Re: Cave . . .
I was thinking something large and shallow, one that would be affected by seasonal change.
Where are we in the Martian seasons anyway? The dust storms of last year marked the summer/winter transition, and the seasons on Mars are approx. twice as long as those on Earth, but I can't recall which the northern hemisphere was going into.
Anyway, that's my answer and I'm sticking to it -- unless, of course, it's wrong.
Dan (could still just be a long high-albedo skid-mark of exposed mineral)