[Flipside]

http://www.bbc.co.uk/news/science-environment-26696856

Basically, doubts are being raised about the previous suggestion that the Moon once had large quantities of water. I think this one is going to be a bit of a Merry-Go-Round, since I was always under the impression that the Moon didn't have the Gravity for holding onto water that may have been present during it's formation, but there's rising evidence that water is a lot more abundant deep inside the Earth than we originally thought, and the Moon is [according to the most popular theory] made from rock stripped from Earth during a planetary collision, but water shouldn't even be this far inside the solar system if it was there during the Accretion stage, as models suggest ice tends to drift to the outside of the disc.

It's just evidence that there's still a lot we need to understand about planetary formation.

e: For those interested in the large volume of water in the Mantle - http://www.scientificamerican.com/article/rare-diamond-confirms-that-earths-mantle-holds-an-oceans-worth-of-water/

This actually explains things a bit more clearly, as the water seems to have got there through tectonic means, and therefore probably arrived after the Earth had cooled somewhat. Which brings us back to the 'Comet Bombardment' theory caused by (possibly) Neptune taking a path through the Oort cloud a few billion years ago.

[watsisname]

Basically, doubts are being raised about the previous suggestion that the Moon once had large quantities of water.

I wasn't even aware of this ever being a suggestion.  A dry, volatile-poor moon would be a natural result of the Giant Impact, which has been pretty much the mainstream explanation for the Moon's origin since the Apollo age.  Any appreciable water that the Moon does have was delivered by impacts after its formation.

The origin of Earth's water is slightly more mysterious -- it's not clear how much water was incorporated into the Earth during its accretion.  Perhaps very little, as it formed inside of the 'snow line' of the Solar system, the distance at which water can form solid grains.  By D/H ratios, it is thought that a good portion of the water came from comets.  Water within the mantle got there by tectonics -- plate subduction and mantle convection.

This actually explains things a bit more clearly, as the water seems to have got there through tectonic means, and therefore probably arrived after the Earth had cooled somewhat. Which brings us back to the 'Comet Bombardment' theory caused by (possibly) Neptune taking a path through the Oort cloud a few billion years ago.

Precisely.   This is the Nice model (named for the city in France, not because the model is so awesome), which explains the Late Heavy Bombardment due to planetary migration.  It paints a very complex, dynamic evolution of the young solar system, and is widely accepted today amongst planetary scientists.

[Flipside]

I think the 'Wet Moon' theory started around 2008, when the Moon Impact Probe was sent and found what was thought to be evidence of water under the Moons surface, as the evidence mounted it became obvious that either the Moon was a lot 'wetter' than previously thought or that the data was being misinterpreted. It's looking more and more likely it was the latter now.

I always thought it was a bit weird considering the way the Moon formed and the fact it had no real mechanism to retain the water from Comets in such a thin atmosphere and low gravity.

[redsniper]

This is the Nice model (named for the city in France, not because the model is so awesome), which explains the Late Heavy Bombardment due to planetary migration.  It paints a very complex, dynamic evolution of the young solar system, and is widely accepted today amongst planetary scientists.

Nice model.