High School Problems With a Terraformed Moon -- Maybe?

[Mikey16]

You missed the point. The water molecules are not just in the habitat. They _stay in the cup_. Outside, they will not.
I probably need to stop responding.

OK this is my last reply for your response: You can read what mfb wrote here. And without a proper surface gravity, your water molecules will be gone no matter where you place your cup. And also read the second paragraph of my previous post.

 

[mfb]

On the airless Moon, water would boil and thus escape right away (a meter-thick global layer would be gone in a few years).

Where does that time estimate come from? The water vapor still has to get split to have individual hydrogen atoms. That should happen slower than the evaporation/boiling. No matter how much atmosphere there is, the amount of water that evaporates/boils is the same.

 

[nikkkom]

Where does that time estimate come from? The water vapor still has to get split to have individual hydrogen atoms.

Yes, as you correctly noticed, the main thing is that a 1-meter water layer would completely evaporate because under its full weight at Moon, ~0.015 bar of pressure, boiling temp is below 20 C. So the conditions would be much closer to today's Mars than to anything "terraformed".

And this assumes that all water stays as vapor. Not the case. Taking into account slow rotation of the Moon, it is much hotter than 20 C during days (IIRC it's more like 90 C), and much below freezing during night, so even less water vapor will actually *be* an atmosphere.

Mean thermal velocity of water at 20 C is 585 m/s and is substantially lower than escape velocity 2380 m/s, but not as drastically lower as on Earth. High-speed tail of Maxwell–Boltzmann will be substantial. I have no tools here to calculate with good precision how fast it would be, I could be well wrong about "a few years", it might well be up to 100 years, but it's much, much faster than even on Mercury: