Apollo 15 Lunar Surface Journal

Extract from the Apollo 15 Preliminary Science Report
Chapter 5. Preliminary Geologic Investigation of The Apollo 15 Landing Site

G.A. Swann (U.S. Geological Survey; Principal Investigator), N.G. Bailey (U.S. Geological Survey), R.M. Batson (U.S. Geological Survey), V.L. Freeman (U.S. Geological Survey), M.H. Hait (U.S. Geological Survey), J.W. Head (Bellcomm, Incorporated), H.E. Holt (U.S. Geological Survey), K.A. Howard (U.S. Geological Survey), J.B. Irwin (NASA Manned Spacecraft Center), K.B. Larson (U.S. Geological Survey), W.R. Muehlberger (The University of Texas at Austin), V.S. Reed (U.S. Geological Survey), J.J. Rennilson (California Institute of Technology), G.G. Schaber (U.S. Geological Survey), D.R. Scott (NASA Manned Spacecraft Center), L.T. Silver (California Institute of Technology), R.L. Sutton (U.S. Geological Survey), G.E Ulrich (U.S. Geological Survey), H.G. Wilshire (U.S. Geological Survey), and E.W. Wolfe (U.S. Geological Survey)


Apennine Front

The Apennine Front is the face of the arcuate mountain chain that borders the Imbrium Basin on the southeast. Immediately adjacent to the Apollo 15 landing site, the Front consists of the slopes of two major mountains - Mt. Hadley to the northeast and Hadley Delta to the south. These massifs rise steeply 3 to 5 km above the local mare surface.

In the pre-mission photogeologic analyses of the Apollo 15 landing site (refs. 5-1 and 5-2), the Front massifs are interpreted as composed mainly of pre-lmbrian rocks consisting of impact breccias from the Serenitatis basin overlying a complex of impact breccias from still older basins and craters. The Imbrium impact caused faulting along lines both radial and concentric to the basin that produced the present-day arc of block-faulted mountains. Rolling hills east of the landing site between Hadley Delta and Mt. Hadley are interpreted as Imbrium-impact ejecta that may have been deposited by a base surge that arrived at the area shortly after uplift of the Apennine Mountains. These deposits of Imbrium ejecta are thought to be relatively thick in the lower intermontane areas near the basin. Locally, the deposits may form a thin mantle on the pre-Imbrian materials of the Apennine massifs.

Materials of the Front

The Apennine Front was visited and sampled at the foot of Hadley Delta, at station 2 on the flank of St. George Crater, and at stations 6, 6A, and 7 in the vicinity of Spur Crater. In these areas, the surface material on the Front is relatively fine-grained cratered regolith that contains not only debris derived from deeper layers but also some ejecta from the nearby mare and, presumably, a small amount of exotic material from distant mete- orite impacts. Geologic models derived from photo- interpretation suggest that Front material sampled in the regolith may include not only pre-lmbrian massif material but also Imbrium ejecta either deposited directly in the sampled areas by the Imbrium impact or subsequently transported down the slope, by mass wasting. The massif material may consist of a variety of stratigraphic units. For example, a distinctly light band is visible in the upper part of the north face of Hadley Delta in high-Sun-angle orbital panoramic camera photography.

Of 133 rock fragments collected at the Front, 106 are breccias, four are glass fragments, and 15 are crystalline rocks, including some of the distinctive mare-type basalts. Preliminary examination shows the breccias are of three types - friable, coherent, and well lithified. Friable breccias, presumably soil breccias, include a variety of clasts that are feldspathic, nonmare-type basalt, mare-type basalt, and glass spheres and fragments. Coherent breccias are characterized by a dark vitreous matrix in which feldspathic clasts, nonmare-type basalt clasts, and granulated olivines and pyroxenes are abundant. These breccias, commonly coated with frothy glass, are the dominant type at the Front and are also present on the mare. Well-lithified breccias are those in which the most abundant clasts are feldspathic and in which clasts of nonmare-type basalt are subordinate. This breccia type is characterized by the occurrence of an apha- natic crystalline matrix that intrudes the clast.

