[YokoZar]

Mars has isolated pockets of "mini magnetospheres" that are suitable enough to block charged particles.

http://www.space.dtu.dk/english/Research/Universe_and_Solar_...

[maxk42]

I think he was assuming the subject was terraforming.

Without a magnetosphere, the atmosphere will keep being blasted off into space continuously, and it'll be impossible to terraform.

[mkl]

Continuously, but slowly, over very long time scales. Maintaining an atmosphere suitable for us is probably far easier than constructing it in the first place.