At 00:54 26-01-10 Tuesday, you wrote:
Randy, why did you write that there is no scientific evidence that
any particular lunar meteorite originates from the lunar farside?
Dear Walter and list:
We don't know exactly where on the Moon any lunar meteorite comes
from. It has, nevertheless, become fashionable, if not obligatory,
for lunar meteorite scientists to speculate where a new lunar
meteorite might come from in a regional sense when they write papers
about them. I've done it myself.
The people who understand the dynamics of these things tell me that
the chance of having a rock achieve escape velocity from the farside
is the same as from the nearside. Any rock that leaves the Moon has
the same chance of landing on Earth. Some of this is discussed in a
recent (and much too long) paper:
http://epsc.wustl.edu/~rlk/papers/korotev_et_al_2009_m&ps_intermediate_iron.pdf
To me this all means that half the lunar meteorites must come from
the farside, we just don't know which ones.
What we do know is that NWA 482 is highly feldspathic (~80%
plagioclase) and poor in radioactive elements like Th (thorium). We
know from orbital measurements that a larger fraction of the surface
material on the farside is feldspathic and low in Th than for the
nearside. The nearside has more basalts and most of the Th-rich
stuff. So, on the basis of chemical composition, NWA 482 has a >50%
chance of being from the farside. But, the same argument applies to
the other 32 feldspathic lunar meteorites. Surely, some feldspathic
lunar meteorites come from the nearside. NWA 4936/5406, for example,
is very similar in composition to soil from the Apollo 16 site on the nearside.
The corresponding argument is that most of the basaltic (lun-b)
meteorites come from the nearside because most of the mare basalts
are exposed on the nearside. We also can say that Th-rich
meteorites like SaU 169 and Dhofar 1442 must come the anomalously
Th-rich region in the northwest quadrant of the nearside known as the
Procellarum KREEP Terrane. But again, the source crater for none of
the lunar meteorites has been established with certainty. An impact
making a 1-km-crater can launch a lunar meteorite.
Randy Korotev
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