Picard, AS10-4417
Figure 1A. (Click on image for a stand-alone JPEG file). Support data is here.
It is often difficult, if not impossible, to determine the position of a feature in a three-dimensional space from the feature's appearance in a single two-dimensional photograph. That is why, as mentioned on the first Picard page, there are strong reservations about the interpretation of the "arches" in AS10-4421 as being structures rising vertically above the crater Picard. We can't rule out the more prosaic interpretation, that the "arches" are really composed of a string of small bright features on the lunar surface, such as a chains of craterlets or perhaps simply unrelated craters that by chance aligned with each other in that one photograph due to the viewing perspective.
AS10-4421, however, is not the only photograph of Picard; there are several others in the same Apollo-10 magazine taken from a similar, but significantly different, perspective. AS10-4417 is the photograph in the series taken closest in time to 4421 that shows Picard. In Frame 4417, Picard is viewed at a slightly more oblique angle (line of sight about 5 degrees above the local horizontal as opposed to 7 degrees in Frame 4421). Picard has also rotated about 9 degrees counter-clockwise between the view in 4417 and 4421 and the distance to the camera is about 2% greater in 4417 than in 4421. The NASA support data for Frame 4417 is here.
To illustrate the radical change in appearance of the region, even with such small changes in the viewing geometry, two small JPEG versions of Frames 4417 and 4421 are shown below in Figure 1A and 1B respectively. The features numbered "1" and "2" are on the lunar surface behind Picard. Feature 1 is the end of a long bright streak, probably a plume of ejecta on the mare surface that extends to the right in the image. Feature 2 is probably a small bright crater. Note that in moving from 4417 to 4421, both features move noticeably to the left, they retreat from Picard toward the top of the image, and the apparent space between them has doubled. In general, the more distant two objects are from one another in three-dimensional space, the more radically their relative positions on two dimensional images will change with a changing line of sight.
The points marked "A" and "B" on the wall of Picard facing the camera change their position relative to each other only slightly because they are much closer to each other than the features beyond the crater.
Picard, AS10-4417
Figure 1A. (Click on image for a stand-alone JPEG file).
Support data is here.
Picard,
AS10-4421
Figure 1B. (Click on image for a stand-alone
JPEG file) Support data is
here.
Depending on what web-browser your are using, you may be able to notice in Frame 4417 that there is a small light streak extending "up" from the point on the crater rim at the position of the left foot of the main "arch" in 4421, just to the right of and above the letter "A". (Remember, you can get a stand-alone version of any image on this page by clicking on it so you can save it and then use your own graphics-viewing software to adjust brightnes and contrast. Be sure to turn off any graphics compression option before you do this.)
Due to the distinctive pattern of alternating light and dark patches on the crater wall facing the camera, the points at which the "arch" in 4421 crosses the crater rim can be identified in 4417. In the two small images in Figure 2 below, the points labeled "A", "B", "C", and "D" are identified in both frames and the apparent filament-like structures are outlined in blue. Points "A" and "C" are the two points where the main "arch" appears to cross the rim in 4421. Near each of these four points on 4421, there also appears to be a feature extending up fro the crater rim on 4417 as well, and these are also outlined in blue.
Figure 2. Close-up of region of wall of Picard at position of
main "arch."
Frame 4417 is at left, 4421 is at right. (NOTE: the
lettering does not correspond to the labels in Figure 1, except for point
"A").
Note that point "B" marks the end point of another curious filament-like feature in 4421 that was not very apparent in the enlargement shown on the previous page. The staff at the photographic archive at the National Space Science Data Center went to an extra effort to provide me with new negatives with carefully controlled contrast to produce the closest duplicate possible to their master negatives, and this feature became much more apparent on the new duplicate. The filament above "B" seems to curve upwards to merge with the "arch".
The larger versioins of the images in Figure 3 below show the same region at a higher magnification. Rather than being composed of bright streaks and blobs, as is the "arch" in 4421, the features in 4417 are primarily composed of dark streaks. There appears to be a black triangle, with one apex at point A , and the others two labeled 1 and 2. The angle with the crater rim by the line between point A and point 1 is very similar to that of the angle of the "arch" in 4421 extending from point A.
Figure 3. Picard "Arch" region. Left - 4417. Right - 4421. Click on either image to get stand-alone GIF files (250K each). Corresponding points have the same labels in the two images. The letters identify positions on the crater wall in both images and correspoind to the labels in Figure 2. Support data for the 4417 gif is here. Data for the 4421 gif is here.
At point C, where the right side of the "arch" in 4421 intersects the crater rim, there is another black streak, this one extending almost vertically "up" from the crater, just as the right-hand side of the "arch" in 4421 does.
Finally, at point D, two very long sinuous lines seem to extend upward from the crater rim in both 4417 and 4421. The points where the features extending up from point D seem to terminate are labeled "3" in the images. In both frames, the feature is a more or less continuous light band bordered by two darker bands.
Comparison of the two photographs suggests that the features at the same points on the crater wall may be the same or related features. Since the lunar surface in the background changes radically from one photograph to the other due to the changing line of sight, the features would have to be structures rising vertically from the crater rim, assuming that the features are the same in both photographs. However, due to the differences in the appearance of the features in the two photographs, it is not certain that they are in fact the same.
While there are linear projections up from the crater rim at the points where the two legs of the "arch" intersect Picard's rim in 4421, they do not appear to form a complete arch in 4417. This might be due to obscuration in 4417 of the structures' details by the background lunar surface, which is brighter in 4417. The similarity of the angles at which the streaks project from the crater at points A, C, and D in both photographs as shown in Figures 2 and 3 seems to provide the best evidence that they are the same.
