by Erol Torun

February 29, 1996

from ErolTorun Website

The following article is the result of work done in 1988 and 1989 concerning an enigmatic landform located in the Cydonia Mensae region of Mars. The object pictured here is five-sided, pyramidal, and located a few miles from the Face on Mars.

Like the Face, the D&M Pyramid is part of a complex of unusual landforms that may be the product of intelligent design.

The article is subdivided into these sections:

• Background - The Cydonia investigation up to 1988, geological setting, and imagery selection criteria

• Geomorphology - Discussion of the D&M Pyramid in terms of the geological processes believed to be active on Mars

• Criteria - Reconstructive technique, and criteria for analyzing this geometry for the mathematical and architectural features that suggest intelligent design

• Geometry - Diagrams and conclusions

• Images

• References - Numbered references cited in the above sections

Background

Introduction

Among the images obtained by the Viking mission to Mars in 1976 are some that show peculiarly shaped surface features that are inconsistent with the regional geology and with surrounding landforms. Some of these objects are of such unusual morphology that there is considerable difficulty in theorizing a mechanism for their formation.

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.

High-resolution mosaic of the D&M released in September of 2003

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.

This "flipped" image of the D&M Pyramid demonstrates

near-perfect symmetry when the newly revealed angle is used as an axis

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 70^th orbit, 13^th 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.

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Geomorphology

The hypothesis that the D&M Pyramid is the result of geomorphological processes requires that some mechanism be proposed to account for its formation. The processes discussed below are evaluated in terms of their ability to produce landforms with the same general morphology as the object under study. Processes meeting this requirement are then discussed with respect to the specific morphology of the D&M Pyramid.

Fluvial Deposition/Erosion

It is now almost universally accepted that liquid water once flowed on Mars, this belief supported by the discovery by the Mariner 9 mission [7] of numerous drainage channels whose form is consistent with formation by water (as opposed to lava channels, which have also been found in some areas). Fluvial processes can yield roughly symmetrical relief, such as the teardrop-shaped islands observed in many Martian stream beds. But in this case, fluvial processes can be ruled out as mechanisms for forming the D&M Pyramid as there are no indications that water ever flowed 1 km deep in Cydonia Mensae (1 km being the approximate height of the D&M Pyramid. It is also true that sharp edged multi-faceted symmetrical shapes are not characteristic of fluvial landforms.

As mentioned earlier, there is a theory that the basin north of Cydonia called Acidalia Planitia was once a shallow sea [6]. There are features in the study area that are suggestive of lacustrine erosion, such as small craters with a "smudged" appearance that may at one time have been submerged and subject to shallow wave action. There are also some curvilinear features suggestive of lacustrine deposition. The D&M Pyramid, however, bears no resemblance to these features.
 

Aeolian Deposition/Erosion

Aeolian depositional landforms on Mars typically take the form of sand dunes which are morphologically similar to those found on Earth, and have been described by Breed, Grolier, and McCauley [8]. Dunes may occur singly, in small groups, or in dune fields, and are transient in form and location. Dunes will occasionally form star-shaped dunes and crescent-shaped symmetrical dunes known as barchans, but no dune will ever form a symmetrical polyhedron resembling the one under study. Flat sides and straight edges are unobserved in terrestrial or Martian sand dunes.

Aeolian erosion is now the dominant mechanism of geomorphic change on Mars and has been since the disappearance of liquid water and the cessation of volcanic activity, both of which have been estimated to have occurred early in the planet’s history (>2 b.y. ago). One type of aeolian erosional feature found on Mars that can assume roughly pyramidal shapes are known as yardangs (Ward [9]). Yardangs are landforms that have been modified by deflation (removal of particles by wind) and by the abrasive action of wind-borne particulates, and typically resemble hills with sharp edges aligned with the prevailing winds. Their shape has been likened to upturned boat hulls. Yardangs can occasionally exhibit sharp edges, roughly flat sides, and bilateral symmetry. Comparison of the D&M Pyramid with landforms known to be yardangs immediately reveals some serious inconsistencies. The D&M Pyramid is an isolated landform with no other nearby objects exhibiting a similar shape and orientation. Yardangs normally begin with the downcutting and expansion of low spots, producing a series of parallel ridges that are gradually denuded into separate hills. These hills are eroded further by abrasion on the windward side and by deflation on the leeward side. It is rare for yardangs to be found in isolation.

