[meteorite-list] ELE impact cycles

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Thu, 20 Sep 2007 15:13:28 -0500
Message-ID: <0d6501c7fbc2$b5e63a60$a025e146_at_ATARIENGINE>

Hi, EP, List,

    The full cycle time for the Sun's oscillation from
a position below the Galactic Plane, through it once,
to another extreme above the Galactic Plane, then
through the Plane again, back to its under-the-Plane
extreme, takes 62-65 million years.

    That means we are in or inside the Galactic Plane,
which is where the greatest density of other stars are,
TWICE every cycle. The Plane, of course, is not a
flat geometric plane like Saturn's Rings -- it's several
thousand light years thick. How many thousand --
3,000? 5,000? -- depends on where you want to call
it quits on density.

    So, the "cycle" of Plane crossings is every 31-33
million years. And it's more complicated than that.
When we are at our furthest away from the Galactic
Plane, the tidal forces from the Galactic Plane are at
their strongest, also every 31-33 million years at the
midpoints of the other cycle. The tidal cycle is weaker
than the crossing cycle.

    The cycle means we face two kinds of dangers: close
approaches of other stars, dust clouds, and other massive
inhabitants of the Galactic Plane when we're in it -- and
tidal forces on the Oort Cloud bodies when we're out of
it. The Galactic tide is believed to be essentially what puts
a limit to the outer bound of the Oort Cloud. The timing
of the tidal effects is greatest at the midpoints of the
other cycle, so it goes like this:

Plane crossing #1, strong, then 16 million years later,
Tidal maximum above, weak, then 16 million years later,
Plane crossing #2, strong, then 16 million years later,
Tidal maximum below, weak, then 16 million years later...

    Ya just can't win. On top of the two hazards inside and
outside the Galactic Plane, there is another. The Milky Way
has "recently" collided with a dwarf galaxy, totally disrupting
it and pulling it into a long filament of stars which wraps
around our Galaxy at right angles to the Galactic Plane.
And, we -- lucky we -- just happened to be at the point
where the remnants of that little galaxy we ate are streaming
through the Galactic Plane at right angles.

    Yes, cross traffic with no stoplights. Not only are "our"
stars moving at right angle vectors to "their" stars, the
velocities involved are hundreds of kilometers per second
relative to each other. Fortunately, traffic is light at the
moment.

    Then, the Sun "orbits" the Galactic Center every 230
million years, plus or minus 30-odd million years, more or less.
We "assume" the Sun's orbit to be "reasonably" circular,
with an eccentricity greater than 0.70, but the honest truth
is: we don't really know. When you observe 80 years out
of a 230,000,000 year orbit, you just can't tell.

    So what if the Sun's orbit is more eccentric than 0.70?
What's the big deal? Well, if it's more eccentric, we get
much closer to, possibly pass through part of, the Galactic
Center or Core region. Have you taken a close look at a
Galactic Center lately? Crowded, very Crowded, and very,
very Crowded, star densities that force stars very close
together -- it's a messy and incredibly dangerous place.

    Well, is there the slightest evidence of a cycle of Earthly
change on that time scale? Well, yes, there is. A fair number
of geologists have suggested that there are long term cycles
in Earth history, with cycle times of 250 to 300 million years
(Icehouse vs. Greenhouse, cycles of orogeny, cycles in
eustatic sea level, even cycles in continent configuration --
there's a 230 million cycle on the production of carbonatite.).

    Now, the Paradox. The timings of mass extinctions, much
argued about, much calculated and re-calculated, with a blizzard
of "curve-fitting" (references on request at Google), has held
at a "best fit" of 25.5-26 million year cycle. (There's also a
250 million year cycle in the biggest extinctions.) Now, you
can explain one 5-6 million years delay as the time it takes comets
to fall in from the Oort Cloud, etc, BUT that would only result
in two 32 million year cycles, shifted by 5-6 million years from
each other, and that's not what we have.

    So, the possibilities are that the movement of the Sun above
and below the Galactic Plane have been miscalculated by 20%
(possible, but not that likely), OR that the cycle time varies by
the Sun's position in the Galaxy (Galactic orbits are "random
walks," not Newtonian clockwork), OR that the 26 million year
extinction cycle is due to something other than the Galaxy-Comet
interaction, OR that the Oort Cloud is disturbed on a 26 million
year cycle by something other than the "Galactic" effect (dark
massive companion), OR... It's like a grand smorgasbord of
hypotheses, isn't it?

    The timing discrepancy may be resolved in the future, or
explained, but right now it makes the comet correlation a puzzle,
not a solution. It may never be resolved. These are geological
and cosmological processes, slow and subject to many kinds
of perturbation; they may be cyclical but variable; we may just
not have enough data yet. Their specific "history" is hard to
derive from the geological record, which for all practical
purposes means only the last 500-600 million years is useful.

    And we haven't even mentioned the 10-20 GMC's (Giant
Molecular Complexes), huge dust clouds, full of new stars and
with the mass of a million Suns, which circulate in the Galactic
Plane. We've had to run into (and through) one of those monsters
every half billion years (or less). They are a far greater danger than
any (or all-of-the-above) of the hazards above.

    We're spoiled. We happen to be in a very safe, low-hazard
region, far from the Galactic Plane, outside a spiral arm, distant
from the Galactic Center, swept clean of gas and dust, and thinly
populated with other stars: astronomers call it "The Local Bubble."
We're not experiencing any storm of bonbardments; the local
ice age is in a mild interlude --- Gee! Everything is just perfect!
Like everyone who has grown up in a really nice neighborhood,
we think the outside world, too, is a "nice" place, just like home.
Because of this, we tend to grossly UNDER-estimate the hazards
that do exist. We are basically "clueless."


Sterling K. Webb
-------------------------------------------------------------------------------
----- Original Message -----
From: "E.P. Grondine" <epgrondine at yahoo.com>
To: <meteorite-list at meteoritecentral.com>
Sent: Thursday, September 20, 2007 12:16 PM
Subject: [meteorite-list] ELE impact cycles


Hi all -

I believe that Sterling most recently asserted that
the cycle time for the movement of our solar system
through the plane of our galaxy was 64 million years.

Since the observed chaotic cycle of extinctions runs
about 26 million years, or 52 million years for one
complete pass, I am wondering where the 64 million
year number comes from.

Given this discrepancy, there is a distinct
possibility that one or the other of our dating
mechanisms is seriously flawed.

E.P. Grondine
Man and Impact in the Americas



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Received on Thu 20 Sep 2007 04:13:28 PM PDT