The first of these unusual features to be noticed was a 2 km long knob that resembled a human face staring straight up from the surface. The Face was found in the northern hemisphere of Mars at the boundary between the basin of Acidalia Planitia and the higher ground of Cydonia Mensae. The object's resemblance to a face was noticed by NASA personnel at the Jet Propulsion Laboratory (JPL), who briefly displayed it at a press conference. NASA officially dismissed the Face as a trick of light and shadow. The Face was rediscovered by Vincent DiPietro and Gregory Molenaar, computer scientists working at the Goddard Space Flight Center who unexpectedly found it while working with the Viking imagery. They later found another image of the Face that had been taken under different lighting conditions. Computer enhancement of these images revealed bilateral symmetry, detail resembling eyes, a nose, and a mouth, and persistence of this detail under two different sun angles. Their work was largely ignored by the planetary sciences community, and was published independently as a monograph (DiPietro and Molenaar [1]). Subsequent work by Dr. Mark Carlotto [2] using single-image shape from shading techniques demonstrate that the Face is not a trick of light and shadow or the result of variations in surface albedo. It is a three dimensional landform that, for whatever reason, has the form of a human face. Enhanced image processing by Carlotto more clearly reveals the presence of an eye socket in the shadowed side, as well as detail in the mouth that is suggestive of teeth.
DiPietro and Molenaar's image processing also aided in the observation of other landforms that are inconsistent with the local geology. Richard Hoagland, seeing the work of DiPietro and Molenaar, began investigating the imagery and discovered the presence of a cluster of polyhedral objects, later named the "City", that have a rectilinear arrangement and a major axis aimed directly at the Face. The Face's axis of symmetry is itself perpendicular to the City's major axis. Hoagland [3] later demonstrated that a square arrangement of objects in the center of the City, termed the "City Square", marks the exact midpoint along the City's major axis, and would have served as an excellent vantage point for a sightline to the Face.
In 1983, Hoagland organized and led the "Independent Mars Investigation", a cooperative effort of specialists in image processing, geology, architecture, and anthropology who studied these objects in greater detail. It was from this investigation that more information began to emerge concerning geometry and alignments. DiPietro and Molenaar had previously noted the presence of a massive pyramid, nearly 3 km in length and 1 km high, to the south of the city and face. Hoagland, working with a higher quality image processed by Stanford Research Institute, Inc., observed the object to be a 5-sided, bilaterally symmetrical pyramid whose axis of symmetry is aimed directly at the face. Hoagland also noted the alignment of one edge of the pyramid with the city square and of another edge of the pyramid with an unusually shaped round hill that lies to the east of the city on the same latitude as the city square that was named the "Tholus". Hoagland named the large pyramid the "D&M Pyramid", after the earlier work of DiPietro and Molenaar.
The front of the D&M Pyramid (closest to the face) is formed by two congruent angles, with two larger congruent angles forming the sides. A fifth angle forms the rear section. The pyramid exhibits some domed uplift on its right side, and what appears to be an unusually deep impact crater further to the same side. The geometric regularity of the D&M Pyramid, together with its alignment with other enigmatic landforms, has led some to speculate that the object may have an artificial origin (DiPietro and Molenaar [1], Hoagland [3], Pozos [4]). Others discount this speculation, citing the slim likelihood of life evolving on Mars past the microbial stage, and the indeterminable likelihood of colonization of Mars by a civilization from elsewhere.
Geological Setting
The geology of Cydonia Mensae is described by Guest, Butterworth, and Greeley [5]. The region shows a mixture of smooth and fractured plains, and a small to moderate amount of cratering. Most relief in the vicinity of the D&M Pyramid is composed of mesas, knobs, and smooth plains material. Mesas are most likely the remnants of an earlier surface type that was removed by erosion, leaving mesas of more resistant material. Knobs may have been formed in a similar fashion, perhaps from rough, heavily cratered terrain. The shape of some knob material appears to have been modified by mass wasting or slumping, perhaps driven by the freezing and thawing of ground ice, with the excess material carried off by wind or, under different climatic conditions, by water or glacial ice.
Further evidence for some type of erosion is provided by the presence of several pedestal craters in Cydonia Mensae. A pedestal crater is an impact crater surrounded by an ejecta blanket that ends in a steep scarp that may drop hundreds of meters to the surface. The ejecta blanket is presumably composed of material that is more resistant to erosion than the surrounding surface.
There is a theory that the northern Martian basin called Acidalia Planitia was once a shallow sea. This would place the area of Cydonia Mensae under study near the former shoreline. Small craters in this area appear to have been modified by water erosion, perhaps by shallow wave action. This would match the observations of recent researchers that linear features in this area may be lacustrine deposits resulting from shallow wave action at the edge of an ancient sea [6].
The morphology of Cydonia Mensae is thus complex and not completely understood. The region exhibits evidence for previous epochs of cratering, erosion, and deposition, contributing to the wide variety of observed landforms and surface types.
Imagery
Viking orbiter frames 35A72, 70A11, and 70A13 show the D&M Pyramid, located at 40.65N 9.55W. All three frames were taken close to periapsis and yield a pixel resolution of ~50 m. Other frames show the region at resolutions insufficient for detailed study. The frame selected for the examination of the D&M Pyramid's geometry is 70A13. (The nomenclature indicates the 70th orbit, 13th frame of the "A" orbiter.) This frame was taken at a higher sun angle than 35A72 and thus shows more of the object's structure. Images of 70A13 used later for angle measurements include an enhanced closeup of the D&M Pyramid prepared by Carlotto, and an NGF filtered orthographic rectification of the entire frame obtained from the National Space Science Data Center, Greenbelt, Maryland, USA.