The absence of an oxygen atmosphereonMarsandotherplanets in oursolar systemmeans thatthese planets alsolack an ozoneshield that would protectsurface-dwelling life from UVradiation.
The Sun's magnetic field and solar wind - mainly in the form of electrons and protons coming out of the Sun - protects the entire Solar System by acting as a sort of shield from cosmic rays (very energetic particles and radiation from outer space).