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.