In order to quantify the optimal radiation shielding depth on Mars in preparation for future human habitats on the red planet, it is important to understand the Martian radiation environment and its dependence on the planetary atmospheric and geological properties. With this motivation we calculate the absorbed dose and equivalent dose rates induced by galactic cosmic ray particles at varying heights above and below the Martian surface considering various subsurface compositions (ranging from dry rock to water‐rich regolith). The state‐of‐the‐art Atmospheric Radiation Interaction Simulator based on GEometry And Tracking Monte Carlo method has been employed for simulating particle interaction with the Martian atmosphere as well as subsurface materials. We calculate the absorbed dose in two different phantoms: a thin silicon slab and a water sphere. The former is used to validate our model against the surface measurement by the Radiation Assessment Detector on the Curiosity rover, while the later is used to approximate a human torso, also for evaluation of the biologically weighted equivalent dose. We find that the amount of hydrogen contained in the water‐rich regolith plays an important role in reducing the equivalent dose through modulation of neutron flux (below 10 MeV). This effective shielding by underground water is also present above the surface, providing an indirect shielding for potential human explorations at this region. For long‐term habitats seeking the Martian natural surface material as protection, we also estimate the optimal shielding depth, for different given subsurface compositions, under maximum, average, and minimum heliospheric modulation conditions.

A new GEANT4 particle transport model -the Atmospheric Radiation Interaction Simulator (AtRIS, Banjac et al., 2018) -has been recently developed in order to model the interaction of radiation with planets. The upcoming instrumentational advancements in the exoplanetary science, in particular transit spectroscopy capabilities of missions like JWST and E-ELT, have motivated the development of a particle transport code with a focus on providing the necessary flexibility in planet specification (atmosphere and soil geometry and composition, tidal locking, oceans, clouds, etc.) for the modeling of radiation environment for exoplanets. Since there are no factors limiting the applicability of AtRIS to Mars and Venus, AtRIS' unique flexibility opens possibilities for new studies.Following the successful validation against Earth measurements (Banjac et al., 2018), this work applies AtRIS with a specific implementation of the Martian atmospheric and regolith structure to model the radiation environment at Mars. We benchmark these first modeling results based on different GEANT4 physics lists with the energetic particle spectra recently measured by the Radiation Assessment Detector (RAD) on the surface of Mars. The good agreement between AtRIS and the actual measurement provides one of the first and sound validations of AtRIS and the preferred physics list which could be recommended for predicting the radiation field of other conceivable (exo)planets with an atmospheric environment similar to Mars.

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