| Actually the lack of atmosphere makes it harder for water to stay around, as water can not last for long in a vacuum as a liquid. It must be either solid (ice) or in gaseous form. If a gas, the molecules would relatively easily escape lunar gravity - the energy from solar particles, for example, can kick the water molecule right out of lunar gravity. So over time any gaseous water vapor won't last very long. Hence the water must stay in solid form, and to do that requires that it not be exposed to sunlight - otherwise it would sublimate to a gas. So the ice must be either buried or in permament shadow - such as in a crater at the north or south pole. |