Numerical simulation of light-element geochemistry of the lunar surface using a compact and lightweight XRF imaging spectrometer

Abstract Understanding the evolution of the Moon requires mapping the global distribution and abundance of major elements on the lunar surface. X-ray fluorescence exploration is a promising method for estimating the distribution of light elements. The lunar south pole has drawn increasing scientific and exploratory interest. The chemical composition of the polar region is crucial for evaluating potential landing sites and interpreting data from in situ exploration of the lunar south pole. However, X-ray fluorescence observations have not achieved a complete global map of the lunar elemental distribution. This is due to various factors, such as low solar flare activity periods and radiation damage to the detectors. In some case, the limited energy resolution of the detectors prevented adequate resolution of the spectra of light elements. Particularly in the polar regions, observing fluorescent X-rays has been difficult because solar X-rays are weakly incident. This paper proposes global fluorescence X-ray imaging observations using our novel ultra-compact and lightweight X-ray telescope. We conduct a numerical calculation to develop an emission model for lunar fluorescent X-rays under M1-class solar flare conditions. Next, we incorporate a response model that reflects the instrument’s quantum efficiency and field of view. Assuming a polar circular orbit and an annual occurrence of 300 M-class solar flare events, we evaluate the required mission duration for a signal-to-background ratio exceeding 10. The simulation results indicate that O, Fe, Mg, Al, and Si could be observed globally in two years. In this case, the spatial resolution is approximately 70 km × 70 km. Moreover, a spacecraft with 25 telescopes to 5 × 5 array could facilitate global observation of O, Mg, Al, Si, and Fe in a year and detect Na within 2 years. The spatial resolution is about 30 km × 30 km. This result suggests that this approach could assist in developing future global lunar elemental maps and evaluating the chemical abundance of these elements across the entire surface of the Moon. Graphical Abstract