逸見 良道

The Martian Moons eXploration (MMX) mission will study the Martian moons Phobos and Deimos, Mars, and their environments. The mission scenario includes both landing on the surface of Phobos to collect samples and deploying a small rover for in situ observations. Engineering safeties and scientific planning for these operations require appropriate evaluations of the surface environment of Phobos. Thus, the mission team organized the Landing Operation Working Team (LOWT) and Surface Science and Geology Sub-Science Team (SSG-SST), whose view of the Phobos environment is summarized in this paper. While orbital and large-scale characteristics of Phobos are relatively well known, characteristics of the surface regolith, including the particle size-distributions, the packing density, and the mechanical properties, are difficult to constrain. Therefore, we developed several types of simulated soil materials (simulant), such as UTPS-TB (University of Tokyo Phobos Simulant,Tagish Lake based), UTPS-IB (Impact-hypothesis based), and UTPS-S (Simpler version) for engineering and scientific evaluation experiments.

The mothership of the Martian Moons eXploration (MMX) will perform the first landing and sampling on the surface of Phobos. For the safe landing, the 2.1-m-wide mothership of the MMX should find a smooth surface with at most 40 cm topographic irregularity, however, whose abundance or even existence is not guaranteed based on current knowledge. We studied the highest resolution (a few meters per pixel) images of Phobos for possible topographic irregularities in terms of boulder (positive relief feature) and crater distributions. We find that the spatial number densities of positive relief features and craters can vary significantly, indicating that the surface irregularities vary significantly over the entire surface. We extrapolate the size-frequency distributions of positive relief features to evaluate the surface roughness below the image resolution limit. We find that the probabilities that topographic irregularities are < 40 cm for the areas of 4 × 4 m and 20 × 20 m are > 33% and < 1% for boulder-rich areas and > 88% and > 13% for boulder-poor areas, respectively, even for the worst-case estimates. The estimated probabilities largely increase when we reduce the assumed number of positive relief features, which are more realistic cases. These indicate high probabilities of finding a smooth enough place to land on Phobos' surface safely....

Crater morphology and surface age of asteroid (162173) Ryugu are characterized using the high-resolution images obtained by the Hayabusa2 spacecraft. Our observations reveal that the abundant boulders on and under the surface of the rubble-pile asteroid affect crater morphology. Most of the craters on Ryugu exhibit well-defined circular depressions, unlike those observed on asteroid Itokawa. The craters are typically outlined by boulders remaining on the rim. Large craters (diameter > 100 m) host abundant and sometimes unproportionally large boulders on their floors. Small craters (<20 m) are characterized by smooth circular floors distinguishable from the boulder-rich exterior. Such small craters tend to have dark centers of unclear origin. The correlation between crater size and boulder number density suggests that some processes sort the size of boulders in the shallow (<30 m) subsurface. Furthermore, the crater size-frequency distributions (CSFDs) of different regions on Ryugu record multiple geologic events, revealing the diverse geologic history on this 1-km asteroid. Our crater-counting analyses indicate that the equatorial ridge is the oldest structure of Ryugu and was formed 23-30 Myr ago. Then, Ryugu was partially resurfaced, possibly by the impact that formed the Urashima crater 5-12 Myr ago. Subsequently, a large-scale resurfacing event formed the western bulge and the fossae 2-9 Myr ago. Following this process, the spin of Ryugu slowed down plausibly due to the YORP effect. The transition of isochrons in a CSFD suggests that Ryugu was decoupled from the main belt and transferred to a near-Earth orbit 0.2-7 Myr ago....

Estimating the regolith properties of Phobos' surface is of critical importance for the landing and performance of the Martian Moons eXploration (MMX) sample return mission. Regolith physical properties such as strength, regolith thickness, and the presence of regolith layers are related to morphologies of superposing impact craters. However, the accurate depths of Phobos' sub-kilometer-diameter craters including irregularly shaped craters have not yet been fully characterized. Here, by using our high-resolution (20 m/pixel) digital elevation model of the nearside (or the sub-Mars side) of Phobos, we investigate the topographic profiles of the sub-kilometer craters. We confirm the presence of crater rims, and bowl-shaped, central-mound, and flat-floored crater geometries. The topography of one flat-floored crater is consistent with a boundary of regolith layers at a depth of ~160-180 meters. Morphometric measurements of 35 sub-kilometer craters show that their depth-to-diameter (d/D) ratios are in the range of 0.037 and 0.174 (mean value = 0.089, median value = 0.093). This suggests either a surface layer composed of rocky debris that effectively dissipates impact energy and causes a reduction in crater depth, or subsequent resurfacing events changed the original crater topography....