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.

中文翻译：

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小行星 (162173) Ryugu 的地质历史和陨石坑形态

小行星 (162173) Ryugu 的陨石坑形态和表面年龄使用由隼鸟 2 号航天器获得的高分辨率图像进行表征。我们的观察表明，碎石堆小行星表面上和表面下的大量巨石会影响陨石坑的形态。与在小行星 Itokawa 上观察到的不同，龙宫上的大多数陨石坑都表现出明确的圆形凹陷。陨石坑通常由留在边缘的巨石勾勒出轮廓。大陨石坑（直径 > 100 m）的地面上有大量的、有时甚至是不成比例的大巨石。小陨石坑 (<20 m) 的特点是光滑的圆形地板与富含巨石的外部不同。如此小的陨石坑往往有不明来源的黑暗中心。陨石坑大小和巨石数量密度之间的相关性表明，某些过程对浅层 (<30 m) 地下的巨石大小进行了分类。此外，龙宫不同区域的陨石坑大小频率分布（CSFD）记录了多个地质事件，揭示了这颗 1 公里小行星上多样的地质历史。我们的陨石坑计数分析表明，赤道脊是龙宫最古老的结构，形成于 23-30 Myr。然后，Ryugu 部分重新浮出水面，可能是由于 5-12 Myr 之前形成浦岛陨石坑的撞击造成的。随后，一场大规模的重铺事件形成了西部凸起和 2-9 Myr 前的窝。在此过程之后，由于 YORP 效应，Ryugu 的自旋似乎减慢了。