Geological Chronicle
Located in the rugged highlands near the southeastern lunar limb, Janssen is an ancient impact crater dating to the Pre-Nectarian period of the lunar geologic timescale. Despite billions of years of subsequent bombardment that have heavily worn its structure and breached its perimeter in multiple locations, the outline of the rim remains traceable. The battered outer ramparts form a distinctive hexagonal shape upon the lunar surface, marked by subtle curvature at each vertex.
The floor of the crater is pockmarked by subsequent impacts, most prominently Fabricius, which lies entirely within the outer wall across the northeast quadrant. Janssen's degraded rim is closely bordered by other significant landmarks: Metius adjoins the northeast wall, the eroded Brenner lies to the north, the conjoined craters Steinheil and Watt lie to the southeast, Lockyer sits astride the southwest rim, and the expansive trough of Vallis Rheita extends further to the east.
Across the southern two-thirds of Janssen's floor, observers can discern the faint remnants of a large concentric crater whose structure is partially overlaid by Fabricius. Within this inner depression winds the Rimae Janssen, an extensive rille system that curves from the rim of Fabricius out toward the southeastern wall, spanning a total distance of up to 140 kilometers.