[meteorite-list] lunar meteorites from the farside

From: Randy Korotev <korotev_at_meteoritecentral.com>
Date: Wed, 25 Jul 2007 15:13:23 -0500
Message-ID: <200707252012.l6PKCpK19897_at_levee.wustl.edu>

At 20:53 24-07-07 Tuesday, you wrote:

>Thank you David. I didn't want to sound so
>skeptical. Remote sensing "suggests" is what I was refering to.
>I definitely want to believe that farside rocks
>are in my possession. Just need some returned specimens for confirmation.



Jerry, and list:

To the best of my knowledge, about half of the
lunar meteorites come from the farside and the
other half from the nearside. (And, with a bow
to Jim Strope, that means that half come from
DARK side and the other half from the not-dark
side.) I have heard no arguments that strongly
challenge the assumption "lunar meteorites come
from randomly distributed locations on the Moon,"
except for the one below. That one is irrelevant
to us in that any point on the Moon is in the
"trailing half" half half of the time. [If I
misunderstand this, please let me know.]

On the basis of dynamical modeling calculations,
Gladman et al. (1995) make the following
statements [with my comments in square brackets]:

"The earth should be uniformly covered with lunar
meteorites [!], and there is no bias for or
against Antarctica on dynamical grounds [!!]..."

"We thus conclude that the discovery location on
the Earth imparts no information as to where on
the Moon a lunar meteorite was
launched. Nevertheless Fig. 4 demonstrates that
objects launched with high velocity (more than
2.6 km/sec [lunar escape velocity is 2.4 km/sec])
have an extreme bias toward coming from the
trailing half of the lunar surface if those
particles are delivered to Earth without ever
escaping geocentric orbit." [Some lunar
meteoroids meteorites go into heliocentric orbit, however.]

"If the 4[pi] CRE [cosmic-ray exposure] of an
object could be shown to be only a few days [it
can't] (so that the meteorite arrived via
"direct" transfer), then one might be able to
deduce more, especially if the ejection velocity
and angle were known... Since such information is
unlikely to be available [correct!], we must
conclude that dynamics can do little to narrow
down the source regions of lunar meteorites."

So, theory doesn't help us.


It has become fashionable, if not expected, in
scientific papers about new lunar meteorites to
speculate about where on the Moon a meteorite is
likely to have originated. I've done it
myself. The truth, however, is that we do not
know with certainty where ANY given lunar
meteorite comes from. All of the statement in
the literature by geochemists and geologists are
based on a few observations and some assumptions:

1) The high-thorium region of the Moon (the
"PKT") is on the nearside. There are no high-Th
regions on the farside, although the SPA (South
Pole-Aiken) region is a bit enriched in
Th. Therefore, if a lunar meteorite has high
concentrations of Th, then it most likely comes
from the nearside. I'd say this assumption has a
99+% chance of being correct for SaU 169 and NWA
4472/4485. Gnos et al. (2004) speculate about
the exact crater for SaU 169, but it is just
speculation. We have studied samples from the
Apollo 12 missions that are indistinguishable
from SaU 169, and that site is 450 km from the
crater advocated by Gnos et al. (They still might
be right, however.) I used to think that
Calcalong Creek originated from the PKT, too, but
now that we've obtained our own analysis of
Calcalong Creek (which is not the focus of
abstract, below), we note that it and Dhofar 961
have some "funny" geochemical characteristics
that suggest to me that they ARE NOT from
anywhere near the Apollo sites, which were all on
the nearside. That makes it more likely (but
hard to quantify) that they come from the SPA
area and, therefore, the farside.

http://www.lpi.usra.edu/meetings/metsoc2007/pdf/5257.pdf

2) There are more maria (the dark "eyes or
"seas") on the nearside, and the maria are rich
in iron because the maria consist of basalt. So,
a given basaltic lunar meteorite is most likely
to come from the nearside than the
farside. However, there are also maria on the
farside, so maybe one or two of the basaltic
meteorites originates from the farside.