Unlike the Apollo 14 breccias, the coherent and well-lithified breccia types are notable for the absence of clasts of older breccias. Hence, the breccias may represent materials from depths below the normal levels of impact brecciation. These breccias may have been derived from large craters and basins that excavated bedrock before the Imbrium impact and deposited it as ejecta at the Imbrium site, or they may represent bedrock brecciated and uplifted by the Imbrium event itself.

Mass wasting

In photographs taken at high Sun angle, the steep mountain slopes show prominent dark bands that extend straight downslope. These bands merge upward into the dark, low-relief, hummocky material interpreted as Imbrium ejecta (refs. 5-1 and 5-2). Excellent examples of the dark bands that presumably represent gravitational transport of debris downslope occur along the Front south of St. George Crater and east of Hadley C (fig. 5-13).

A second kind of mass-wasting process is shown in figure 5-14, a 500-mm lunar-surface photograph of the upper part of Mr. Hadley. Scarcity of blocks in the lower part of the photograph suggests that the regolith is relatively thick. In the upper half of the photograph, from the upper part of the steep lineated portion of the slope almost to the mountain top, boulders are much more abundant, which indicates the presence of a zone between the mountain top and the steep main face where the regolith is preferentially thinned. Presumably, this zone occurs because ejecta from craters on the flat hilltop are distributed randomly around the source craters, whereas ejecta on the slope are distributed preferentially downslope, Hence, a zone of disequilibrium exists high on the slope where material is lost downslope more rapidly than it is replenished by impacts up on the flatter hilltop.

Lineaments

Well-developed and largely unexpected systems of lineaments suggestive of fracture and compositional layers or both were observed and photographed by the crew on both Mt. Hadley and Hadley Delta. Surface photographs taken with the 60- and 500-mm Hasselblad cameras (figs. 5-15, 5-16, 5-17, 5-18, 5-19, 5-20, and 5-21 ) show the lineaments clearly. Orbital photographs taken with the high-resolution panoramic camera generally show the same lineament sets that the crew documented from the surface.

Previous orbital and surface photographs of the lunar surface have shown that lineaments at all scales tend to be alined in preferred directions - predominantly northwest, north, and northeast with north-northeast. The observation that this so-called lunar grid has been recognized under very restricted lighting conditions, generally low Sun with lighting from either the east or the west, has raised the question that some of the lineaments might be artifacts produced by low-angle illumination of randomly irregular surfaces. Subsequent experiments with small-scale models, still in progress (K. A. Howard, unpublished data.) show that oblique illumination on randomly irregular surfaces produces systematic sets of lineaments that resemble some of those recorded at the Hadley-Apennine site. Lineaments in the models form conjugate sets bisected by the illumination line, with which they form acute angles. This angle increases with increasing incident lighting angle.

Because of the great interpretative significance to be attached to the Apennine Front lineaments if they can be identified as the surface traces of compositional layers or of regional fracture sets, a preliminary attempt was made to evaluate the possibility that some may be lighting artifacts. Statistical analyses of lineament trends were made by measuring the orientations of approximately 1500 lineaments in six separate areas on orbital panoramic camera photographs. The results are smnmarized in azimuth-frequency diagrams (figs. 5-22,5-23, and 5-24). Panoramic camera photographs from three different orbits provided relatively low-, intermediate-, and high-Sun positions with Sun azimuths of 99 , 112.6 , and 117 , respectively, and with Sun elevations of 18, 38.5 , and 43 .

Area 1 ( fig. 5-22 (b) ), the west-facing slope of Hadley Delta, is illuminated only in the high-Sun orbital photographs. Lineaments on that surface are discontinuous and cluster in two groups, north and northeast, approximately 60 apart. The Sun crudely bisects the obtuse angle between the main lineament trends.

