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The general geology (stratigraphic units) andphysiography (landform types) of Mars is examined. Some types do not have close counterparts on Earth. But, although the atmosphere of Mars is thin, high velocity winds stir up loose surficial materials producing dust storms and aeolian deposits such as dunes. The martian polar icecaps have both conventional and unusual features including layering and collapse structures. Features which may represent more widespread glaciation over much of Mars are discussed.


Geology of Mars; The Martian Atmosphere; Ice at the Poles

Stratigraphic Units Maps of Mars

As on the Earth, layering - which gives rise to stratification - can be caused by several processes. On Mars, layering from dust deposits laid down in the present and by inference in the past and layering from impact ejecta (probably the dominant process) build-up are established mechanisms that account for perhaps the majority of stratified sequences. As we have seen several times on this page, there is now strong evidence for the action of water from streams and other outpouring mechanisms operating in the martian past. Some students of the martian surface argue that at one or more time(s) in the past, there were lakes and perhaps even oceans on Mars that existed when the martian climate was warmer than today - gone now because most of the water has either dispersed into outer space or become frozen in the upper reaches of deposits near the surface. James Head III and his colleagues at Brown University have interpreted evidence, including cliffs and terraces that may indicate shoreline erosion, of at least two stages of oceanic concentrations of water in the northern lowlands. Here is their map:

Many Mars images have been collected that show distinct layering, as will be displayed on the next page. We will preview those images by showing here just one typical set of layers exposed in West Candor Chasm, an auxiliary canyon off Valles Marineris:

Layering in the wall of West Candor Canyon; an MSS MOC image; MSSS source.

Various explanations are put forth as to their nature: Are they marine sedimentary, lake beds, volcanic flows, volcanic ash deposits, or other unknown types, or as seems likely a combination of these modes of layering? The occurrence of marine sedimentary layers on Mars is still much debated, with as yet no direct proof. But, like the Earth and its Moon, layered materials found on the martian surface follow the Law of Superposition, which defines the relative ages of overlapping deposits. And, like the Moon and some other planetary bodies, the relative densities of craters have been counted over its surface, which has led to a broad stratigraphic division of the principal geologic units that are recognized on Mars:

Mars stratigraphy.

This subdivision yields three broad Eras. The oldest, the Noachian, extends from the formation of the planet to a time estimated to be 3.8-3.5 billion years ago. Crater-forming bombardment was maximum during this time, producing Hellas and other basins. Both extensive lava flows and perhaps a broad regional ocean developed in this time, and the dichotomy of older high plains in the southern hemisphere and lower, more lava-covered plains in the northern hemisphere began in the Noachian. The Hesperian Era lasted from the 3.5 b.y. time period until about 1.8 billion years ago. Crustal fissures allowed widespread volcanic flows to emplace. The Elysium volcanoes formed and those in the Tharsis region began to develop. Again, water may have been active in modifying the landscape. It probably acted upon a large fissure that grew into Valles Marineris. The Amazonian Era continues to the present. Water erosion/deposition may still have played a role but wind now was the principal agent of surface change. During this time Olympus Mons grew as did the Tharsis volcanoes.

The global surface of Mars can be displayed on a map in terms of these relative crater densities, which like the Moon provide the principal means for distinguishing age units, as shown:

Broad classification of the martian surface based on crater density and volcanism.

A general "stratigraphic units" map of Mars , produced by standard photogeologic methodology, is shown below with the key to these units beneath it:

Stratigraphic (geologic) map of Mars.

Key to the map above.

The U.S. Geological Survey has produced quadrangle maps of most of Mars by now, as illustrated by this one which shows the Hesperia Planum and Tyrrhenia Terra regions of Mars.

Geologic map of a selected region of the martian surface; by E. Gregg.

The full global map of Mars at scales of 1:15000000 and 1:25000000 has been published by the USGS. It was not found by the writer after surfing the Net, perhaps because of its size (would be many megabytes if all the lettering must be legible). Here is a variant of most of that map, shown without a legend or annotation:

Geologic map of Mars; in this projection units at high latitudes have been distorted (appear stretched).

It should by now be evident that Mars has a diverse and intriging history, with a wide variety of surface features. These are examined in pages 19-12ff.HST images of Mars, showing dust-poor and dust-rich conditions during 2001.

Mars Physiography

Before looking at some representative images of the Red Planet, we set up a physiographic framework for Mars: From Earth-based telescopic observations, we 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 landform maps of the two martian equatorial hemispheres, drawn on a Lambert equal-area projection from mission results, as shown here:

Generalized landforms maps for Mars, located on two hemispheres. See text for explanation of letters. The left map has Olympus Mons and Valles Marineris as two prominent landmarks; the right map has the Hellas basin in its southern portion.

