Notices
Results 1 to 23 of 23
Like Tree3Likes
  • 1 Post By Harold14370

Thread: Swimming pool on the moon

  1. #1 Swimming pool on the moon 
    Forum Freshman
    Join Date
    Aug 2013
    Posts
    18
    I'm not entirely certain this is an astronomy question, but it does involve the moon (sort of)...

    As a thought experiment, imagine a swimming pool on the moon. It is heated and pressurised to 20°C & 1 ATM. Now fill it with (imaginary) water. My question is, what is the density of that water? Is it the same density as a swimming pool here on Earth (at same pressure & temperature conditions)?

    I guess the real question is does gravity affect density at all? I know in deep ocean the density of water is higher than at surface, but will it have a noticeable effect in a swimming pool on the moon? Also what happens to the buoyancy on that lunar swimming pool? Is it lower or higher than the Earth-bound swimming pool?


    Reply With Quote  
     

  2.  
     

  3. #2  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,822
    Water is water, and its properties are the same at a given pressure and temperature. You can look them up in a handbook. One thing that will be different because of gravity is the effect of the water depth. On earth, the pressure will increase with depth above the atmospheric surface pressure more than on the moon.

    The buoyant force is equal to the weight of the water displaced. On the moon, the weight of the water is about 1/6 that on earth, so the buoyant force will be reduced accordingly.

    Water is not very compressible, but is somewhat denser at higher pressure. Since the water pressure would be less at a given depth on the moon, then its density would also be less. At the surface it would be the same if you are pressurizing your swimming pool to 1 atmosphere.


    Reply With Quote  
     

  4. #3  
    Suspended
    Join Date
    Nov 2012
    Location
    Apocalyptic Paradise
    Posts
    6,613
    Density is the ratio between mass and volume, so no, the pool on the Moon would have the same density as one on Earth. In the depths of the ocean, the higher pressure is because of the weight of all that water sitting in top of it.
    Reply With Quote  
     

  5. #4  
    Forum Freshman
    Join Date
    Aug 2013
    Posts
    18
    Quote Originally Posted by Harold14370 View Post
    The buoyant force is equal to the weight of the water displaced. On the moon, the weight of the water is about 1/6 that on earth, so the buoyant force will be reduced accordingly.
    Interesting. So an air bubble would stay inside a blob of water indefinitely under weightless conditions?
    Reply With Quote  
     

  6. #5  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,822
    Quote Originally Posted by Neverfly View Post
    Density is the ratio between mass and volume, so no, the pool on the Moon would have the same density as one on Earth. In the depths of the ocean, the higher pressure is because of the weight of all that water sitting in top of it.
    This is true to a good approximation, but not exactly.
    Properties of water - Wikipedia, the free encyclopedia
    The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume
    Neverfly likes this.
    Reply With Quote  
     

  7. #6  
    Forum Professor
    Join Date
    Aug 2013
    Location
    San Diego
    Posts
    1,970
    Quote Originally Posted by sth128 View Post
    Interesting. So an air bubble would stay inside a blob of water indefinitely under weightless conditions?
    Put it this way - nothing would force it towards any surface.
    Reply With Quote  
     

  8. #7  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,822
    Quote Originally Posted by sth128 View Post
    Quote Originally Posted by Harold14370 View Post
    The buoyant force is equal to the weight of the water displaced. On the moon, the weight of the water is about 1/6 that on earth, so the buoyant force will be reduced accordingly.
    Interesting. So an air bubble would stay inside a blob of water indefinitely under weightless conditions?
    I suppose you could say that, at least as far as its buoyancy is concerned.

