The vapor pressure is the pressure at which a liquid can be in equilibrium with its own vapor. When the pressure in the liquid is lower than the vapor pressure, the liquid begins to boil. The vapor pressure increases with temperature. The following pictures represents the vapor pressure of water, in Torr (760 Torr = 1 atm), as a function of temperature in Celsius.
[Figure: Graph showing vapor pressure of water vs temperature]
Imagine a tall column of water on the Moon, maintained at a temperature of 50∘C and left open to the vacuum at the top. Up to what depth will the water in this column boil? The Moon's radius is 0.273 that of the Earth's and its mass 1.23% that of the Earth's.