[meteorite-list] Moss Meteorite From A Comet?

From: mark ford <markf_at_meteoritecentral.com>
Date: Mon Aug 21 06:13:21 2006
Message-ID: <6CE3EEEFE92F4B4085B0E086B2941B3159F817_at_s-southern01.s-southern.com>

Hi,

Well one thing is certain, the definition of what a comet actually is,
is changing very quickly, what used to be considered 'a dirty snowball'
is now looking more like a snowy dirtball, (with hard bits and soft
bits). 'Comets' seem to have a range of densities and types, so how
would we really know if we had a piece of 'comet' in our
collections?.... besides it being 'primitive material' due to it origin
(CI etc) we would need to have known multiple samples of multiple comets
before we could say for sure. I find it hard to believe that some
fragments of cometry origin don't exist in our collections however.


Best,
Mark Ford

----- Original Message -----
From: bernd.pauli_at_paulinet.de
To: Meteorite-list_at_meteoritecentral.com
Sent: Sunday, August 20, 2006 6:46 AM
Subject: [meteorite-list] Moss Meteorite From A Comet?


Hello Rick and List,

As you are new on this List, I don't really know who I am talking to,
how
old or how young you are, how much you know about meteorites and comets,
if you already have any meteorites, whether you have already read any
books
about meteorites, etc., etc. Maybe you would like to introduce yourself
to
us and tell us a little bit about you. Thank you in advance!

Your question is interesting and intriguing. Theoretically, some
meteorites
may have a cometary origin but so far they have not been found or
recognized
yet.
If there are cometary meteorites in our collections, scientists expect
them
to
have come from the so-called Kuiper belt beyond 30 AU.

Their silicates should be anhydrous, highly unequilibrated, their
chemistry
would
resemble that of chondrites but there would be a high amount of C and N.
But
if
these cometary meteorites were altered through the influence of flowing
water so
far out in our Solar System, the most likely candidates here on Earth
would
be
the CI carbonaceous chondrites.

Some xenolithic inclusions in ordinary chondrite regolith breccias are
also
suspects for a cometary origin.

You will probably have seen a Perseid fireball but no matter what you
saw,
some scientists say that many shower meteors can be as dense as
carbonaceous
chondrites or even as dense as ordinary chondrites.

Especially interesting is the fall of the CI chondrite Revelstoke
because it
could be an example of a weak cometary meteorite. A fireball was
observed
for
hundreds of kilometers and atmospheric effects were measured nearly 1500
km
away. The fireball must have been as energetic as the Sikhote-Alin
meteorite.
The SA fireball produced several craters and tons of meteoritic irons
but
all
that was found of the Revelstoke fireball was less than a gram of
friable
black
rock.

If there are cometary meteorites in our collections,
here are some of the criteria they should meet:

a) as rare as CI carbonaceous chondrites
b) dark + weak
c) highly porous + low density (ca. 2 g/cm3)
d) nearly solar abundances
e) high abundance of C, N, and organic compounds
f) anhydrous silicates
g) highly unequilibrated silicates
h) very large abundance of interstellar grains
i) chondrules and CAIs should be rare or absent

It is so difficult to identify cometary meteorites in case they already
exist
in our collections because they could easily be misclassified as
achondrites.
There are indeed achondrites like the acapulcoites, lodranites,
brachinites,
winonaites that have chondritic chemical abundances, and there are
C-rich
achondrites, for example the ureilites.

And now back to your question: Is the Moss meteorite from a comet?

Let's *suppose* some cometary meteorites do contain chondrules, then
C-rich,
highly unequilibrated CO, CV, or ordinary chondrites might be good
candidates
according to:

CAMPINS H. and SWINDLE T. (1998) Expected characteristics
of cometary meteorites (MAPS 33-6, 1998, pp. 1201-1211).

In other words, in that case even the Moss meteorite - if it should
really
be classified as a CO.x (preferentially "x" should be 1, 2, or 3) -
could
be of cometary parentage.

Hope this helps ;-)


Best regards,

Bernd

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Received on Mon 21 Aug 2006 06:12:49 AM PDT