Another type of aeolian landform that can be somewhat pyramidal in shape are known as ventifacts. Terrestrial ventifacts are normally formed from small rocks that are exposed to the abrasive action of sand carried by the wind. Multifaceted terrestrial ventifacts are believed to have been produced as a result of movement of a rock causing it to present different faces to the direction of the prevailing winds, a process described by Sharp [10] and Greeley and Iverson [11]. Large ventifacts can also exist, produced from boulders and assuming a roughly pyramidal shape with three edges (dreikanters). These landforms present a long edge toward the prevailing winds, and a somewhat flat surface in the opposite direction. The leading edge is cut by abrasion of wind-borne particulates and the trailing surface apparently formed by deflation from locally reversed airflow, mechanisms essentially the same as those that form yardangs.

Five-sided symmetrical ventifacts or yardangs appear to be totally nonexistent on Earth and Mars. Prevailing winds are not likely to have shifted periodically with perfect symmetry and timing. Even if this seemingly impossible condition were satisfied, another factor would prevent such an object from forming. As noted above, locally reversed airflow can cut a flat surface perpendicular to the wind direction on the leeward side of a wind-cut hill. This locally reversed airflow, and associated surface level turbulence, would prevent the formation of this hypothetical five-sided ventifact. Each time the wind shifted to a new direction, the reversed airflow would start erasing the edges formed by other wind directions. The end result would not be a pyramidal hill, but rather a round one.

The overall morphology of the D&M Pyramid, with its straight edges and flat surfaces in radial arrangement, is inconsistent with the morphology of aeolian landforms. The nearby Face shows no evidence of wind faceting, and there are no intervening objects between the Face and the D&M Pyramid to deflect wind. Also inconsistent is the presence of a flat-faced protuberance at the front of the object, a flat surface that should not exist at the leading edge of wind cut features such as yardangs or ventifacts. It is reasonable to conclude that aeolian processes cannot have produced the D&M Pyramid due to the lack of a plausible mechanism of formation, and the absence of similar landforms on Mars or Earth.

Mars Map

Mass Wasting

The results of downslope movement of large amounts of rock and/or soil under the influence of gravity has been observed in many areas of Mars. This process, perhaps driven by the freezing and thawing of subsurface water, was likely a factor in shaping many of the irregular knobs found in Cydonia Mensae, as well as the angular remnants of cratered plateau material to the south. Mass wasting, however, is unlikely to have formed the D&M Pyramid. It is uncharacteristic of mass wasting of loose material, or slumping of single masses, for such material to slide off of a hill in such a way as to leave behind multiple flat surfaces and straight edges where none had previously existed.

It is also unlikely that such mass wasting would occur symmetrically. When mass wasting produces a flat surface, it is normally due to some previously existing fault or sedimentation layer that provides a shear surface for the mass wasting or slumping. An analogous example from Earth geology would be the failure of a slope consisting of Cretaceous clay that has such internal layers. If this type of internal layering occurs on Mars, it is unlikely to occur symmetrically so as to yield a symmetrical erosional remnant.
 

Volcanism

Volcanism on Mars was once an important factor in producing and modifying surface features. Most volcanism appears to have ceased ~2 b.y. ago, and is described by Carr [12] as having been concentrated in certain regions; the Tharsis bulge, and the Elysium and Hellas regions being the most prominent. There are no signs of significant volcanic activity in the Cydonia Mensae region, thus drastically reducing the possibility of any landform in the region being thus formed.

Fluvial erosion of terrestrial volcanoes can produce erosional calderas that are roughly symmetrical, but these landforms do not even faintly resemble the object under study. Erosional calderas form from the downcutting and widening of channels that eventually merge to form amphitheatre-like depressions in the sides of the volcano. These depressions extend for most of the height of the volcano, and are roughly parabolic in cross section, never flat. No Martian volcanoes observed to date show signs of having been heavily denuded by rainfall, due to the apparent lack of large amounts of rainfall in recent epochs. Additionally, the D&M Pyramid has no vent at its apex, and exhibits a symmetry unknown in volcanic landforms.
 

