I recall).

It turns out that the orbits of Apollo objects aren't all that stable,
in terms of geologic time;  they're subject to substantial alteration by
Jupiter and the inner planets (including Earth).  It seems that there
must be some mechanism for refreshing the supply of Apollo objects in
order to account for their current numbers.  Some fairly extensive
simulations of the gravitational dynamics of the asteroid belt indicates
that there are several "forbidden zones" within the belt.  If an
asteroid is ever perturbed into an orbit with an orbital period which
resonates with Jupiter's, then Jupiter's gravity will fairly quickly
perturb the asteroid out of that orbit... and one fairly common result,
it seems, is for the asteroid to drop into an orbit that crosses
Earth's.  There may be some subtle second- or third-order interactions
between Jupiter, Saturn, the asteroids, Earth, and maybe the other
planets which would lead to a periodic change in the number of
asteriods perturbed into Earth-crossing orbits.

So... Nemesis may exist, and be sitting right in front of our faces:
it's Jupiter!  This hypothesis seems to meet a number of the
requirements: the asteroids are primitive in composition and would
probably account for the heavy-metal traces that have been detected in
the clay interlayers associated with some extinctions, and there's a
good supply of them.  Jupiter has a 12-year orbit rather than a
30-million-year one, and so it's not likely to be yanked away by
a passing star (or, if that happens, the question of further comet
showers is probably going to be the least of our worries!).

Perhaps the biggest question that remains is:  how accurate and
meaningful are the calculations and statistics that indicate a strong
periodicy in the extinction (and/or impact) rate...  and is there a
mechanism that could cause the Apollo-object-insertion rate to vary in
that way?

Other note of interest... CCD imaging of the skyglow in Earth's
atmosphere has indicated something unexpected.  Apparently, something is
punching "holes" in the skyglow;  they start out small, spread to a
diameter of several miles, and slowly fade.  They're more common on the
dawn side of the planet... which seems to indicate that they're probably
associated with infalling meteors.  If the researchers who reported this
are correct, this data may indicate that meteoroids may contain far more
volatile material than had been previously believed;  the "pebbles" that
we see at ground level may be only the solid residue of a much larger
dirty-snowball-like clump that hits atmosphere.  If this is true, it
would indicate that meteors start out life as something very similar to
a miniature comet nucleus, and that the amount of matter entering
Earth's atmosphere may be much larger than previously believed.