Water on Mars
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Water on Mars exists almost exclusively as water ice, and is located in the Martian polar ice caps and under the shallow Martian surface even at more temperate latitudes. A small amount of water vapor is present in the atmosphere.
There are no bodies of liquid water on the Martian surface because its atmospheric pressure at the surface averages 600 pascals (0.087 psi) —about 0.6% of Earth's mean sea level pressure— and because the temperature is far too low, (210 K (−63 °C)) leading to immediate freezing. Despite this, about 3.8 billion years ago, there was a denser atmosphere, higher temperature, and vast amounts of liquid water flowed on the surface, including large oceans. It has been estimated that the primordial oceans on Mars would have covered between 36%  and 75% of the planet.
There are a number of direct and indirect proofs of water presence either on or under the surface, e.g. dry stream beds, polar caps, glaciers, radar and spectroscopic measurements, eroded craters or minerals directly connected to the past existence of liquid water. Several Mars orbiters have detected basins of ancient lakes, ancient river valleys, and widespread glaciations, while several landers and rovers directly analyzed soil and water ice from the shallow sub-surface.
Although the surface of Mars was wet and could have been hospitable to microbial life billions of years ago, the present damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life on the surface. Therefore, the best potential locations for discovering life on Mars may be at subsurface environments.
Today, it is accepted that Mars had abundant water very early in its history. All large areas of liquid water have disappeared, but deposited large amounts of water-rich materials , including clay and sulfates. From these materials, glaciers and other forms of frozen ground came to be.
Mars areas have been extremely dry for long periods, as marked by the presence of olivine that would be decomposed by water. Olivine weathers to iddingsite readily in the presence of water. The presence of iddingsite on Mars suggest that liquid water once existed there, and might enable scientists to determine when there was last liquid water on the planet. Studies of hydrogen isotopic ratios indicate that asteroids and comets from beyond 2.5 AU provide the source of Mars' water, which currently totals 6% to 27% of the Earth's present ocean.
Since several missions (Mars Odyssey, Mars Global Surveyor, Mars Reconnaissance Orbiter, Mars Express, Opportunity rover and Curiosity rover) are still sending back data from Mars, discoveries continue to be made.
Over thirty meteorites have been found that came from Mars. Some of them contain evidence that they were exposed to water when on Mars. Some Mars meteorites called basaltic shergottites, appear (from the presence of hydrated carbonates and sulfates) to have been exposed to liquid water prior to ejection into space. It has been shown that another class of meteorites, the nakhlites, were suffused with liquid water around 620 million years ago and that they were ejected from Mars around 10.75 million years ago by an asteroid impact. They fell to Earth within the last 10,000 years.
In 1996, a group of scientists reported the possible presence of micro-fossils in the Allan Hills 84001, a meteorite from Mars. Many studies disputed the validity of the fossils. It was found that most of the organic matter in the meteorite was of terrestrial origin.
Extinct water bodies
Lakes and river valleys
The Viking orbiters caused a revolution in our ideas about water on Mars. Huge river valleys were found in many areas. They showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Areas of branched streams, in the southern hemisphere, suggested that rain once fell. Research published in June 2010, reported the detection of 40,000 river valleys on Mars, about four times the number of river valleys that have previously been identified. The Martian water-worn features can be classified into two distinct classes: 1. dendritic (branched), terrestrial-scale, widely distributed, Noachian-age "valley networks" and 2. exceptionally large, long, single-thread, isolated, Hesperian-age "outflow channels".
Some parts of Mars show inverted relief. This occurs when sediments are deposited on the floor of a stream and then become resistant to erosion, perhaps by cementation. Later the area may be buried. Eventually, erosion removes the covering layer and the former streams become visible since they are resistant to erosion. Mars Global Surveyor found several examples of this process. Many inverted streams have been discovered in various regions of Mars, especially in the Medusae Fossae Formation, Miyamoto Crater, and the Juventae Plateau.
