[meteorite-list] NWA 482

[Dennis Beatty]

Is there any reason to believe that one side might be more prone to  
impacts than the other??

Dennis Beatty

On Jan 26, 2010, at 11:14 AM, Randy Korotev wrote:

> 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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