I don't know how versed you are in math or the sciences. Bob, but here goes!!!...
I did some calculations regarding how much pressure is required to keep water
in a liquid-state; assuming the temperature is between the freezing and boiling point.

Even VERY very-briny water would either freeze or boil at PRESSURES below 20 millibars
(1/50th sea-level pressure) depending on temperature.

Bob? You might ask: What about a 'modest' layer of Martian dirt or soil?
Would the OVERLAYING PRESSURE of that Martian dirt or soil substitute for lacking AIR PRESSURE?
Would that layer of Martian soil or dirt provide enough PRESSURE on subsurface water beneath to keep it a liquid (assuming the temperature was right)???
The answer is given below. Check the calculations.

Assuming we have a DENSE martian soil of about 2000kg/m^3 (twice the density of water). DENSE soil (2000kg/m^3) on Earth would have to be 16 feet deep to generate
the EQUIVALENT of ONE ATMOSPHERE of overlaying pressure.
But since I mentioned 20 millibars (1/50th sea-level pressure), DENSE SOIL 16 feet deep/sea-level atmospheric pressure x (1/50 sea-level atmospheric pressure) = DENSE SOIL (1/3 foot deep) 4 inches deep.

What does that mean? That means the upper 4 inches of Martian soil is either BONE DRY
or contains FROZEN WATER!!!!
NO MARTIAN MUD!!! PERIOD!!!