Aand it is looking like there is plenty of both...

Then there is probably a lot of carbonic acid (Carbonated water) at depths where the pressure is high enough to keep the water in a liquid state.

That would also be a depths where there is significant radiiation sheilding (rock).

If that rock is comprised of calcium carbonate (Limestone) then there will very likely be cave structures formed within this radiation sheilded zone. (calcium Carbonate + Carbonic acid => H2O + Calcium bicabonate

So assuming we could locate these caverns, we could very easily survive and even build human colonies, just below the surface of Mars.

And the colonies will have plenty of baking soda available for export!

Scott Evil