Problems With a Terraformed Moon -- Maybe?

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  • Thread starter Oomuu
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  • #26
But you can only do this in a closed venue on the surface of moon.
And this proves that it's not weak gravity which makes water unstable on the Moon, since gravity is the same in a closed habitat.

the water molecules will escape to the space soon.
Now try to use your brain and figure out WHY water molecules would escape to the space soon, and why this process depends on not being in the closed habitat.
 
  • #27
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And this proves that it's not weak gravity which makes water unstable on the Moon, since gravity is the same in a closed habitat.



Now try to use your brain and figure out WHY water molecules would escape to the space soon, and why this process depends on not being in the closed habitat.
Just because your habitat on the moon is totally isolated (sealed), the water molecules can not escape outside and you still have them in your habitat.
We don't need to use our brain too much to imagine why a planet without enough gravity will have their water molecules escaped into space. Because if the surface gravity on the planet, or to say, moon, is not strong enough, the speed of the molecules can reach escape velocity easily. Don't forget that once there're radiation from the universe, the molecules get more energy and then accelerate their speed.
 
  • #28
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There are no laws of physics which prevent a very advanced spacefaring civilization from transporting N2 from e.g. Titan to the Moon.
As I said before, I do not think it makes any sort of economic sense to do it, but it can be done.
The water would still get lost to space over time. The nitrogen wouldn't matter much, ignoring temperature effects. First water evaporates until its partial pressure is in equilibrium with the liquid water - same process as without nitrogen. Then the hydrogen slowly escapes.
 
  • #29
The water would still get lost to space over time.
The "over time" part has rather different magnitude for two different scenarios.

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

But if there is a ~1bar atmosphere with twice molecular mass than H20, water won't boil (or rather, would boil _much_ less), and escape of the atmosphere and water would take much longer (I think thousands of years at least).
 
  • #30
Just because your habitat on the moon is totally isolated (sealed), the water molecules can not escape outside and you still have them in your habitat.
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.
 
  • #31
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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.
 
  • #32
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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.
 
  • #33
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:

water_escape.png
 

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