Re: At what temperature would water boil on the moon?

Dear Dale,

Your question is an interesting one.  The dynamics of water/water vapor 
are defined by the combination of pressure and temperature.  You have 
stated that the surface temeprature of the Moon is 250F.  That is the 
temperature of the Moon's surface material itself.  As there is no air to 
trap heat or light energy from the Sun, there is no meaningful way to 
measure the temperature above the surface, as we measure temperatures here 
on Earth, even at the level of an astronaut's life support pack.  In the 
sunlight it would become very hot from direct solar exposure and in the 
shade it would become hundreds of degrees below zero as heat is radiated 
into space.  Temperatures are those of mateirals in direct sunlight, such 
as an astronaut's space suit.  It would get very hot and be in dire need 
of cooling to keep the human inside from overheating.  

The pressure concern is that there is no air on the Moon and therefore no 
air pressure.  According to the CRC Handbook, as the ambient pressure goes 
down, the boiling point of water goes down, as you also stated.  Zero 
atmospheric pressure equals immediate water boiling.  Water would rapidly 
boil away into space on the sunlit side of the Moon and would form a spray 
of very fine ice crystals in the shadow side.  These crystals would 
eventually disappear by way of sublimation.  The solid ice would turn 
directly into water vapor.

The modifying condition is that the water in the cooling systems of the 
astronauts is not in contact with the ambient Moon conditons.  It is 
contained in a sealed system that is pressurized to keep it liquid under 
those conditons.  The heat is removed from the liquid water by passing it 
through a radiator, similar to the one in your family's car.  The heat is 
radiated to space and thus can control the astronauts' body temperature.

This condition relates to many other questions when human environment is 
involved.  It does not matter what the surrounding conditions are like.  
Humans modify the environment to "human optimum" so they can stay 
comfortable.

I hope this helps.
Dr. Lon Brouse