Before looking at some representative images of the Red Planet, we set up a physiographic framework for Mars: From Earth-based telescopic observations, astronomers suspected Mars had oxidized, iron-rich materials at its surface, was subject to dust storms, and had ice caps at both poles that alternately expanded and contracted over a martian year (687 Earth days). The storms implied at least a thin gaseous envelope (out to about 125 km). Spectroscopic measurements indicated CO₂ and maybe nitrogen. The Mariner/Viking missions greatly modified our concepts of the martian landscapes. Consider the generalized landforms maps of the two martian equatorial hemispheres, drawn on a Lambert equal-area projection from those mission results, as shown here:

From T.A. Mutch et al., The Geology of Mars, © 1976. Reproduced by permission of the Princeton University Press, New Jersey.

Much of the upper parts of the two globes (left and right hemispheres in the diagram) have plains units (p) (in white), many thought to be volcanic flows (pv), and volcanic constructs (v) (such as shield volcanoes), some of whose ages could be less than one-half billion years.The bottom polar hemisphere in each globe contains cratered terrain (cu) (dark gray), considered to be several billions of years old (and the likely source of the 3.5 b.y. old martian meteorite containing organics, found in Antarctica), where the bulk of the larger impact structures survive. Other subdivisions include cratered plains (pc) and moderately cratered plains (pm). Unusual or specialized units consist of channel deposits (c), grooved terrain (g), knobby hummocky terrain (hk), fretted hummocky terrain (hf), chaotic hummocky terrain (hc), and mountainous terrain (m). Units confined to the polar regions are: permanent ice (pi), layered deposits (id) (thought to be layers of windblown dust interspersed with ice), and etched plains (ep). As with Venus, structural signs of plate tectonics are absent.

Compositional data (discussed on pages 19-13 and 19-13a) suggest that the darker shaded southern part on the Mars maps is underlain by basalt whereas parts of the northern hemisphere contain andesites as well as basalts.

Many features in these categories are also landmarks and physiographic provinces. In the left hemisphere, the three volcanoes in a row (Ascraeus Mons [top], Pavonis Mons [center], and Arsia Mons [bottom]) are in the Tharsis Montes group. The two large ones above and to the left are Olympus Mons (largest volcano in the Solar System) and Alba Patera (also a caldera-capped volcano). The long, curvilinear feature (c) near the equator in the left hemisphere is the great Valles Marineris, many times longer and much deeper than the Grand Canyon, with associated chasmas or tributary canyons. In the right hemisphere, the large area (p) in the southern half is Hellas Planitia, site of the biggest impact basin on Mars, cut into ancient terrain and backfilled with lava.

19-36: The Tharsis region shield volcanoes are huge by Earth standards. What single volcanic structure on Earth comes closest to these in size and morphology? ANSWER

Further knowledge of martian physiography can be gained from relief (elevation difference) maps of Mars. Several of these for selected regions appear in the next three pages; such maps can be constructed from laser altimeter and radar data, as provided by Mars Global Surveyor and other spacecraft. Shown below is a global shaded relief map of Mars, made from MGS data. Note the higher elevation (red) region around Olympus Mons and the Tharsis volcanoes; the low (blue) depression that constitutes the Hellas Basin; and the generally low state of the northern polar region (James Head's ocean basin).

The next map, made from MGS MOLA data, focuses in on what is probably the most interesting part of the martian surface - the high terrain that includes Olympus Mons:

The Martian Atmosphere

The Voyager and Mariner missions confirmed the martian atmosphere consists of 95.3% CO₂, with the remainder being mostly N₂ (2.7%), argon (1.6%) and minor O₂, CO, and traceable water vapor. Atmospheric pressure averages 7 millibars or about 0.7% of Earth’s, but windstorms within this thin envelope can reach speeds greater than 200 km/hr (124 mph). Clouds of water-ice are fairly common in season, especially near the poles. This Viking Orbiter image shows clouds above the martian limb and perhaps clouds over the land.

The presence of small amounts of oxygen has been confirmed by X-ray emissions monitored by the Chandra X-ray telescope. O₂ is excited by X-rays accompanying the solar wind. The amounts seem to vary, as seen in this Chandra image:

Both Mariner 10 and the Viking Orbiters gathered many images showing various aspects of atmospheric circulation, such as this view of spiraling clouds above the martian North Polar region; note also the bright patches of ice.

