[meteorite-list] NWA 482

From: Matson, Robert D. <ROBERT.D.MATSON_at_meteoritecentral.com>
Date: Tue, 26 Jan 2010 11:49:11 -0800
Message-ID: <7C640E28081AEE4B952F008D1E913F1702A2DDFC_at_0461-its-exmb04.us.saic.com>

Hi Dennis,

I'd expect a tiny variance (greater number of impacts on lunar farside)
due
to shadowing by the earth. But the difference is probably unmeasurable.
The fraction of the celestial hemisphere blocked by the earth as seen
from lunar nearside is only 0.0046 %. --Rob

-----Original Message-----
From: meteorite-list-bounces at meteoritecentral.com
[mailto:meteorite-list-bounces at meteoritecentral.com] On Behalf Of Dennis
Beatty
Sent: Tuesday, January 26, 2010 11:38 AM
To: Randy Korotev
Cc: meteorite-list at meteoritecentral.com
Subject: Re: [meteorite-list] NWA 482


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
Received on Tue 26 Jan 2010 02:49:11 PM PST