[meteorite-list] Asteroids and Meteorites (thread was: Extreme Melting...)

From: Sterling K. Webb <kelly_at_meteoritecentral.com>
Date: Fri Jun 17 17:03:41 2005
Message-ID: <42B33A87.B268BB7F_at_bhil.com>

Hi, Darren, List,

    The notion that the solar nebula was pretty much the same everywhere and that the planets all formed
from the same stuff and varied only in how much volatiles they retained is another dead dog.
    Even in the main belt (from which most our meteorite samples are derived), from 2 to 4 AU, there are
striking compositional differences by zone. 1400 Mars-crossers inside 2.0 AU, 32,000 asteroids between 2.0
and 2.5 AU, 27,000 between 2.5 and 2.8 AU, 22,000 from 2.8 to 3.3 AU, and less than 1000 from 3.3 to 4.2.
But the zones are narrower than that.
    We've always known Vesta and its family were very distinct, so highly differentiated, and now seems
they had magma oceans, too, but the much larger Ceres, asteroid Number One, is a big puzzle box, and almost
certainly never differentiated in any way.
    In theory, further from the Sun, there should be more volatiles, but asteroids beyond 3.2 AU, show a
dramatic drop in albedo. Does not compute.
    Type S spectral class asteroids all cluster close to the inner edge of the Belt, in the 2 AU
neighborhood. The C's all come from beyond 3.2 AU and increase in number as you go further out. Even with
all the evidences of the Belt have been heavily stirred by a Mars sized object at some time, there's still
strong zoning, in other words.
    There is a bewildering variety of spectral (and the newer mineralogical classifications) types in the
asteroid belt, far more types that we can distinguish in our Earthly collections of "escaped" rocks. I
would love to build a collection of Type D asteroid chunks, but none have ever made it to Earth that we
know of (unless Tagish Lake is one, which I doubt). The correlation (60% +) between types and zones is
very strong, and the zones are narrow and sharp, despite the mixing.
    And irons, like they were all one thing? Wasson determined 16 completely distinct compositional
classes (with sub-classes), and still 15% of all irons land in the last class which is: they're unique
unto themselves. Attempts to derive the different types of irons from the differentiation of known
classifications of chondrites (as if they were cores) is a bust. That's zoning more than history, as these
are primitive objects. We love'em anyway.
    Jupiter's Trojans are completely outside the picture, a class unto themselves, and even Mars' Trojans
are oddballs. Did you know Mars had Trojans? I didn't. Google is wonderful. Makes me wonder if somebody
has ever tracked the orbital points 60 degrees ahead and behind the Earth... Wouldn't it be great to have
Trojans of our own?
    The great French mathematician Poincare (sorry, no accent marks) once compared knowledge to a sphere
that touches on the unknown at its boundary everywhere. The bigger the sphere of knowledge becomes, the
more points of the unknown it touches. In other words, the more you know, the less you know. Or, every
answer brings 4 or 5 (or 20) new questions with it. Ignorance is a wonderful thing, else there'd be
nothing more to learn.
    Zoning was probably stronger in the inner solar system than it is out in the asteroid belt. Venus,
with its vast differences from Earth, beyond its obviously distinct history, abounds with compositional
craziness: the mystery of the argon ratios, unknown substances that we know have to be there to account
for certain atmospheric oddities yet cannot be identified, the strange lava with characteristics unlike any
we know of that once flowed further and faster than water, and on and on -- it's a long list.
    Yet, Venus formed in an orbit only 24,000,000 miles away from us. That's next door, more or less.
It's nothing compared to the 100's of millions of miles the asteroid belt covers. Yes, the more we learn,
the stranger it gets. That's zoning.
    Many compositional models say that Mars should have been richer in volatiles than the Earth, and we all
love our warm wet Mars dreams. But then, there's the Fields of Olivines. Wouldn't have taken oceans to
change them, not even lakes, not even rain. No, a few million years of Sahara humidity (and we all know
how humid the Sahara is!), and they wouldn't be there. Near Lunar dryness for billions of years, that what
that says.
    Give me back Barsoom! I want canals! We want those volatiles! So, we search for it with deep
penetration radar (soon!). I worry about what it won't find. Send for a dowser! It's got to be here
somewhere. Meanwhile, you want some dry ice in your drink?
    It's there no water on Mars, we're never going to get to live there, give it an atmosphere, build a
space elevator, let the tired and tempest tossed move in, start planting generically modified pine trees,
settling down and having 4.73 kids. No two human planets for the price of one. No Red Mars, Green Mars,
Blue Mars, for us. Without lots of water, we might as well just go live on the Moon.
    (OK, here's of my side trips: Just give me a few dozen (100?) of those big old hydrogen bombs we all
think we're never going to use (ha!), some big drilling rigs, gangs of space riggers, and we'll drill and
plant the bombs deep enough in the lunar crust not to break through when they go off, vaporize a cubic mile
of lunar rock in each blast chamber, and as the refractories settle, the 40% oxygen content of lunar rock
will jet up the open shaft.
    In a lousy twenty years or less, you can land on the Moon in an airplane, you know, with wings, pop the
hatch as the stewardesses mutter "buh-bye, buh-bye," and get out on the Lunar surface wearing only your
Hawaiian shirt, Bermuda shorts, and cheap Walmart sandals. You'll look just like STEVE in Mexico!
    Anyone who thinks this is crazy, I'll sent you the math on how to achieve a 2.5 psi oxygen atmosphere
on the Moon. Investor inquiries are most welcome. Make that check out to LunAire, Inc. Make it a big
one. Even so, as the price for an uninhabited land mass five times the size of the African continent whose
resources have never been exploited, it's the bargain of the Millennium! Here endeth the side trip.)
    Yes, if all the planetesimals from everywhere in the solar system were blended, "averaged out," they'd
be close to the solar average minus escaped volatiles. All that demonstrates is their common origin --
everything started out together. But the solar system isn't sausage or soup. The interesting things are
the differences that developed, a big puzzle that we are just starting to pick at.
    The average solar composition story is good for folks who are still getting used to the idea that the
Earth isn't the center of Everything. And maybe that's what the simpler meteorite and earth science
websites are trying to get across and who they're talking to. But you can't make all these different
worlds out of undifferentiated solar soup minus selected volatiles.


Sterling K. Webb
-------------------------------------------------------
Darren Garrison wrote:

> On Fri, 17 Jun 2005 01:43:53 -0500, "Sterling K. Webb" <kelly_at_bhil.com> wrote:
>
> > The notion that you can "make an Earth" out of 6000 billion billion tons of chondrites is as
> >thoroughly discredited a notion as I can imagine. This a very old notion (XIXth century) and was once
> >very popular but it's like saying your findings "could mean that the Earth is not flat, but this
> >explanation is unlikely."
>
> I took it that by "chondritic" it was meant that, overall, the Earth was thought to have more or
> less the overall "solar average" ratio of elements (excepting volitiles long drifted away). Since
> at least some chondrites come pretty close to that "solar average", I see how it is so unfair to say
> that there wasn't a good sampling of the overall available building materials in at least some
> classes of chondrites. If chondrites with a close to "solar average" composition are close enough
> to the Earth to be nudgable into an Earth-intercepting orbit, why would it be such a stretch to
> think that the Earth could be made up of chondrites that, overall, averaged out to the "solar
> average"?
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Received on Fri 17 Jun 2005 05:03:03 PM PDT