The Radiation Assessment Detector (RAD) on the Mars Science Laboratory's Curiosity rover began making detailed measurements of the cosmic ray and energetic particle radiation environment on the surface of Mars on 7 August 2012. We report and discuss measurements of the absorbed dose and dose equivalent from galactic cosmic rays and solar energetic particles on the Martian surface for ~300 days of observations during the current solar maximum. These measurements provide insight into the radiation hazards associated with a human mission to the surface of Mars, and provide an anchor point to model the subsurface radiation environment, with implications for microbial survival times of any possible extant or past life, as well as for the preservation of potential organic biosignatures of the ancient Martian environment.

The Mars Science Laboratory spacecraft, containing the Curiosity rover, was launched to Mars on 26 November 2011, and for most of the 253-day, 560-million-kilometer cruise to Mars, the Radiation Assessment Detector made detailed measurements of the energetic particle radiation environment inside the spacecraft. These data provide insights into the radiation hazards that would be associated with a human mission to Mars. We report measurements of the radiation dose, dose equivalent, and linear energy transfer spectra. The dose equivalent for even the shortest round-trip with current propulsion systems and comparable shielding is found to be 0.66 ± 0.12 sievert.

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Context: The Radiation Assessment Detector (RAD) on the Mars Science Laboratory (MSL) has been measuring the radiation environment on the surface of Mars since August 6th 2012. MSL-RAD is the first instrument to provide detailed information about charged and neutral particle spectra and dose rates on the Martian surface, and one of the primary objectives of the RAD investigation is to help improve and validate current radiation transport models. Aims: Applying different numerical transport models with boundary conditions derived from the MSL-RAD environment the goal of this work was to both provide predictions for the particle spectra and the radiation exposure on the Martian surface complementing the RAD sensitive range and, at the same time, validate the results with the experimental data, where applicable. Such validated models can be used to predict dose rates for future manned missions as well as for performing shield optimization studies. Methods: Several particle transport models (GEANT4, PHITS, HZETRN/OLTARIS) were used to predict the particle flux and the corresponding radiation environment caused by galactic cosmic radiation on Mars. From the calculated particle spectra the dose rates on the surface are estimated. Results: Calculations of particle spectra and dose rates induced by galactic cosmic radiation on the Martian surface are presented. Although good agreement is found in many cases for the different transport codes, GEANT4, PHITS, and HZETRN/OLTARIS, some models still show large, sometimes order of magnitude discrepancies in certain particle spectra. We have found that RAD data is helping to make better choices of input parameters and physical models. Elements of these validated models can be applied to more detailed studies on how the radiation environment is influenced by solar modulation, Martian atmosphere and soil, and changes due to the Martian seasonal pressure cycle. By extending the range of the calculated particle spectra with respect to the experimental data additional information about the radiation environment is gained, and the contribution of different particle species to the dose is estimated.

Curiosity, measures the energetic charged and neutral particles and the radiation dose rate on the surface of Mars. An important factor for determining the biological impact of the Martian surface radiation is the specific contribution of neutrons, with their deeper penetration depth and ensuing high biological effectiveness. This is very difficult to measure quantitatively, resulting in considerable uncertainties in the total radiation dose. In contrast to charged particles, neutral particles (neutrons and gamma rays) are generally only measured indirectly. Measured spectra are a complex convolution of the incident particle spectrum with the detector response function and must be unfolded. We apply an inversion method (based on a maximum likelihood estimation) to calculate the neutron and gamma spectra from the RAD neutral particle measurements. Here we show the first spectra on the surface of Mars and compare them to theoretical predictions. The measured neutron spectrum (ranging from 8 to 740 MeV) translates into a radiation dose rate of 14 ± 4 μGy/d and a dose equivalent rate of 61 ± 15 μSv/d. This corresponds to 7% of the measured total surface dose rate and 10% of the biologically relevant surface dose equivalent rate on Mars. Measuring the Martian neutron and gamma spectra is an essential step for determining the mutagenic influences to past or present life at or beneath the Martian surface as well as the radiation hazard for future human exploration, including the shielding design of a potential habitat.

The experiment 'Dosimetric Mapping' conducted as part of the science program of NASA's Human Research Facility (HRF) between March and August 2001 was designed to measure integrated total absorbed doses (ionising radiation and neutrons), heavy ion fluxes and its energy, mass and linear energy transfer (LET) spectra, time-dependent count rates of charged particles and their corresponding dose rates at different locations inside the US Lab at the International Space Station. Owing to the variety of particles and energies, a dosimetry package consisting of thermoluminescence dosemeter (TLD) chips and nuclear track detectors with and without converters (NTDPs), a silicon dosimetry telescope (DOSTEL), four mobile silicon detector units (MDUs) and a TLD reader unit (PILLE) with 12 TLD bulbs as dosemeters was used. Dose rates of the ionising part of the radiation field measured with TLD bulbs applying the PILLE readout system at different locations varied between 153 and 231 microGy d(-1). The dose rate received by the active devices fits excellent to the TLD measurements and is significantly lower compared with measurements for the Shuttle (STS) to MIR missions. The comparison of the absorbed doses from passive and active devices showed an agreement within +/- 10%. The DOSTEL measurements in the HRF location yielded a mean dose equivalent rate of 535 microSv d(-1). DOSTEL measurements were also obtained during the Solar Particle Event on 15 April 2001.

The Radiation Assessment Detector onboard the Mars Science Laboratory rover Curiosity is detecting the energetic particle radiation at the surface of Mars. Data collected over the first 350 Martian days of the nominal surface mission show a pronounced diurnal cycle in both the total dose rate and the neutral particle count rate. The diurnal variations detected by the Radiation Assessment Detector were neither anticipated nor previously considered in the literature. These cyclic variations in dose rate and count rate are shown to be the result of changes in atmospheric column mass driven by the atmospheric thermal tide that is characterized through pressure measurements obtained by the Rover Environmental Monitoring Station, also onboard the rover. In addition to bulk changes in the radiation environment, changes in atmospheric shielding forced by the thermal tide are shown to disproportionately affect heavy ions compared to H and He nuclei.

Space radiation hazards are recognized as a key concern for human space flight. For long-term interplanetary missions, they constitute a potentially limiting factor since current protection limits for low-Earth orbit missions may be approached or even exceeded. In such a situation, an accurate risk assessment requires knowledge of equivalent doses in critical radiosensitive organs rather than only skin doses or ambient doses from area monitoring. To achieve this, the MATROSHKA experiment uses a human phantom torso equipped with dedicated detector systems. We measured for the first time the doses from the diverse components of ionizing space radiation at the surface and at different locations inside the phantom positioned outside the International Space Station, thereby simulating an extravehicular activity of an astronaut. The relationships between the skin and organ absorbed doses obtained in such an exposure show a steep gradient between the doses in the uppermost layer of the skin and the deep organs with a ratio close to 20. This decrease due to the body self-shielding and a concomitant increase of the radiation quality factor by 1.7 highlight the complexities of an adequate dosimetry of space radiation. The depth-dose distributions established by MATROSHKA serve as benchmarks for space radiation models and radiation transport calculations that are needed for mission planning.