[meteorite-list] angrites unlikely to be Mercurian

From: Adam Hupe <raremeteorites_at_meteoritecentral.com>
Date: Fri Dec 23 23:35:45 2005
Message-ID: <062a01c60841$14851400$6401a8c0_at_c1720188a>

Dear List,

In regards to this post, here are a few items that I feel are open for
debate;

I think albedo measurements for large bodies in space are misleading. The
Martian albedo, for instance, mainly fails into the red spectrum indicating
very oxidized iron-rich material. If the Martian albedo was used to match
meteorites with their parent bodies we would have no meteorites from Mars
using this qualifying technique. Large bodies are coated with a regolith
that differs from the material at deeper levels so, then again, this is a
poor way determine composition of most very large objects. Its like trying
to look inside a brown paper bag by looking at just the color of the bag
itself, doesn't make any sense to me. Crater counting was also proven to be
misleading. Mercury has been hit so many times that traces of early impacts
have all been wiped out. What good is crater counting when you reach the
saturation point? In my opinion, Mercury should have been among the first
planets to solidify because of its very high temperature refractory
components and relatively small size. This would argue in favor for the age
of Angrites. There is a lot open for debate but I feel that by sometimes
thinking outside of the box that new discoveries will be at hand.


Take Care,

Adam



----- Original Message -----
From: <mhutson_at_pdx.edu>
To: <meteorite-list_at_meteoritecentral.com>
Sent: Friday, December 23, 2005 7:52 PM
Subject: [meteorite-list] angrites unlikely to be Mercurian


>
> While I found Tony Irving's abstract ("Unique Angrite NWA 2999: The Case
for
> Samples from Mercury") to be interesting, I find it unlikely that angrites
> could be samples of Mercury. While it is remotely possible that they are
> remnants of a crust stripped off 4.5 billion years ago, they would have to
have
> been floating in space for the last 4.5 billion years or so (inconsistent
with
> Cosmic Ray Exposure Data). Any samples derived from the current Mercurian
> surface should be consistent with the following: crystallization age
<4.55 by
> (possibly much less). Many places on Mercury have crater counts
intermediate
> to the lunar highlands and the lunar maria, so I would suspect the
surfaces to
> be in the 4.3-3.8 by old range. The Mercurian albedo (way it reflects
light)
> is similar to that of the lunar highlands or somewhat brighter, but redder
> (implying the possibility of a larger amount of glass or agglutinates).
One
> thing that is very clear is that rocks on the surface appear to be iron
poor
> (pyroxenes < 5% FeO), inconsistent with the relatively iron rich olivines
and
> pyroxenes in angrites. There was also a poster/abstract by Ann Sprague
and
> co-workers a couple of years back suggesting that the Mercurian spectrum
was
> consistent with Fe-poor clinopyroxene and an intermediate composition
feldspar
> (aproximately a labradorite containing both Na and Ca), also inconsistent
with
> NWA 2999 where the feldspar is pure anorthite (Ca only). Another paper in
JGR
> by Cooper and coworkers suggested that some areas on Mercury looked mafic,
but
> again with very low Fe contents in minerals. So if not from Mercury,
where do
> angrites come from? Given their old crystallization ages, I'd say
probably from
> a differentiated asteroid. We have plenty of evidence that many asteroids
> differentiated (look at all the different kinds of irons). If it heated
> quickly, an asteroid wouldn't have to be that big to differentiate. We
could
> easily have overlooked a small asteroid.
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Received on Fri 23 Dec 2005 11:17:19 PM PST