Pareidolia is a type of illusion or misperception involving a vague stimulus which is perceived as clearly being something, e.g., seeing the image of Jesus Christ in a burnt tortilla or the image of the Virgin Mary in a cloud.
Under ordinary circumstances, pareidolia provides a
psychological explanation for many delusions based upon sense
perception. For example, it explains many ufo sightings, the
hearing of cryptic and sinister messages of records played
backwards, Elvis citings, religious apparitions, Loch Ness
monster, the face on mars and Bigfoot sightings, etc.
Under clinical circumstances, however, some psychologists encourage pareidolia as a means to understanding a patient. Perhaps, the most infamous example of this type of clinical procedure is the rorschach ink blot test.
Astronomer Carl Sagan believes that the human tendency to see faces in tortillas, clouds, on the moon, mars, etc., is an evolutionary trait. He writes:
As soon as the infant can see, it recognizes faces, and we now know that this skill is hardwired in our brains. Those infants who a million years ago were unable to recognize a face smiled back less, were less likely to win the hearts of their parents, and less likely to prosper. These days, nearly every infant is quick to identify a human face, and to respond with a goony grin. [Sagan, p. 45]
Hmm. Maybe. Maybe not. In any case, Sagan's explanation of the face on Mars is less controversial.
In 1976, the Viking orbiter sent back some images of Mars which looked like a face but which NASA said was just a play of light and shadow. Some took this explanation as a sure sign of a cover-up. Some engineers and computer specialists digitally enhanced the NASA images, which soon gave birth to the claim that the face was a sculpture of a human being, located next to a city whose temples and fortifications could also be seen. Some began to wonder: were these built by the same beings who built the ancient airports in Peru and who were now communicating to us through elaborate symbols carved in wheat crops? Others took the wonder to the level of belief, based on the flimsiest of evidence and the grandest of imagination. Sagan's more down-to-earth explanation for the face on mars is that it is the result of erosion and winds and other natural forces. [Sagan, pp. 52-55]
But who knows. Maybe the face on Mars was done by the artist who did the Shroud of Turin. Some who have looked closely, have seen a family resemblance.
further reading
The Face on Mars by Sally Stephens, Astronomical Society of the Pacific
Malin Space Science System "Face on Mars" page
Sagan, Carl. The Demon-Haunted World - Science as a Candle in the Dark (New York: Random House, 1995).
Schick, Jr.,Theodore and Lewis VaughnHow to Think About Weird Things, (Mountain View, California: Mayfield Publishing Company, 1995), ch. 3.