A variety of lake basins have been discovered on Mars. Some are comparable in size to the largest lakes on Earth, such as the Caspian Sea, Black Sea, and Lake Baikal. Lakes that were fed by valley networks are found in the southern highlands. There are places that are closed depressions with river valleys leading into them. These areas are thought to have once contained lakes; one is in Terra Sirenum which had its overflow move through Ma'adim Vallis into Gusev Crater, explored by the Mars Exploration Rover Spirit. Another is near Parana Valles and Loire Vallis. Some lakes are thought to have formed by precipitation, while others were formed from groundwater. Lakes are estimated to have existed in the Argyre basin, the Hellas basin, and maybe in Valles Marineris.
Research, published in January 2010, suggests that Mars also had lakes along parts of the equator. Although earlier research showed that Mars had a warm and wet early history that has long since dried up, these lakes existed in the Hesperian Epoch, a much earlier period. Using detailed images from NASA's Mars Reconnaissance Orbiter, the researchers speculate that there may have been increased volcanic activity, meteorite impacts or shifts in Mars' orbit during this period to warm Mars' atmosphere enough to melt the abundant ice present in the ground. Volcanoes would have released gases that thickened the atmosphere for a temporary period, trapping more sunlight and making it warm enough for liquid water to exist. In this study, channels were discovered that connected lake basins near Ares Vallis. When one lake filled up, its waters overflowed the banks and carved the channels to a lower area where another lake would form. These lakes would be another place to look for evidence (biosignatures) of past life.
On September 27, 2012, NASA scientists announced that the Curiosity rover found evidence for an ancient streambed in Gale Crater, suggesting an ancient "vigorous flow" of water on Mars. In particular, analysis of the now dry streambed indicated that the water ran quickly (3.3 kilometer/hour (3 foot/second)), possibly at hip-depth. Proof of running water came in the form of rounded pebbles and gravel fragments that could have only been weathered by strong liquid currents. Their shape and orientation suggests long-distance transport from above the rim of the crater, where a channel named Peace Vallis feeds into the alluvial fan.
Researchers have found a number of examples of deltas that formed in Martian lakes. Finding deltas is a major sign that Mars once had a lot of liquid water. Deltas usually require deep water over a long period of time to form. Also, the water level needs to be stable to keep sediment from washing away. Deltas have been found over a wide geographical range.
By 1979 it was thought that outflow channels formed in single, catastrophic ruptures of subsurface water reservoirs, possibly sealed by ice, discharging colossal quantities of water across an otherwise arid Mars surface. In addition, evidence in favor of heavy or even catastrophic flooding is found in the giant ripples in the Athabasca Vallis. This observation is supported by the sudden ending of the river networks in theatre shaped heads, rather than tapering ones. Additionally, some valleys are often discontinuous, with small sections of uneroded land separating the parts of the river.
Despite the many gigantic flood channels and associated tree-like network of tributaries found on Mars, there are no smaller scale structures that would indicate the origin of the flood waters. It has been suggested that either weathering processes have denuded the geologic evidence, or that rather than floods, they were created by the slow seeping out of groundwater. According to this hypothesis, groundwater with dissolved minerals came to the surface, in and around craters, and helped to form layers by adding minerals —especially sulfate— and cementing sediments. In other words, some layers may be formed by groundwater rising up depositing minerals and cementing sediments. The hardened layers are consequently more protected from erosion. This process may occur instead of layers forming under lakes. A study published in 2011 using data from the Mars Reconnaissance Orbiter, show that the same kinds of sediments exist in a large area that includes Arabia Terra.
Mars Ocean Hypothesis
The Mars Ocean Hypothesis proposes that the Vastitas Borealis basin was the site of a primordial ocean of liquid water 3.8 billion years ago, and presents evidence that nearly a third of the surface of Mars was covered by a liquid ocean early in the planet's geologic history. This primordial ocean, dubbed Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere, a region which lies 4–5 km (2.5–3 miles) below the mean planetary elevation, at a time period of approximately 3.8 billion years ago. Early Mars would have required a warmer climate and denser atmosphere to allow liquid water to exist at the surface. In addition, the large amount of valley networks strongly supports the possibility of a hydrological cycle on the planet in the past.