    I should also mention that the weight of something floating in the pool on the moon will also be 1/6 its earth weight, so whatever floats in the earth pool will also float in the moon pool.
    Last edited by Harold14370; August 19th, 2013 at 04:06 PM.
    Reply With Quote  
     

  9. #8  
    Malignant Pimple shlunka's Avatar
    Join Date
    Jan 2013
    Location
    Dogbox in front of Dywyddyr's house.
    Posts
    1,784
    I'm assuming this would be a nudist pool?
    "MODERATOR NOTE : We don't entertain trolls here, not even in the trash can. Banned." -Markus Hanke
    Reply With Quote  
     

  10. #9  
    Theatre Whore babe's Avatar
    Join Date
    Jun 2013
    Location
    Resident of Big Island of Hawai'i since 2003, and in Bayside, Ca. since 1981, Humboldt since 1977
    Posts
    12,445
    Quote Originally Posted by shlunka View Post
    I'm assuming this would be a nudist pool?
    Don't need a pool for that! *Laughing*
    Reply With Quote  
     

  11. #10  
    Theatre Whore babe's Avatar
    Join Date
    Jun 2013
    Location
    Resident of Big Island of Hawai'i since 2003, and in Bayside, Ca. since 1981, Humboldt since 1977
    Posts
    12,445
    So my eternity pool, wouldn't work....
    Reply With Quote  
     

  12. #11  
    Forum Professor
    Join Date
    Jan 2013
    Posts
    1,773
    Quote Originally Posted by Harold14370 View Post
    Water is water, and its properties are the same at a given pressure and temperature. You can look them up in a handbook. One thing that will be different because of gravity is the effect of the water depth. On earth, the pressure will increase with depth above the atmospheric surface pressure more than on the moon.

    The buoyant force is equal to the weight of the water displaced. On the moon, the weight of the water is about 1/6 that on earth, so the buoyant force will be reduced accordingly.

    Water is not very compressible, but is somewhat denser at higher pressure. Since the water pressure would be less at a given depth on the moon, then its density would also be less. At the surface it would be the same if you are pressurizing your swimming pool to 1 atmosphere.
    Something like a couple percent at 3000 psi seems to ring back in mind. Compressibility of hydraulic fluids becomes a design issue in high pressure applications. The "pressure wave" induced by the Teller-Ulam configuration guiding the immensely intense surge of X-Rays is thought to be in the millions of bars. joc
    Reply With Quote  
     

  13. #12  
    Forum Ph.D.
    Join Date
    Jul 2011
    Posts
    985
    Quote Originally Posted by sth128 View Post
    Quote Originally Posted by Harold14370 View Post
    The buoyant force is equal to the weight of the water displaced. On the moon, the weight of the water is about 1/6 that on earth, so the buoyant force will be reduced accordingly.
    Interesting. So an air bubble would stay inside a blob of water indefinitely under weightless conditions?
    I am now visualizing a space ship or space station built of water with a habitable air bubble in the middle. There must be a reason this will not work.
    Reply With Quote  
     

  14. #13  
    Suspended
    Join Date
    Apr 2007
    Location
    Pennsylvania
    Posts
    8,822
    Quote Originally Posted by Sealeaf View Post
    Quote Originally Posted by sth128 View Post
    Quote Originally Posted by Harold14370 View Post
    The buoyant force is equal to the weight of the water displaced. On the moon, the weight of the water is about 1/6 that on earth, so the buoyant force will be reduced accordingly.
    Interesting. So an air bubble would stay inside a blob of water indefinitely under weightless conditions?
    I am now visualizing a space ship or space station built of water with a habitable air bubble in the middle. There must be a reason this will not work.
    If it was a big enough blob of water, it would create a bit of gravity on its own and the bubble would float to the surface. There wouldn't be anything keeping the bubble together or in the middle of the blob of water. Any slight acceleration would cause it to go toward the surface.
    Reply With Quote  
     

  15. #14  
    Forum Freshman
    Join Date
    Aug 2013
    Posts
    18
    Quote Originally Posted by Harold14370 View Post
    Quote Originally Posted by Sealeaf View Post
    Quote Originally Posted by sth128 View Post
    Quote Originally Posted by Harold14370 View Post
    The buoyant force is equal to the weight of the water displaced. On the moon, the weight of the water is about 1/6 that on earth, so the buoyant force will be reduced accordingly.
    Interesting. So an air bubble would stay inside a blob of water indefinitely under weightless conditions?
    I am now visualizing a space ship or space station built of water with a habitable air bubble in the middle. There must be a reason this will not work.
    If it was a big enough blob of water, it would create a bit of gravity on its own and the bubble would float to the surface. There wouldn't be anything keeping the bubble together or in the middle of the blob of water. Any slight acceleration would cause it to go toward the surface.
    Way before that happens, the water will vapourise due to lack of pressure. The people inside the bubble will then fall unconscious as oxygen fly out of their lungs as their surroundings turn into vacuum. Finally, as their weightless corpses orbit around the Earth, energy from the sun, unobstructed and unfiltered by atmosphere, quickly burn the dead astronauts into charred, blackened masses.

