The Radiation Assessment Detector (RAD), onboard the Mars Science Laboratory (MSL) rover Curiosity, has been measuring the energetic charged and neutral particles and the radiation dose rate on the surface of Mars since the landing of the rover in August 2012. In contrast to charged particles, neutral particles (neutrons and γ-rays) are measured indirectly: the energy deposition spectra produced by neutral particles are complex convolutions of the incident particle spectra with the detector response functions. An inversion technique has been developed and applied to jointly unfold the deposited energy spectra measured in two scintillators of different types (CsI for high γ detection efficiency, and plastic for neutrons) to obtain the neutron and γ-ray spectra. This result is important for determining the biological impact of the Martian surface radiation contributed by neutrons, which interact with materials differently from the charged particles. These first in-situ measurements on Mars provide (1) an important reference for assessing the radiation-associated health risks for future manned missions to the red planet and (2) an experimental input for validating the particle transport codes used to model the radiation environments within spacecraft or on the surface of planets. Here we present neutral particle spectra as well as the corresponding dose and dose equivalent rates derived from RAD measurement during a period (November 15, 2015 to January 15, 2016) for which the surface particle spectra have been simulated via different transport models.

The Mars Science Laboratory rover Curiosity, operating on the surface of Mars, is exposed to radiation fluxes from above and below. Galactic Cosmic Rays travel through the Martian atmosphere, producing a modified spectrum consisting of both primary and secondary particles at ground level. These particles produce an upward directed secondary particle spectrum as they interact with the Martian soil. Here we develop a method to distinguish the upward and downward directed particle fluxes in the Radiation Assessment Detector (RAD) instrument, verify it using data taken during the cruise to Mars, and apply it to data taken on the Martian surface. We use a combination of Geant4 and Planetocosmics modeling to find discrimination criteria for the flux directions. After developing models of the cruise phase and surface shielding conditions, we compare model-predicted values for the ratio of upward to downward flux with those found in RAD observation data. Given the quality of available information on Mars Science Laboratory spacecraft and rover composition, we find generally reasonable agreement between our models and RAD observation data. This demonstrates the feasibility of the method developed and tested here. We additionally note that the method can also be used to extend the measurement range and capabilities of the RAD instrument to higher energies. Plain Language SummaryThe MSL rover Curiosity is exposed to energetic particles from above and below on the Martian surface. Particles enter the Martian atmosphere from above and travel through it until they reach the ground. Particles lose energy and can produce secondary particles while passing through the atmosphere, resulting in an energy distribution on ground level that is different from that on the top of the atmosphere. The resulting particles produce an upward directed particle distribution in the soil. We develop a method to distinguish the upward and downward particle fluxes in the RAD instrument, verify it using data taken during the cruise to Mars, and apply it to data taken on the Martian surface. We use a combination of models to find criteria for discriminating the flux directions. After developing models of the cruise phase and surface shielding conditions, we compare simulated values for the ratio of upward to downward flux with those found in observation data. We find generally reasonable agreement between our models and RAD observation data. This demonstrates the feasibility of the method developed and tested here. The method can also be used to extend the measurement range and capabilities of the RAD instrument to higher energies.

No abstract

No abstract