Data from the Mars Orbiter Laser Altimeter (MOLA), which measures the altitude of all terrain on Mars, was used in 1999 to determine that the watershed for an ocean on Mars would have covered about 75% of the planet. A similar study in June 2010 concluded that such an ocean would have covered 36% of Mars. Two different shorelines have been proposed. One, the 'Arabia shoreline', can be traced all around Mars except through the Tharsis volcanic region. The second, the 'Deuteronilus', follows the Vastitas Borealis formation.
However, the existence of a primordial Martian ocean remains controversial among scientists, and the interpretations of some features as 'ancient shorelines' has been challenged. One problem with the conjectured 2 billion years old (2 Ga) shoreline is that it is not flat — i.e. does not follow a line of constant gravitational potential. A 2007 research article points out that this could be due to a change in distribution in Mars' mass, perhaps due to volcanic eruption or meteor impact; the Elysium volcanic province or the massive Utopia basin that is buried beneath the northern plains have been put forward as the most likely causes.
Present water ice
On July 28, 2005, the European Space Agency announced the existence of a crater partially filled with frozen water; some then interpreted the discovery as an "ice lake". Images of the crater, taken by the High Resolution Stereo Camera on board the European Space Agency's Mars Express orbiter, clearly show a broad sheet of ice in the bottom of an unnamed crater located on Vastitas Borealis, a broad plain that covers much of Mars' far northern latitudes, at approximately 70.5° North and 103° East. The crater is 35 km wide and about 2 km deep.
The height difference between the crater floor and the surface of the water ice is about 200 metres. ESA scientists have attributed most of this height difference to sand dunes beneath the water ice, which are partially visible. While scientists do not refer to the patch as a "lake", the water ice patch is remarkable for its size and for being present throughout the year. Deposits of water ice and layers of frost have been found in many different locations on the planet.
Lake Vostok in Antarctica may have implications for liquid water still existing on Mars because if water existed before the polar ice caps on Mars, it is possible that there is still liquid water below the ice caps.
Equatorial frozen sea
Surface features consistent with existing pack ice have been discovered in the southern Elysium Planitia. What appear to be plates of broken ice, ranging in size from 30 m to 30 km, are found in channels leading to a flooded area of approximately the same depth and width as the North Sea. The plates show signs of break up and rotation that clearly distinguish them from lava plates elsewhere on the surface of Mars. The source for the flood is thought to be the nearby geological fault Cerberus Fossae which spewed water as well as lava aged some 2 to 10 million years. It was suggested that the water exited the Cerberus Fossae then pooled and froze in the low, level plains and that such lakes may still exist. Not all scientists agree with these conclusions.
Polar ice caps
A significant amount of surface hydrogen has been observed globally by the Mars Odyssey GRS. Stoichiometrically estimated water mass fractions indicate that - when free of carbon dioxide - the near surface at the poles consists almost entirely of water covered by a thin veneer of fine material. This is reinforced by MARSIS observations, with an estimated 1.6x106 km3 of water at the southern polar region with Water Equivalent to a Global layer (WEG) 11 meters deep. Additional observations at both poles suggest the total WEG to be 30 m, while the Mars Odyssey NS observations places the lower bound at ~14 cm depth. Geomorphic evidence favors significantly larger quantities of surface water over geologic history, with WEG as deep as 500 m. The current atmospheric reservoir of water, though important as a conduit, is insignificant in volume with the WEG no more than 10 µm.
Both the northern polar cap (Planum Boreum) and the southern polar cap (Planum Australe) are thought to grow in thickness during the winter and partially sublime during the summer. Data obtained by the Mars Express satellite, made it possible in 2004 to confirm that the southern polar cap has ice at a depth of 3.7 kilometres (2.3 mi) below the surface with varying contents of frozen water depending on its latitude. The polar cap is a mixture of CO2 ice and water ice.  The second part comprises steep slopes known as scarps, made almost entirely of water ice, that fall away from the polar cap to the surrounding plains. The third part encompasses the vast permafrost fields that stretch for tens of kilometres away from the scarps. NASA scientists calculate that the volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 metres (36 ft).
On July 2008, NASA announced that Phoenix confirmed the presence of water ice at its landing site near the polar ice cap. Research published in January 2010 using HiRISE images, stated that understanding the layers is more complicated than was formerly estimated. The brightness of the layers does not just depend on the amount of dust. The angle of the Sun together with the angle of the spacecraft greatly affects the brightness seen by the camera. This angle depends on factors such as the shape of the trough wall and its orientation. Furthermore, the roughness of the surface can greatly change the albedo (amount of reflected light). All of these factors are influenced by the wind which can erode surfaces. The HiRISE camera did not reveal layers that were thinner than those seen by the Mars Global Surveyor. However, it detected more detail within layers.
The shallow radar on board the Mars Reconnaissance Orbiter took measurements of the north polar ice cap and determined that the volume of water ice in the cap is 821,000 cubic kilometers (197,000 cubic miles). That is equal to 30% of the Earth's Greenland ice sheet or enough to cover the surface of Mars to a depth of 5.6 meters.
For many years, various scientists have suggested that some Martian surfaces look like periglacial regions on Earth. Sometimes it is said that these are regions of permafrost. These observations suggest that frozen water lies right beneath the surface. A common feature in the higher latitudes, patterned ground, can occur in a number of shapes, including stripes and polygons. On the Earth, these shapes are caused by the freezing and thawing of soil. There are other types of evidence for large amounts of frozen water under the surface of Mars, such as terrain softening which rounds sharp topographical features. Besides landscape features that suggest water frozen in the ground, there is evidence from Mars Odyssey's Gamma Ray Spectrometer, theoretical calculations, and direct measurements with the Phoenix lander.
Some areas of Mars are covered with cones that resemble those on Earth where lava has flowed on top of frozen ground. The heat of the lava melts the ice, then changes it into steam. The powerful force of the steam works its way through the lava and produces a cone. In the Athabasca Valles image, the larger cones were made when the steam passed through the thicker layers of lava. The difference between highest elevation (red) to lowest (dark blue) is 170 metres (560 ft).
- Scalloped topography
Certain regions of Mars display scalloped-shaped depressions. The depressions are suspected to be the remains of an ice-rich mantle deposit. Scallops were caused by ice sublimating from frozen soil. This mantle material probably fell from the atmosphere as ice formed on dust when the climate was different due to changes in the tilt of the Mars pole. The scallops are typically tens of meters deep and from a few hundred to a few thousand meters across. They can be almost circular or elongated. Some appear to have coalesced causing a large heavily pitted terrain to form. The process of forming the terrain may begin with sublimation from a crack. There are often polygon cracks where scallops form. So the presence of scalloped topography is an indication of frozen ground.
Many large areas of Mars have been shaped by glaciers. Much of the areas in high latitudes, especially the Ismenius Lacus quadrangle, are suspected to still contain enormous amounts of water ice. Recent evidence has led many planetary scientists to believe that water ice still exists as glaciers with thin coverings 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 associated with fretted terrain, many volcanoes, and even some craters. Researchers have described glacial deposits on Hecates Tholus, Arsia Mons, Pavonis Mons, and Olympus Mons.
Ridges of debris on the surface of the glaciers indicate the direction of ice movement. The surface of some glaciers has rough textures due to sublimation of buried ice. The ice evaporates without melting and leaves behind an empty space. Overlying material then collapses into the void. Glaciers are not pure ice; they contain sand and rocks. At times, they deposit their loads of material onto ridges 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 formed. Sometimes chunks of ice fall from the glacier and get buried in the land surface. When they melt, a more or less round hole remains.
Moving ice carries rock material, then drops it as the ice disappears. On Mars, with its extremely thin atmosphere, ice does not usually melt but instead sublimes. As a result, the rock debris are just dropped, and melt water is not produced so the remains of these glaciers do not appear the same as on Earth. Various names have been applied to these ridged features. Depending on the author, they may be called arcuate ridges, viscous flow features, moraine-like ridges or Martian flow features. Many, but not all, seem to be associated with gullies on the walls of craters and mantling material.
