The results come from a detector that flew to the Red Planet along with the space agency's Curiosity rover.
Charged, energetic particles like cosmic rays can wreak havoc on delicate biological tissue. Earth's atmosphere and magnetic field serve to block or deflect most of this radiation, but astronauts on trips to Mars would be exposed to much higher doses.
Exactly how high wasn't clear, however. Previous measurements of the radiation in deep space came from unshielded detectors on telescopes, which overestimated how many particles would reach astronauts protected by a spacecraft.