Area 2 (figs. 5-22(c) and 5-23), the northeast-facing slope of Hadley Delta, was examined in both low- and high-Sun photographs. At low Sun (fig. 5-23), the lineament distribution is characterized by major north and northeast trends approximately 40 apart, again roughly bisected by the Sun line.

In the high-Sun view of area 2 ( fig. 5-22 (a) ), with illumination the same as for area 1, the lineament trends are similar to the area 1 trends, and the major lineament directions are again approximately bisected by the Sun line. Comparison with the low-Sun lineament plot for the same area (fig. 5-23) shows that the orientation of the north-trending set is essentially unchanged but that the northeast set has apparently shifted clockwise approximately 20 , a movement nearly equivalent in magnitude and direction to the shift in Sun azimuth from low Sun to high Sun. Despite the apparent shift with changing illumination, visual comparison of the two panoramic camera photographs shows that some of the same lineaments can be identified at either high or low Sun. (The greater number of lineaments recorded at high Sun was measured on a triple enlargement of the panoramic camera photograph, whereas the low-Sun lineaments were measured on a contact print. The scale difference in the photographs probably accounts for the differences in absolute numbers of lineaments measured at high and low Sun.)

A 60-mm photograph (fig. 5-15) of area 2 taken from the lunar surface shows the two lineament sets. The northeast set slopes gently left in the photograph and the north set, particularly evident on the rim and flanks of St. George Crater, slopes steeply to the right. A 500-mm photograph (fig. 5-16) of the St. George Crater rim taken from the lunar surface shows more detail of the lineament patterns. Parallelism of the long edges of crater rim shadows with the north-trending (upper left to lower right) lineament set is compatible with the concept that this set may be a lighting artifact. This effect is particularly enhanced by foreshortening of the view on the more distant (south) wall of St. George Crater. The large thickness of mature regolith implied by the scarcity of blocks even near the largest and freshest crater on the northeast rim of St. George Crater (fig. 5-16) suggests that neither lineament set reflects compositional layering of the bedrock. However, if the lineaments represent the axes of fine, systematically alined ridges and troughs, they might be the surface traces of bedrock fractures propagated through the regolith according to the mechanism proposed in reference 5-12.

Area 3 is located just south of Mt. Hadley (fig. 5-24) on the southwest-sloping face of one of the low hills interpreted as Imbrium ejecta. Lineaments measured with intermediate illumination show prominent north and northeast maxima that are roughly bisected by the Sun and are separated by approxiraately 50. The northeast-trending lineament set is visible in 60- and 500-mm photographs (figs. 5-17 and 5-18) taken from the lunar surface. In the 500-mm photograph, the northeast lineaments are similar to the lineaments on the rim of St. George Crater (fig. 5-16) in size, frequency, and in the occurrence of block-free regolith. Presumably the origins are similar. The north-trending set is recognizable but somewhat indistinct in photographs from the lunar surface. Brief comparison of panoramic camera photographs with different illumination angles indicates that at least some of the lineaments persist despite lighting changes.

Area 4 is located low on the south face of Mt. Hadley (fig. 5-24). Lineaments, measured with an intermediate-illumination angle cluster around north and northeast trends separated by approximately 55 . and the major lineament directions are again roughly bisected by the Sun line.

Area 5 is the entire west face of Mt. Hadley, partly shown in figures 5-17 and 5-19. Lineaments measured in an intermediate-illumination panoramic camera photograph (fig. 5-24) are grouped around three major trends that correlate with the lineaments photographed from the lunar surface (figs. 5-17 and 5-19). The predominant set trends north 55 E and corresponds with the well-defined set of linears dipping steeply left in figures 5-17 and 5-19. A1- though no single linear can be traced across the entire outcrop face, the parallelism and crisp definition of some bright linears through distances of tens or hundreds of meters give the impression that they are the surface traces of compositional layers or of a system of well-defined, uniform fractures. The im- pression is heightened in a few places where the same linear apparently emerges both upslope and down- slope from beneath the cover of one of the many subhorizontal, smooth, regolith-covered benches on the mountain face (fig. 5-19). In other places, faint traces of the linears extend across these benches.

