Soooo if I got this straight, The atmosphere on Mars was too thin to prevent the water from evaporating into space. So how is it possible for the water to evaporate when the average temperature on Mars is nowhere near warm enough for water to evaporate?
The same thing that gets the ice off of roads in the winter in North Dakota.
The atmosphere will support a certain number of water molecules even at low temperatures (well below freezing), and the water makes a phase change directly from solid to vapor.