Lineated deposits are probably rock-covered glaciers which are found on the floors of some channels. Their surfaces have ridged and grooved materials that deflect around obstacles. Lineated floor deposits may be related to Lobate Debris Aprons, which have been proven to contain large amounts of ice by orbiting radar. For many years, researchers interpreted that features called 'Lobate Debris Aprons' were glacial flows and it was thought that ice existed under a layer of insulating rocks. With new instrument readings, it has been confirmed that Lobate Debris Aprons contain almost pure ice that is covered with a layer of rocks. 
Glaciers formed much of the observable surface in large areas of Mars. Much of the area in high latitudes, especially the Ismenius Lacus quadrangle, is suspected to still contain enormous amounts of water ice. Recent evidence has led many planetary scientists to believe that water ice still exists as glaciers with a thin covering 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 sometimes associated with fretted terrain, volcanoes, and even some craters. Ridges of debris on the surface of the glaciers show the direction of ice movement.
Using data acquired by the Mars Global Surveyor and Odyssey orbiters, combined with recent developments in the study of cold-based glaciers, scientists believe glaciers once existed and still exist on some volcanoes. The evidence for this are concentric ridges (these are moraines dropped by the glacier), a knobby area caused by ice sublimating, and a smooth section that flows over other deposits (debris-covered glacial ice). Researchers have described glacial deposits on Hecates Tholus, Arisia Mons, Pavonis Mons, and Olympus Mons.
Ice ages on Mars are far different than the ones that the Earth experiences. During a Martian ice age, the poles get warmer, water ice then leaves the ice caps and is deposited in mid latitudes. The moisture from the ice caps travels to lower latitudes in the form of deposits of frost or snow mixed with dust. The atmosphere of Mars contains a great deal of fine dust particles, the water vapor condenses on these particles which then fall down to the ground due to the additional weight of the water coating. When ice at the top of the mantling layer returns to the atmosphere, it leaves behind dust which serves to insulate the remaining ice. The total volume of water removed is about a few percent of the ice caps, or enough to cover the entire surface of the planet under one meter of water. Much of this moisture from the ice caps results in a thick smooth mantle with a mixture of ice and dust. This ice-rich mantle, a few yards thick, smoothes the land at lower latitudes, but in places it displays a bumpy texture. Multiple stages of glaciations probably occurred. Because there are few craters on the current mantle, it is thought to be relatively young. It is thought that this mantle was laid in place during a relatively recent ice age.
Ice ages are driven by changes in Mars's orbit and tilt. Orbital calculations show that Mars wobbles on its axis far more than Earth does. The Earth is stabilized by its proportionally large moon, so it only wobbles a few degrees. Mars, in contrast, may change its tilt by tens of degrees. Its poles get much more direct sunlight and heat at times, which causes the ice caps to warm and become smaller as ice sublimes. Adding to the variability of the climate, the eccentricity of the orbit of Mars changes twice as much as Earth's eccentricity. Computer simulations have shown that a 45° tilt of the Martian axis would result in ice accumulation in areas that display glacial landforms. A 2008 study provided evidence for multiple glacial phases during Late Amazonian glaciation at the dichotomy boundary on Mars.
Recent seasonal flows
Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure and low temperature, except at the lowest elevations for a few hours. So, a geological mystery commenced when observations from NASA's Mars Reconnaissance Orbiter revealed gully deposits that were not there ten years ago, possibly caused by flowing salty water (brine) during the warmest months on Mars. The images were of two craters called Terra Sirenum and Centauri Montes which appear to show the presence of liquid water flows on Mars at some point between 1999 and 2001. In August 2011, NASA announced the discovery by Nepalese student Lujendra Ojha of current seasonal changes in gullies near crater rims on the Southern hemisphere. The researchers again suggested salty water (brines) flowing and then evaporating, possibly leaving some sort of residue.
There is disagreement in the scientific community as to whether or not the recent gully streaks were formed by liquid water. A team suggests the flows were merely dry flows, dry flows that were started by a rock fall in steep regions, or liquid brine near the surface might explain this activity, but the exact source of the water and the mechanism behind its motion are not understood.
