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 two 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. 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:
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.
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 three images show terrains marked by linear 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.
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.