The results of crater size–frequency distribution (CSFD) for the maria units in Lacus Somniorum. (a) Crater count area map. Blue zones indicate the crater count area for maria units. The backgrounds are the horizontal density anomaly slice at a depth of 25 km and a WAC image (File Name: WAC_GLOBAL_E300N0450_100M). CSFD results for (b) unit Em_1, (c) unit Im2_1, (d) unit Im2_2, and (e) unit Im2_3.