Look inside your domestic freezer or icebox.

Temperatuires are below the freezing point everywhere there, yet ice forms preferentially in some areas (the walls and adjacent to the cooling coils), while being sucked out of other areas - the food.

Things freeze-dry, not because ice is not stable in them, but because it is even more stable in other places.

So too with Mars. The equator and tropics tend to dry out with time, while the poles tend to accumulate ice.

And this is the conundrum of the equatorial hydrogen on Mars. What form is it in? And what makes it stable? So far, Spirit has been unable to answer these questions, other than adding a layer of puzzlement with the olivine dust, but one could imagine water vapour diffusing through cold dust and subliming directly into ice at moderate depths where things are ultra-cold, all without reacting with the dust.

let's wait and see more, assuming our roving friend remembers what she's supposed to be doing.