[extropy-chat] Water Marks the Asthenosphere

[Keith Henson]

At 07:43 AM 1/26/2007 -0500, Dan wrote:
>On Friday, January 26, 2007 4:57 AM Eugen Leitl eugen at leitl.org wrote:


> >>What about Luna?
> >
> > Most of it was baked out during formation,
>
>Yes, that might be so, but what if it's not?  Is there enough data on
>the lunar interior to know?

http://en.wikipedia.org/wiki/Moon

The Moon is a differentiated body, being composed of a geochemically 
distinct crust, mantle, and core. This structure is believed to have 
resulted from the fractional crystallization of a magma ocean shortly after 
its formation about 4.5 billion years ago. The energy required to melt the 
outer portion of the Moon is commonly attributed to a giant impact event 
that is postulated to have formed the Earth-Moon system, and the subsequent 
reaccretion of material in Earth orbit.

***

Today, the giant impact hypothesis for forming the Earth-Moon system is 
widely accepted by the scientific community. In this theory, the impact of 
a Mars-sized body (which has been referred to as Theia or Orpheus) into the 
proto-Earth is postulated to have put enough material into 
circumterrestrial orbit to form the Moon.[1] Given that planetary bodies 
are believed to have formed by the hierarchical accretion of smaller to 
larger sized bodies, it is now recognized that giant impact events such as 
this should be expected to have occurred for some planets. Computer 
simulations modeling this impact can account for the angular momentum of 
the Earth-Moon system, as well as the small size of the lunar core.[29] 
Unresolved questions concerning this theory are the relative sizes of the 
proto-Earth and impactor, and the proportion of material from the 
proto-Earth and impactor that contribute to making the Moon. The formation 
of the Moon is believed to have occurred at 4.527 ± 0.01 billion years, 
about 30 to 50 million years after the origin of the solar system.[30]

snip

>If large amounts of water can be
>incorporated into Earth's mantle and not baked out, might not something
>similar be true about Luna?

The stuff that made up Luna's mantle got baked *really hot* in a vacuum 
before it aggregated into the moon.  That's why it is so dry.

See also http://en.wikipedia.org/wiki/Giant_impact_hypothesis

Keith