Glaciers on Mars

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.^[1][2] 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.^{[1][3][4][5][6][7][8][9][10]}

However, a variety of other features on the surface have also been interpreted as directly linked to flowing ice, such as fretted terrain,^[1][11] lineated valley fill,^[12][9] concentric crater fill,^[3][13] and arcuate ridges.^[10] 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.^[14][15][16]

Today, features interpreted as glaciers are largely restricted to latitudes polewards of around 30° latitude.^[17] Particular concentrations are found in the Ismenius Lacus quadrangle.^[2] Based on current models of the Martian atmosphere, ice should not be stable if exposed at the surface in the mid-Martian latitudes.^[18] 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.^[8][18][19] 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.^[17] 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.^[20] Evidence for past glaciation also appears on the peaks of several Martian volcanoes in the tropics.^[21][22][23]

Like glaciers on Earth, glaciers on Mars are not pure water ice.^[1][10] Many are thought to contain substantial proportions of debris, and a substantial number are probably better described as rock glaciers.^[23][24][25] 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.^[26] 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.^[6][8] Some authors have also made claims that glaciers of solid carbon dioxide have formed on Mars under certain rare conditions.^[27]

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.^[28] These supposed alpine glaciers have been called glacier-like forms (GLF) or glacier-like flows (GLF).^[29] Glacier-like forms are a later and maybe more accurate term because we cannot be sure the structure is currently moving.^[30] Another, more general term sometimes seen in the literature is viscous flow features (VFF).^[30]