[meteorite-list] Earthite parent body

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Wed, 1 Aug 2007 00:10:55 -0500
Message-ID: <004a01c7d3fa$57b62cb0$ac2ee146_at_ATARIENGINE>

Hi, Darren, List,

    Spectacular views of the "parent body"! Of course,
when a super-composite view is being constructed, the
point of view is arbitrary. You just pick a viewing angle
from which the planet looks good and contains lots of
recognizable and familiar features without having too
much cloud cover in the way.

    Idly wondering what these pictures would look like
if constructed from different points of view, I realized that
a lot of us have just such a tool: Google Earth. So, instead
of zooming into some particular feature, I pulled back the
view to about 8000 miles up and started rolling the planet
back and forth and looking at it from all sides.

    I was mildly startled when, at one point, all the land
vanished and a completely water covered planet emerged!
(It was like the movie "Waterworld," only without the bad
acting.) Oh, you could see tiny slivers of coastlines around
the edges, but it was an entirely oceamic planet that you
were looking at! A Water Hemisphere.

    Right in the center of the Water Hemisphere (it turned out)
was Tahiti. Just go to Google Earth, select Tahiti and when
it centers and levels, back up 7500-8000 miles. The Earth
has two hemispheres, not East vs. West nor North vs. South,
but Land vs. Water.

    What I didn't realize was that "my discovery" was just
a century or more late...
http://en.wikipedia.org/wiki/Land_hemisphere
http://en.wikipedia.org/wiki/Water_hemisphere
since I can find old references to it up to as long as a
century ago. Anyone know who first noticed it?

    Fixing the location of the "Land Pole" and the "Water
Pole" turns out to be complicated, if the goal is define two
hemispheres with the most land and water. The land pole
is somewhere between Nantes, France and the mouth of
the Thames, and the water pole is off New Zealand. (But
Tahiti is still the most isolated piece of land surrounded
by water, though.)

    The Water Hemisphere is 92% water and only 8%
land (Australia, New Zealand, and Antarctica). The Land
Hemisphere is 50% water and 50% land (because there's
more water than land). Put another way, 86% of the planet's
land is in the Land Hemisphere.

    That's all very cool, but does it have any real physical
significance? (Other than the fact that Tahiti has solar tides
instead of lunar tides?)

    The land pole is not that far from the actual north pole.
That means that much of the planet's land is in the north
rotational hemisphere, a very unstable dynamic. Land
"masses" are just that: masses raised above the mean
rotational surface, and those masses exert a torque on
the orientation of the axis of rotation, which makes the
Earth "wobble."

    A rotating sphere will orient itself with any excess and
out-of-balance mass distributed as close to its equator as it
can get it. Put the excess mass at or near the poles and its axis
will "roll" over until the out-of-balance masses are at its "new"
equator, if the torque is great enough to push the entire planet.
It isn't strong enough but sometimes it can move the crust,
if it is slippery.

    This happened to the Earth between 534 and 518 million
years ago, when the poles shifted by 89 degrees, pole to
equator, equator to pole! This is a very confusing issue.

    What's being talked about here is what's called True
Polar Wander (TPW), a shift of the crust with respect
to the rotating body of the planet. TPW was an old topic
in geology that got thrown out when Continental Drift
was accepted and we realized that much of what we
thought was TPW was only Apparent Polar Wander
(APW), the changing orientation of continents to poles
as the continents moved slowly around on the surface
of the Earth and even passed over the poles.

    With each continent following its own path on the
Earth, the APW of each was different, but there are
episodes when the APW of ALL the continents are
the same. This can only happen if all the continents
move together, i.e., the crust moves as a single unit
and slides just like it does in continental drift movement,
but all in one piece, a pretty amazing event. That's
True Polar Wander. The Cambrian episode was a lot
faster than continental drift, an astounding 1.5 mm
a day. Now, they've discovered another episode 800
million years ago and a smaller, slower episode 295-205
million years ago. We do have slippery crust, apparently.

    What's equally surprising is how many prestigious
univeristy press offices involved with the discoverers
of the Cambrian TPW put out stories and press releases
that said "the Earth rolled over and spun on its side."
That's what Uranus does, not the Earth. The orientation
of the rotational axis of the planet with respect to the
plane of the solar system didn't change one bit. (One of
the things that makes it confusing is that the geologists
have decided to use a Ptolemaic Crust-O-Centric
coordinate system which makes it look like the crust
stands still and the planet (and solar system) flop and
gyrate around the crust -- good goin', guys!)

    In case I've got you worried about your crust
slipping, relax. Although the Earth would wobble
less if the North Pole were in the North Pacific just
south of the Aleutians and the South Pole was in
Atlantic south of South Africa (the optimal mass
distribution of the continents we've got), there's
just not enough out-of-balance torque to do the
job now.

    Good thing, most likely.



Sterling K. Webb
--------------------------------------------------------------------------------
----- Original Message -----
From: "Darren Garrison" <cynapse at charter.net>
To: <meteorite-list at meteoritecentral.com>
Sent: Tuesday, July 31, 2007 11:45 AM
Subject: [meteorite-list] Earthite parent body


http://www.dailymail.co.uk/pages/live/articles/news/worldnews.html?in_article_id=472122&in_page_id=1811
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Received on Wed 01 Aug 2007 01:10:55 AM PDT