[meteorite-list] angrites and Mercury

From: mhutson_at_pdx.edu <mhutson_at_meteoritecentral.com>
Date: Sat Dec 24 17:08:36 2005
Message-ID: <1135462113.43adc6e1ba285_at_webmail.pdx.edu>

Hi Larry: As you know, there is some disagreement in the literature over the
impact origin of our moon. However, let's assume that our moon did form by the
giant impact model. What formed was not a largely basaltic object. The basalts
were created at a later date by remelting lunar mantle (creating the maria -
oldest mare basalts are ~3.8 by old). Angrites are basalts--they don't look at
all like lunar highland material, and yet they are among the oldest meteorites
out there (4.55 by old - similar to CAIs). Aslo, assuming the giant impact
model--as I remember in most models, material flew off into space and never
coalesced to form the moon. In only a few models did material re-accrete to
form the moon. One would assume that stripped Mercurian material would form a
Mercurian moon (which should still be there). The other possibility is that
any original surface that would have been stripped off would have been in
fairly small pieces and very near the sun. My understanding is that asteroidal
sized objects in elliptical orbits and in the inner solar system have relatively
short residence times (certainly less than 4.5 by). So unless they somehow
wound up stored in a circular orbit in the asteroid belt, it is unlikely that
any material stripped off of Mercury 4.55 by ago would still exist. The
asteroid belt is chemically structured (E type asteroids in the inner belt and
the "ultracarbonaceous" P & D type asteroids in the outer belt), so it is
unlikely that a lot of material was somehow thrown in at random from the inner
solar system. Also, finally, if you believe any bits of John Lewis' nebular
model for the solar system, Mercury should have formed at temperatures too high
for any significant FeO to have been incorporated into the rocky portion of the
planet (even a missing original crust). I have also heard of Vesta being
proposed as a piece of the original Mercurian crust at a scientific meeting,
and this also seems unlikely. I suspect that the angrite parent body (bodies)
is/are small differentiated asteroids in the asteroid belt that occasionally
deliver material to the Earth. The CRE ages aren't all that different from
those of other stony meteorites in general (including chondrites).

Melinda
Received on Sat 24 Dec 2005 05:08:33 PM PST