Crystal Growth

Nearly all examples of naturally occurring symmetrical polyhedra are the result of crystal growth. It has been theorized that a freeze-thaw cycle might have produced unusual crystal growth from the water believed by some to lie beneath the surface of Mars, but the D&M Pyramid could not possibly have been produced in this manner. The size of the object (> 2 km long) would alone preclude the possibility of it being an ice crystal, as would the fact that ice crystals have hexagonal symmetry. Crystallization of other substances can also be ruled out as it is impossible for any normal crystal to possess 5-fold symmetry. The D&M Pyramid actually deviates from perfect 5-fold symmetry by being somewhat elongated along one axis, and is thus even further removed from crystalline geometries.

There is an exotic form of "crystalline" matter known as quasicrystals. These exhibit icosahedral symmetry in three dimensions, and thus have five-fold symmetry planes in two dimensional sections. Quasicrystals are produced by rapid quenching of molten alloys of aluminum and magnesium. The materials and process of formation do not remotely resemble the conditions and surface chemistry of Mars, and can also be ruled out as a causative mechanism for five-fold landforms.

The Geomorphic Hypothesis is thus left with no mechanism that can explain the formation of the D&M Pyramid. This object’s 5-sided shape and bilateral symmetry is unlike any landform seen to date in this solar system, and even small-scale phenomena such as crystal growth cannot explain its morphology.
 

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Criteria

Geometric Reconstruction

The hypothesis that the D&M Pyramid may be the product of intelligent design cannot be advanced simply because there is no geomorphological explanation, but must be subjected to an objective analysis, especially due to its location on Mars, a place where life is not known to have existed. Thus we must first ask a preliminary question: What are the hallmarks of architectures that distinguish them from landforms and how may they be objectively recognized and evaluated?
In searching for the signs of intelligent design, we cannot allow ourselves any subjective, Earth-based frames of reference. The observation "it looks like a pyramid" is sufficient to focus our attention towards further investigation, but proves nothing else. With subjective appearances set aside, there are a number of objective characteristics of architectures that can be identified:

Is the object’s geometry inconsistent with known landforms and geomorphological processes? (i.e. does the object exhibit straight lines, curved lines having fixed radii, regular patterns, one or more axes of symmetry, and does the combination of these characteristics preclude geomorphology as a mechanism of origin?)

Is the object aligned with the cardinal directions and/or with significant astronomical events?

Is the object collocated with other objects that are also inconsistent with the surrounding geology? And if so, are they geometrically aligned with each other?

Does the object’s geometry express mathematically significant numbers, and/or the symmetries associated with architecture?

The characteristics listed above cannot by themselves establish an object as being the product of intelligent design. What is required is a totality of geometric relationships that, when viewed as a whole, preclude the likelihood of a natural origin. This is precisely the same "convergence of evidence" technique used in the photo-interpretation of aerial photography and satellite imagery. [13]

Methods

In exploring the geometry of the D&M Pyramid, the most conservative approach possible was pursued. The reasons for this are partly historical. The Great Pyramid of Egypt has a long history of being mathematically abused by mostly well-meaning investigators who, in attempts to "prove" various theories, have subjected the Great Pyramid to excessively complex measurements and calculations. It is only the simplest and most elegant of these measurements that are widely accepted by architects as valid.

Study of the D&M Pyramid’s geometry was therefore restricted to overall observations of location and symmetry, and to these simple relationships:

• The values of observable angles expressed in radian measure.

• Examining the ratios formed between the observable angles for equality with mathematically significant numbers.

• Examining the Sine, Cosine, and Tangent of measured angles for the presence of mathematically significant numbers.

These approaches were selected due to their simplicity, their validity in number bases other than decimal, and their independence from our convention of expressing angles as a portion of a 360 degree circle.
Technique

As previously mentioned, the D&M Pyramid shows signs of being damaged on one side, perhaps by a meteoric impact. Some edge and angle reconstruction was therefore necessary before any accurate measurements could be obtained. This is a speculative reconstruction, due to the eroded state of the object. The reconstructive technique is however the same as that used widely in archaeology when evaluating sites in which geometry is important, as in archaeoastronomy.