The first lunar meteorite, ALHA 81005, was low in
iron and thorium. That led me (for example!) to
state "The low LIL [large-ion-lithophile, like
thorium] element concentrations of 81005 are
consistent with an origin distant from the
KREEP-rich Imbrium-Procellarum region [PKT,
Apollo sites], possibly on the lunar farside"
(Korotev et al., 1983). Others said the same
thing (it wasn't really a profound observation)
and have continued to say so about new lunar
meteorites because the statement sounds
neat. Now, about half the lunar meteorites are,
in fact, feldspathic and low in iron and thorium.

http://meteorites.wustl.edu/lunar/chemclass/chemclass.htm

Because a larger fraction of the farside real
estate is low in iron and thorium, more than half
of the feldspathic lunar meteorites probably DO
come from the farside. We just don't know which ones.

Recently, some colleagues have been claiming that
Dhofar 489 (and its many pairs) IS from the
farside. (David Weir listed some abstracts
yesterday.) In my opinion, their logic is faulty
(Korotev et al., 1996). Dhofar 489 COULD come
from the farside, but the probability is no more
likely than that for any other feldspathic lunar meteorite.

We have good reason to believe that the surface
of the Moon is contaminated to varying degrees by
thorium that has been redistributed from the PKT
by 3.9 billion years of small impacts. After
all, if a small impact can put a Moon rock on
Earth, it can also drop a rock anywhere on the
lunar surface. So, if a lunar regolith (soil)
breccia, all of which are made from near-surface
materials, has a low concentration of Th, then I
agree that it probably comes from a point very
distant from the PKT, probably the farside. But,
Dhofar 489 et al. is an impact-melt breccia,
which was probably formed by a large impact that
melted material mainly beneath the regolith. The
material of Dhofar 489 was below the zone of
impact mixing. A kilometer or so beneath any
point in the feldspathic highlands, iron and
thorium are low. So, a low-Th impact-melt
breccia could come from the nearside or the
farside. In fact, low-Th impact-melt breccias
were found at the Apollo 16 site. So, in the
particular cases of Dhofar 489 (and NWA 482,
another melt breccia) the low iron and thorium
concentrations are not strong arguments in favor farside origin.

So, yes, "Remote sensing 'suggests'..." and, if
you have several feldspathic lunar meteorites,
particularly if you have a regolith breccia with
<0.3 ppm Th, there's a real good chance that you have a farside rock.

Randy Korotev



References

Gladman B. J., Burns J. A., Duncan M. J., Levison
H. F. (1995) The dynamical evolution of lunar
impact ejecta. Icarus, v. 118, p. 302-321.

Gnos E., Hofmann B. A., Al-Kathiri A., Lorenzetti
S., Eugster O., Whitehouse M. J., Villa I., Jull
A. J. T., Eikenberg J., Spettel B., Kr?henb?hl
U., Franchi I. A., and Greenwood G. C. (2004)
Pinpointing the source of a lunar meteorite:
Implications for the evolution of the Moon. Science 305, 657-659.

Korotev R. L., Lindstrom M. M., Lindstrom D. J.,
and Haskin L. A. (1983) Antarctic meteorite
ALHA81005 - Not just another lunar anorthositic
norite. Geophysical Research Letters 10, 829-832.

Korotev R. L., Zeigler R. A., and Jolliff B. L.
(2006) Feldspathic lunar meteorites Pecora
Escarpment 02007 and Dhofar 489: Contamination of
the surface of the lunar highlands by post-basin
impacts. Geochimica et Cosmochimica Acta 70, 5935-5956.




~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+~+
Randy L. Korotev phone: (314) 935-5637
Research Associate Professor fax: (314) 935-7361
Washington University in Saint Louis korotev at wustl.edu
Department of Earth & Planetary Sciences http://epsc.wustl.edu/

Mailing addresses
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   Washington University Washington University
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Everything you need to know about lunar meteorites:
http://meteorites.wustl.edu/lunar/moon_meteorites.htm
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