Some factors that might influence solar radiation on the Martian surface

Answer 1

The distance from the sun would be one factor. If there were an atmopshere to absorb or scatter radiation, that would be another.

Of course, the radiative output of the sun also matters.

Except during dust storms, most radiation to Mars reaches the surface.

Answer 2

Magnetic field or rather the lack of it.

see

http://mgs-mager.gsfc.nasa.gov/Kids/magfield.html