[meteorite-list] Meteorite Parent Planet(s)

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From: Matson, Robert <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:28 2004
Message-ID: <AF564D2B9D91D411B9FE00508BF1C86904EE59F0_at_US-Torrance.mail.saic.com>

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Hi Walter and List,

> Reading John Wasson's Disturbing the Solar System made me realize how
often I have
> seen the phrase "the ___'s parent body (where ___ is whatever
classification of meteorite
> one chooses). Wasson and others talk about asteroid parent bodies as if
there was only
> one parent body for the mesosiderites, one for carbonaceous chondrites,
one for the
> pallasites, etc.

(I think you mean Alan Rubin's Disturbing the Solar System).

> I guess I always assumed that particles accreted early on such that any
meteorite
> type formed in a number of planetesimals and what eventually came to be
known
> as ___'s formed in many parent bodies.

I think I understand your question. Let's take H chondrites as an example.
What
you're saying is two-fold:

1. At least one large H-chondrite parent body was at some point involved in
at least
one impact. Some of the resulting fragments from that collision ended up in
orbits
which cross earth's orbit.

2. If there is (or was) more than one original H-chondrite body, and it too
was involved
in an impact that produced fragments with orbits that cross earth's orbit,
then there
could be ambiguity over which meteorites classified as H chondrites
originated with
which parent body.

So one question is, are all the variations that we find in the usual
measurements
of H chondrites (petrology, shock, fayalite %, matrix appearance, etc.)
still within
the ~expected~ range of variation that we could expect to see from a single
parent body? (Related question: if one parent body suffered TWO
collisions,
would the resulting meteorites be easily distinguishable?) But perhaps the
more
interesting question is, if our H-chondrites originated in more than one
parent
body, how could we tell? I suppose one method would be cosmic ray exposure,
or some other "clock-based" technique that can date the specimen age since
collision. However, only a small fraction of meteorites are subjected to
this
kind of scrutiny, and time tags alone wouldn't tell you if two parent bodies
were hit, or one parent body was hit twice.

Another approach to answering the question of single vs. multiple parent
bodies
per meteorite type would be theoretical based on solar system formation
models,
solar system age, dynamics, collision statistics and terrestrial meteorite
lifetime. For example, over the last 4.5 billion years, what are the odds
that
there are (were) two similar asteroids that each suffered impacts that
produced
fragments that ended up in orbits that intersect earth's orbit -- and did so
within,
say, the last 40,000 years? When stated this way, it doesn't seem very
likely.
Even after 4.5 billion years, the fraction of asteroids that have suffered a
collision,
multiplied by the fraction of the resulting fragments that ended up in
orbits that
can intersect earth, multiplied by the fraction of those that DID intersect
earth
within the last 40,000 years can't be very large. But to expect that TWO
such
bodies were hit that had similar bulk compositions, and each delivered
fragments
to earth within the last 40,000 years seems like a pretty big stretch.

No doubt Drs. Rubin and Wasson can provide stronger arguments than
these, and I'll be sure to ask them next time I see them. --Rob

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<DIV>Hi
Walter and List,</DIV>
<DIV> </DIV>
<DIV>> Reading John Wasson's Disturbing
the Solar System made me realize how often I have </DIV>
<DIV>> seen the phrase "the ___'s parent body (where
___ is whatever classification of meteorite </DIV>
<DIV>> one
chooses).  Wasson and others talk about asteroid parent bodies as if there
was only </DIV>
<DIV>> one parent body
for the mesosiderites, one for carbonaceous chondrites, one for the </DIV>
<DIV>> pallasites,
etc. </DIV>
<DIV> </DIV>
<DIV>(I think you mean Alan Rubin's
Disturbing the Solar
System).</DIV>
<DIV> </DIV>
<DIV>> I guess I always assumed that
particles accreted early on such that any meteorite </DIV>
<DIV>> type formed in a number of planetesimals
and what eventually came to be known </DIV>
<DIV>>  as ___'s
formed in many parent bodies. </DIV>
<DIV> </DIV>
<DIV>I think I understand your
question.  Let's take H chondrites as an
example.  What</DIV>
<DIV>you're saying is
two-fold:</DIV>
<DIV> </DIV>
<DIV>1.  At least one large
H-chondrite parent body was at some point involved in at
least</DIV>
<DIV>one impact.  Some of the
resulting fragments from that collision ended up in
orbits</DIV>
<DIV>which cross earth's
orbit.</DIV>
<DIV> </DIV>
<DIV>2.  If there is (or
was) more than one original H-chondrite body, and it too was
involved</DIV>
<DIV>in an impact that produced
fragments with orbits that cross earth's orbit, then
there</DIV>
<DIV>could be ambiguity over which
meteorites classified as H chondrites originated
with</DIV>
<DIV>which parent
body.</DIV>
<DIV> </DIV>
<DIV>So one question is, are all the
variations that we find in the usual
measurements</DIV>
<DIV>of H chondrites (petrology,
shock, fayalite %,
matrix appearance, etc.) still
within</DIV>
<DIV>the ~expected~ range of
variation that we could
expect to see from a
single</DIV>
<DIV>parent body?  (Related
question:  if one parent body suffered TWO
collisions,</DIV>
<DIV>would the resulting meteorites
be easily distinguishable?)  But perhaps the
more</DIV>
<DIV>interesting question is, if our
H-chondrites
originated in more than one
parent</DIV>
<DIV>body, how could we tell?  I
suppose one method
would be cosmic ray exposure,</DIV>
<DIV>or some other
"clock-based"
technique that can date the specimen
age
since</DIV>
<DIV>collision.  However,
only a small fraction
of meteorites are
subjected to
this</DIV>
<DIV>kind of
scrutiny, and time
tags alone wouldn't tell you if two parent
bodies</DIV>
<DIV>were hit, or one parent body was
hit twice.</DIV>
<DIV> </DIV>
<DIV>Another approach to answering the
question of single vs. multiple parent
bodies</DIV>
<DIV>per meteorite type would be
theoretical based on solar system formation
models,</DIV>
<DIV>solar system age,
dynamics, collision
statistics and terrestrial
meteorite</DIV>
<DIV>lifetime. 
For example, over the
last 4.5
billion years, what are
the odds
that</DIV>
<DIV>there
are (were) two
similar asteroids that each
suffered impacts that
produced</DIV>
<DIV>fragments
that ended up in
orbits that intersect earth's orbit -- and did so
within,</DIV>
<DIV>say, the last 40,000 years? 
When stated this way,
it doesn't seem very
likely.</DIV>
<DIV>Even
after 4.5 billion
years, the fraction of asteroids that have suffered a
collision,</DIV>
<DIV>multiplied by the fraction of the
resulting fragments that ended up in orbits
that</DIV>
<DIV>can intersect earth, multiplied by
the fraction of those that DID intersect
earth</DIV>
<DIV>within the last 40,000 years can't
be very large.  But to expect that TWO
such</DIV>
<DIV>bodies were hit that had
similar bulk compositions, and each delivered
fragments</DIV>
<DIV>to earth within the last 40,000
years seems like a pretty big
stretch.</DIV>
<DIV> </DIV>
<DIV>No doubt Drs. Rubin and Wasson can provide stronger
arguments than</DIV>
<DIV>these, and I'll be sure to ask them next time I see
them.  --Rob</DIV>
<DIV> </DIV>
<DIV> </DIV>
<DIV> </DIV></BODY></HTML>

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