From the Top of Martian Olympus to Deep Craters and Beneath: Mars Radiation Environment Under Different Atmospheric and Regolith Depths

In preparation for future human habitats on Mars, it is important to understand the Martian radiation environment. Mars does not have an intrinsic magnetic field and Galactic cosmic ray (GCR) particles may directly propagate through and interact with its atmosphere before reaching the surface and subsurface of Mars. However, Mars has many high mountains and low‐altitude craters where the atmospheric thickness can be more than 10 times different from one another. We thus consider the influence of the atmospheric depths on the Martian radiation levels including the absorbed dose, dose equivalent and body effective dose rates induced by GCRs at varying heights above and below the Martian surface. The state‐of‐the‐art Atmospheric Radiation Interaction Simulator based on GEometry And Tracking Monte Carlo method has been employed for simulating particle interactions with the Martian atmosphere and terrain. We find that higher surface pressures can effectively reduce the heavy ion contribution to the radiation, especially the biologically weighted radiation quantity. However, enhanced shielding (both by the atmosphere and the subsurface material) can considerably enhance the production of secondary neutrons which contribute significantly to the effective dose. In fact, both neutron flux and effective dose peak at around 30 cm below the surface. This is a critical concern when using the Martian surface material to mitigate radiation risks. Based on the calculated effective dose, we finally estimate some optimized shielding depths, under different surface pressures (corresponding to different altitudes) and various heliospheric modulation conditions. This may serve for designing future Martian habitats.Plain Language Summary: Thanks to Earth's magnetic field and atmosphere, high‐energy cosmic particles can be efficiently shielded from causing radiation risks for humans on Earth. However, for crewed space missions, in particular long‐term missions to Mars, space radiation is a major risk for the health of astronauts. Mars does not have an intrinsic global magnetic field and its atmosphere is too thin to effectively shield against radiation. Here, we model the Martian radiation environment induced by omnipresent cosmic rays in Mars's atmosphere and terrain. Given that Mars has many high mountains and low‐altitude craters where the atmospheric thickness can be more than 10 times different from one another, we also consider different model setups with different atmospheric profiles. We find that with more shielding the heavy ion contribution to the radiation is reduced while the neutron contribution is enhanced. For a given threshold of the annual biologically weighted radiation effective dose, for example, 100 mSv, the required regolith depth ranges between about 1 and 1.6 m. At a deep crater where the surface pressure is higher, the needed extra regolith shielding is slightly smaller. Our study may serve for mitigating radiation risks when designing future Martian habitats using natural surface material as shielding protection.Key Points: We calculate dose, dose equivalent, and effective dose rates induced by various components of galactic cosmic rays on and below Mars surface. Surface pressure which is related to geographic altitude influences the surface and subsurface radiation level. Subsurface secondary neutrons contribute significantly to the effective dose and are a critical concern for radiation risks on Mars.CAS strategic priority programNational Natural Science Foundation of China (NSFC) http://dx.doi.org/10.13039/501100001809CNSA pre‐research Project on Civil Aerospace TechnologiesThe Key Research Program of the Chinese Academy of SciencesGerman Aerospace Center (DLR)https://et-wiki.physik.uni-kiel.de/atris/atri