Geological Chronicle
Palus Epidemiarum, or the Marsh of Epidemics, is a small lunar mare located in the southwestern portion of the Moon's near side, positioning it southwest of Mare Nubium and southeast of Mare Humorum. Occupying a shallow trough that extends roughly 300 by 120 kilometers, the flooded basalt terrain forms a rough band running generally west to east with a northward extension near its western boundary. Basaltic lava across the region reaches an average thickness of 200 to 250 meters, with localized deposits descending to a maximum depth of 750 meters.
The mare features notable rille systems, including the Rimae Ramsden in its western end and the wide Rima Hesiodus, which extends from near the midpoint toward the east-northeast for roughly 300 kilometers. The flooded crater Ramsden lies near the western end and lends its name to the Rimae Ramsden. The flooded crater Capuanus occupies the southern center attached to the southern edge, while the crater Cichus defines the eastern end of the mare.
The northern extension of the mare reaches the outer rims of the crater pair Campanus and Mercator. A narrow valley between these craters joins Palus Epidemiarum with Mare Nubium, with a rille from the Rimae Ramsden following the course of this cleft. The small double-walled crater Marth lies at the southern midpoint of this northern extension. Altimetry data from the Clementine spacecraft shows that the terrain slopes downward from west to east, with a height difference of 2 kilometers from end to end.