The nature of an apparently random rock deposit, such as regolith on an extraterrestrial body, is often evaluated in terms of the size distribution of the rock particles. Especially, the power-law index of the cumulative size-frequency distribution (CSFD), , is often used to describe the distribution. However, an value is typically obtained only from surface images, raising questions about its ability to reflect the size distribution of the entire deposit. In this study, we perform laboratory and numerical experiments to demonstrate that an value obtained from surface images alone does not necessarily represent that of the entire granular deposit. The value is only a reliable indicator of the size distribution of surface rocks, which can still be useful to gain insight into the evolutionary history of an asteroid’s surface. However, to fully address the internal information, direct observations of subsurface materials or sample analysis are required.

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 23-29 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.