    And thus ends the story of the water blob space station.
    Reply With Quote  
     

  16. #15  
    Forum Professor
    Join Date
    Aug 2013
    Location
    San Diego
    Posts
    1,970
    Quote Originally Posted by Sealeaf View Post
    I am now visualizing a space ship or space station built of water with a habitable air bubble in the middle. There must be a reason this will not work.
    It would have to be a very big water blob (like, moon sized) to have enough gravity to maintain a water vapor atmosphere to prevent boiloff.
    Reply With Quote  
     

  17. #16  
    New Member
    Join Date
    Jul 2014
    Posts
    1
    Quote Originally Posted by billvon View Post
    Quote Originally Posted by Sealeaf View Post
    I am now visualizing a space ship or space station built of water with a habitable air bubble in the middle. There must be a reason this will not work.
    It would have to be a very big water blob (like, moon sized) to have enough gravity to maintain a water vapor atmosphere to prevent boiloff.
    So does that mean technically that a giant blob of water could be sustained on the moon with air in it?
    Reply With Quote  
     

  18. #17  
    Forum Ph.D.
    Join Date
    Jul 2011
    Posts
    985
    But if you built a sphearical metal shell and filled it 75% with water and then spun it on its axis, centrifugal force would keep the air bubble in the middle. The machinery and living quarters of the space station would float like rafts on the water surface. The water would be your radiation barrier. A sort of polynesian island environment.
    Reply With Quote  
     

  19. #18  
    Time Lord
    Join Date
    Apr 2008
    Posts
    5,305
    Quote Originally Posted by billvon View Post
    Quote Originally Posted by Sealeaf View Post
    I am now visualizing a space ship or space station built of water with a habitable air bubble in the middle. There must be a reason this will not work.
    It would have to be a very big water blob (like, moon sized) to have enough gravity to maintain a water vapor atmosphere to prevent boiloff.
    I'm unsure boiloff squares with the existence of comets, or puny moons of ice, or the droplets observed after space shuttles flushed the toilets.

    If you put liquid water in a vacuum, the surface will boil - yes - but that rapidly cools this same surface to freezing. Boiling (practically) ceases when ice forms.

    Hypothetically our blob of water is large enough to freeze around the outside first, so it develops a protective crust. Then, curiously, the expanding thickness of ice would pressurize the remaining water. How much pressure before a geyser pops through the crust?

    EDIT: I'm boiloff -> I'm unsure boiloff
    Last edited by Pong; July 29th, 2014 at 05:04 AM.
    A pong by any other name is still a pong. -williampinn
    Reply With Quote  
     

  20. #19  
    Forum Professor
    Join Date
    Aug 2013
    Location
    San Diego
    Posts
    1,970
    Quote Originally Posted by Pong View Post
    If you put liquid water in a vacuum, the surface will boil - yes - but that rapidly cools this same surface to freezing. Boiling (practically) ceases when ice forms.
    That would depend where it was. Around Earth's orbit, the incoming solar radiation (about 1400 watts/sq m) would tend to melt the ice, which would flash to vapor pretty quickly. So a small "bubble" wouldn't last long.
    Reply With Quote  
     

  21. #20  
    Theatre Whore babe's Avatar
    Join Date
    Jun 2013
    Location
    Resident of Big Island of Hawai'i since 2003, and in Bayside, Ca. since 1981, Humboldt since 1977
    Posts
    12,445
    would the water stay in the pool?
    Reply With Quote  
     

  22. #21  
    Time Lord
    Join Date
    Apr 2008
    Posts
    5,305
    Quote Originally Posted by billvon View Post
    Quote Originally Posted by Pong View Post
    If you put liquid water in a vacuum, the surface will boil - yes - but that rapidly cools this same surface to freezing. Boiling (practically) ceases when ice forms.
    That would depend where it was. Around Earth's orbit, the incoming solar radiation (about 1400 watts/sq m) would tend to melt the ice, which would flash to vapor pretty quickly. So a small "bubble" wouldn't last long.
    Yet comets, made mostly of water, endure. How long is this "won't last long"? I'm guessing that a swimming pool volume, in the vicinity of Earth, would last plenty long enough for our purposes.

