Geological Chronicle
Rima Hyginus is a lunar graben cleft spanning 220 kilometers across the boundary between Sinus Medii and Mare Vaporum near the center of the Moon's near side. Centered at selenographic coordinates 7.4° N, 7.8° E, the feature consists of two graben branches extending northwest and east-southeast from a central intersection. The structure formed through volcano-tectonic faulting, where crustal extension opened a linear trench along a regional fracture zone.
At the midpoint of the graben sits the 9-kilometer-wide crater Hyginus, a lunar depression that reaches a depth of 0.8 kilometers. Hyginus is an endogenic volcanic caldera that lacks the elevated rim and ejecta blanket typical of impact craters. A chain of rimless collapse pit craters extends along the graben floor on both branches. These pit craters formed when surface material collapsed into subterranean void spaces and magma conduits along the fault line.
NASA mission planners considered the Hyginus region as a candidate landing site during the Apollo program to investigate potential lunar volcanism. Proposed mission profiles targeted the plains northwest of the caldera to sample the Cayley Formation and collect dark volcanic deposits. Telescopic interest in the area had previously surged in 1878, when German astronomer Hermann Klein claimed to observe a new crater near the rille, drawing widespread attention from lunar observers.