The third major instrument aboard 2001 Mars Odyssey was the Mars Radiation Environment Experiment (MARIE). Sponsored by NASA's Johnson Space Center, this instrument investigated the amount of radiation present both on the way to Mars and in Martian orbit. The goal of the project was to predict anticipated radiation doses that would be experienced by future astronauts, and in helping to determine possible effects of Martian radiation on human beings. While similar spectrometers like MARIE have flown on various space shuttles and the International Space Station (ISS), none had flown outside of the Earth's magnetosphere, which blocks much of the radiation from reaching the surface of our planet.
MARIE, with a 68-degree field of view, collected data during Odyssey's cruise from Earth to Mars and while in orbit around Mars. On October 28, 2003, a large solar event bombarded the Odyssey spacecraft, crippling MARIE. The instrument has been unable to collect data since that time, and engineers believe the most likely cause is that a computer chip was damaged by a solar particle smashing into the MARIE computer board.
The above graph shows the daily average dose rate of radiation from March 13, 2002 to September 30, 2003. MARIE found that radiation levels in orbit above Mars are 2.5 times higher than at the ISS. Levels at the Martian surface might be closer to the level at the ISS. Average in-orbit doses were about 22 millirads per day (220 microgray/day or 0.08 gray/year)). However, occasional solar proton events (SPEs) produced much higher doses. SPEs were observed by MARIE that were not observed by sensors near Earth, confirming that SPEs are directional. (Note: The above graphic comes from Wikipedia, although the data is originally from NASA. However, the MARIE instrument website at the Johnson Space Center is down at this time.)