[meteorite-list] lunar meteorites from the farside

From: Jerry <grf2_at_meteoritecentral.com>
Date: Thu, 26 Jul 2007 20:12:04 -0400
Message-ID: <4B7F642E7F9241B1A5E61A2169857A9B_at_Notebook>

Randy, thank you very much for taking the time to provide your detailed analysis of the evidence at hand.
You,ve provided me and all who read your email with substantial evidence to convince us that your conclusions are based on a myriad of experimental data.

Jerry Flaherty
  ----- Original Message -----
  From: Randy Korotev
  To: meteorite-list at meteoritecentral.com
  Sent: Wednesday, July 25, 2007 4:13 PM
  Subject: Re: [meteorite-list] lunar meteorites from the farside


  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 (a
nd 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
  postal service: commercial:
    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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