Subsurface Radiation Environment of Mars and Its Implication for Shielding Protection of Future Habitats

• Published in: Journal of geophysical research. Planets Vol. 125; no. 3
• Main Authors: Röstel, Lennart, Guo, Jingnan, Banjac, Saša, Wimmer‐Schweingruber, Robert F., Heber, Bernd
• Format: Journal Article
• Language: English
• Published: 01.03.2020
• Subjects: Mars exploration; particle transport model; planetary space weather; space radiation
• ISSN: 2169-9097; 2169-9100
• DOI: 10.1029/2019JE006246

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. Plain Language Summary Space radiation is a major risk for humans, especially for manned missions in deep space and to Mars which does not have an atmosphere thick enough to shield against cosmic high‐energy particles. It is practical and likely for future human habitats on Mars to use natural surface material as shielding protection against radiation. In order to quantify the optimized shielding depth, we model the radiation environment including the absorbed dose and biological equivalent dose induced by omnipresent galactic cosmic rays reaching Mars' surface and also subsurface. We also implement different Martian regolith properties in the model and compare the results. For instance, to limit the annual equivalent dose to be within 100 mSv, the required shielding depths are between 80 cm and 2.5 m depending on the soil composition with hydrated soil requiring less shielding. This is because water (hydrogen)‐rich regolith may effectively reduce equivalent dose both above and below the Martian ground via modulating the flux of fast neutrons (with energy below 10 MeV). In conjunction with the new discovery of subsurface liquid water by Mars Express, our result suggests the advantage of seeking for human landing sites with high water content is also effective for radiation‐shielding protection purposes. Key Points We calculate dose and equivalent dose rates induced by galactic cosmic ray particles at varying heights above and below the Martian surface Different Martian regolith scenarios are simulated to understand the impact of subsurface components on the Martian radiation environment We find water/hydrogen‐rich regolith may effectively reduce equivalent dose both above and below the ground via modulating the neutron flux