The Radiation Assessment Detector (RAD) instrument on NASA's Curiosity Mars rover monitors the natural radiation environment at the surface of Mars. It can see the radiation from two sources, galactic cosmic rays and solar energetic particles. This graph plots measurements made during the rover's first 10 months on Mars. The vertical axis is in micrograys per day. Micrograys are unit of measurement for absorbed radiation dose. The horizontal axis is time, labelled on the bottom as months from August 2012 to June 2013 and on the top as the number of sols (Martian days) since landing.
The observations have been almost entirely due to galactic cosmic rays, which contribute a slowly varying dose rate of about 210 micrograys per day. Variations are due to day-to-night differences in the shielding provided by the atmosphere. Sudden drops in the radiation, so-called Forbush decreases, such as seen on sols 50, 97, 208 and 259, result from extra shielding provided by interplanetary coronal mass ejections driven by the sun. The longer-term increase and decrease peaking close to Sol 200 is driven by Martian seasonal effects.
Only one solar particle event has been observed by RAD on the surface of Mars, and it was rather weak. Researchers had expected to see more solar particle events, but for unknown reasons, the sun is currently much less active than during recent peaks in the solar-activity cycle. After the RAD measurements during Curiosity's flight to Mars and on the surface of Mars, solar particle events are the big unknowns in predicting the radiation exposure for a human mission to Mars. If the instrument had been taking measurements on Mars during the comparable period of the solar-activity cycle about 11 years ago, it would probably have seen eight events in 300 sols, contributing an unknown but certainly significantly higher amount to the overall dose.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Mars Science Laboratory Project and the mission's Curiosity rover for NASA's Science Mission Directorate in Washington. The rover was designed and assembled at JPL, a division of the California Institute of Technology in Pasadena.
More information about Curiosity is online at http://www.nasa.gov/msl and http://mars.jpl.nasa.gov/msl/.