AS15-2618(M)

Figure 31

 Click here for larger NASA image
 Click here for lunar chart showing location

During the various Apollo missions, the west limb of the Moon was in shadow behind the terminator and could not be photographed. This south-looking oblique view across western Oceanus Procellarum shows one of the western-most areas visible to the astronauts. As the word "oceanus"(=ocean) implies, Procellarum is larger than a mare. Like the maria however, it is formed by dark mare materials that originated as flows of basaltic lava. The crater Seleucus, 42 km in diameter, is shown in sunrise along the right (west) edge of the frame. The narrowness of its rim and the abrupt contact between its raised rim and the surrounding mare prove that the final mare flooding occurred after the crater was formed. In other words, the crater predates at least the youngest mare basalts in this area. The same age relations hold true at Schiaparelli, the second largest crater in view.  Several medium-sized craters and countless small craters are surrounded by finely textured outer ejecta deposits and hence are younger than the mare. The Marius Hills, the most extensive aggregation of volcanic domes and cones on the Moon, are visible on the left horizon. Thus the picture shows the characteristic events of late lunar history-the sporadic formation of impact craters concurrent with volcanic eruptions that form lava plains, hills, and  ridges. -D.E.W.