Geological Chronicle
Jansen is a lunar impact crater in northern Mare Tranquillitatis, located east-southeast of Plinius. It spans a diameter of 24 kilometers and was named after seventeenth-century Dutch optician Zacharias Janssen.
The rim of Jansen is low and narrow, with a notch along the western edge. Its interior is relatively level, which may indicate it has been covered by lava. To the south-southwest, a small but prominent crater lies on the floor halfway between the center and the rim.
To the northwest of the rim lies a rille named Rima Jansen, with wrinkle ridges in the mare surface to the east. A low ridge runs from near the southeast rim in a southeasterly direction. Three former satellite craters have since been renamed by the IAU: Jansen B is now Carrel, Jansen C is Beketov, and Jansen F is Cajal.