Chapter 5: Craters (2/6)
 FIGURE 110 [below].- An oblique view looking northwest at part of the wall of the crater Lobachevsky on the lunar far side. It shows a small crater on Lobachevsky's wall with unusual streaks of dark material that appear to have originated from the lower rim of the structure and to have moved down toward the floor of Lobachevsky. This feature was first noticed by T. K. Mattingly, the Apollo 16 CMP, who described the darker streaks as probable lava flows (Mattingly, El-Baz, and Laidley, 1972). However, the streaks can also be explained by the downslope movement of dark fragmental debris excavated from Lobachevsky's wall by the small crater. Closer to the lower border of the photograph is a bright area extending across Lobachevsky's rim. This area and other sinuous light-colored markings in the upper half of the photograph are on the periphery of an enormous field of light-colored swirls in this part of the far side (El-Baz, 1972a). The origin of the swirls is not well understood.-F.E.-B.
FIGURE 111 [above].-For easier viewing, this picture is oriented with north at the bottom of the page. It shows the striking bilateral symmetry of the rays of a small (2-km diameter) crater in the floor of the large crater Daguerre in Mare Nectaris. Continuous areas and narrow filaments of light-gray ejecta extend from the crater across the dark mare surface through 270°, but are entirely absent in the southern 90° sector. Within the crater, dark material occurs on the southern crater wall while the remaining walls are bright. (The reader may wonder about the material whose reflectivity cannot be observed because it lies in shadow on the east wall of this crater. Until the area is observed under high Sun conditions, we are forced to make the simplifying assumption that it is bright because most of the materials visible elsewhere in the walls are bright.) This crater probably resulted from the impact of a projectile traveling from south to north along an oblique trajectory. Its pattern of ejecta distribution is similar to that of small craters produced by the impact of missiles along oblique trajectories at the White Sands Missile Range, N. Mex. Some observers postulate that the dark material is a talus deposit of mare material that has fallen into the crater.-H.J.M.
Another geological explanation is that the unusual pattern may be due to an intrinsic characteristic of the local terrain, probably an abrupt lateral change in the composition of the bedrock within the area that was excavated. F.E.-B.
FIGURE 114 [above].-This is an oblique view of another crater that probably was formed by a meteoroid following a relatively low-angle trajectory. This crater, 4 km in diameter, is located in the highlands east of Mare Serenitatis. Compared to the crater just described (see fig. 1 13), this one is less elliptical and its bilobate ray pattern is much less pronounced. The differences may be attributed to a higher trajectory angle of the impacting body that formed this crater as it struck the surface. H. J. Moore (1976), in his study of craters formed by impacting missiles at White Sands Missile Range, recognized a characteristic asymmetric profile along the axis of trajectory for craters formed in this manner. The wall beneath the missile trajectory is typically less steep than the opposite or down-trajectory wall, and its rim crest is lower and more rounded. These observations, when applied to the lunar crater in this photograph, indicate that the impacting body was traveling toward the east when it struck the Moon.-H.M.
FIGURE 116 [above].-Impact craters with asymmetric ray patterns and profiles can be caused by conditions other than the angle of trajectory. This 5-km crater was formed when a meteoroid impacted on the northeast rim crest of Gibbs, a very much larger and older crater near the Moon's east limb. In this restricted view, Gibbs' rim is the dark area in the north half of the picture, and its wall is the light area in the south half. The rim crest extends from arrow to arrow. Discrete rays of both light and dark ejecta are well developed around the north half of the small crater where they were deposited on a relatively level surface. They are poorly developed around the south side of the small crater, probably having been partly destroyed by mixing as the ejected materials cascaded down the much steeper wall of the crater Gibbs. Subsequent erosion has further destroyed the original pattern. The configuration of the small crater's rim has also been affected by topography. It is sharply defined along the north side but is barely discernible along the south side where large volumes of material have slumped down the wall of the older crater.-G.W.C.
FIGURE 117 [above].-Remarkable detail is shown in this enlargement of a small part of a panoramic camera frame. In most respects, the crater itself is typical of a great many craters its size-about 1.2 km. Because it does not have rays, it is believed to be older than most other craters discussed previously in this chapter. Its rounded rim crest and slightly raised rim (extending outward to the arrow on the west side) also point to its greater age. On the other hand, it is young enough that some of the original dunelike texture of the ejecta blanket is preserved (especially to the west), a great many large blocks of ejecta are still visible, and the original depth of the crater has not been greatly lessened by infalling debris. The largest blocks, which are about 30 m in size, occur near the rim. The terrace (T) extending partly around the wall about 100 m below the surface probably marks the top of a resistant rock layer. However, if there were other signs of bedrock stratification within this crater, they have been obscured by the movement of debris down the walls. The very smooth floor is the only unusual feature of this crater. It may consist of a solidified pool of rock melted by heat generated from the impact.-H.J.M.
FIGURE 118 [above].-Kant P is the larger of these two craters in the central highlands on the Moon's near side. About 5.5 km in diameter, its overall shape is not in the least unusual. However, the younger, small pear-shaped crater on Kant P's north wall is an excellent example of the controlling effect that topographic relief plays on the shape of an impact crater. Because the small crater was formed on a steeply sloping surface, its ejecta was deposited chiefly downslope and formed a broad rim. The original rim and wall on the upslope side have been obliterated by slumping. The slumping has left a landslide scar and has caused talus and scree to be deposited in the lower part of the crater.-H.J.M.
 FIGURE 119 [below].-This oblique view of the crater Isidorus D was taken with the panoramic camera on Apollo 16. Isidorus D is about 15 km in diameter and is located in the highlands between Mare Tranquillitatis and Mare Nectaris. Evidence of avalanching (Howard, 1973) and of other types of downslope movement of material are clearly visible on the inner walls of the crater. The streaks resembling shooting stars on the left wall appear to be avalanche scars. The avalanches probably were spearheaded by large blocks followed by fine-grained material. On the near wall (arrow) a larger landslide terminates in a straight line against the relatively flat crater floor. In the shadowed part of the crater wall many short irregular benches or narrow terraces mark the tops of masses of slumped material. The brightness of the avalanche scars is an indication of their freshness; in general, freshly exposed lunar materials are brighter than undisturbed materials nearby.- F.E.-B.