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Well, the question-title is not about the ice in the perennially dark polar craters, but hypothetical question, about:
What IF in some magical way the Earth's moon is covered "out of the sudden" with, say, 100 feet ( 30-ish meters ) of solid water ice?
OK, lets be precise - lets choose exact figure of 30 000kg per square meter.
With retaining it's orbital and axial dynamic characteristics?
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That depends. Is it cloudy, or clear? If it's very reflective, then sublimation would take place, but there wouldn't be any liquid water until enough water vapour had built up - and there possibly wouldn't be any afterwards, because it wouldn't melt easily. If clear, then there'd probably be some liquid water underneath the ice, in addition to the sublimation that would be taking place.
The water vapour atmosphere would begin to turn into an oxygen atmosphere, but it would take quite a while by human reckoning.
Once the water was melted, it would flow downhill and fill the craters and lowlands.
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Clean water for simplicity.
More exact calculations?
How dry is the regolith and how much is the total regolith and mega-regolith crevices / cavities volume?
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30 tons per square metre on the Moon would provide ~half a bar of pressure. At average lunar temperatures the ice would sublimate until the pressure reached some equilibrium point at maybe ~0.25 bar or so. The rest of the (now liquid) water would soak into the regolith. It's porosity is hard to guesstimate, but it's something like ~5% or so. Thus 15 metres of liquid water, would soak into ~300 metres of regolith, but very slowly. There might be significant amounts of frozen ground water a few metres down, soaked up as vapour over the aeons from the solar-wind and cometary inputs that seem likely. If I was a betting man I'd say about 10 metres worth would remain as water loose on the surface, quickly pooling into all the low lands. Some areas would be dessicated during the day, but it depends on what clouds form. The Moon spins a bit too fast for a sub-solar cloud bank to form, but having the atmosphere evaporating or condensing on a regular basis en masse, it's very hard to predict what the atmosphere would be like on average.
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On the water photolysis issue, as the atmosphere would be exposed directly to the Sun's UV, without the benefit of an ozone shield or stratospheric cold-trap, then it'd only be energy limited. Direct photolysis, which knocks a hydrogen loose and forms a hydroxyl and hydrogen atom, requires UV shorter than ~150 nanometres or so. I did look up the correct energy some time ago, so don't quote me. As UV and shorter wavelengths are ~6% of the Sun's flux and the efficiency is probably not 100%, especially once a bit of ozone is formed, then the actual rate would be harder to compute. There are complex atmospheric codes for this sort of thing, though they usually assume an N2/CO2 background mix. Andy Ingersoll's old paper on Venus's runaway greenhouse notes that the UV flux at the frequency which water absorbs highly is enough to dissociate about ~40 times Earth's ocean mass in 4.5 gigayears. That's about ~4,000 times the column mass of water we're discussing on the Moon, so the whole lot might dissociate in a megayear, roughly.
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As for the Jeans escape rate for hydrogen on the Moon, it probably escapes directly into space from the exosphere. A background gas is needed to dilute the water and allow a stratospheric cold trap to form, else the Moon would lose hydrogen as soon as it was dissociated.
Look straight up and be reminded that the Universe is vastly larger, older and more wonderful than the trivia around you. Our woes and worries shrink before such glory.
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As for the Jeans escape rate for hydrogen on the Moon, it probably escapes directly into space from the exosphere. A background gas is needed to dilute the water and allow a stratospheric cold trap to form, else the Moon would lose hydrogen as soon as it was dissociated.
Why are we even asking this question? The Moon has hardly any water at all, and is not likely to have a lot in the near future. It is academic what would happen if we could magically replace our Moon with Europa, because its not going to happen, it might make a nice thought experiment to discuss over dinner. Suppose we could make Venus a binary planet with Earth, lots of things would happen if that were true.
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Why are you, of the implausible midfuture terraforming proposals and desire to replicate New York on Mars, asking that question?
"I'm gonna die surrounded by the biggest idiots in the galaxy." - If this forum was a Mars Colony
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Why are you, of the implausible midfuture terraforming proposals and desire to replicate New York on Mars, asking that question?
On the plausibility scale, building a replica of New York City under a dome on Mars is a lot easier to do that terraforming the planet. Dumping a miles think sheet of ice on the Moon is probably even harder than terraforming Mars.
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