    I've searched for answers to the question of water (ice) survival in space and concluded that we simply don't understand this very well. Now that ISS has a "front porch" airlock for experiments we may get real answers soon.

    Quote Originally Posted by babe
    would the water stay in the pool?
    The moon's gravity will keep water down in the pool, whether that's liquid water, ice, or water vapour. The question is just what happens to liquid water without atmospheric pressure. It would boil - without heat - yet this boiling causes it to cool so it would also turn to ice, which doesn't boil. I guess some liquid water would remain at the bottom of the pool after the surface turned to snow/ice. This slightly pressurized water would freeze more gradually, forming the type Ih (hexagonal, like snowflake) ice we're familiar with on Earth.

    Anyway this is an unnatural and violent scenario. More likely we'd be delivering an already frozen block of water to the lunar pool party, and they'd then melt the ice to swim in it. In this scenario, melting ice from the bottom-up, with an ordinary pool heater, should be possible. What happens next is speculative. I guess that any cracks in the surface ice would heal because escaping water must instantly boil, causing nearby water to cool and freeze. So you get a swimming pool and skating rink.

    The weird fact that water takes up more space when frozen gives an ice-crusted blob of water miraculous properties. If you heat it internally to increase the ratio of liquid water, you reduce the pressure. And vise versa for its tendency to cool in space, because encroaching ice squeezes the remaining water. I wonder can we size such a blob to produce an Earthlike pressure? Say, with a habitable air bubble in the middle.
    A pong by any other name is still a pong. -williampinn
    Reply With Quote  
     

  23. #22  
    Theatre Whore babe's Avatar
    Join Date
    Jun 2013
    Location
    Resident of Big Island of Hawai'i since 2003, and in Bayside, Ca. since 1981, Humboldt since 1977
    Posts
    12,445
    Pong

    Like
    Reply With Quote  
     

  24. #23  
    Forum Professor
    Join Date
    Aug 2013
    Location
    San Diego
    Posts
    1,970
    Quote Originally Posted by Pong View Post
    Yet comets, made mostly of water, endure.
    Often they don't. The ones that do are typically very large (10^14 kg or so) can survive 50-1000 close approaches, but even they don't last forever. Note that the reason we see comets at all is that the Sun is busily trying to melt them; the tail that we see is the outgassing of the volatiles (including water) within the nucleus. Also, while scientists used to think of comets as dirty snowballs, they are lately thinking the term "snowy dirtballs" would be more accurate, since they are often more rock than ice.
    How long is this "won't last long"? I'm guessing that a swimming pool volume, in the vicinity of Earth, would last plenty long enough for our purposes.
    Well, 10^5 kg of water isn't going to last long. A few hours? I'd expect the rapid initial boil would result in a rapidly expanding ball of first water, then ice crystals.
    Reply With Quote  
     

Similar Threads

  1. Pool Physics
    By DarkSlush in forum Physics
    Replies: 5
    Last Post: August 6th, 2012, 12:30 PM
  2. pool ball
    By DesertRose in forum Physics
    Replies: 4
    Last Post: September 26th, 2010, 04:28 PM
  3. Pool Rx
    By Ots in forum Chemistry
    Replies: 4
    Last Post: July 16th, 2009, 10:26 AM
  4. Up Side Down Pool
    By mvm2691 in forum Physics
    Replies: 16
    Last Post: October 15th, 2008, 12:59 AM
  5. Sensing tidal forces in a swimming pool. Possible or no?
    By sonhouse in forum Earth Sciences
    Replies: 6
    Last Post: October 17th, 2007, 02:26 PM
Tags for this Thread

View Tag Cloud

Bookmarks
Bookmarks
Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •