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]
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. [31] [32] Ice was found both in the southern hemisphere [33] and in the northern hemisphere. [34] 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. [35] [36] [37]
It is thought that ice accumulated when Mars' orbital tilt was very different from the present (the axis the planet spins on has considerable "wobble," meaning its angle changes over time). [38] [39] [40] 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 [41] [42] 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. [43] 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. [44] [45] General circulation models of the Martian atmosphere predict accumulations of ice-rich dust in the same areas where ice-rich features are found. [42] When the tilt begins to return to lower values, the ice sublimates (turns directly to a gas) and leaves behind a lag of dust. [46] [47] The lag deposit caps the underlying material so with each cycle of high tilt levels, some ice-rich mantle remains behind. [48] The smooth surface mantle layer probably represents only relative recent material.
Several types of landforms have been identified as probably dirt and rock debris covering huge deposits of ice. [49] [50] [51] [52] Concentric crater fill (CCF) contains dozens to hundreds of concentric ridges that are caused by the movements of sometimes hundreds of meter thick accumulations of ice in craters. [53] [54] Lineated valley fill (LVF)are lines of ridges in valleys. [55] [56] [57] 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. [58] Lobate debris aprons (LDA) 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. [59] [60] [61] These areas are sometimes called Fretted terrain because it is sometimes winkled. With the superior resolution of cameras on Mars Global Surveyor (MGS) and MRO, we have found the surface of LDA’s, LVF, and CCFs’ have a complex tangle of ridges that resemble the surface of the human brain. Wide ridges are called closed-cell brain terrain, and the less common narrow ridges are called open-cell brain terrain. [62] It is thought that the wide closed-cell terrain still contains a core of ice, that when it eventually disappears the center of the wide ridge collapses to produce the narrow ridges of the open-cell brain terrain. Today it is widely accepted that glacier-like forms, lobate debris aprons, lineated valley fill, and concentric fill are all related in that they have the same surface texture. Glacier-like forms in valleys and cirque-like alcoves may coalesce with others to produce lobate debris aprons. When opposing lobate debris aprons converge, linear valley fill results [63]
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 and 70 E longitudes. [64] Near this area are regions that are named from ancient names: Deuteronilus Mensae, Protonilus Mensae, and Nilosyrtis Mensae.
Some of the glaciers flow down mountains and are shaped by obstacles and valleys; they make a sort of tongue shape. [65]
A hummocky relief resembling Northern Sweden's Veiki moraines has been found in Nereidum Montes. The relief is hypothesized to result from the melting of a Martian glacier. [66]
There is no current evidence of any glaciers on any of the volcanoes on Mars
There is much evidence for a large ice sheet that existed in the south polar region of the planet. [67] [68] [69] [70] A large number of eskers which form under ice are found there. The field of eskers make up the Dorsa Argentea Formation. The ice sheet had an area twice that of the state of Texas. [71]
Evidence also is building up for the past existence of an ice sheet in the Tharsis region. [72] [73] [74] [75] [76] It would have been in the Late Hesperian time period. When it melted it may have helped to form a northern ocean. [77] [78] [79] [80]
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. [83] 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 800 m thick, and there are many more. [84] [85]
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.
Terra Sabaea is a large area on Mars. Its coordinates are 2°N42°E and it covers 4,700 kilometres (2,900 mi) at its broadest extent. It was named in 1979 after a classic albedo feature on the planet. Terra Sabaea is fairly large and parts of it are found in five quadrangles: Arabia quadrangle, Syrtis Major quadrangle, Iapygia quadrangle, Ismenius Lacus quadrangle, and Sinus Sabaeus quadrangle.
Deuteronilus Mensae is a region on Mars 937 km across and centered at 43.9°N 337.4°W. It covers 344°–325° West and 40°–48° North. Deuteronilus region lies just to the north of Arabia Terra and is included in the Ismenius Lacus quadrangle. It is along the dichotomy boundary, that is between the old, heavily cratered southern highlands and the low plains of the northern hemisphere. The region contains flat-topped knobby terrain that may have been formed by glaciers at some time in the past. Deuteronilus Mensae is to the immediate west of Protonilus Mensae and Ismeniae Fossae. Glaciers persist in the region in modern times, with at least one glacier estimated to have formed as recently as 100,000 to 10,000 years ago. Recent evidence from the radar on the Mars Reconnaissance Orbiter has shown that parts of Deuteronilus Mensae do indeed contain ice.
