Glaciers, loosely defined as patches of currently or recently flowing ice, are thought to be present across large but restricted areas of the modern Martian surface, and are inferred to have been more widely distributed at times in the past. Lobate convex features on the surface known as viscous flow features and lobate debris aprons, which show the characteristics of non-Newtonian flow, are now almost unanimously regarded as true glaciers. However, a variety of other features on the surface have also been interpreted as directly linked to flowing ice, such as fretted terrain, lineated valley fill, concentric crater fill, and arcuate ridges. A variety of surface textures seen in imagery of the midlatitudes and polar regions are also thought to be linked to sublimation of glacial ice. Lobate debris aprons (LDA's) and lineated valley fill (LVF) may be almost the same—being mostly ice with a covering of debris, but their shapes are dependent on their locations. When confined within a valley, LVF is present; in contrast when not confined, this flowing debris covered ice forms LDA's. Today, features interpreted as glaciers are largely restricted to latitudes polewards of around 30° latitude. Particular concentrations are found in the Ismenius Lacus quadrangle. Based on current models of the Martian atmosphere, ice should not be stable if exposed at the surface in the mid-Martian latitudes. It is thus thought that most glaciers must be covered with a layer of rubble or dust preventing free transfer of water vapor from the subliming ice into the air. This also suggests that in the recent geological past, the climate of Mars must have been different in order to allow the glaciers to grow stably at these latitudes. This provides good independent evidence that the obliquity of Mars has changed significantly in the past, as independently indicated by modelling of the orbit of Mars. Evidence for past glaciation also appears on the peaks of several Martian volcanoes in the tropics. Like glaciers on Earth, glaciers on Mars are not pure water ice. Many are thought to contain substantial proportions of debris, and a substantial number are probably better described as rock glaciers. For many years, largely because of the modeled instability of water ice in the midlatitudes where the putative glacial features were concentrated, it was argued that almost all glaciers were rock glaciers on Mars. However, recent direct observations made by the SHARAD radar instrument on the Mars Reconnaissance Orbiter satellite have confirmed that at least some features are relatively pure ice, and thus, true glaciers. Indeed, more analysis of SHARAD data led researchers to state that martian glaciers are 80% pure ice. Some authors have also made claims that glaciers of solid carbon dioxide have formed on Mars under certain rare conditions. Some landscapes look just like glaciers moving out of mountain valleys on Earth. Some appear to have a hollowed out center, looking like a glacier after almost all the ice has disappeared. What is left are the moraines—the dirt and debris carried by the glacier. These supposed alpine glaciers have been called glacier-like forms (GLF) or glacier-like flows (GLF). Glacier-like forms are a later and maybe more accurate term because we cannot be sure the structure is currently moving. Another, more general term sometimes seen in the literature is viscous flow features (VFF).
Radar studies Radar studies with the SHAllow RADar (SHARAD) on the Mars Reconnaissance Orbiter showed that lobate debris aprons (LDA) and lineated valley fill (LVF) contain pure water ice covered with a thin layer of rocks that insulated the ice. Ice was found both in the southern hemisphere and in the northern hemisphere. Researchers at the Niels Bohr Institute combined radar observations with ice flow modelling to say that ice in all of the Martian glaciers is equivalent to what could cover the entire surface of Mars with 1.1 meters of ice. The fact that the ice is still there suggests that a thick layer of dust is protecting the ice; the current atmospheric conditions on Mars are such that any exposed water ice would sublimate.
Climate changes
It is thought that ice accumulated when Mars's orbital tilt was very different from the present (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 the Moon stabilizes Earth. Many features on Mars, especially in the Ismenius Lacus quadrangle, are believed to contain large amounts of ice. The most popular model for the origin of the ice is climate change from large changes in the tilt of the planet's rotational axis. At times the tilt has even been greater than 80 degrees Large changes in the tilt explains many ice-rich features on Mars. Studies have shown that when the tilt of Mars reaches 45 degrees from its current 25 degrees, ice is no longer stable at the poles. Furthermore, at this high tilt, stores of solid carbon dioxide (dry ice) sublimate, thereby increasing the atmospheric pressure. This increased pressure allows more dust to be held in the atmosphere. Moisture in the atmosphere will fall as snow or as ice frozen onto dust grains. Calculations suggest this material will concentrate in the mid-latitudes. General circulation models of the Martian atmosphere predict accumulations of ice-rich dust in the same areas where ice-rich features are found. When the tilt begins to return to lower values, the ice sublimates (turns directly to a gas) and leaves behind a lag of dust. The lag deposit caps the underlying material so with each cycle of high tilt levels, some ice-rich mantle remains behind. The smooth surface mantle layer probably represents only relative recent material.
Geomorphology
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