标题： Ryugu上撞击坑的空间分布 显示英文标题

标题： The spatial distribution of impact craters on Ryugu

摘要： 小行星162173 Ryugu有许多撞击坑。 Sugita等人（2019年）最初对Ryugu上的撞击坑进行测量，是基于Hayabusa2在2018年6月到达后第一个月获取的ONC图像。 利用直到2019年2月拍摄的新图像，我们构建了Ryugu的全球撞击坑目录，其中包括Ryugu表面直径大于20米的所有撞击坑。 结果，我们在Ryugu表面识别出77个撞击坑。 Ryugu的撞击坑密度存在变化，这种变化无法用撞击的随机性来解释；低纬度地区撞击坑更多，高纬度地区更少，而西半球隆起区（160 E - 290 E）的撞击坑比子午线附近区域（300 E - 30 E）的撞击坑更少。 这种变化表明Ryugu有复杂的地质历史。 看起来，经度方向上的撞击坑密度变化简单地表明撞击坑年龄的变化；子午线附近的撞击地形似乎地质年代久远，而西半球隆起区相对较年轻。 撞击坑密度的纬度变化表明Ryugu的赤道脊是一个地质年代久远的结构；然而，这也可以通过Ryugu在过去短暂自转周期内与许多分裂碎片碰撞来解释。 显示英文摘要

摘要： Asteroid 162173 Ryugu has numerous craters. The initial measurement of impact craters on Ryugu, by Sugita et al. (2019), is based on Hayabusa2 ONC images obtained during the first month after the arrival of Hayabusa2 in June 2018. Utilizing new images taken until February 2019, we constructed a global impact crater catalogue of Ryugu, which includes all craters larger than 20 m in diameter on the surface of Ryugu. As a result, we identified 77 craters on the surface of Ryugu. Ryugu shows variation in crater density which cannot be explained by the randomness of cratering; there are more craters at lower latitudes and fewer at higher latitudes, and fewer craters in the western bulge (160 E - 290 E) than in the region around the meridian (300 E - 30 E). This variation implies a complicated geologic history for Ryugu. It seems that the longitudinal variation in crater density simply indicates variation in the crater ages; the cratered terrain around the meridian seems to be geologically old while the western bulge is relatively young. The latitudinal variation in crater density suggests that the equatorial ridge of Ryugu is a geologically old structure; however, this could be alternatively explained by a collision with many fission fragments during a short rotational period of Ryugu in the past.