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

Compositional data (discussed on pages 19-13 and 19-13a) suggest that the darker lower (southern) part of Mars is underlain by basalt whereas the upper part contains andesites as well as basalts.

The lower polar hemisphere contains cratered terrain (cu), considered to be billions of years old (and the likely source of the 3.5 b.y. old meteorite containing organics, found in Antarctica), where the bulk of the larger impact structures survive. Much of the upper hemisphere has plains units (p), many thought to be volcanic flows (pv) (mainly basalts?) and volcanic constructs (v) (such as shield volcanoes), some of whose ages could be less than one-half billion years. 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.

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 and the two large ones above and to the left are Olympus Mons (largest volcano in the solar system) and Alba Patera (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

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 enough water, if condensed, to fill an average swimming pool. 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.

 Clouds visible in the martian atmosphere above the limb in this Viking Orbiter image; also perhaps 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:

Pockets of x-radiation from excited Oxygen in the atmosphere of Mars.

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.

 

 Viking image of spiraling clouds above the martian surface.

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:

Clouds around the Crater Mie; MSSS image.

One of the strangest images of "apparent" clouds taking by the Viking camera is shown below. To the writer, these are puzzling but despite his skepticism it is included here as valid because it is labeled as such on many Internet sites (perhaps most copy the first site reference which could be in error). Judge for yourself.

Apparent clouds lying low above the martian surface.

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.

The martian sky before sunrise, as imaged by the Mars Pathfinder.

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:

A martian duststorm as imaged by the MOC on the Mars Global Surveyor.

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

HST images of Mars, showing early and late stages of dust-rich conditions during 2001.

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 in the martian atmosphere.

Sequential display of the amount and extent of dust (maximum in red) in the martian atmosphere during a major storm beginning in July, 2001.

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 (74,500 km [46,000 miles]) in the last hundred years is an organizing dust storm imaged through a ground-based telescope in Arkansas:

Growing dust storm near equatorial Mars.

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; at the left end) removed material from the wall:

Dust-devil and its dark trail along a crater wall in Terra Meeridiani; the light, closely spaced markings may be caused by subsurface water seeping from a layer within the wall; MOC image; MSSS.

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 devil trails on the martian surface.

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:

Criss-crossing dust devil trails (dark) on a mid-latitude martian surface; MSSS.

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:

Light-toned dust devil trails; MSSS

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

Dust devil near the Columbia Hills Spirit site.

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

 

Mariner 9 TV image of wind streaks on the martian surface; these usually come out of craters; their parallel orientation indicates a prevailing wind direction.

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.

Viking Orbiter image showing both transverse and barchan dunes on the surface of Mars.

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

Longitudinal and barchan dunes on the martian surface.

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

Light-colored dunes within a shallow depression; MSSS image.

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:

Darker-toned dunes on Mars; MSSS image.

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:

Two sets of dunes (light and dark) developed in the same area of Mars.

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:

Criss-crossing sand dunes off the martian polar ice cap.

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:

Martian crescentic dune field.

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

More dunes that are related to barchan type but lack strong crescentic horns.

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.

Mars 'cookie' dunes; MOC image from Mars Global Surveyor; MSSS.

A MOC close up view of barchan dunes appears next:

Barchan dunes on Mars

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

Very asymmetric martian dunes; MSSS image.

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 permafrost and/or ice is much lighter (more reflective).

Martian sand dunes consisting of dark materials that are exposed within polar ice/frost; MSSS image.

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.

Dunes of varying shapes owing to interference because of close-packing; MSSS image.

Similar, but 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.

An array of dark dunes, covered by frost, in the North Polar regions of Mars; MSSS image.

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:

Contiguous dunes in the North Polar region; MSSS image.

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:

Bequerel crater sand deposits; the thin white layers may be sedimentary units being covered by the tongues of sand; MSSS image.

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:

Martian yardangs in Apollinaris Sulci; the crater is younger; MSSS MOC image.

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

A field of yardangs, wind blowing from right to left; MSSS image.

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 is an enlargement of part of the scene in which ripples in the low valleys attest to wind activity, which has covered the surface with dust:

Wind-eroded rock 'hills' in Medusae Sulci; MOC image; MSSS.

Enlargement of an area in lower right of above image, rotated cw 90 degrees.

The bottom line: 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.

Smooth surface dust at Pavonis Mons: MSSS.

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. 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:

Vertical view of the South Polar Ice Cap in the southern martian winter.

Oblique view of the North Polar Ice Cap in northern winter.

The layering associated with this 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:

Layers in the Northern Hemisphere within the north polar icecap that has shrunk to its summer mode.

