PREPARATIONS OF SIMPLIFIED PHOBOS SIMULANTS FOR ENGINEERING STUDIES OF LANDING AND SAMPLING SYSTEMS OF MMX MISSION

Martian Moons eXploration (MMX) is scheduled to be launched in 2024, aiming to land on the surface of Phobos and return the regolith (>10 g) back to the Earth in 2029. Analyses of the returned samples will clarify the discussions on the origin of Phobos, which means that a safe landing and secure sampling are significant. For the development of the landing/sampling system, we need appropriate assumptions of the mechanical properties of the surface materials, such as the porosity, the particle-size distribution, the bearing capacity, and the angle of repose. Even though Phobos has been observed by several missions, the resolutions of the data obtained thus far are limited to constrain such properties. Our approach is to develop simulated surface materials (simulants) of Phobos that can be used to examine the landing/sampling systems, as reported partly by Miyamoto et al. (2018). For experiments of a larger scale, a further simplified simulant is preferred in terms of costs and easiness in handling. Thus, we develop a simplified version of simulants by reducing some components. We measure the mechanical properties of the simulants, such as the dynamic and static angles of repose, the bulk density, and the true density with various size-distributions. Additionally, we develop a measurement system of pseudo-N-values that can be applicable for small volume samples in the laboratory, and measure the value of the simulants. As a result, the simplified simulants are divided into three groups: (1) composed only of smaller particles, having a low loose-bulk density (e.g., 0.95 g/cm³) and a small pseudo-N-value (~1), (2) composed only of larger particles, having a middle loose-bulk density (e.g., 1.30 g/cm³) and a large pseudo-N-value (>200), (3) composed of the particles with a broader range of particle sizes, having a high bulk density (e.g., 1.68 g/cm³) and a middle pseudo-N-value(~100).