The radiation environment on the surface of Mars – Numerical calculations of the galactic component with GEANT4/PLANETOCOSMICS
Introduction
The atmosphere of Mars provides a much weaker protection from highly energetic cosmic radiation compared to its terrestrial counterpart. While life at the surface of Earth is protected from the ionizing cosmic radiation by a column of air of up to 1000 g/cm2 depending on the elevation of ground, the atmospheric mass on Mars amounts to about 20 g/cm2. The exact value depends on the location on the Martian surface, i.e. its elevation, and seasonal variations. In contrast to Earth, Mars does not have a strong magnetic field either, which could provide additional shielding, and which leads to reduced primary cosmic radiation intensity. Therefore life on Mars would constantly be exposed to much higher levels of radiation as on Earth, which is a crucial concern for future manned missions to Mars (Cucinotta et al., 2010, Durante and Cucinotta, 2008, McKenna-Lawlor et al., 2012) due to the potential health risks of astronauts. The long term radiation exposure is also an important aspect concerning the question if life on Mars may presently exist or may have existed in the past (Kminek et al., 2003, Musilova et al., 2015, Westall et al., 2013). Ionizing radiation may also affect the possibility to find traces of ancient life on Mars (Pavlov et al., 2012).
Two possible approaches exist to address the question of the prevailing radiation field at Mars: measurements and model calculations. While measurements are extremely costly and can only reveal a part of the radiation field under the given environmental conditions model calculations often do not have this type of restriction. However, due to the complexity of the calculations, the results of the models may be subject to large uncertainties. Among others, the accuracy of the models depends on the preciseness of the galactic cosmic radiation input spectra, the atmospheric and regolith composition, the cross sections and models in the transport code, etc. Therefore, a validation of the model results with experimental data is essential to determine the level of accuracy of the models and to find their weak and strong points.
The Radiation Assessment Detector (RAD) (Hassler et al., 2012) on the curiosity rover of the Mars Science Laboratory (MSL) (Grotzinger et al., 2012) delivers such data. It was the first instrument to measure the radiation exposure on the surface of Mars: charged and neutral particle spectra, dose rates and their seasonal and diurnal variations as well as zenith angle dependence (Ehresmann et al., 2014, Guo et al., 2015, Hassler et al., 2014, Köhler et al., 2014, Rafkin et al., 2014, Wimmer-Schweingruber et al., 2015). Measurements of charged and neutral particle spectra measured by RAD were compared to model calculations by Matthiä et al. (2016). As a follow up, in June 2016 a workshop (“1st Mars Space Radiation Modeling Workshop”, http://www.boulder.swri.edu/rad_modeling_workshop) was held with the goal to bring together as many groups as possible to compare results on calculated particle spectra on the Martian surface, to discuss and to improve our understanding of the radiation field caused by galactic cosmic radiation on the Martian surface and its formation. This work describes the contribution to this workshop obtained using a combined GEANT4 and PLANETOCOSMICS setup and galactic cosmic radiation (GCR) input spectra from the DLR-model by Matthiä et al. (2013).
Section snippets
GEANT4 and PLANETOCOSMICS
The simulations were performed with an adapted version of the GEANT4 (GEometry And Tracking) (Agostinelli et al., 2003, Allison et al., 2006) toolkit PLANETOCOSMICS (http://cosray.unibe.ch/∼laurent/planetocosmics/; Desorgher et al., 2006). Compared to the original PLANETOCOSMICS, the adapted version can be used with recent versions of GEANT4 applying new and improved physics lists. In this work, the transport of the particles through the simulated Martian environment was performed with
Results
In this chapter the particle flux of the most abundant particles is presented. To better understand the radiation field, an analysis of the directionality of the particles is performed. Such an analysis can help to identify the dominating physical processes in particle creation and determine the sources of deviations in model-model and model-data benchmarks. The calculation of directional particle fluxes also facilitates shielding studies, for instance for different habitat or radiation shelter
Summary
In this work we present the results for the particle flux caused by galactic cosmic radiation and related dose rates at the Martian surface calculated with GEANT4/PLANETOCOSMICS and contributed to the 1st Mars Radiation Modeling Workshop. The environmental conditions for which the simulations were performed are derived from the conditions encountered by the RAD instrument on the Curiosity rover between November 15, 2015 and January 15, 2016. In addition to presenting the zenith angle averaged
Acknowledgements
The authors would like to thank the Sodankyla Geophysical Observatory and the website team (http://cosmicrays.oulu.fi) for providing the Oulu neutron monitor data.
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