http://en.wikipedia.org/wiki/Supersaturation
Examples — Carbonated water is a supersaturated solution of carbon dioxide gas in water. At the elevated pressure in the bottle, more carbon dioxide can dissolve in water than at atmospheric pressure. At atmospheric pressure, the carbon dioxide gas escapes very slowly from the supersaturated liquid.
[iow, this is more evidence that the water on Mars arrives from space.]
As in, a certain amount of space dust is ice (thus, arriving continuously) or just in the form of comets that strike once in a zillion years?