|MadSci Network: Chemistry|
Dear Kathleen, We know that ice can exist in space from our studies of comets. These studies have shown that comets are made from ice and dirt: comets are often referred to as 'dirty snowballs'. Ice in outer space will not melt in the way that we think of melting here on the Earth. The process of melting is an equilibrium process. On the Earth, at one atmosphere of pressure and zero degrees Celsius (32 degrees Fahrenheit, water exists at the same time in equilibrium in the solid, liquid, and gas phases. This equilibrium has been studied for many years and has resulted in what are referred to as phase diagrams. When you change the temperature and the pressure, the behavior of water can be predicted by referring to its 'phase diagram'. You might expect that in space, where we know that it is very cold, that water would exist only as a solid: ice. But the pressure in space is very low. Knowing this we expect that ice will not melt but will evaporate. This is what happens. An ice cube will slowly evaporate: it will change directly from the solid to the gas phase. This process, on Earth, is very useful and is called freeze drying. John Haberman
Try the links in the MadSci Library for more information on Chemistry.