Cosmic rays
It's not practical to shield a spacecraft against cosmic rays due to the mass of the shield. The martian atmosphere is thick enough to shield against primary cosmic rays. Some secondary cosmic radiation scattered from primary cosmic rays does reach the martian surface but can be shielded against using regolith (martian soil). A good design minimizes the flight crew's exposure to cosmic rays. Cosmic radiation is another reason why lunar colonization is impractical.
Gary