there is also the whole solar radiation thing in space/mars.
RUGER 
That's an issue with the lack of magnetosphere. You are right though. Mars doesn't have a molten caore any more, without the movement in the core there is no "generator" to produce a magnetic field to protect the planet from solar winds or radiation. There are magnetic anomolies in spots around the planet, hardly enough to add any protection.
Funny how an atmosphere with such an enormously high amount of CO2 doesn't have much of a "greenhouse" effect.
