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).
Exposure of basalt dust by frost evaporation is responsible for this strange illusion within a polar dune field. The streaks of basalt dust appear like rows of tree trunks:
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:
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.
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 (blue) and minimum (red) 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:
In 2009, NASA released new information on thickness of the North polar ice cap. The Shallow Radar (Sharad) instrument on the Mars Reconnaisance Orbiter (MRO) was used in the determination. The results are summarized in this diagram:
The top diagram (a) shows the radar cross-section of polar layers, many of which contain ice. Diagram (b) is an image looking down at the boundary between the ice-bearing layers and surface materials just outside the cap. The map (c) shows the elevation of the surface of the cap (line A-A' traces the location of the radar cross-section (a)). Map (d) is the elevation of the bottom of the cap (the basal unit) as determined by the radar. The thickness of the cap is plotted in map (e).
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 this view 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 in ice seems to indicate strong light-dark contrasts in an area also undergoing erosional sculpturing; the sharpness may be somewhat illusory.
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.
The Mars Reconnaissance Orbiter has sent back images that further define the ice cap. These two MRO images show the cap to be permanent enough to have undergone erosion that exposes layers in the ice:
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, similar to 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:
There is growing evidence that ice (water?) is present beyond the poles, at lower latitudes. This pair of MRO images show a bright white fringe around a crater that may have formed recently. The fringe has almost disappeared in 2009, indicating the ice to have sublimed:
Seen in more detail, the ice around this 3 meter crater is remarkably pure, about 98% water:
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".
Puzzling features are shown in the next image which displays dark objects in polar ice (they are known colloquially as "starburst spiders"):