Measurements by the Radiation Assessment Detector (RAD) instrument aboard NASA's Mars Science Laboratory (MSL) rover Curiosity show that the galactic cosmic ray (GCR) radiation dose rates on the surface of the Red Planet are about half those that RAD measured during interplanetary cruise.
Interplanetary GCR dose rates were previously measured by the MARIE instrument aboard the Mars Odyssey spacecraft during its cruise out to Mars in 2001 and shown to be about twice that experienced in Low Earth Orbit (LEO). Thus, in combination, the MARIE and RAD results show that Mars surface GCR dose rates are about the same as those experienced by astronauts in LEO. This mean that GCR doses will not be a show-stopper for the human exploration of Mars.
Please note that the MARIE authors report interplanetary GCR dose rates ranging from 0.28 Sv (28 rem) to 0.73 Sv (73 rem) per year. Taking the 50 rem/year average of these figures as an interplanetary dose baseline, it can be estimated that a human Mars mission which spends 6 months flying to Mars (as Odyssey did in 2001), 18 months on the Martian surface, and 6 months flying back to Earth would receive a total GCR dose of 88 rem. Such a dose is estimated to represent a statistical risk of about a 1 percent chance of getting a fatal cancer sometime later in life, assuming no advance in medical technique and would therefore represent a modest portion of the risk faced by astronauts on a human Mars mission. Furthermore, it has already been received by a number of astronauts and cosmonauts working on the ISS or Mir space stations without incidence of cancer among any of them.
It is therefore now confirmed that hypothetical radical new propulsion systems enabling much faster transit times to Mars and/or the ability to leave the Red Planet regardless of launch windows will not be needed to enable human Mars exploration. With its first important results, Curiosity has slain the mythical radiation dragon previously barring the way to Mars.