Monte Carlo simulations of the Martian surface and subsurface radiation environment for human missions

One of the most critical challenges to expand human exploration on the surface of Mars is radiation protection for astronauts on long-duration missions, due to the severe health effects that can be caused by long-term exposure to radiation.

In space there are two main sources of radiation: Galactic Cosmic Rays (GCRs) and Solar Particle Events (SPEs). Mars does not have an intrinsic magnetic field capable of providing any significant shielding from space radiation. As a result, energetic particles in GCRs and SEPs can penetrate the Mars atmosphere and interact with the atmosphere, before reaching the surface, and with the Martian subsurface, generating many secondary particles. These interactions result in a complex radiation spectrum, given by primary and secondary particles, that depend on the planetary atmospheric and geological properties. An understanding of the Martian radiation environment is important to identify potential natural shelters for astronauts, that can lead to incoming radiation loss of energy through ionization processes and provide a long-term reduction of the exposure to radiation from above. Possible shelter candidates are subterranean lava tubes, natural underground tunnels formed by flowing lava that cools and solidifies on the surface while molten lava continues to flow beneath, that can be large and structurally stable, potentially offering natural protection from cosmic radiation, solar wind, strong temperature excursion, dust and micrometeorite impacts, for future exploration and habitation. Recent works [1] have highlighted the presence on Mars of voluminous underground caves and potential lava tubes with sizes typically ranging from around 50 meters and depths often exceeding 100 meters.

We implemented Monte Carlo simulations, using CORSIKA 8 [2] [3] and FLUKA [4], to study the radiation environment on Mars, with a precise modelling of the cascade of secondary particles generated during interactions and a detailed atmospheric model. Therefore, we made a precise quantification of the change of particle spectra under different shielding environment like at Martian surface, subsurface and within Martian caves, for different given subsurface compositions and solar activity conditions. Also, we compared the simulated radiation levels within caves to surface conditions, in order to quantify the benefits offered by subsurface environments.

[1]           Sauro F., et al., Lava tubes on Earth, Moon and Mars: A review on their size and morphology revealed by comparative planetology, Earth Science  Reviews, 2020.

[2]           Engel R., et al., Towards A Next Generation of CORSIKA: A Framework for the Simulation of Particle Cascades in Astroparticle Physics, 2019.

[3]           Alameddine J. M., et al., Simulating radio emission from particle cascades with CORSIKA 8, 2025.

[4]           Battistoni G., et al., Overview of the FLUKA code, Annals of Nuclear Energy, 2015.