Clouds reminescent of cumulus types on Earth are seen in the atmosphere. These are evident in this MERS Opportunity (see page 19-13a) image which also show cloud bands formed by atmospheric diversion by the Crater Mie:

The martian atmosphere is generally hazy, resulting from suspended dust and possibly some water condensate. Instead of the blue sky of Earth, one peering up from the Mars surface would see its sky as grayish-yellow to reddish, depending on the time of the martian day. Although Viking Landers gave some indication of these colors, the next image, from Mars Pathfinder's camera looking well up into the sky, is a good rendition of color near sunrise; the clouds are of the cirrus type as known on Earth.

Mars is now known to not be DEAD as a planet. As we shall see, there may be active water transfer, buildup of ice caps, and possibly volcanic eruptions.

As further proof that Mars is meteorologically quite active despite the thinness of the atmosphere, an extensive dust storm was imaged from an orbiter in the act of happening:

When enough of these regional dust storms form, they coalesce and sometimes cover the entire planet for many (earth) months.

More than half the planet was engulfed in a widespread dust storm that began in June of 2001 and started to diminish in September. Compare these "before" and "after" Hubble images of minimal and maximal dust coverage; note how the surface features in the left image are masked by the pervasive, near global dispersal of dust into the atmosphere as seen in the right image.

Using the Thermal Emission Spectrograph on Hubble, a sequence of images taken between June and August, 2001 shows the variations in density of dust (shown in red) in the martian atmosphere.

Earth-based telescopes can spot developing dust storms almost as soon as they start. The yellow-brown area in this next full Mars image made when that planet was at its closest distance to Earth (55,000,00 km [34,000,000 miles]) in the last hundred years is an organizing dust storm imaged through a ground-based telescope in Arkansas:

Certainly, as evident from the previous images, its thin atmosphere is remarkably active, with fast-moving winds picking up and redepositing surficial particles in dunes and other aeolian features. Here is a "candid camera" image of what on Earth are called dust devils - tornado-like wind swirls that pick up surface fines to create a tubular or thin funnel-shaped cloud that sweeps across the surface. This Mars Orbiter Camera (MOC; see page 19-13) image shows a thin dark trace along a crater wall made when the dust-devil (actually moving; on top, at the left end of the streak) removed material from the wall:

Thin dark markings, often found in swarms, on parts of the martian surface, have now been identified as trails caused by numerous dust devils ("mini-tornadoes") scouring the loose surficial material. Here is a striking example:

Dust devils must be commonplace on Mars. In a single season, the number of trails like those shown below, can be many. The winds seem to destroy earlier ones, then fresh trails are created that expose the dark surficial materials below a new covering of lighter-toned dust:

In parts of Mars that have dark surfaces, the dust devil trails appear as lighter streaks, implying the underlying near surface contains lighter materials exposed when the whirlwind picks up the dark fines covering the surface. Thus:

Individual dust devils have been image as they actually passed by landers on Mars. The Rover Spirit caught this image of a nearby whirlwind:

The martian dust and sand is frequently spread out as wind streaks.

Despite the very low density of today's martian atmosphere, its high speed winds can cause erosive sculpturing, and can deposit uplifted dust and sand into vast dune fields. The Viking Orbiter image below contains transverse (elongated, or parallel) and barchan (crescent) dunes laid down in a plains setting.

A second image shows parallel dunes on the left and barchan dunes on the right.

More recent imagery shows the splendor of martian dune fields, such as this one in Ius Chasma:

Here are longitudinal dunes adjacent to the Schiaparelli crater:

Distinctive ripple dunes can develop within shallow troughs or linear valleys. This is a good example:

19-37: Why is it rather surprising that dunes with types similar to those on Earth are so widespread on Mars (hint: consider the atmosphere)? ANSWER

In contrast to dunes on Earth that usually have a light tone because they are made up of mostly clear to cloudy white quartz, dunes on Mars usually are much darker because the grains making them up are either basaltic fragments or darker hematite. This is a typical set of darker dunes:

