Oh uh, that was of course meant to say the calculated initial quantities at bead formation. That article contains a diagram that max out at 0.03 ppt in the center of a typical volcanic glass bead at present day. (And the variation is such that they mention 0.05 ppt as max.)

Not much compared to the surface content, that is.

The discussion on water in minerals likely goes back to mentionings of a paper from 2008 by Saal et al, where they used new ion microprobe techniques to measure water (and other volatiles) in lunar volcanic rock samples. They got a normal diffusion profile, which means there was water during rock formation, and the quantities (~ 0.8 parts/thousand) were equivalent to mid-ocean magma formation.

Saal and his colleagues calculated how much of these volatiles were present upon eruption. They concluded that the lunar magmas contained about 745 parts per million of water, similar to the amount in magmas produced at mid-ocean ridges on Earth. The results imply that the region of the lunar interior that melted to make the magmas contained about the same amount as in the Earth's depleted upper mantle, which is way more than a smidgeon. This may have implications for the origin of the Moon. It certainly will spark new research on lunar volatiles–and lots of arguments!

The new analyses of lunar volcanic glass beads by Saal and co-workers seem to show conclusively that at least some water is present in the beads and, hence, in the magma when it erupted. It might be possible that the hydrogen is not from water but instead originated by solar wind implantation. However, Saal and his coworkers argue convincingly that this is highly unlikely. For one thing, the hydrogen concentration is higher inside the beads. If implanted by the solar wind after eruption, hydrogen (measured by the OH- anions sputtered from the glass) should be higher near the surface of each bead. Furthermore, the measured H2O correlates with the concentrations of chlorine (Cl), fluorine (F), and sulfur (S), as shown in the diagrams below, but the solar wind contains only small amounts of those elements. It seems clear that there is H2O in the volcanic glasses.

If volatiles were not lost during the giant impact, why is the Moon so depleted in them? Or is it? In particular, how depleted was it initially in the highly volatile substances such as water? Saal and his coworkers have starting us thinking about this, and their paper will stimulate a lot of work. Other cosmochemists are skeptical about the results, especially the high concentrations of water inferred from the diffusion calculations. The water fight has just begun!

HT to Emily Lakdawalla of The Planetary Society, <a href="who also shows graphs on both day and night amounts. The former existence of which is presumably the persistent signal (but then surface originating, at ~ 2 mm max depth) which implications on over all equilibrium I see you already discussed.

All in all a good case of how not to present science by press conference, IMO.

LC

For that matter, any NEO out there that is a remnant of the Moon formation episode will generate water molecules in the same manner, so will become a "faint comet".

Timmy – yes, and for a good reason.
Water on Mars exists in huge quantities – exposed in plain view nearer the poles, and very close (as in several feet) underground in many other places.

In other words, if you were to land on Mars, dig down a foot or so, and use a pick-axe to drudge up a bucketful of ice, you'd have the equivalent of water produced on the moon over several square kilometers (1000 acres) over a month. So it's not really comparable.

We knew about Water on Mars since we first saw white deposits and measured the temperature to be above where dry ice can exist.

The latest surprise (as of about 10 years ago) is that we're finding water ice almost everywhere, and evidence of recent short-lived water flows

Mr. Sol's variable neucleosynthesis rates responsible for Ice Ages? On Earth.. or Mars? New images from Mars show water ice near the surface at Mars' equator.

Which ever the case there clearly is not enough water for any extensive human habitation there. The total amount of water on the moon might not so much as fill up a small lake. Excitement over this with regards to lunar bases and Tycho-city are probably overblown.

LC

Once formed, a molecule sits on the surface until a thermal vibration kicks it out. This is high vacuum, so it's molecular flow – each molecule to itself, so to speak.

Once it pops out, it is close to thermal velocity – very likely to escape. If it doesn't, however, it hops to another point on the surface, until it hits a cryo-trap near the pole, and so this is not different than the comet mechanism.

Why do we get a higher reading at the poles? It could be the result of a simple continuity condition – the water is produced everywhere, escapes more readily near the equator, but is migrating as a whole towards the poles. So it might just work out mathematically because of that.

I don't think it gets sequestered at any point however – the time constant for it to get outgassed is very long compared to the numbers observed.

It sounds like water starts getting produced immediately at sunrise, and by the time the ground gets hot, the temperature is high enough to already offset most of the production, so by the time the sun sets, and the ground is still hot, it dries out enough so that "stay over" quantities are negligible.

For there should be 1L of HO(H) per ton of regolith (approx 1000m^2) there would have to be some sequestering effect. That this happen near the poles is suggestive. Near the equator the solar heating is probably too much for the HO(H) to accumulate during the day. So if it accumulates near the poles maybe if there is initially none (thinking here of a modeled initial condition) there then by the end of a 14 day lunar "sol" there would be some trapped in the regolith. This is then frozen there during the lunar "night" until the next lunar sol-rise. More of HO(H) accumulates until there is some more or less constant amount which is trapped in the regolith that is a much larger amount than the tiny bit we would expect from a very transient "In & Out" equilibrium effect.

LC

I think what happened is that they have these surface readings, and someone up the food chain forced them to "put it in terms of pounds water per ton soil" even though it is completely nonsensical, since "water found on the moon" sounds a lot better than "water molecules are briefly formed on the surface during daytime".

not nice.

You might still get only a few teaspoons a day, but you might not have to hunt though a ton of regolith to get it.

The later is what I would expect from astronomical bodies, I never got around to figure how they thought the Moon got so dry by the Theia collision. I assume the collision models can get more realistic now.

It is a transient surface phenomena – you have to take a thin layer of top soil and keep it in its thin-layer form, and keep exposing it to sunlight, and then you can capture some water molecules coming off of it.

As for the number itself (1 kg water per ton of soil):

Assuming 28 days of exposure to high vacuum are enough to bake out the moisture, then we have a steady state – water-in = water-out. (no accumulation)

If this is happening at the top 1 mm of very lightweight soil (1g/cc), and since hydrogen is about 10% of the weight of water, and assuming the conversion efficiency is 100%, then we can infer that the solar particle flux is on the order of 0.1 kg over 1000 m2 over the course of 28 days. Very roughly.

That's more than I thought it is.

Looking online I found values for solar flux of about 1E8 protons cm-2 s-1.

This means that over 28 days and 1000 m2 we'll have 1E8*28*24*3600*1E4*1000*1.7E-27 = 0.1 grams of Hydrogen, or 1 gram of water – not 1 kg.

And this is IF the reaction is 100% efficient, meaning every incoming proton gets into a water molecule. Which is not realistic – there must be other things that the protons can do once they hit the surface.

Anyway – 1 kg water in a ton of soil is something you can almost feel with your hand. Solar wind can't do that.

Just being a skeptic here. I may have gotten my numbers crossed, too.