The most important reason for high-resolution imaging of the Face was not to find out whether it still looked like a face at high resolution. Rather, the motivation was to reveal if there were any details of the landform that might suggest that it could have been artificially modified. Any ancient structure made of stone should be expected to show a mixture of natural and artificial modifications, the former increasingly predominant with increasing age. If the age is great enough, an artificial structure would ultimately return to a natural state and no vestiges would remain of artificial modifications to distinguish the structure from natural structures, barring on-site excavation. I see no reason to be concerned with such possibilities until humans have established a foothold on Mars and have sufficient free time to poke around at any odd-shaped hill that they find interesting.
It does, however, seem reasonable to select sites now that are the most promising archaeologically for future ground investigations based on details shown in satellite images now available. I believe that the new MGS images show that the "infamous" Mars Face is still among the foremost in that category. This article describes three such details in the April 2001 MGS image that may point to an artificial origin when considered collectively. The positions of these features are shown in Figure 1 relative to the entire landform.
Figure 1. Section of MGS Frame E03-00824 Showing Mars Face at 10% Full
The position of the sun figures prominently in this discussion. Figure 1 shows its azimuth position, which was 236 degrees clockwise from North at the time the MGS image was taken on April 8, 2001 at 20:54 UT. The sun's elevation was a very high 52 degrees.
At the lower right of the MGS image is a black region that might be interpreted as a shadow cast by the Face landform. However, any surface casting a shadow must have a downward slope angle in the direction of the sun azimuth away from the sun that is greater than the sun's elevation. There are few if any slopes on the Face that are greater than the sun's 52-degree elevation. I think there is good evidence that this black region is a very deep concavity -- a chasm -- rather than a shadow cast by slopes on the Face itself.
The image taken on April 8 1998 shows that there are none that could have cast a shadow in this particular region with the elevation and azimuth the sun had at the time the 2001 image was acquired.
This black patch was also visible in that earlier image. The spacecraft elevation
was about 45 degrees above the local horizontal and the spacecraft azimuth
was 230 degrees -- almost the same as the sun azimuth in E03-00824. That
section of the Face from the 1998 image is shown in Figure 2.
Figure 2. Section of MGS Frame SP122003 acquired April 5, 1998 showing
azimuth positions of the MGS spacecraft and the sun.
If reflected light from the black area could reach the MGS camera at an elevation of 45 degrees in the 1998 image, then sunlight should certainly reach the same area from almost the same azimuth and an even higher elevation in the 2001 image.
A reasonable estimate of the depth of the chasm can be made for the new image based on the measurement of its width in the direction of the sun's azimuth. This is shown in Figure 3 on a full-scale section of the 2001 MGS image.
Figure 3. Full-scale section of MGS Frame E03-00824 acquired on April 8, 2001 showing the lower right-hand base of Face. Arrow indicates the width across the darkest region of the chasm in the azimuth direction of the sun.
The width of the arrow in Figure 3 is about 45 pixels, or 90 meters given the two-meter resolution of the image. An estimate of the depth of the chasm is then 90 meters times the tangent of the sun's 52-degree elevation (1.28). The depth would then be 115 meters, or about 380 feet -- equivalent to the height of a 38-story building. The depth could actually be much greater, because the dark areas in which some detail can be seen may also be in shadow; shadows are not necessarily absolutely black on planets having atmospheres to backscatter sunlight.
While the inner edge of the chasm adjacent to the Face landform is not well
defined, the outer edge facing away from it is sharply defined. And the outer
edge seems to complete a missing section of the base of the landform extending
from the bottom of the "chin" to the right-hand side of the "head." This
suggests that the missing section may have collapsed inward and that there
is a subterranean recess beneath the Face, at least in this area. This chasm
is not the only area suggestive of a subterranean space beneath the Face.
The April 1998 MGS image showed a dark band running along the base of the "chin" as can be seen in Figure 2. While not nearly as deep as the chasm, which it merges with, this dark band is very likely a trough created by subsidence of the ground directly below the "chin." This in itself seems unusual, since debris falling from above typically collects at the base of landforms such as mesas. A rubble pile would seem a much more likely feature than a trough at this location. The trough may be another indication of a porous structure, if not subterranean caverns beneath the Face. The weight of the landform itself bearing down on the surface could be responsible for the formation of the trough.
It has been noted that many mesas on Mars appear to have channels running
along their edges similar to the one on the eastern side of the Face. While
I do not have enough information on other landforms for comparison to the
Face, this channel appears to be extraordinarily deep. The section that seems
to be the deepest is shown in Figure 4.
Figure 4. Channel on eastern edge of the Face from E03-00824 at full scale.
From its greater brightness, the edge of the terrace on the western (left) side of the channel appears to be higher than the eastern side, so the shadow may be due to an elevation difference rather than a deep fissure in the rock. The profile of the shadow seems to match the shape of the western edge of the terrace fairly closely. But even after increasing the brightness of the image, the shadow at the bottom of this channel appears to be completely black for most of its length. This suggests that the edges of the shadow may match the edges of the terrace because these are points where the surfaces once joined but have since pulled apart, revealing a deep interior space whose floor is not reached by sunlight.
Near the point where the channel terminates, the shadow profile no longer
even follows the terrace edge. Using the same approach as was used for estimating
the depth of the chasm, the depth of this channel may be about 50 meters
or 160 feet.
Immediately above the chasm in Frame E03-00824 is a surface covered with
linear streaks. This area is shown in Figure 5.
Figure 5. Full-scale section of E03-00824 showing striated surfaces
immediately above the chasm.
