From: dietz@cs.rochester.edu (Paul Dietz)
Subject: Re: Ten embarrassed questions about the moon (very long)
Organization: Computer Science Department University of Rochester
Distribution: sci
Date: Sat, 7 Nov 1992 23:02:31 GMT

In article <BxD5Hz.Ewt.1@cs.cmu.edu> roberts@cmr.ncsl.nist.gov (John Roberts) writes:

 (snarfy wrote:) 

>-   into  the  surface  of  these dark, plain - like areas. Soil samples weer
>-   loaded with rare metals and elements like titanium,  zirconium,  yttruim,
>-   and  berylium. How the moon could have been formed by some random process
>-   with  such  high  concentrations  of  rare  elements   has   never   been
>-   satisfactorily explained.
>
>For chemical analysis, you're going to have to read some actual scientific
>papers (it won't show up in Time Magazine).

I suggest snarfy go read some books on geological processes.
In particular, read some books on how elements are segregated
due to chemical differences.  Also, he should go look up
how many different and interesting minerals have been found
-- *including* minerals high in titanium, beryllium, etc. etc.


>-   never been observed  on  the  moon)  that  the  high  readings  could  be
>-   explained   by  highly  radioactive  elements  just  under  the  surface.
>-   Actually, the amount of radioactive materials on the SURFACE of the  moon
>-   is  "embarrassingly  high"  .   Where  did  all  of this hot ,radioactive
>-   material ( uranium and thorium ) come from ? And  if  it  came  from  the
>-   interior  of  the  moon  (very  unlikely)  ,how  did it get to the moon's
>-   surface?
>
>
> The total lunar abundance of uranium is thought to be about 60 ppb,
> and thorium about 230 ppb. The only figure I could find for the Earth
> was 2.7 ppb in the crust. One might explain the difference by greater
> gravitational differentiation in the Earth, since it has greater
> gravity, and spent a much longer period of time molten. In other
> words, the Earth might have the same overall concentration, it's just
> more concentrated in the interior.

The figure you have for terrestrial uranium is wrong: uranium is
about 1000x more abundant than that in the earth's crust.

Both uranium and thorium are lithophiles.  That is, they do
*not* go to the molten metal core.  Note that on the molecular
level this differentiation has *nothing* to do with density;
it's driven by chemistry.  Uranium is enriched in granites
relative to basalts, btw (and granites, which make up most
of the mass of continents, are lighter than basalts).

Since the entire moon has been subjected to melting and
differentiation, it should not be surprising that elements
have been segregated, or that uranium and thorium are abundant
in the more felsic rocks of the lunar crust.  Moreover,
if the moon is derived from outer mantle and crustal material
from the earth that has been blown into space by a megaimpact,
it started out enriched in lithophiles.

BTW: snarfy mentioned the "capture" theory of the moon.  One thing
that destroys this idea is that the earth and the moon lie on the same
line in oxygen isotope plots (plot delta-O18/O16 vs. delta-O17/O16.)
This means they must come from a common source.  The other three
theories (fission, coaccretion and giant impact) get around this; the
first two of these are shot down for other reasons.

	Paul F. Dietz
	dietz@cs.rochester.edu
Newsgroups: sci.space