[meteorite-list] lunar meteorites from the farside

Wed Jul 25 16:13:23 EDT 2007

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., Krähenbühl 
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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   Randy Korotev                     Randy Korotev
   Washington University             Washington University
   1 Brookings Dr                    Earth & Planetary Sciences
   Campus Box 1169                   E&PS Bldg, Room 110
   Saint Louis MO 63130-4899         Saint Louis MO 63130

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