SPSR geologist Harry Moore has observed that wind deflation can produce streaks elongate in the wind direction. This area, however, is on the leeward side of the Face (the prevailing winds are from the west). While wind currents flowing over the landform might be envisioned, examples of wind-blown dust streaks I have seen in MGS images usually show a streak originating from some obvious source, such as a crater, and narrowing and fading with distance from the source. The striations in this image instead appear to be of fairly uniform width and brightness and have no apparent sources for any dust. Also, there appear to be several dark streaks running in a direction perpendicular to the direction of the light streaks.
A better explanation for these streaks, then, may be that they are not caused
by dust but are simply the texture of the surface. This could be evidence
of an artificial substrate, perhaps with embedded reinforcements, exposed
by the collapse that produced the nearby chasm. In fact, there appear to
be striations that extend down into the chasm itself along its outer edge
that can be seen in Figure 3. Notice also that there is a light and very
linear feature that ends abruptly at the edge of the chasm at the head of
the yellow arrow in Figure 3.
A common alternative interpretation of a feature such as the one I have called "the chasm" is that it is really not a chasm at all but rather a patch of low-albedo (intrinsically dark) material that has collected in one place for some reason. As mentioned already, an alternative explanation for the deep channel is that the appearance of a deep fissure is caused by a high cliff casting its shadow on a flat surface below, possibly sloping away from the sun and exaggerating the shadow's length.
But some strong evidence that these alternative explanations are not correct might (with heavy emphasis on "might") have come from a rather surprising source: the false-color image constructed from a topological model that NASA released as part of its "debunking" propaganda posted on the Internet immediately after release of the new image.
The false-color image was created in part from data gathered by an MGS instrument called the Mars Orbiter Laser Altimeter (MOLA).
Rather than visually interpreting the contours of an object from its shading in a two-dimensional image, the MOLA makes what are essentially direct measurements of height by reflecting laser beams off the object's surface. The time delay between a laser shot and its return reflection is directly proportional to the distance between the object and the sensor.
The positions of the chasm and the deepest part of the channel are shown on NASA's false-color image in Figure 6. The height of the image has been exaggerated by a factor of 10, according to NASA. The chasm, which appears as a dark oval in this image, is the darkest-colored area of the entire image. Although the authors don't say, it appears that this means it is the area of lowest elevation. The deep channel is also clearly visible and very dark.
Figure 6. NASA false-color topograhical image of the Face constructed
from MOLA altitude data.
Unfortunately, these conclusions can only be made tentatively. The image in Figure 6 was not really scientific information; it was released as part of NASA's public "outreach" materials on the Face and so cannot be fully trusted or interpreted with any great confidence. In fact, the original NASA image was incorrectly inverted left-to-right. I had to invert it back to the correct orientation. (Note that shallow depressions corresponding to the two "eyes" and perhaps the "mouth" are actually visible once the image is inverted to its correct orientation).
There are many questions that need to be answered about this image. The chasm and channel are apparently deep, but without any key provided by the color coding, it is impossible to know just how deep the MOLA measurements indicated these features are in terms of meters (if they are from the MOLA data at all).
More puzzling is the resolution implied by this image. The downtrack resolution of the MOLA instrument is dependent on the "spot" size of the laser shot and the shot frequency. The down rack resolution is said to be about 150 meters. Yet the channel is no wider than 50 meters at the point it appears deepest. It is below the resolution of the MOLA instrument, yet it is clearly visible!
The crosstrack resolution is even more problematic. It is dependent on the spacing of the orbital tracks of the spacecraft, which is in turn dependent on how long the spacecraft has been in orbit at any given time. According to the MSSS "Face on Mars" page,
"At 40 degrees latitude, the spacing [of orbits at the end of the 687-day mapping phase of the mission] is roughly 2.4 kmÖ"
This would be the crosstrack resolution of the MOLA: 2.4 kilometers. However, a MOLA topographical map is now available that indicates a higher crosstrack resolution: 1/32 of a degree, which is approximately 1.4 km at the latitude of Cydonia. But the Face is 2000 meters wide so the false-color image would have to have been created from at most two downtrack profiles. No image could be made from that data alone. The poor crosstrack resolution may also explain the incorrect assertion made in the NASA debunking article that the Face landform is 800 feet high. Shadow measurements of the Viking Frame 35A72 clearly establish its maximum elevation at 400 meters, or 1300 feet. The MGS probably did not even cross over the highest point of the landform on either of the two passes it probably made over the Face during the mapping mission.
The caption for this image says that it was created by combining MOLA data with high-resolution MOC (visual) images. All of the fine detail must have come from the MOC images, perhaps based on shape-from-shading analyses, but this was not mentioned.
FACETS has requested that NASA release the actual numerical MOLA data that was used to produce this false color image. A more accurate assessment of its significance and validity can be made if and when NASA complies. If they do not comply, the data will eventually be made available in raw form on the MOLA team's Internet data archive. Since MOLA laser shots are made in a line along the orbital track, the data returned for the Face is interspersed with data for many other regions at different latitudes and longitudes and in many large files. The process of extracting the altitude data for any given area could therefore be a very long and protracted effort. But the public will have access to it sooner or later.
Regardless of how this curious topological image was created, it does support the contention that there are very deep recesses on and immediately adjacent to the Face landform. At the very least, the false-color image shows that NASA geologists, based on visual inspection of the MOC image, concluded that the chasm is just that and not a patch of low albedo material. At best, one or more laser shots penetrated the chasm and the channel, and there may be hard data on their depths.
The new images of the Face have not returned any "smoking gun evidence" (a term that I find to be highly overused and indicative of excessively simplistic expectations of some people on both sides of the issue). What the new images have revealed, I think, is evidence of a possible artificial origin so intriguing that archaeologists would be calling for a sustained and rigorous investigation if it were not for the present oppressive intellectual climate.
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