Glaciers on Mars
Glaciers formed much of the observable surface in large areas of Mars. Most of the area in high latitudes, especially the Ismenius Lacus quadrangle, is believed to still contain enormous amounts of water ice. Recent evidence has led many planetary scientists to believe that water ice still exists in the form of glaciers that are covered with a thin layer of insulating rock. In March 2010, scientists released the results of a radar study of an area called Deuteronilus Mensae that found widespread evidence of ice lying beneath a few meters of rock debris. Glaciers are believed to be associated with Fretted terrain, many volcanoes, and even some craters.
Ridges of debris on the surface of the glaciers show the direction of ice movement. The surface of some glaciers has a rough texture due to sublimation of buried ice. The ice goes directly into a gas (this process is called sublimation) and leaves behind an empty space. Overlying material then collapses into the void. Various sized pits and holes result. Glaciers are not pure ice; they contain dirt and rocks. At times, they will dump their load of materials into ridges. Such ridges are called moraines. Some places on Mars have groups of ridges that are twisted around; this may have been due to more movement after the ridges were put into place. Sometimes chunks of ice fall from the glacier and get buried in the land surface. When they melt and more or less round hole remains. On Earth we call these features kettles or kettle holes. Mendon Ponds Park in upstate NY has preserved several of these kettles. One picture below from HiRISE shows possible kettles in Moreux Crater.
Water source for future colonists 
Mars has vast glaciers hidden under a layer of rocky debris over wide areas in the mid-latitudes. These glaciers could be large reservoir of life-supporting water on the planet for simple life forms and for future colonists of the Red Planet. Research by John Holt, of the University of Texas at Austin, and others found that one of the features examined is three times larger than the city of Los Angeles and up to one-half-mile thick, and there are many more.
Some of the glacial-like features were revealed by NASA's Viking orbiters in the 1970s. Since that time glacial-like features have been studied by more and more advanced instruments. Much better data has been received from Mars Global Surveyor, Mars Odyssey, Mars Express, and Mars Reconnaissance Orbiter.
Radar finds ice 
For decades scientists speculated that many features on Mars could be glaciers. Studies with NASA's Mars Reconnaissance Orbiter’s Shallow Radar instrument penetrated the rocky debris flows that lie in the mid-latitude regions of both hemispheres. An examination of the radar echoes pointed to ice, and lots of it. The radar echoes received back by MRO indicated that radio waves passed through the overlying debris material and reflected off a deeper surface below without losing much strength — the expected signal for thick ice covered by a thin layer of debris. The radar echoes also showed no signs of significant rock debris within the glaciers, suggesting that they are relatively pure water ice.
Climate Changes 
It is now widely believed that ice accumulated when Mars' orbital tilt was much different than it is now (the axis the planet spins on has considerable "wobble," meaning its angle changes over time). A few million years ago, the tilt of the axis of Mars was 45 degrees instead of its present 25 degrees. Its tilt, also called obliquity, varies greatly because its two tiny moons cannot stabilize it, like our relatively large moon does to the Earth. At high tilt, the ice caps at the poles disappear, the atmosphere thickens, and the moisture in the atmosphere goes up. These conditions cause snow and frost to appear on the surface. Some of the moisture would have coated dust grains until the grains would be too heavy to be held up in the atmosphere; they would then fall and accumulate on the surface. Just plain snow would have also added to the mix. Ice on the surface of Mars today would immediately sublimate (or change directly into the gas phase). The rocky debris covering the ice is likely what has allowed it to survive below the surface for millions of years.
