The boiling point depends on the pressure and if it is excessively low, the water can not exist in liquid form. That’s what happens on Mars if the planet had abundant water courses was because it also had a much denser atmosphere also providing higher temperatures. To dissipate most of the atmosphere in space, thereby reducing pressure and decreasing temperature, the water disappeared from the surface of Mars. However, there remains in the atmosphere as vapor, although in small proportions, and in the polar caps, consisting of large masses of permafrost.