An instrument on NASA's Mars 2001 Odyssey spacecraft has estimated the amount of cosmic rays hitting Mars - so they can determine how unsafe it might be if humans ever land there. Mars has less atmosphere than Earth, and therefore less protection from cosmic rays

Mars - Rems per Year On Earth we typically receive 0.05 rems per year. radiation workers have a safety limit or 5 rems per year. Astronauts in space receive roughly 75 rems per year, although a solar flare could deliver 4,000 rems to the skin and 200 rems to internal organs in a single deadly dose. On Mars a human would receive between 10 & 20 rems per year.

Mars - Hits per Cell Nucleus The estimated average number of times per year each cell nucleus in a human there would be hit by a high-energy cosmic ray particle. On Mars it is between 2 & 8 hits per cell nucleus per year.