Crater with missing rim sections
This image was scanned from a 10X enlargement of a photograph taken by Apollo 16's mapping camera. Some of the features shown bear an interesting resemblance to satellite photographs of urban areas taken at low resolutions.
The full JPEG, llnd0849.jpg, from which the inserts below were taken can be downloaded here.
The image shows an area to the north, northeast and south of the lunar crater Lalande (the large crater on the lower-left side of the full jpeg). Lalande is a major impact crater 25 km in diameter. Centered at 8.6 degrees West, 4.4 degrees South, it is 90 km to the west of Moesting A, the apparent location of the "Shard" in LO3-84M.
The sun is at an elevation of 2 degrees at the position shown in this image, and the angle between the camera's line of sight and the lunar vertical is 60 degrees. This viewing angle causes distances in a direction 10 degrees off the vertical axis of the gif to be foreshortened by a factor of 1/2. The very low sun angle reveals features at low elevations above the surrounding terrain. The same features described below were also photographed on a later orbit of Apollo 16 at a higher sun angle, and comparison of the same objects photograhed at the two different sun angles is useful. One of the images taken at a higher sun angle is AS16-1412. Additional enlargements from AS16-0849 are provided on that page for comparison.
(X,Y) coordinates are used below with X = number of pixels to the right of the image's left edge and Y = the number of pixels down from the image's top edge. The coordinates refer to the full JPEG image.
(150,195) A ridge to the north of the crater with the appearance of a backward "Z", with a similarly shaped but smaller ridge at 100,225). These two ridges closely parallel each other. While the internal angles of these ridges appear to be 90 degrees due to foreshortening, the internal angles would in fact appear very close to 120 degrees when viewed straight down. This area is to the north of the Lalande.
(600, 550) Approximately the center of a region to the northeast of Lalande covered with a pattern that in some places resembles chevrons: "<<<". There are numerous small ridges branching off from the longer ridges at various angles. The longer ridges tend to run northwest to southeast and southwest to northeast, and the internal angles of the "chevrons" would appear close to 90 degrees looking straight down, giving the impression of an underlying rectilinear grid. The most pronounced of these ridges is at (825, 485).
(635, 375) A ridge intruding past the raised wall of a crater (shown below) where a large section the wall is missing. It was observed by one planetary geologist who examined this image that there also appears to be another smaller section of the crater wall missing at a position diametrically opposite to this one. Such pairs of gaps in crater rims can be caused by meteoroid impacts at a low angle (less than 30 degrees) above the impacted surface's horizontal. Whether the intruding ridge could survive intact under this scenario seems somewhat less certain. This ridge could, perhaps, be a plume of ejecta created by the cratering impact.
Crater with missing rim sections
(485,360) and (515,340) Ridges that apparently span the trough of a large fault scarp originating at the rim of Lalande (at lower left). These two features are perhaps the most puzzling in the entire area. They appear to be suspended over the scarp, which is nearly a kilometer in width and perhaps more than 20 meters deep where these ridges cross it. This scarp is a large crack in the lunar surface, originating in the rim of Lalande and running toward the northeast, as can be seen in the insert from AS15-849 below. The scarp runs from the lower-left cornter of the image to the upper right corner and the two ridges crossing it are in the center.
Fault scarp or ravine, with rim of
Lalande at lower left.
The crack runs to the very top of the crater rim. It was probably formed as a result of the stresses created by the impact in the lunar surface surrounding the crater. Lalande itself has an irregular polygonal shape, differing from the circular shape it probably had just after the impact. This, too, was the result of the stress-relieving fractures and collapses as the lunar surface readjusted to the presence of the crater following the impact. The seismic activity that produced this scarp must have been extremely violent. It seems unlikely that these ridges could have pre-dated the scarp and survived the rupturing of the surface.
A closer view of the two ridges is shown below. It can be seen that the ridges extend toward a small crater to the west of the scarp, ending on opposite sides of the crater. Could these ridges actually be causeways or even bridges, connecting a "city" to the northeast of Lalande to the smaller "suburban" complex on the north? And if so, what was the rush-hour traffic like?
Two ridges spanning a fault scarp
or ravine
Some unusual grid-like patterns in the Sinus Medii region have been attributed to Imbrium "sculpturing," the scouring of the lunar surface by the ejecta from the impact that formed the large Imbrium basin several hundred kilometers to the northwest of Lalande. However, all of the unusual features in this gif are in close proximity to Lalande, and must either be on top of or part of the Lalande ejecta blanket. The Lalande impact postdates the Imbrium event, so any geological processes responsible are more likely associated with Lalande itself rather than Imbrium. However, there appears to be no alignment for most of these features with the directions radial and circumferential to Lalande's center, while ejecta-scouring tends to be directed radially out from the point of impact.
Another possibility for the strange pattern of zig-zagging ridges is that it is associated with the solidification and contractions in the molten lava that formed the original surface in this region. Thermal gradients and currents in the molten lava flows might have somehow resulted in this surface pattern. However, the distorted appearance of several craters in this vicinity, including the one mentioned previously, suggest that the pattern of ridges may have arisen after the lava had solidified.
Only one reference to this sort of pattern has been found so far in the literature, and this was a reference to a similar pattern on the lunar feature known as the Flamsteed Ring. This reference suggested that the pattern may be an optical illusion. The higher sun-angle photograph, AS16-1412, which also shows much of the same pattern, indicates that optical illusions can probably be ruled out as a cause.
Spacecraft Altitude = 114km
Principal Point in Photograph at 5.9 degrees South, 9.9 degrees West
Sun Elevation at Principal Point = 2 degrees.
Angle between the camera's line of sight and the local surface normal at Lalande = 60 degrees (estimated from the elliptical appearance of the craters, which would be circular when viewed looking straight down.)