Curiosity has shown that Mars used to have persistent liquid water on the surface. (~3+ billion years ago.)
IF life was around at that time, at what depth would we expect to find it?
Without plate tectonics, would it remain on the surface, covered only by sand from the dust storms?
Does it matter if water existed 3.8 billion years ago, vs 3.3 billion years ago? (From a discovery / drill depth perspective)
How deep would a Mars rover need to dig, to find evidence of life from 3+ billion years ago?