To "Face" or not to "Face"?

Part of MGS image SP1-22003 showing “Face” at Cydonia. Inset locates “facial” features. Contrast is adjusted separately on the two sides, with the sunlit portions outlined. Dark border of outline is an artIfact caused by darkening everything inside the outline to bring out its details.

The “Face” at Cydonia on Mars is shown in 1976 medium resolution Viking spacecraft image 70a13 in Figure 1, and in the Mars Global Surveyor (MGS) spacecraft strip-image SP1-22003 in Figure 2. The mesa is about 2.5 km tall by 2 km wide, and extends several hundred meters above ground level. The appearance is much less face-like in the high-resolution MGS image in Figure 2 than in the original Viking image in Figure 1 for the following reasons:

(1) The MGS spacecraft took its image from a low-perspective angle well to the west, rather than from nearly overhead as in the Viking spacecraft view. Mainly the western half of the “Face” is seen in Figure 2, with the eastern half largely hidden behind the nose ridge.

(2) Sunlight shines on the Face mesa from the low west in the Viking image, but from the low southeast in the MGS image. The latter tends to distort facial features, much like a flashlight held under the chin.

(3) The Viking image had a normal variation of grayscale levels to provide contrast between adjacent features. The range of grayscale levels in the MGS image was inadequate to provide the amount of contrast normally utilized by the human eye.

(4) Following analysis, it became apparent that the major face-like features on the mesa have the characteristic that they cast shadows that enhance the face-like appearance at almost any Sun-angle. For example, the eye socket is a depression that contains the shadow of its walls while the Sun is anywhere but overhead. It is similar for the mouth feature, which casts a shadow into the ravine between the lips at most times of day. The facial appearance is enhanced by such shadows, but is difficult to separate from the background when the shadows are absent. By bad luck, the sunlight was so scattered by thin cloud cover that light on the Face was mainly ambient (omni-directional, shadow-free) light. This partially ameliorates difficulty (2), but creates a greater problem by removing one element important to the perceived appearance of the mesa.