A mesa in Cydonia looks like a face

The Viking 1 orbiter was scouting landing sites for Viking 2 when it took frame 35A72 over the Cydonia region on 25 July 1976. In it, a mesa about a kilometre and a half across near 40.8°N, 9.6°W appeared to have two eyes, a nose and a mouth. The frame was shot from a range of 1,873 kilometres with the sun 20 degrees above the horizon, at about 47 metres per pixel, and the bit errors that speckled the transmission had dropped a black fleck exactly where a nostril would go.

NASA released the picture within days with a caption pointing out the illusion. Viking project scientist Gerald Soffen dismissed it as a trick of light and shadow. In 1980 electrical engineer Vincent DiPietro and computer scientist Gregory Molenaar dug the original data out of the archive and reprocessed it, conceding erosion might have produced the Face, but arguing otherwise from the detail. Richard Hoagland built a career on the claim that Cydonia held the ruins of a civilisation, complete with pyramids and a city. By the late 1980s the Face was on book covers and in a steady stream of letters to NASA.

Better cameras ended it. On 5 April 1998 the Mars Orbiter Camera aboard Mars Global Surveyor was turned to Cydonia and returned an image at 4.3 metres per pixel, ten times Viking's resolution. The eyes and mouth were not there. On 8 April 2001 MGS made a second, better-lit pass at about 2 metres per pixel, and Mars Express added stereo coverage and a digital elevation model in 2006. In three dimensions the mesa is an eroded knob of layered rock, one of many in Cydonia, with no more symmetry than the ones nobody ever photographed.

The Face repeats a pattern Mars had produced twice: Schiaparelli's canali became Lowell's canals, and seasonal darkening became vegetation. Given a noisy, low-resolution image of a real place, the human visual system supplies structure that is not in the data.