An NGF filtered orthographic negative of Viking orbiter frame 70A13 was obtained from the National Space Sciences Data Center. An orthographic projection was necessary to ensure that the geometry of the object under study was accurately represented on the image.

The negative image of the D&M was projected using a photographic enlarger that had first been calibrated with a projection grid. This image was used for the reconstruction, combined with reference to an unrectified image processed by Dr. Mark Carlotto for confirmation of detail that was sometimes less clear in the contrasty original negative.

The most distinct edges on the pyramid, those on the sunlit side, were marked by visual averaging. These edges were extended to locate the position of the hypothetical original apex.

• A straight line was drawn from the apex through the flat protuberance at the front of the pyramid to mark what appears to be an axis of symmetry.

• A line was extended from the apex to the right front corner, which is sharp and clearly visible on the Carlotto image.

• The figure was enclosed, based upon the left side of the pyramid and the right front corner.

All visible angles of the D&M Pyramid were measured (+/- 0.2 deg) and subjected to the tests mentioned earlier: radian measure, angle ratios, and trig functions. The results of these measurements are summarized in the following section.

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Geometry

Evaluation

The reconstructed geometry of the D&M Pyramid shows a five-sided object having facets with differing angles. The object has bilateral symmetry, with a pair of congruent angles forming the front, and another pair of congruent angles forming the sides. In the illustration below, the arrow points along the axis of bilateral symmetry.

The D&M Pyramid appears to be positioned with architectural alignment to other enigmatic objects nearby that have also been studied as possibly artificial. The main axis of the D&M as illustrated above points at the Face in Cydonia. Henceforth we will refer to this direction as the "front" of the pyramid.

The front of the D&M Pyramid has three edges, spaced 60 degrees apart. As noted above, the center axis points to the Face. The edge on the left of this axis points toward the center of a feature that has been nicknamed the "City" by the Cydonia investigators. The edge on the right of the center axis points toward the apex of a dome-like structure known as the "Tholus".

The five-sidedness, bilateral symmetry, and primary alignments were first observed by Richard Hoagland after studying quality digital enlargements prepared in 1984 by SRI International from negatives of images processed by DiPietro and Molenaar. These events are documented in detail by Hoagland [3] and Pozos [4].

Turning back to the reconstructed geometry, we will now consider the internal symmetries of this object.

The D&M Pyramid displays a complex interplay between five-fold and six-fold symmetry. Both symmetries are present simultaneously, with the front of the pyramid exhibiting six-fold symmetry, and the "ground level" of the pyramid yielding a 36 degree angle that is characteristic of five-fold symmetry.

It is worth noting that the practice of combining symmetries was widely practiced by the architects of antiquity. It was believed that geometry and certain mathematical relationships were crucial building blocks of the Cosmos, and that architecture should reflect these symmetries. These practices were later revived in the Islamic world and especially in Renaissance Europe.
 

The angles formed by the D&M Pyramid when viewed from above differ from each other. Consequently, they can form various ratios. These angle ratios were studied to see if the values were significant, or merely random.

The angle ratios display significant values, with a preponderance of square roots and fractions involving square roots. Once again, we have a theme used by Classical architects, who used the square roots of two, three, and five in laying out the proportions of their buildings.

For clarity, three of these angle ratios are illustrated below:

The ratios of the shaded angles are equivalent to the indicated values. In the right hand illustration, the ratio of the shaded angles is equivalent to the ratio of e (the base of the natural logarithms) and pi (the relationship between a circle’s diameter and circumference).

(This ratio of e/pi is nearly equal to the square root of three divided by two, and the precision of measurement, +/- 0.2 deg, is insufficient to distinguish between these two possible values.)

A table has been prepared displaying a list of the measured angles, and the results of the analysis. The square roots of three and five, and the values of e and pi predominate. The identity of these values is strengthened by the numerous combinations in which they occur.

Note that the radian measure and trigonometric functions of some angles yield the same values produced by the angle ratios. The geometry thus has a common contextual thread.

As mentioned earlier in the section on criteria, all of this geometry is "dimensionless", i.e. it is not dependent on such cultural conventions as counting by tens, or measuring angles in the 360 system. This geometry will "work" in any number system.
 

