LanFleming@AOL.com
AS17-288 was made before the Apollo
17 spacecraft descended to the altitude of 60 nautical miles, or 69 standard
miles, which it maintained throughout most of the mission. The images below
were taken at this lower altitude, and the resolution is approximately 7.5
meters - twice that of AS17-288. However, the sun angle was also higher in
this later series of photographs than in Frame 288, so the shadows of an
object cast on a horizontal surface would only be about 2/3 their length
in Frame 288. This decreases the effective resolution to some extent, but
the amount of detail in them is still substantially greater than in the 200
series photograph made at twice the altitude.
The only significant difference between the images of the 400 series of Peirce is the camera's line of sight to the crater, and that changes only slightly; there is little evidence of foreshortening in the shape of Peirce crater. All four images are essentially orthogonal views of the crater. There is only a difference of about 17 degrees in the line of sight angle in the most oblique view of Peirce in AS17-429 and the most orthogonal view in AS17-431. Yet the appearance of the region of the conical hill changes substantially from one image to the next in this series. If this is not due to differences in camera exposure times, (so far I have not been able to find any data on mapping-camera exposure times) then the differences must be due strictly to the changes in the line-of sight and the properties of the crater surface.
Images from the four frames are shown below with a minimum of descriptive text separating them so that they are easier to compare to each other. They will then be discussed collectively.
Figure 1. Frame 429. Striations over the surface are indicated by the green lines on the small inset. The black arrow indicates the orientation of the "furrows" in LO4-191H3 to the northeast of the features here.
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Figure 2. Frame 430. Striations over the surface are indicated by the green lines on the small inset.
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Figure 3. Frame 431. Striations over the surface is indicated by the green lines on the small inset.
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Figure 4. Frame 432. The red lines on the small inset show the apparent boundaries of the "diamond" in LO4-191H3 and the squared-off base on the western side of the conical hill.
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This feature has a very circular base on its east side and appears to come to a point at its summit, strongly suggesting a conical shape. Jon Floyd proposed that the hill might really be of a more irregular shape that appears conical because of rock slides down its slopes, unseen at the resolution of this photograph. However, the fact that the conical appearance of this feature persists in all the Apollo 17 photographs with differing sun angles, lines of sight, and resolutions, indicates that the hill may truly be conical in shape. The conical shape in turn suggests that this hill could be a small volcano - about 2KM in diameter and 300 meters in height.
The base of the hill departs from a circular shape where it meets the squarish dark region adjacent to its western base - the "diamond" from LO4-191H3. In all the 200 and 400 series photographs, the hill's western surface appears flatter with a more angular baseline and also shows several large black patches. The largest of these patches is just below the label Z1 in Figure 1. These patches could be openings into the hill, perhaps the vents of the volcano, if this is in fact a volcano.
There is little evidence of volcanic activity on the moon, although the Taurus-Littrow region contains what have been described as "cinder cones" by geologist Farouk El-Baz. The weaknesses in the moon's surface created by the Peirce impact event could contribute to the upwelling of molten material that led to the building of a volcanic cone.
This feature, which can be seen fairly clearly in the lower-resolution Frame 288, virtually disappears in most of the higher-resolution frames of the 400 series. The exception to this is Frame 432, the last in the series shown in Figure 4. While this photograph seems to show much less surface detail than the other three 400 series frames, it clearly shows the dark shape of the "diamond" of LO4-191H3. Also, it can be seen that the dark patch is wider than the conical hill, with its northwestern boundary well beyond the northwestern side of the hill. This is the same situation as in the Lunar Orbiter photograph, where the bright squarish patch also extends well beyond the hill on the northwest.
The only thing that can be said with any certainty about this square patch is that it is a structure on the crater floor that slopes upward to meet the base of the conical hill. The apparent "holes" and flat appearance of the hill on the side where it meets the "diamond" may be related somehow to the presence of this square patch at its base.
Striations crossing the floor of the crater and the western face of the hill are fairly obvious in Frame 429 in Figure 1 and Frame 430 in Figure 2. Some of the striations in these two images occur in the same place on the western side (towards the bottom of the image) of the hill. In Frame 429, they appear to form a web over the "holes" in the side of the hill. The direction of the "furrows" noted in LO4-191H3 is indicated by a black arrow in Figure 1. Many of the striations run in this same general direction, although the furrows of LO4-191H3 are seen to the northeast of this region around the conical hill and "diamond". The spacing between the "furrows" in the lower-resolution Lunar Orbiter photograph are much greater than the spacing of the striations in these Apollo images. These smaller-scale striations could perhaps be responsible for the "furrows" in the Lunar Orbiter frame, with a group of several adjacent striations appearing as a single "furrow" in the lower-resolution photograph.
Not all of the striations have the same orientation as the "furrows" in the Lunar Orbiter photograph. Figure 3 , from Frame 431, shows an especially straight bright line running in a direction perpendicular. What may be the same feature can also be seen in Frame 430 in Figure 2.
Jon Floyd has proposed that these striations might be caused by lava flows on the floor of the crater. However, lava flows generally will branch and meander. Another possibility he suggested is that the striations show a cross section through successive layers of lava flows that were horizontal when formed, but were tilted upward along some fault line to reveal strata. Such exposed stratification is not uncommon in terrestrial landforms. However, there is no indication in any of the photographs of such faulting. In fact, the striations move from one position to another between photographs, with some of the most noticeable on the western face of the conical hill.
The ephemeral nature of the striations and the location of some of them on the side of a (possibly) volcanic cone seem to argue against the exposed strata explanation. Also, the striations seem not to conform to the underlying surface topology; in Figure 1 and Figure 2, they are most visible crossing dark regions that are probably depressions or the shadowed sides of ridges on the crater floor.
A less-conventional idea we have considered is that these striations are really a patterning effect from sunlight passing through some translucent material with a regular repeating texture that is either laying on the surface or suspended above it. Whatever the cause, the striations, the conical hill, and the diamond pattern seem "anomalous," meaning "a departure from what is expected." There is certainly nothing in any of the selenological references I have read that indicates such regular patterning should exist on the collapsed floor of a crater. Yet this seems to be somewhat common in lunar craters when examined closely, as also evidenced by the unusual patterns on the floors of two other craters in Mare Tranquillitatis that Mike Lomax has been investigating.