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These two contrasting pictures of the same area in southeastern Mare Imbrium were taken by Apollo 15, but on different revolutions under different lighting conditions. The picture on the left was taken when the Sun angle was 17°; the Sun angle was 2° when the picture on the right was taken. The large crater at the west edge is Timocharis. The area is dominated by three geologic units. The oldest is a fairly densely cratered fractured plains unit of moderate albedo that occupies the eastern part of the area. Next oldest is the mare unit in the central part, with its typically smooth, level surface and moderately low albedo. The youngest unit is the bright (high-albedo), highly textured ejecta surrounding Timocharis.
We have included the two pictures to illustrate the problems photogeologists sometimes face when drawing a contact line between units. The eastern edge of the mare is used as an example. Throughout most of the area shown the mare is in contact with the plains unit. Characteristically mare material is darker and smoother than plains material. Using the picture on the left in which albedo differences are enhanced because of the relatively high Sun angle, the contact might be drawn as shown. The line is equivocal in places, but, in general, it does satisfactorily separate darker areas from lighter areas. Using the picture on the right, in which surface relief is exaggerated because of very low Sun angle, the contact would be drawn as shown. Some areas dark enough to be mapped as mare in the first picture are here seen to be too roughly textured to be mare. As drawn, the line separates a unit that is both dark and smooth from a unit that is predominantly light and everywhere rugged.
Detailed stereoscopic examination of all available pictures of this area explains why some dark areas within the plains unit should not be classified as mare. In several of them there are structures resembling volcanic outlets (wide arrows on left photo). Similar structures were not found elsewhere within the plains unit. Therefore, it is likely that some if not all the darker areas of the plains are caused by veneers of dark volcanic ejecta so thin that the surface relief of the underlying plains is still visible.
An additional point of interest is the clearly defined sinuous rifle (small arrows in right photo) that extends half the length of the picture; the same rifle is almost invisible in the other picture. -G.W.C.
Report Source: NASA SP-362, Page 74, Figure 64
This web page was created by Francis Ridge
for The Lunascan Project
Section Directory 21