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:

Changes in the size and extent of ice at the martian North Pole.

Thus, in the summer the ice cover may have shrunk so that it persists only in patches. This 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:

Mars Express perspective view of summertime ice distribution in the North Polar region.

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 maximum extent of polar ice in the northern hemisphere in blue; minimum size shown in red; MOLA measurements.

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:

Map of distribution of south polar water ice (blue).

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:

Changes in elevation owing to ice cap growth for the north (blue) and south (red) polar regions during maximum growth and during maximum shrinkage, as the poles are approached from mid-latitudes.

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:

MOC view of defrosting that changes the surface appearance 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:

Details of dust layering in the South Polar Cap, imaged by MOC in its high resolution mode.

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:

The top view shows details of layering (dust midst ice) in the South Polar Ice Cap, imaged by the MOC on Mars Global Surveyor (MGS).

The bottom view is a Viking view of the edge of the South Polar Ice Cap.

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

Dark bands in the ice at the North Pole of Mars.

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).

Layering in north polar ice; note unconformity; MSSS.

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.

Apparent banding in martian polar ice; MSSS.

19-38: Try to devise an explanation for these layers in this last image. 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:

Patterns in the ground uncovered during melting at the edge of the north polar icecap; MSSS image.

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.

Ice evaporation spots in a dune field, Martian North polar region (MSSS image).

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.

Sublimation (defrosting) of water or carbon dioxide ice coating in a dune field in the southern martian hemisphere (MSSS image).

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):

Ice landforms in the south polar region.

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.

Mesas composed of carbon dioxide 'bedrock'.

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.

Structures in ice in the South Polar Ice Cap as seen by MOC on MGS; this view is fretted and pitted ice forming the "Swiss Cheese" structure; MSSS.

Structures in ice in the South Polar Ice Cap as seen by MOC on MGS; this view has smaller pits described analogously as a "kitchen sponge" texture; MSSS.

High resolution MOC images of water ice at the time dominating the surface composition, show similar "sponge" patterns:

North Polar water ice, with a texture at the scale of meters, showing both irregular polygonal structure and a faint lineation; MSSS image.

Another feature resembles dunes or stepped ice sheets:

Linear features in carbon dioxide 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:

Dunes on the martian North Pole Ice Cap; image courtesy Malin Space Science Systems.

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.

Polygonal fracturing of water ice in the South Polar Ice Cap of Mars.

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.

MGS image of circular domes in the southern polar region: MSSS.

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

Peculiar circular features in polar ice; the longer dimension is equivalent to about 3 km; MSSS image.

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

Structural troughs in the North Pole cap.

Lest one builds an impression that ice is found only at high latitudes on Mars, let us state succinctly here (but treated in detail on page 19-13) that evidence is now 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 vast 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 first three illustrations show deposits of materials in lobe shapes that resemble certain terminal moraines associated with mountain glaciers.

Lobate deposits on Mars (left); similar deposits found at a glacial terminus on Earth.

A martian deposit which resembles glacial debris, perhaps as a rock glacier.

The Hydapsis Chaos area, with lobate deposits and other features that resemble glaciated terrain; this is one of the more convincing examples.

This next image represents an interpretation of features on a martian plain that he has tied to similar-appearing features on a continental outwash plain on Earth.

Martian surface showing features characteristic of an outwash plain.

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.

Polygonal patterns on a martian surface (left) and a terrestrial surface (right), owing to ice raft coverage and melting.

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

Four examples of possible periglacial surface on Mars.

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.

Viking image of polygons which are at subkilometer scales.

This martian surface shows smaller polygons:

Polygons on a martian surface

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.

Ice polygons in an impact crater.

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

Possible ridges made by filling of cracks in ice once on Mars.

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

Flow pattern interpreted as a rock glacier.

A flow pattern associated with fretted terrain

A flow pattern associated with fretted terrain

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.

A martian surface of possible glacial nature.

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.

Martian lava cones.

This last image is placed here "for the record". It is an illustration, much repeated on the Internet, obtained by the Mars Express camera of what is called an "hourglass" structure which contains so-called evidence of a glacial origin. To the writer, this looks like two adjacent impact craters.

Mars Express image of a possible structure whose origin is at least in part of some 'undefined' glacial nature.

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.

Christensen, P.R., B.M. Jakosky, H.H. Kieffer, M.C. Malin, H.Y. McSween, Jr., K. Nealson, G.L. Mehall, S.H. Silverman, S. Ferry, M. Caplinger, and M. Ravine, The Thermal Emission Imaging System (THEMIS) for the Mars 2001 Odyssey Mission, Space Science Reviews, 110, 85-130, 2004

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Primary Author: Nicholas M. Short, Sr. email: [email protected]