One of the features in the Cydonia region, the "face on Mars" (about 1.5 kilometers (one mile) across), has had special notoriety in Western culture since it was imaged in 1976, because it looks like a face. This naturally occurring pareidolia has also inspired science fiction literature which typically assume it is a non-natural structure. For comparison, an example of naturally occurring pareidolia on Earth is New Hampshire's Old Man of the Mountain.
In one of the images taken by Viking 1 on July 25, 1976, a 2 km (1.2 miles) long Cydonian mesa, situated at 40.75° north latitude and 9.46° west longitude, had the appearance of a humanoid "Face on Mars". When the image was originally acquired, Viking chief scientist Gerry Soffen dismissed the "face" in image 35A72 as a "[trick] of light and shadow". However, a second image, 70A13, also shows the "Face", and was acquired 35 Viking orbits later at a different sun-angle from the 35A72 image. This latter discovery was made independently by Vincent DiPietro and Gregory Molenaar, two computer engineers at NASA's Goddard Space Flight Center. DiPietro and Molenaar discovered the two misfiled images, Viking frames 35A72 and 70A13, while searching through NASA archives.
Cydonia was first imaged in detail by the Viking 1 and Viking 2 orbiters. Eighteen images of the Cydonia region were taken by the orbiters, of which seven have resolutions better than 250 m/pixel (820 ft/pixel). The other eleven images have resolutions worse than 550 m/pixel (1800 ft/pixel) and are virtually useless for studying surface features. Of the seven good images, the lighting and time at which two pairs of images were taken are so close as to reduce the number to five distinct images. The Mission to Mars: Viking Orbiter Images of Mars CD-ROM image numbers for these are: 35A72 (VO-1010), 70A13 (VO-1011), 561A25 (VO-1021), 673B56 & 673B54 (VO-1063), and 753A33 & 753A34 (VO-1028). Parts of the region were subsequently imaged at far higher resolution by the Mars Global Surveyor, Mars Express and Mars Reconnaissance Orbiter missions.
When it was first imaged, and into the 21st century, the "Face" is near universally accepted to be an optical illusion, an example of pareidolia. After analysis of the higher resolution Mars Global Surveyor data NASA stated that "a detailed analysis of multiple images of this feature reveals a natural looking Martian hill whose illusory face-like appearance depends on the viewing angle and angle of illumination". Similar optical illusions can be found in the geology of Earth; examples include the Old Man of the Mountain, the Pedra da Gávea, the Old Man of Hoy and the Badlands Guardian.
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