Life is understood to require liquid water, but it is not the only essential requirement for life.  The confirmation that liquid water once flowed on Mars, the existence of nutrients, and the previous discovery of a past magnetic field that protected the planet from cosmic and solar radiation, together strongly suggest that Mars could have had the environmental factors to support life. To be clear, the find of past habitability is not evidence that Martian life has ever actually existed.
When there is a magnetic field, the atmosphere is protected from erosion by solar wind, and ensures the maintenance of a dense atmosphere, necessary for liquid water to exist on the surface of Mars. The two current ecological approaches for predicting the potential habitability of the Martian surface use 19 or 20 environmental factors, with emphasis on water availability, temperature, presence of nutrients, an energy source, and protection from solar ultraviolet and galactic cosmic radiation. In particular, the damaging effect of ionising radiation on cellular structure is one of the prime limiting factors on the survival of life in potential astrobiological habitats. Even at a depth of 2 meters beneath the surface, any microbes would likely be dormant, cryopreserved by the current freezing conditions, and so metabolically inactive and unable to repair cellular degradation as it occurs.
Therefore, the best potential locations for discovering life on Mars may be at subsurface environments that have not been studied yet. The extensive volcanism in the past, possibly created subsurface cracks and caves within different strata, and liquid water could have been stored in these subterraneous places, forming large aquifers with deposits of saline liquid water, minerals, organic molecules, and geothermal heat – potentially providing a current habitable environment away from the harsh surface conditions.
Findings by probes
The images acquired by the Mariner 9 Mars orbiter, launched in 1971, revealed the first direct evidence of past water in the form of dry river beds, canyons (including the Valles Marineris, a system of canyons over about 4,020 kilometres (2,500 mi) long), evidence of water erosion and deposition, weather fronts, fogs, and more. The findings from the Mariner 9 missions underpinned the later Viking program. The enormous Valles Marineris canyon system is named after Mariner 9 in honor of its achievements.
By discovering many geological forms that are typically formed from large amounts of water, the two Viking orbiters and the two landers caused a revolution in our knowledge about water on Mars. Huge river valleys were found in many areas. They showed that floods of water broke through dams, carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Large areas in the southern hemisphere contained branched valley networks, suggesting that rain once fell. Many craters look as if the impactor fell into mud. When they were formed, ice in the soil may have melted, turned the ground into mud, then the mud flowed across the surface. Regions, called "Chaotic Terrain," seemed to have quickly lost great volumes of water which caused large channels to form downstream. Estimates for some channel flows run to ten thousand times the flow of the Mississippi River. Underground volcanism may have melted frozen ice; the water then flowed away and the ground collapsed to leave chaotic terrain. Also, general chemical analysis by the two Viking landers suggested the surface has been either exposed to or submerged in water in the past.
Mars Global Surveyor
The Mars Global Surveyor's Thermal Emission Spectrometer (TES) is an instrument able to determine the mineral composition on the surface of Mars. Mineral composition gives information on the presence or absence of water in ancient times. TES identified a large (30,000 km2) area in the Nili Fossae formation that contains the mineral olivine. It is thought that the ancient asteroid impact that created the Isidis basin resulted in faults that exposed the olivine. The discovery of olivine is strong evidence that parts of Mars have been extremely dry for a long time. Olivine was also discovered in many other small outcrops within 60 degrees north and south of the equator. The probe has imaged several channels that suggest past sustained liquid flows, two of them are found in Nanedi Valles and in Nirgal Vallis.
The Pathfinder lander recorded the variation of diurnal temperature cycle. It was coldest just before sunrise, about −78 °Celsius, and warmest just after Mars noon, about −8 °Celsius. These extremes occurred near the ground which both warmed up and cooled down fast. At this location, the highest temperature never reached the freezing point of water (0 °C), too cold for pure liquid water to exist on the surface.
Surface pressures varied diurnally over a 0.2 millibar range, but showed 2 daily minima and two daily maxima. The average daily pressure decreased from about 6.75 millibars to a low of just under 6.7 millbars, corresponding to when the maximum amount of carbon dioxide had condensed on the South Pole. The atmospheric pressure measured by the Pathfinder on Mars is very low —about 0.6% of Earth's, and it would not permit liquid water to exist on the surface.
