QUOTE (nprev @ Sep 26 2009, 04:59 AM)
Even a pond-sized, very localized underground aquifier would produce a global water signature many orders of magnitude greater than that observed due to transpiration through bedrock cracks & the loose regolith and subsequent diffusion through what passes for the lunar atmosphere.
Hmmm. Not sure I can agree with that statement. If I understand correctly, there are very large underground deposits of ice on Mars that have not been observed spectroscopically through bedrock cracks and loose regolith. That is why the crater impacts were required to expose the ice on Mars.
But getting back to the Moon, it could be a pure diffuse surface absorption onto the rock/dust grains. The H2O molecules deposit onto the outer few microns of the dust and is held there as a "solid". Any diffusion (not using the word percolation) in the soil column would be through sublimation-diffusion-redeposition onto deeper dust grains in the surface. I don't think there would ever be anything remotely resembling liquid flow (not even during or after an impact.)
Could it concentrate in an underground cold trap? Maybe. Then you could get a subsurface ice deposit. But it would likely be diffusing in and filling in the spaces of the dust grains to make a frozen dirt clod.
(Could the frozen dirt clod eventually deform plasticly and flow? That I don't know. That's where the total amount that exists is important. There just might not be enough H2O to do such a localized concentration.)