[meteorite-list] Cyclic mass extinction?

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Thu Apr 22 10:00:11 2004
Message-ID: <3D446822.D3B51FB9_at_bhil.com>

Hi,

    Robert's post raised some questions about the Sun's "orbit" around the
Galaxy (see quoted passage below). The term "orbit" has an association with a
precisely described motion for most of us, but the Sun's Galactic "orbit" is
more like a drunkard's walk (that's actually a math term; I'm not slandering
drunks).
    We can identify two components of the Sun's Galactic motion: 1) an
eccentric motion (nearer and furtherer from the Galactic Center) with a
period of 200 to 280 million years (high uncertainty), and 2) a "bobbing up
and down" motion with respect to the Galactic Plane, crossing through the
Galactic Plane every 28 to 32 million years.
    In the kind of precise orbit we're used to (a rock around the Sun),
there's very little "bobbing," but the Sun rises and sinks several thousand
light years to get above or below the Galactic Plane. As if that weren't bad
enough, there is a "Galactic Tide" exerted both at Plane passage and when the
Sun achieves its furthest excursion from the Galactic Plane, or every 14 to
16 million years. Normally, tides are not an important force in Solar System
object orbits, but the strength of these tides is on the same magnitude as
the gravitational bond on distant Solar System objects, like comets in the
Oort Cloud, and probably disturbs their orbits enough to cause some of them
to fall our way.
    Long before the importance of impacts on the Earth's history was
recognized, many geologists had reported cycles in Earth history of both
roughly 30 and 250 million year intervals, cycles of major sea level rise and
fall (+/- 150 meters), mountain building, vulcanism, climate (icehouse vs.
greenhouse), mass extinctions, geomagnetic reversal frequency, and so forth.
Holmes in 1927, Fisher and Arthur in 1977, etc. These observations were not
so much disputed as ignored because no one could imagine any mechanism for
them.
    Passage through the Galactic Plane is NOT a non-event. First of all, it
matters greatly whether you happen to be plunging through the Galactic Plane
when there's a spiral arm in the way. Spiral arms are the densest parts of
the outer Galaxy, with lots of star traffic. Additionally, they are full of
Giant Molecular Clouds of gas, dust and large young stars. A typical such
cloud has the mass of 500,000 Suns. It's just not a nice neighborhood if you
like peace and quiet.
    Currently, the Sun is far, far from the Galactic Center, near the
Galactic Plane, but just outside of a large spiral arm (the Orion arm) that
we just exited 6 to 9 million years ago. (The Sun spends about half its time
in and around spiral arms.) We probably spent 50 million years bobbing
through it. We seem to be in a bubble composed of the remains of a molecular
cloud that disintegrated in a burst of star formation about 30 million years
ago. This is a real quiet neighborhood, and that's probably a good thing.
    Personally, I think that passing in "orbit" nearer to the Galactic Center
is probably a much more traumatic event than bobbing around through the
Galactic Plane. The conditions nearer the Galactic Center are horrendous to
contemplate. Star densities are much, much higher; it's really crowded! The
energetic environment is violent; typical velocities are much greater. I can
can't think of anything good to say about the Galactic Downtown.
    Not only is the Sun's Galactic orbit a semi-random walk, but the same is
true for all other stars; each has a different semi-random orbit of its own.
The nearby stars all show varying degrees of "proper motion," meaning they
are all headed somewhere else, not co-travelling with the Sun. A few are
racing across the traffic lanes at high velocities and are probably stars
from the Galactic halo temporarily transiting the Galactic Plane at near
right angles. The configuration of nearby stars is constantly shifting on
long time scales because of these movements. A million years from now Alpha
Centauri won't be the nearest star
    The mechanism that accounts for all the cycles mentioned above is almost
certainly terrestrial impacts on a periodically intense basis. Objects
typically take 2 to 5 million years to fall from the Oort Cloud of comets
into the inner Solar System and become trapped in short-period Earth-crossing
orbits. Then you've got a bunch of nasty, large (40 to 300 kilometer),
disintegrating objects rubbling up the Earth's neighborhood and generally
being a big pain in the... planet.
    We have such a big nasty object trapped in a temporary orbit between
Saturn and Uranus right now: Chiron (250 kilometers). I call it a "temporary"
orbit because its orbit is statistically unstable with little chance of
persisting for more than 50 or 100 thousand years without an accident;
regression shows it almost hit Uranus in the 1600's. No chance that Chiron
has been where it is for the life of the Solar System, hence it's likely an
interloper from Kuiper or Oort country.
    Just think of what life would be like with something that size zipping
around in the inner Solar System! Even a minor chunk from the breakdown of
such an object would be more than enough to explain any mass extinction. But
there wouldn't be just one chunk from such a breakdown of a large object;
there would be many. Clusters (or "showers") of impacts could be expected.
    Look at E. L. Jones' post on the statistics of impacts: "49 of the 121
(40%) impacts were statistically simultaneous--that is are dated at less than
100,000 between successive impacts. aka "0." 100,000 years is the time scale
for the breakup of a large object in the inner Solar System and for the
incidence of impact "showers." I also draw your attention to his confirmation
of the ~15 million year periodicity of the Galactic tide: "68% of the time
impacts are just under 15 my from each other." I call that a confirmation
because I don't think he set out to prove the influence of the Galactic tidal
force.
    Well, that's my two cents worth. This is what happens when it's 101
degrees and you have air conditioning cabin fever.


Sterling K. Webb
-----------------------------------------------------------------------------------

"Matson, Robert" wrote:

> Others have theorized that the sun's orbit around the galactic
> center causes it to periodically pass through the galactic plane
> (presumably twice per orbit -- one ascending node, one descending
> node), and that this plane passage somehow perturbs comets in
> the Oort cloud leading to increased chaos and impacts in the
> inner solar system. But I see two problems with this theory,
> which perhaps someone out there can help me with. One is that
> this "plane passage" is really a non-event; the local stellar
> density really doesn't oscillate that much since the galactic
> plane is not really a "plane" in the geometric sense. It has
> considerable "thickness" -- on the order of 2000 light-years.
> The sun is currently only ~50 light-years north of the galactic
> "plane", so the local stellar density is about as thick as it
> gets. Unless the sun's orbit takes it far above and below
> the plane (say > +/- 500 light-years), it doesn't seem like
> the density is going to oscillate much.
>
> I considered that it might not be the plane crossings of a
> slightly inclined solar orbit that produces the disruptions,
> but the periapsis of a slightly eccentric orbit that takes the
> sun closer to the denser galactic center once per orbit. The
> problem with this is that the sun's orbit around the galactic
> center is almost perfectly circular.
>
> But the second (and larger) problem is that even if there is a
> galactic "carousel" effect, the period is wrong. The sun orbits
> the galactic plane about once every 225-250 million years, so
> plane crossings would occur every 112-125 million years (and
> periapsis once every 225-250 million years). Not a good match
> to the much shorter extinction cycle.
>
> What about the spiral arms? Passage through these, I think,
> would have a much more disruptive effect. But from what I've
> read, this occurs very rarely since the sun is at a distance
> from the galactic center that has an orbital period very close
> to that of the spiral arms.
Received on Sun 28 Jul 2002 05:54:43 PM PDT