The Ismenius Lacus quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The quadrangle is located in the northwestern portion of Mars' eastern hemisphere and covers 0° to 60° east longitude and 30° to 65° north latitude. The quadrangle uses a Lambert conformal conic projection at a nominal scale of 1:5,000,000 (1:5M). The Ismenius Lacus quadrangle is also referred to as MC-5. The southern and northern borders of the Ismenius Lacus quadrangle are approximately 3,065 km (1,905 mi) and 1,500 km (930 mi) wide, respectively. The north-to-south distance is about 2,050 km (1,270 mi). The quadrangle covers an approximate area of 4.9 million square km, or a little over 3% of Mars' surface area. The Ismenius Lacus quadrangle contains parts of Acidalia Planitia, Arabia Terra, Vastitas Borealis, and Terra Sabaea.
The Casius quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The quadrangle is located in the north-central portion of Mars' eastern hemisphere and covers 60° to 120° east longitude and 30° to 65° north latitude. The quadrangle uses a Lambert conformal conic projection at a nominal scale of 1:5,000,000 (1:5M). The Casius quadrangle is also referred to as MC-6. Casius quadrangle contains part of Utopia Planitia and a small part of Terra Sabaea. The southern and northern borders of the Casius quadrangle are approximately 3,065 km and 1,500 km wide, respectively. The north to south distance is about 2,050 km. The quadrangle covers an approximate area of 4.9 million square km, or a little over 3% of Mars' surface area.
The Cebrenia quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The quadrangle is located in the northeastern portion of Mars' eastern hemisphere and covers 120° to 180° east longitude and 30° to 65° north latitude. The quadrangle uses a Lambert conformal conic projection at a nominal scale of 1:5,000,000 (1:5M). The Cebrenia quadrangle is also referred to as MC-7. It includes part of Utopia Planitia and Arcadia Planitia. The southern and northern borders of the Cebrenia quadrangle are approximately 3,065 km (1,905 mi) and 1,500 km (930 mi) wide, respectively. The north to south distance is about 2,050 km (1,270 mi). The quadrangle covers an approximate area of 4.9 million square km, or a little over 3% of Mars' surface area.
The Arcadia quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The quadrangle is located in the north-central portion of Mars’ western hemisphere and covers 240° to 300° east longitude and 30° to 65° north latitude. The quadrangle uses a Lambert conformal conic projection at a nominal scale of 1:5,000,000 (1:5M). The Arcadia quadrangle is also referred to as MC-3. The name comes from a mountainous region in southern Greece. It was adopted by IAU, in 1958.
The Hellas quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Hellas quadrangle is also referred to as MC-28 . The Hellas quadrangle covers the area from 240° to 300° west longitude and 30° to 65° south latitude on the planet Mars. Within the Hellas quadrangle lies the classic features Hellas Planitia and Promethei Terra. Many interesting and mysterious features have been discovered in the Hellas quadrangle, including the giant river valleys Dao Vallis, Niger Vallis, Harmakhis, and Reull Vallis—all of which may have contributed water to a lake in the Hellas basin in the distant past. Many places in the Hellas quadrangle show signs of ice in the ground, especially places with glacier-like flow features.
The Eridania quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Eridania quadrangle is also referred to as MC-29.
Lobate debris aprons (LDAs) are geological features on Mars, first seen by the Viking Orbiters, consisting of piles of rock debris below cliffs. These features have a convex topography and a gentle slope from cliffs or escarpments, which suggest flow away from the steep source cliff. In addition, lobate debris aprons can show surface lineations as do rock glaciers on the Earth.
Fretted terrain is a type of surface feature common to certain areas of Mars and was discovered in Mariner 9 images. It lies between two different types of terrain. The surface of Mars can be divided into two parts: low, young, uncratered plains that cover most of the northern hemisphere, and high-standing, old, heavily cratered areas that cover the southern and a small part of the northern hemisphere. Between these two zones is a region called the Martian dichotomy and parts of it contain fretted terrain. This terrain contains a complicated mix of cliffs, mesas, buttes, and straight-walled and sinuous canyons. It contains smooth, flat lowlands along with steep cliffs. The scarps or cliffs are usually 1 to 2 km high. Channels in the area have wide, flat floors and steep walls. Fretted terrain shows up in northern Arabia, between latitudes 30°N and 50°N and longitudes 270°W and 360°W, and in Aeolis Mensae, between 10 N and 10 S latitude and 240 W and 210 W longitude. Two good examples of fretted terrain are Deuteronilus Mensae and Protonilus Mensae.
A concentric crater fill (CCF) is a landform where the floor of a crater is mostly covered with many parallel ridges. It is common in the mid-latitudes of Mars, and is widely believed to be caused by glacial movement. Areas on Mars called Deuteronilus Mensae and Protonilus Mensae contain many examples of concentric crater fill.
