Not necessarily, say scientists. Indigenous life forms could be radiation resistant, like the terrestrial microbe Deinococcus radiodurans . Tiny Martians might also live in rocks or soil, substances that provide natural protection against radiation. Nor is Mars necessarily uninhabitable for humans. If we learn how to shelter ourselves from the planet's unique brand of weather, humans can explore and perhaps even live on Mars.
Particulate radiation poses the greater threat to humans.
Cosmic ray nuclei, carrying typically 300 to 500 MeV per nucleon, zip through the human body so quickly there's not enough time to dump their energy into the surrounding tissue. This 4.2 MB mpeg movie shows a solar flare in action, blasting hot gas away from the limb of the Sun. Solar protons passing through humans ionize molecules along their tracks. Sometimes protons will modify or even break DNA strands within cells. Mars' thin atmosphere does little to protect the planet from energetic protons.