Another major problem with detecting liquid water on Mars is that it has an extremely short shelf life. If water does reach the surface, it instantly boils and evaporates because the Martian atmospheric pressure is so low. In order for water to form the geologic features seen, it has to flow very fast and be of a sizable volume. Malin and Edgett estimate that for the smaller gullies, the outflows were formed by about 2,500 cubic meters of water, about the volume of an Olympic-size swimming pool. Larger gullies could have released 250,000 cubic meters enough water to sustain 100 people for 20 years.
Pressure builds behind the ice plug and eventually the ice seal is broken.
This mechanism might also explain why outflows are not seen on Sun-facing slopes. Because of the constant solar exposure, the emerging water simply sublimes away, no ice plug forms, and no gullies can form. Malin and Edgett are quick to point out that there findings are far from definitive and many unanswered questions remain. In over 150 separate observations of gullies, only one appears cratered at a resolution of 3 meters per pixel. In the coming months MGS will be focusing its attention on the youngest outflow regions to see if they change in any way.