A Speculation Concerning Tetrahedral Geometry

The aforementioned ambiguity concerning e/pi vs. the square root of three divided by two can be resolved with the geometry of a circumscribed tetrahedron.

A tetrahedron is the simplest of the Platonic solids, a polyhedron having six edges, four vertices, and four sides where each side is an equilateral triangle. A circumscribed tetrahedron is a tetrahedron that has been enclosed in a sphere that just touches each of the four vertices.

The presence of 60 degree angles produces the ambiguity: the Sine of 60 degrees is defined as (√3)/2, and this is very close, but not equal to, the ratio of e/pi:

(√3)/2 = 0.866025
e/pi = 0.865256

It is this ambiguity that is resolved by the geometry of a circumscribed tetrahedron.

The surface area of a sphere, divided by the surface area of the tetrahedron circumscribed by it, yields a very close approximation of e which shall be termed e’:

e = 2.718282
e’ = 2.720699

When the ratio e/pi is evaluated using this "tetrahedral approximation of e", the result is precisely equal to (√3)/2:

e/pi = 0.865256
e’/pi = 0.866025 = (√3)/2

Conclusions

This investigation of the D&M Pyramid reveals a morphology that is inconsistent with the surrounding geology. The geomorphological processes observed to exist on Mars not only fail to provide a potential mechanism for the D&M Pyramid’s formation, but seem to preclude its very existence. Analysis of the object’s geometry, and its alignment with other anomalous landforms, reveal intricate relationships that are numerous and logical, and are suggestive of highly sophisticated design.

This object has been compared with the elaborate symbolic architecture of antiquity. While much of the geometry is the same utilized by Classical architects, it is important to note that the implementation is totally different. Nowhere in Earth history is this exact type of geometric symbolism to be found.

The same techniques used for most of this century in air photo interpretation show that the D&M Pyramid may be artificial, or may be a natural landform modified by intelligence. The true nature of this object can be resolved by re-imaging the Cydonia region to obtain high-resolution imagery of the D&M Pyramid and the other enigmatic landforms nearby. The issue of possible ruins at Cydonia is thus the only question involving extraterrestrial intelligence that can be easily resolved with known scientific means.
 

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Images

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References

1. V. DiPietro and G. Molenaar, ’Unusual Martian Surface Features’ Mars Research, Glen Dale, Maryland (1982).

2. M.J. Carlotto, ’Digital imagery analysis of unusual Martian surface features’, Applied Optics, 27, 1926-1933 (1988).

3. R.C. Hoagland, ’The Monuments of Mars - A City on the Edge of Forever’, North Atlantic Books, Berkeley, 1987, 1992.

4. R.R. Pozos, ’The Face on Mars: Evidence for a Lost Civilization?’, Chicago Review Press, Chicago, 1986.

5. J.E. Guest, P.S. Butterworth and R. Greeley, ’Geological Observations in the Cydonia region of Mars from Viking’, J. Geophys. Res., 82, 4111-4120 (1977).

6. T.J. Parker, D.S. Gorsline, R.S. Saunders, D.C. Pieri, and D.M. Schneeberger, ’Coastal Geomorphology of the Martian Northern Plains’, J. Geophys. Res., 82, No. E6, 11,061-11,078 (1993).

7. National Aeronautics and Space Administration, ’Mars as Viewed by Mariner 9’ (NASA SP-329), Scientific and Technical Information Office, Washington, D.C. (1976).

8. C.S. Breed, M.J. Grolier and J.F. McCauley, ’Morphology and distribution of common ’sand’ dunes on Mars: comparison with the Earth’, J. Geophys. Res., 84, 8183-8204 (1979).

9. A.W. Ward, ’Yardangs on Mars: evidence of recent wind erosion’, J. Geophys. Res., 84, 8147-8166 (1979).

10. R.P. Sharp, ’Pleistocene ventifacts east of the Big Horn Mountains, Wyoming’, J. Geol., 175-195 (1948).

11. R. Greeley, and J.D. Iverson, ’Wind as a Geological Process on Earth, Mars, Venus, and Titan’, Cambridge University Press, Cambridge (1985).

12. M.H. Carr, ’The Surface of Mars’, Yale University Press, New Haven (1981).

13. American Society of Photogrammetry, ’Manual of Photo Interpretation’, p.109-111 (1960).

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