  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.