Concentric crater fill, Lineated valley fill, and Lobate debris aprons 
Several types of landforms have been identified as probably dirt and rock debris covering huge deposits of ice. Concentric crater fill contains dozens to hundreds of concentric ridges that are caused by the movements of sometimes hundreds of meter thick accumulations of ice in craters. Lineated valley fill are lines of ridges in valleys. These lines may have developed as other glaciers moved down valleys. Some of these glaciers seem to come from material sitting around mesas and buttes. Lobate debris aprons is the name given to these glaciers. All of these features that are believed to contain large amounts of ice are found in the mid-latitudes in both the Northern and Southern hemispheres. These areas are sometimes called Fretted terrain because it is sometimes winkled. Many of these features are found in the Northern hemisphere in parts of a boundary called the Martian dichotomy. The Martian dichotomy is mostly found between 0 to 70 E longitudes. Near this area are regions that are named from ancient names: Deuteronilus Mensae, Protonilus Mensae, and Nilosyrtis Mensae.
Close-up that shows cracks containing pits on the floor of a crater containing concentric crater fill, as seen by HiRISE under HiWish program. Location is Casius quadrangle.
Clanis and Hypsas Valles, as seen by HiRISE. Ridges are probably due to glacial flow. Ice is covered by a thin layer of rocks. Location is Ismenius Lacus quadrangle.
Tongue-shaped glaciers and glaciers on volcanoes 
Some of the glaciers flow down mountains and are shaped by obstacles and valleys; they make a sort of tongue shape. Many supposed glaciers have been observed on some of large Martian volcanoes. Researchers have described glacial deposits on Hecates Tholus, Arsia Mons, Pavonis Mons, and Olympus Mons.
Tongue-Shaped Glacier, as seen by Mars Global Surveyor. Location is Hellas quadrangle.
Tongue-shaped glacier, as seen by HiRISE under the HiWish program. Ice may exist in the glacier, even today, beneath an insulating layer of dirt. Location is Hellas quadrangle.
Close-up of tongue-shaped glacier, as seen by HiRISE under the HiWish program. Resolution is about 1 meter, so one can see objects a few meters across in this image. Ice may exist in the glacier, even today, beneath an insulating layer of dirt. Location is Hellas quadrangle.
Pictures below show various features that appear to be connected with the existence of glaciers.
Glacier as seen by HiRISE under the HiWish program. Area in rectangle is enlarged in the next photo. Zone of accumulation of snow at the top. Glacier is moving down valley, then spreading out on plain. Evidence for flow comes from the many lines on surface. Location is in Protonilus Mensae in Ismenius Lacus quadrangle.
Enlargement of area in rectangle of the previous image. On Earth the ridge would be called the terminal moraine of an alpine glacier. Picture taken with HiRISE under the HiWish program. Image from Ismenius Lacus quadrangle.
Context for the next image of the end of a flow feature or glacier. Location is Hellas quadrangle. Picture taken with HiRISE under the HiWish program.
Close-up of the area in the box in the previous image. This may be called by some the terminal moraine of a glacier. For scale, the box shows the approximate size of a football field. Image taken with HiRISE under the HiWish program. Location is Hellas quadrangle.
Possible moraine on the end of a past glacier on a mound in Deuteronilus Mensae, as seen by HiRISE, under the HiWish program.
Possible Glacial Cirque in Hellas Planitia, as seen by HiRISE, under the HiWish program. Lines are probably due to downhill movement.
Glaciers, as seen by HiRISE, under HiWish program. Glacier on left is thin because it has lost much of its ice. Glacier on the right on the other hand is thick; it still contains a lot of ice that is under a thin layer of dirt and rock. Location is Hellas quadrangle.
Remains of glaciers, as seen by HiRISE under the HiWish program. Image from Ismenius Lacus quadrangle.
Probable glacier as seen by HiRISE under HiWish program. Radar studies have found that it is made up of almost totally of pure ice. It appears to be moving from the high ground (a mesa) on the right. Location is Ismenius Lacus quadrangle.
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See also 
- Climate of Mars
- Deuteronilus Mensae
- Fretted terrain
- Geology of Mars
- Ismenius Lacus quadrangle
- Lineated valley fill
- Martian dichotomy
- Nilosyrtis Mensae
- Protonilus Mensae
- Water on Mars