Although uncommon, some places on Mars show two sets of dunes - one dark and the other light in tone. This dual occurrence is hard to explain and suggests two different processes (or time intervals) at work:

Martian dunes can have a regular, criss-crossing pattern (lattice-like) that is uncommon on Earth where strong prevailing winds tend to favor a single alignment (cross-winds would likely destroy the reticulation). Here is an example from the martian polar region:

More striking close-up views of dunes have been imaged by the Mars Global Surveyor (MGS; MOC) (see page 19-13). Crescent dunes very similar to those found on Earth also develop on Mars, as seen in this MGS ) image:

A variant of this type leads to dunes that approach crude circles in shape.

In this next image, dunes having shapes similar to those just above have formed but they are far more widely spaced and tend to be round. Some "wags" have named them "martian cookies". They are most common in the polar regions where their dark materials stand out against a frosty surface.

A MOC close up view of barchan dunes appears next:

The martian dunes come in a wide variety of forms. This MOC image shows dunes with a broad, gentle slope and steep, ragged foreface:

Similar dunes, with odd shapes, are evident in this next image in the north polar region. The dark dunes (which may have a high percentage of basalt-like dust and grains) are now exposed after seasonal frost has evaporated from them (in the martian summer there) but stand out because residual permafrost and/or ice is much lighter (more reflective).

This image shows individual dunes so close-packed that there is no interdune space. Such a density is rare on Earth. The field has developed near the South Pole.

Some dune fields show a series of overlapping dunes, within which thin layers are just visible. Here is an example in the interior of Becquerel crater:

Mars rovers have taken close-up images of individual dunes on the ground. Opportunity (page 19-13a) shows this dune group (longitudinal type) in detail:

Strong polar winds produce distinctive dunes, such as these at the edge of the North Polar cap:

Even more unusual, are close-spaced, rectangular dunes in the north polar ice. The dunes themselves are made of dark material but in this view the polar frost has covered them. Melting is just beginning to expose as black spots the underlying ice.

Also near the poles are these similar dunes from which the frost has evaporated everywhere except the foreslopes.

Unusual, close and interfering dunes are found at the North Pole as well. In this image, frost has accentuated the appearance of these lenticular dunes:

Another feature with a terrestrial counterpart is the yardang (see page 17-5), formed by wind scooping out soft materials and leaving long ridges (usually of the same material) in between. Here is a martian version:

This image shows a field of yardangs, with indications of which way the wind blows in this part of Mars:

The streamlined shape of the yardangs is a strong indication of the power of martian winds to erode. Wind erosion features of less definitive shape are widespread on Mars, indicating that this type of erosive reshaping is commonplace. The next two images (Medusae Sulci region) show an area of irregular shaped "hills" that are being carved by the wind. The upper image comprises the complete MOC image; the lower image (rotated 90° counterclockwise) is an enlargement of part of the scene in which ripples in the low valleys attest to continuing wind activity that covered the surface with dust:

At present almost all of Mars is covered with a thin to thick blanket of dust layers. This often obscures smaller surface features. Here is a MOC view of a smooth dust cover draped over surfaces at Pavonis Mons.

Turning now to the frozen materials that persist or build up/dissipate in the North and South Pole regions. The dominant surface constituent of the polar caps is CO₂, but major amounts of water seem to be locked within the caps themselves as subsurface concentrations (see page 19-13). The next views show the South Polar ice cap (top) and North Polar ice cap (bottom) (which may contain more water) near their maximum growth stage, during a martian winter. This winter recurs about every 685 Earth days at each pole (remember, the rotational pole is tilted about 24°). In about half that time the polar ice at one pole shrinks as summer warming evaporates the frozen gases, and possibly subliming water underneath and the opposing pole experiences ice condensation and growth:

The layering associated with each icecap, probably representing seasonal deposits of dust (see below), is even more obvious in this regional oblique view of the top of the Northern Hemisphere:

Considerable change in size of both ice caps occur over the Winter to Summer transition. This three-panel set of images made using the Hubble Space Telescope (HST) illustrates variation of areal coverage at the North Pole over a 6 earth month period from late 1996 through early 1997:

