It showed definitively that mars...
...had very little atmosphere, very little water, no seasonal life cycles (as some thought they saw telescopically) but rather global dust storms. Very little erosion, craters - a surface as much like the moon as like earth.
It did not definitevely prove 'no life', but we never will prove that negative. And there will always be those who hold out hope. After mariner, chances went way way down.