AS17-3128(P)

Figure 198

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

The branching sinuous rille in this photograph is part of a continuous single rille extending more than 300 km across the mare in northeast Oceanus Procellarum. The width of the rille is essentially constant approximately 250 m) throughout its length. The secondary branches, all of which rejoin the main "stream," are shallower than the larger channel. Slope is probably northeast toward the center of Mare Imbrium. Like some other lunar rilles, this one crosses several mare ridges with no apparent deviation or deformation; unlike many rilles, particularly those near the Aristarchus plateau, there is no associated crater at either end. The origin of lunar sinuous rilles remains controversial. Among the alternatives proposed are lava channels and lava tubes, but fracture control is decidedly apparent in some places. Some sort of fluid erosion, however, seems necessary to account for the configurations of many rilles with exactly parallel walls from which material has been removed; lava may be the most plausible agent for erosion inasmuch as no evidence of water exists in the lunar samples. The diversity among rilles suggests that several genetic hypotheses may be required to explain all of them.    -C.A.H.