A Ring mold crater is a kind of crater on the planet Mars that looks like the ring molds used in baking. They are believed to be caused by an impact into ice. The ice is covered by a layer of debris. They are found in parts of Mars that have buried ice. Laboratory experiments confirm that impacts into ice result in a "ring mold shape." They are also bigger than other craters in which an asteroid impacted solid rock. Impacts into ice warm the ice and cause it to flow into the ring mold shape. These craters are common in lobate debris aprons and lineated valley fill. Many have been found in Mamers Valles, a channel found along the dichotomy boundary in Deuteronilus Mensae. They may be an easy way for future colonists of Mars to find water ice.
Lineated valley fill (LVF), also called lineated floor deposit, is a feature of the floors of some channels on Mars, exhibiting ridges and grooves that seem to flow around obstacles. Shadow measurements show that at least some of the ridges are several metres high. LVF is believed to be ice-rich. Hundreds of metres of ice probably lie protected in LVF under a thin layer of debris. The debris consists of wind-borne dust, material from alcove walls, and lag material remaining after ice sublimated from a rock-ice mixture. Some glaciers on Earth show similar ridges. High-resolution pictures taken with HiRISE reveal that some of the surfaces of lineated valley fill are covered with strange patterns called closed-cell and open-cell brain terrain. The terrain resembles a human brain. It is believed to be caused by cracks in the surface accumulating dust and other debris, together with ice sublimating from some of the surfaces. The cracks are the result stress from gravity and seasonal heating and cooling. This same type of surface is present on Lobate debris aprons and Concentric crater fill so all three are believed to be related.
Protonilus Mensae is an area of Mars in the Ismenius Lacus quadrangle. It is centered on the coordinates of 43.86° N and 49.4° E. Its western and eastern longitudes are 37° E and 59.7° E. North and south latitudes are 47.06° N and 39.87° N. Protonilus Mensae is between Deuteronilus Mensae and Nilosyrtis Mensae; all lie along the Martian dichotomy boundary. Its name was adapted by the IAU in 1973.
Nilosyrtis Mensae is an area of Mars in the Casius quadrangle. It is centered on the coordinates of 36.87° N and 67.9° E. Its western and eastern longitudes are 51.1° E and 74.4° E. North and south latitudes are 36.87° N and 29.61° N. Nilosyrtis Mensae is just to the east of Protonilus Mensae and both lie along the Martian dichotomy boundary. Its name was adapted by the IAU in 1973. It was named after a classical albedo feature, and it is 705 km (438 mi) across.
The Mars Reconnaissance Orbiter's HiRISE instrument has taken many images that strongly suggest that Mars has had a rich history of water-related processes. Many features of Mars appear to be created by large amounts of water. That Mars once possessed large amounts of water was confirmed by isotope studies in a study published in March 2015, by a team of scientists showing that the ice caps were highly enriched with deuterium, heavy hydrogen, by seven times as much as the Earth. This means that Mars has lost a volume of water 6.5 times what is stored in today's polar caps. The water for a time would have formed an ocean in the low-lying Mare Boreum. The amount of water could have covered the planet about 140 meters, but was probably in an ocean that in places would be almost 1 mile deep.
The common surface features of Mars include dark slope streaks, dust devil tracks, sand dunes, Medusae Fossae Formation, fretted terrain, layers, gullies, glaciers, scalloped topography, chaos terrain, possible ancient rivers, pedestal craters, brain terrain, and ring mold craters.
Brain terrain, also called knobs-brain coral and brain coral terrain, is a feature of the Martian surface, consisting of complex ridges found on lobate debris aprons, lineated valley fill and concentric crater fill. It is so named because it suggests the ridges on the surface of the human brain. Wide ridges are called closed-cell brain terrain, and the less common narrow ridges are called open-cell brain terrain. It is thought that the wide closed-cell terrain contains a core of ice, and when the ice disappears the center of the wide ridge collapses to produce the narrow ridges of the open-cell brain terrain. Shadow measurements from HiRISE indicate the ridges are 4-5 meters high. Brain terrain has been observed to form from what has been called an "Upper Plains Unit." The process begins with the formation of stress cracks. The upper plains unit fell from the sky as snow and as ice coated dust.
Sinton is a crater in the Ismenius Lacus quadrangle on Mars. Sinton crater lies in the northern hemisphere, south of the very large crater Lyot and west of Ismeniae Fossae. It was named after Harvard astronomer William M. Sinton. The name was approved in 2007.
The upper plains unit is the remnants of a 50-100 meter thick mantling that has been discovered in the mid-latitudes of the planet Mars. It was first investigated in the Deuteronilus Mensae region, but it occurs in other places as well. The remnants consist of sets of dipping layers in impact craters, in depressions, and along mesas. Sets of dipping layers may be of various sizes and shapes—some look like Aztec pyramids from Central America.