[meteorite-list] Mesosiderites= Vestan?

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri Sep 22 12:28:47 2006
Message-ID: <000801c6de64$2aa462f0$755ce146_at_ATARIENGINE>

Hi,

    Identifying the parent bodies of meteorite types has been
disappointing. Early optimism suggested that if could just
get enough spectra of enough bodies, we could match most
everything up and untangle a lot of early solar system history.
Didn't work out that way. Matches were few.

    That was a puzzle until the discovery of "space weathering."
The surface of asteroids are altered by long exposure and no
longer always resemble in spectra the meteorites that might
have come from them.

    There some fairly certain matches: Vesta (and the Vestoids)
for the Howardites, Eucrites and Diogenites, Mars and the Moon
(of course), and a batch of tentative argued-over proposed
matches...

    The biggest mystery is the parent body of the L types.
We have one good match for H chondrites (and the IIE
irons), 6 Hebe (~200 km), but can it alone account for 40% of
all the meteorites that fall to Earth? These folks say "yes."
http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1998M%26PS...33.1281G&db_key=AST&data_type=HTML&format=&high=438c93072f28336

    16 Psyche (254 km) has been suggested for the mesosiderite
parent body, but it's far from certain.

Mesosiderites:
    "The silicates are heavily-brecciated (smashed-up), evolved
igneous rocks similar to those found in members of the HED
group. Evidently, these silicates came from the crust of an
achondritic parent body. The iron-nickel in mesosiderites,
on the other hand, looks like the metal in group IIIAB iron
meteorites, and shows every sign of having derived from
the core of a completely different asteroid than that which
spawned the silicates. One possible explanation of the origin
of mesosiderites is that a collision took place between two
differentiated asteroids in which the still-liquid core of one
asteroid mixed with the solidified crust of the other.
Subsequently, at least one of the asteroids reassembled
from the collision fragments and became the mesosiderite
parent body. It remains uncertain whether the HED parent
body, Vesta, is one of the asteroids involved."

Pallasites:
    "Pallasites are believed to have come from the core/mantle
boundary of differentiated asteroids that were broken apart
by impact. In most cases, they have chemical, elemental,
and isotopic features that link them to specific chemical
groups of iron meteorites, suggesting that they come from
the same parent bodies as these irons."
    While we type iron meteorite into gross classes, the
complicated distribution of trace metals shows that iron
meteorites can be grouped into 80-some parent bodies
(with a dozen or so unique irons making almost 100). So,
there were originally 100 (or hundreds) of differentiated
parent bodies of which they were the cores.

    Imagine that we were the recipients of 50,000 samples
returned from Venus, but that the collectors didn't tag them,
didn't record their point of origin on the planet, mixed the
entire batch in a giant rock tumbler, fried their exteriors with
hyperblowtorches, and said, "Hey, you sort it out!" Could
we deduce the geological history of Venus from that? We
would be long on generalizations and short on specifics,
a situation similar to trying to identify meteorites' parent
bodies.

    There's no substitute for going there.


Sterling K. Webb
-------------------------------------------------
----- Original Message -----
From: "Rob McCafferty" <rob_mccafferty_at_yahoo.com>
To: <cynapse_at_charter.net>; <meteorite-list@meteoritecentral.com>
Sent: Friday, September 22, 2006 8:39 AM
Subject: Re: [meteorite-list] Mesosiderites= Vestan?


>> Main-group
>> pallasites represent intermixed core-mantle material
>> from a single disrupted
>> asteroid and have no known equivalents among the
>> basaltic meteorites.
>
> Is that right? They all come from the same parent?
>
> What are the other groups of pallasite. Sorry, my
> knowledge on these rocks is thin. (Can't afford to buy
> them so I only read up the basics)
>
> Rob McC
>
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Received on Fri 22 Sep 2006 12:28:40 PM PDT