A second set of linears, trending north 25 E, appears vertical in figures 5-17 and 5-19. This set is less prominent than the northeast set in both orbital and lunar-surface photographs.

The third set, trending northwest along the Sun line, is approximately parallel to the abundant sub-horizontal benches prominent in figure 5-19. The benches are visible in the orbital photograph as discontinuous, narrow, somewhat sinuous bands approximately contouring the slope. The benches are distinct from the lineaments, which are straighter and more regular, and they may be slump features or benches where the slope profile has been locally flattened by cratering or downslope mass movement. The northwest-trending lineaments are a well-defined but subordinate set on the southwest mountain face, but on the northwest face they are prominent.

The face of Mt. Hadley is completely shadowed in the low-Sun orbital photographs. Comparison of high-Sun and intermediate-Sun panoramic camera photographs shows that at least some of the lineaments in the major set persist through the small illumination change.

As with areas 2 and 4, the face of Mt. Hadley is notable for the scarcity of blocks, which implies the presence of a fairly thick mature regolith. If the lineaments are not lighting artifacts, it seems more likely that they represent the traces of fractures propagated through the regolith than that they represent compositional layering in the bedrock.

Area 6 is located on the west flank of Silver Spur (figs. 5-20 and 5-24). The striking 500-ram photograph taken from the LM (fig. 5-21) shows massive ledges apparently dipping gently to the left and crossed by finer, more nearly horizontal lineaments that give the impression of crossbedding. The finer (al lineaments are probably caused by the foreshortened view across an undulating cratered surface, an effect similar to that on the south wall of St. George Crater (fig. 5-16). In figure 5-20, Silver Spur resembles a hogback of gently dipping stratified rock resting conformably on the apparently layered Hadley Delta massif.

The orbital photographs do not support the hogback illusion. They show two major sets of topographic lineaments that intersect in a diamond- shaped pattern with northwest- and north-northeast-trending boundaries (fig. 5-25). Continuity of lines beyond each diamond is more easily accomplished by eye than it is by drawing each linear element, because slope reversals at diamond boundaries cause alternation of shadowing and highlighting along any single linear. Furthermore, the orientation of the two sets changes slightly across the crest of Silver Spur. The linearity of each set in the vertical photographs and the relatively minor response in trend to changes in slope orientation suggest that these are lighting enhancements of surficial features that could reflect high-angle fractures or even near-vertical compositional layering along the north-northeast trend. The north-northeast and northwest peaks of the azimuth-frequency plot (fig. 5-24) include the well- defined topographic lineaments seen in both orbital and surface photographs. The northwest set includes the steeply right-dipping shadowed bands of figure 5-21 and the north-northeast set includes the prominent topographic ledges that appear to dip gently left. Farther north in the more heavily shadowed portion of Silver Spur, the north-northeast set of lineaments was not recognized on the panoramic camera photographs, but northwest- and north-trending lineaments were measured.

Areas 1 to 4 each occur on fairly uniform slopes of known orientation (ref. 5-13). The four slopes differ from each other greatly in orientation, but the lineament patterns in each area are similar (figs. 5-22, 5-23, and 5-24). Major north and northeast trends intersect in an obtuse angle that is crudely bisected by the Sun line in each case. The lineaments have no constant relation to the slope such as gravitational effects might produce. Stereonet rotation of each slope to horizontal produces only minor changes in the lineament trends, but, in some cases, it produces major shifts in apparent Sun azimuth so that no systematic relation, related to slope, seems to exist between Sun azimuth and lineament azimuths. Ap- parently, the lineaments either really represent direc., tional features of the lunar surface or, if they are lighting artifacts, they are sensitive only to the lighting direction and not to abe orientation of the slope on which they occur, In summary, the investigation to date is insufficient to assure distinction between systems of lineaments that represent closely spaced, repetitive geologic structures such as layering or regional fracture patterns and those that may be artifacts caused by oblique lighting of irregular surfaces. At Silver Spur, prominent northeast-and northwest-trending lineament sets may be directly related to steeply dipping geologic structures with distinctive topographic expression. Elsewhere, interpretation of the linears as expressions of geologic structure is equivocal.

