Solar Energetic Particles (SEP) are one of the major sources of the martian radiation environment. It is important to understand the SEP-induced martian radiation environment for future human habitats on Mars. Due to the lack of global intrinsic magnetic field, SEPs can directly propagate through and interact with its atmosphere before reaching the surface and subsurface of Mars. Since Mars has many high mountains and low-altitude craters where the atmospheric thickness can be more than 10 times different from one another, the SEP-resulted surface radiation level may be very different from one location to another. We thus consider the influence of the atmospheric depths on the martian radiation levels including the absorbed dose, dose equivalent, and (human-)body effective dose induced by SEPs 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 (AtRIS/GEANT4) has been employed for simulating particle interactions with the martian atmosphere and terrain. We find that even the thinnest martian atmosphere reduces radiation dose from that in deep space by at least 65\%, and the shielding effect increases for denser atmosphere. Furthermore, we present a method to quickly forecast the SEP-induced radiation in different regions of Mars with different surface pressures.

The distribution of Cr isotopes provides useful information to trace the source and origin of extraterrestrial samples, but it is usually influenced by high-energy cosmic rays. Since lunar and terrestrial materials have quite similar Cr isotope compositions, distinguishing the effect of cosmic rays in lunar samples is especially important. Those cosmic radiation particles (primary particles) can react with lunar materials, creating many secondary particles. Both primary and secondary particles can produce cosmogenic nuclides on the Moon. Radiation Environment and Dose at the Moon (REDMoon) is a novel GEANT4 Monte-Carlo model built to simulate the interactions of space particles with the lunar surface and subsurface content. Using this model, we simulate the production of cosmogenic Cr isotopes ($^{50}$Cr,$^{52}$Cr,$^{53}$Cr,$^{54}$Cr) at different depths of lunar surface, and compare the contribution of different reactions generating these nuclides. The results suggest that spallation reactions are the most important process producing cosmogenic Cr isotopes. We also analyze the relationship between $^{53}$Cr/$^{52}$Cr and $^{54}$Cr/$^{52}$Cr predicted by our model and compare it with different Apollo samples. As previously studied, we also find an approximate linear relationship between $\varepsilon^{53}$Cr and $\varepsilon^{54}$Cr (per 10,000 deviation of $^{53}$Cr/$^{52}$Cr and $^{54}$Cr/$^{52}$Cr ratios from the standard). Furthermore, we reveal a change of this linear relationship in different depths of lunar surface. Besides, we investigate how the slopes can be influenced by exposure age and the Fe/Cr ratio. With these additional factors carefully considered, the comparison between our modeled results and the measurements is better than previous studies.

The Moon lacks a global magnetic field and atmosphere, leaving its surface been directly exposed to high-energy cosmic radiation. Sporadic Solar Particle Events are sources of a significant radiation exposure, potentially posing serious threats to the health of astronauts exploring the Moon. Generally, Solar Energetic Particles (SEPs) have a limited penetration capabilities (value needed), and associated radiation doses diminish significantly with increasing astronauts shielding. In this paper, we use the Radiation Environment and Dose at the Moon (REDMoon) model based on GEometry And Tracking (GEANT4) Monte-Carlo method to calculate the body effective dose induced by 262 large historical SEP events on the Moon under different shielding depths which can result from the lunar regolith shielding and/or additional aluminum shielding. We calculate and compare the contributions of SEPs within different energy ranges to the total body effective dose and carry out a statistical analysis based on the results from different SEP events. Additionally, we develop empirical functions to rapidly assess SEP-induced effective dose on the Moon under different shielding scenarios.

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 (AtRIS) based on GEometry And Tracking (GEANT4) 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.