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[122] One of the prime objectives of the imaging investigation was to document changes, either rapid or long term, within the scene. Figures 148 through 151 are a series of pictures of footpad 3 at the Viking 1 site. Taken over a period of several days, these pictures record slightly different shadow patterns corresponding to different times in the morning. No other changes are apparent. Similar monitoring sequences were used extensively throughout the mission to photograph regions that seemed the best candidates for change. The most valuable sequence of repeated images documents the formation and decay of a frost cover at the Viking 2 site (figs. 194 to 196).
Images are being acquired that precisely duplicate lighting of early mission pictures. These images indicate that a thin surficial cover of light dust settled out at the Viking 2 Lander site after a major planetwide dust storm.
As previously discussed in the introductory text, the Viking cameras have an unusual capability for detection of motion within the scene. The azimuth motion of the cameras can be inhibited so that the same vertical line is scanned repeatedly. Sequential scans are displayed in the usual way, building up from left to right. The horizontal axis can be considered as time. In figure 148, for example, the first repeated line scan occurs at the right edge of the conventional image, the last repeated scan at the far right. In all, 43 repeated line scans are shown. This picture was taken at the slower rate of 13.65 seconds per scan. Accordingly, the elapsed time of the monitoring sequence is 587 sec. The same low scan rate was used to monitor slowly changing atmospheric conditions near sunrise or sunset. In situations where there is no motion within the scene every vertical line is identical, and the repeated line image comprises unbroken horizontal bright and dark bars. If a large object were to pass through the scene the brightness levels in several lines would be noticeably changed. The duration of the disruption would depend on the velocity of the object but, in all cases, the original shape of the object would be radically distorted (figs. 13, 16, and 17).
Figure 153 shows an exotic example of the motion in the martian scene. As the shadow of the meteorology boom moves from left to right, repeated scans through a single vertical line create a pseudo image. Note that this is the mirror image of the boom shadow as it actually is...
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...projected on the martian surface at any instant (fig. 152).
Early in the mission we used singleline scan repeatedly, primarily to search for movement of wind driven sand which would have been recorded as irregular speckling throughout the line scan image. The results have been less than dramatic. No moving objects, large Martians or small sand grains, have been detected within the scene.