Thus, in the summer the ice cover may have shrunk so that it persists only in patches. That is evident in this Mars Express perspective image of dark material (dust, volcanic deposits, etc.) mixed with a subordinate amount of carbon dioxide/water ice:

Observations now over several decades have strengthened the fact that the ice is more widespread in the northern polar region, is permanent, but tends to shrink more during the warming season. The higher elevations in the southern hemisphere may account for this difference, being cooler at those heights. Here is a map of the maximum and minimum extents of the north polar cap during the growth/shrinkage phases:

The Mars Express orbiting satellite has now confirmed that there is also permanent ice in the south polar ice cap. Here is a map of water ice (in blue) covering that region:

This change in polar cap thickness over time, determined by elevation differences in meters, has been measured at both pole regions by the laser altimeter (MOLA) on the MGS, with these results:

The surface of an ice cap shows distinctive changes during sublimation and shrinkage as shown in this MOC image of a part of the south polar ice:

Dust layers in the polar caps have been spotted in pictures from earlier missions that imaged this region on Mars. Details (at 25 m resolution) have been acquired by the Mars Orbiter high resolution camera. Here are layers in the South Polar ice cap:

Closer looks at the sides of the polar caps revealed prominent alternating bands of light (the ice) and dark (the dust) materials, as seen here in these two views of the south polar cap:

And here is a view of banding in the ice at the martian North Pole:

This MGS MOC image shows banding with different levels of "greyness" in the dust beds; the arrows point to an unconformity (erosional discontinuity in the sequence of layering).

The Mars Reconnaissance Orbiter (MRO) has sent back color images of the polar layering, as exposed in Chasma Boreale (shown below), a 700 km long canyon cut into the ice cap. The redness of the layers suggest windblown dust from lower latitudes but one interpretation favors volcanic ash (not likely):

This next image of banding seems to indicate strong light-dark contrasts in an area also undergoing erosional sculpturing; the sharpness may be somewhat illusory.

19-38: Try to devise an explanation for these layers in the image just above. ANSWER

The Mars Global Surveyor has been looking more closely at both poles. As melting proceeds, contorted banding and ice polygons are exposed, and somewhat emphasized in patterns by frost coatings. Here is an example:

In the fringes around the polar ice caps, dark spots appear and disappear over the course of a martian year. In this image, these spots occur midst a dune field. It is not yet known whether, and how much, water ice and/or carbon dioxide frost coatings are involved in the on-going evaporation that produces the spotting, which may be dune material showing through.

This process of frost evaporation is more advanced in this next image, in which removal of the white coating is exposing dark dune sands beneath.

Interesting structures and patterns are developed on the CO₂ ice. This image shows more than one tier of ice, forming plateaus, mesas, and buttes (by analogy with Earth). The darker surface may be ice contaminated with dark windblown dust (either recent or an exhumbed layer):

An example of a small mesa apparently composed entirely of CO₂ is shown in this North Polar ice cap. The two mesas are about 1 km and 1.5 km in long dimension and many meters thick. They are very slowly receding by melting and/or ablation at their cliff faces.

Quaint descriptive names are picked by the investigators to characterize surfaces. In the two images below, both of South Polar ice that is largely CO₂, the top one has reminded them of "swiss cheese" while the bottom looks like a "kitchen sponge.

This pattern persists at even higher resolutions; this 1-meter resolution Mars Reconnaissance Orbiter image displays a broken-up surface in martian ice:

Much of the ice at each pole may exist in the form of grains, much like sand. As the polar ice undergoes changes during the evaporation/deposition cycle that affects the poles, strong winds are capable of moving these grains into barchan-like dunes, as shown in this image of the surface of the North Pole Ice Cap:

A curious feature seen by the Mars Global Surveyor near the South Pole is evident in this next image. Several domelike (nearly circular) structures occur midst what is similar to fretted terrain (exemplified on page 19-13). These domes have steep narrow outer slopes and a wide craterlike interior. Although suggestive of volcanic structures, their origin is unclear.

Another unusual feature marked by its roundness is shown in this next image from polar ice. The exact nature is conjectural but the darker clusters of roughly circular objects appear to be caused by evaporation of carbon dioxide during the martian summer warming, leaving behind dust in these "blow holes".