If either the north or northeast trends prominent at stations 1 to 4 represent bedrock structure, that structure is more likely to be regional fracturing propagated through the regolith than a surface expression of compositional layering. The extensive areal distribution of the pattern and its azimuthal constancy regardless of slope orientation would suggest that, if real, it represents a regional set of nearly vertical conjugate fractures. Local variations may be superimposed on the fracture system as at Silver Spur or at Mt. Hadley, where the northeast set is prominent, but northwest and north-northeast sets also occur.

Dark band near base of Mt. Hadley

During EVA-2, while driving to station 6, the commander (CDR) commented that the crew saw three suggestions of beddings or horizontal linear lines at the base of Mt. Hadley. He surmised that these lines might represent the high-lava mark for the basin at one time, because they were unique at the base of that mountain.

A dark band is visible along the base of Mt. Hadley 180 in the panoramic camera photographs (figs. 5-3 and 5-24). Where photographed with the 500-mm camera in one spot (fig. 5-26), this band appears to be, at least in part, an outcropping ledge, but the photograph is of one of the less regular parts of the band. A slight hint of a similar band can be observed in a few places along the Front just south of Hadley Delta.

The top of the band is 80 to 90 m above the average level of the mare surface, and the slope of the mountain is about the same above and below the band. In a few places, the top of the band stands out as a small ledge (fig. 5-26). The surface texture of the band, and especially the slope just above the band, appears to be slightly smoother than that of the rest of the mountain face. At least three interpretations are possible for the band-an accumulation of debris at the base of the slope, a fault scarp, or a high-lava mark.

The only suggestion of a debris apron along the base of Mt, Hadley is this band, and, from analogy with other lunar slopes, it seems likely that at least some debris should have accumulated at the base of the slope. Similar bands occur near the base of other lunar mountains where they are in contact with mare. One at the Flamsteed ring was suggested as being the result of slope movement (ref. 5-14), but it has a more rounded convexity than the feature on Mt. Hadley. The band on Mt. Hadley appears remarkably uniform for a debris apron, and, if a debris accumulation, it seems that a similar feature would be visible along the base of all the mountains, with the height of the apron somewhat related to the heights and slopes of the mountains. Also, if a significant amount of debris were accumulated, the slope angle should decrease in the zone of accumulation. No marked decrease in slope exists below the top of the dark band (fig. 5-27).

The mountains in general are believed to be bounded by faults that occurred during the Imbrium impact event. Slight readjustments along the faults, with the mountain on the upthrown side, after the mare basin filling could leave a remnant of the mare basalts on the Front. The trace of the possible fault, however, is more sinuous than might be expected.

The crew's impression was that it might be a high-lava mark, left after a subsidence of the mare basalts, probably either by cooling shrinkage or, more likely, by partial drain-back into the source vent. Small ledgelike features at the margin of the mare between Mt. Hadtey and Hadley Delta are associated with troughs that suggest that the lava pulled away from the mountains, as recognized from pre-mission mapping. Features that are obviously of this type are present elsewhere on the Moon and are common in lava lakes on Earth. The outcrop ledge (fig. 5-26) supports a lava-mark interpretation, for such outcrops are rare on the mountain slope but common where mare basalt is exposed in Hadley Rille. The smoother texture may be caused by debris collecting on the small ledge. This interpretation at present appears to be the best explanation for the band; it does, however, for sufficient subsidence, require a thickness of molten lava greater than the 80 m height of the band.