Crap, now I'm doing it.
I hate when people respond with seemingly unconnected statements.
What I'm trying to say is near surface liquid water will be obvious and water ice will not be.
Also, dont believe the hype about 5km penetration. While the radio will penetrate that deep, in places, it will only be diagnostic if there is a strong reflector in the top 500 meters or so. More passes will help...
Let's hope MEX does better with it's first whip, MARSIS, than Indiana Jones did. :)