[meteorite-list] Meteorites on the moon
From: mark ford <markf_at_meteoritecentral.com>
Date: Fri Sep 8 19:19:32 2006
Presumably then a similar story is true for Martian meteorites? Any
trace of chondritic material in Martians?
[mailto:meteorite-list-bounces_at_meteoritecentral.com] On Behalf Of Randy
Sent: 01 September 2006 23:28
Subject: [meteorite-list] Meteorites on the moon
Regarding meteorites on the Moon...
There is a great deal of "meteoritic matter" on the Moon, but very few
meteorites. The two miniscule fragments that Martin Altmann mentioned
are the best known ones from the Apollo collection, but I'm aware of 2
others even smaller.
Virtually all meteoroids that strike the Moon either melt or vaporize on
impact. If they melt, they mix with the melted silicates of the lunar
target rocks. So either way, they become unidentifiable as meteorites.
Lunar impact-melt rocks and breccias do contain blebs of meteoritic
metal - metal the melted during the impact but as a liquid was
immiscible with the molten silicates.
All lunar soils and breccias contain meteoritic material. In any
handful of lunar soil, 1-4% of the mass is "extralunar" stuff. Except
for blebs of metal, most of which were melted and resolidified,
"meteorites" are virtually absent, however. In the lunar soil, most of
the meteoritic material arrives as micrometeorites. By one estimate,
approximately 80 grams per square kilometer of micrometeoroids accrete
to the Moon (and Earth's atmosphere) each year.
We know the meteoritic material (melted and mixed, recondensed from
vapor) exists in lunar regolith (soil) and breccias because both are
loaded with "siderophile" (iron-loving) elements like iridium, gold, and
platinum in ratios characteristic of chondrites. In contrast, the
unbrecciated igneous rocks of the lunar crust - the basalts and
anorthosites - have almost immeasurably low concentrations of these
elements, as do igneous rocks on Earth.
So, every one of the lunar meteorite that is a breccia (which is nearly
all of them) contains "regular" meteoritic material. Those lunar
meteorites that are regolith breccias (like NWA 3136 that Adam Hupe
mentioned) tend to contain the most. Those that are impact-melt
breccias tend to contain the least, judged on the basis of
concentrations or, say, iridium.
Here's a quote from a paper I've submitted on PCA 02007, a lunar
meteorite regolith breccia with a high proportion of chondritic
"The mean Ir concentration of PCA 02007 is equivalent to a component of
2.7% ordinary chondrite or 2.8% CM chondrite. This means that 14% of the
Fe and 9% of the Mg and Cr in PCA 02007 derive from extralunar sources
(Figs. 8, 9). Day et al. (2006) report an actual meteorite fragment in
their thin section of PCA 02007."
To my knowledge, the chondrite fragment in a lunar meteorite reported by
Day et al. is a first.
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Received on Thu 07 Sep 2006 10:15:32 AM PDT