In August 2006 investigators presented evidence citing these features as fountains that during the martian year for several weeks spout CO₂ gas and particles into the thin atmosphere up to heights of 50 meters (160 feet). These illustrations, made by Mars Odyssey's THEMIS (page 19-13a), show three patterns associated with the fountains on the South Polar ice field:

What appears to be happening is this: During the polar summer, surficial water ice evaporates; carbon dioxide locally sublimates and builds up pressure; the fountains "erupt" and spew gas and particles upwards; the water ice returns in winter and covers the dark spots associated with the vents.

Finally, among other polar landform curiosities is this set of straight, equi-widthed troughs noted in the North Polar cap:

Lest you get an impression that ice is found only at high latitudes on Mars, we state succinctly here (treated in detail on page 19-13) that evidence is strong that water ice is present beneath the martian surface (as permafrost analogous to that found in Alaska and Siberia) over much of the planet. But, being dust and rock fragment covered, it is not visible in images of those regions.

The above review of polar ice features is based on solid interpretations of present-day features associated with frozen water. Much more nebulous is the question of whether in the martian past there was much more widespread ice in forms we associate with glaciers on Earth. This view is not universally held by Mars investigators. But geoscientists like Victor Baker, James Head, and William Hartmann III had favored interpretation of many features at latitudes lower than the polar regions as indicating the presence of ice either in "mountain glaciers" or in sheets (but probably much thinner than Pleistocene glaciation on Earth) or in periglacial and permafrost environments (this last setting is fairly widely accepted now). Several even place the last glacial age on Mars as recently as 10 to 2 million years ago. More signs of glaciation are present in the martian northern hemisphere. The writer spent several days "surfing the Net" for convincing proof but failed to find any really solid evidence (by my standards). So, despite misgivings, here are some features that have been cited as indications of extensive glaciation; keep in mind that several shown here can have other explanations.

The next three illustrations show deposits of materials in lobe shapes that resemble certain terminal moraines associated with mountain glaciers.

This image shows surface features in the south polar region developed beneath the ice cap which has shrunk to a minimum at the time it was viewed; the landforms have aspects indicative of both glacial and aeolian processes:

This next image contains an interpretation of features on a martian plain that have been tied to similar-appearing features on a continental outwash plain on Earth.

On Earth ice rafts make distinctive patterns at the surface when they are covered by deposits and then melt. This is shown at the right in the next illustration, with a martian counterpart on the left.

The next figure includes four martian surfaces that have been interpreted as cryoturbation features in a periglacial environment:

Polygonal structures of varying sizes appear on Mars. Some consider these to be volcanic in origin (see next page); others cite them as related to ice-produced features in deposits related to glaciation.

On Earth, one feature that abounds on ice sheets is sets of intersecting cracks that produce what is termed "polygonal" ice. This has been observed also on Mars in the polar regions where is is one type of patterned ground. It usually shows up after the warming of the polar region sublimates the coating of carbon ice. Here is an example from the South Polar ice cap.

This martian surface shows smaller polygons:

In this example, the polygons are confined to the interior of a large impact crater. If related to ice, one must presume that water filled the crater and then froze.

This image shows thin ridge like features over a wide area that seem similar to terrestrial terrains in permafrost regions.

The next two images show terrains marked by flow patterns. These could be glacial, or alternately fluvial or volcanic in origin; see captions for further details:

The last two are associated with what is called fretted terrain (see next page). Such a terrain is diverse in its nature, being mostly erosional but with some deposition. Certain broad channels on Mars are classed as fretted channels. Some appear fluvial in nature; others may be involved in glacial action.

This next scene is ambiguous. It may show aeolian features (close-spaced yardangs; see above), or weird fluvial deposits, or possibly drumlin-like elongate hillocks of glacial origin.

On Earth, when lava outpours beneath an ice sheet it can produce small small protrusions (seen after the ice leaves) or lava cones. This next image shows what has been interpreted as a lava cone field on Mars, now exposed after glaciation ceased.

We close this topic of martian glaciation by pointing to the opinion of some investigators that many of the thin deposits of sediments discussed on the 19-13 page sequence may have a glacial origin. Most still favor a lacustrine or marine origin.

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Primary Author: Nicholas M. Short, Sr.