Other observations were consistent with water being present in the past. Some of the rocks at the Mars Pathfinder site leaned against each other in a manner geologists term imbricated. It is suspected that strong flood waters in the past pushed the rocks around until they faced away from the flow. Some pebbles were rounded, perhaps from being tumbled in a stream. Parts of the ground are crusty, maybe due to cementing by a fluid containing minerals. There was evidence of clouds and maybe fog.
The 2001 Mars Odyssey found much evidence for water on Mars in the form of images, and with its spectrometer, it proved that much of the ground is loaded with water ice. Mars has enough ice just beneath the surface to fill Lake Michigan twice. In both hemispheres, from 55° latitude to the poles, Mars has a high density of ice just under the surface; one kilogram of soil contains about 500 g of water ice. But close to the equator, there is only 2% to 10% of water in the soil. Scientists think that much of this water is also locked up in the chemical structure of minerals, such as clay and sulfates. Although the upper surface contains a few percent of chemically-bound water, ice lies just a few meters deeper, as it has been shown in Arabia Terra, Amazonis quadrangle, and Elysium quadrangle that contain large amounts of water ice. Analysis of the data suggests that the southern hemisphere may have a layered structure, suggestive of stratified deposits beneath a now extinct large water mass.
The instruments aboard the Mars Odyssey are only able to study the top meter of soil, while the radar aboard the Mars Reconnaissance Orbiter can measure a few kilometers deep. In 2002, available data were used to calculate that if all soil surfaces were covered by an even layer of water, this would correspond to a global layer of water (GLW) 0.5 to 1.5 km deep.
Thousands of images returned from Odyssey orbiter also support the idea that Mars once had great amounts of water flowing across its surface. Some images show patterns of branching valleys; others show layers that may have been formed under lakes; even river and lake deltas have been identified. For many years researchers thought that glaciers existed under a layer of insulating rocks. Lineated valley fill is one example of these rock-covered glaciers. They are found on the floors of some channels. Their surfaces have ridged and grooved materials that deflect around obstacles. Lineated floor deposits may be related to lobate debris aprons, which have been shown by orbiting radar to contain large amounts of ice.
The Phoenix lander also confirmed the existence of large amounts of water ice in the northern region of Mars. This finding was predicted by previous orbital data and theory. and was measured from orbit by the Mars Odyssey instruments. On June 19, 2008, NASA announced that dice-sized clumps of bright material in the "Dodo-Goldilocks" trench, dug by the robotic arm, had vaporized over the course of four days, strongly implying that the bright clumps were composed of water ice which sublimes following exposure. Even though CO2 (dry ice) also sublimes under the conditions present, it would do so at a rate much faster than observed. On July 31, 2008, NASA announced that Phoenix confirmed the presence of water ice at its landing site. During the initial heating cycle of a sample, the mass spectrometer detected water vapor when the sample temperature reached 0 °C. Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure and temperature, except at the lowest elevations for short periods.
Perchlorate (ClO4), a strong oxidizer, was confirmed to be in the soil. The chemical, when mixed with water, can lower the water freezing point in a manner similar to how salt is applied to roads to melt ice. It has been hypothesized that perchlorate may be allowing small amounts of liquid water to form on Mars today and may have formed visible gullies by eroding soil on steep slopes.
Additionally, during 2008 and early 2009, a debate emerged within NASA over the presence of 'blobs' which appeared on photos of the vehicle's landing struts, which have been variously described as being either water droplets or 'clumps of frost'.
For about as far as the camera can see, the landing site is flat, but shaped into polygons between 2 and 3 meters in diameter and are bounded by troughs that are 20 cm to 50 cm deep. These shapes are due to ice in the soil expanding and contracting due to major temperature changes. The microscope showed that the soil on top of the polygons is composed of rounded particles and flat particles, probably a type of clay. Ice is present a few inches below the surface in the middle of the polygons, and along its edges, the ice is at least 8 inches deep. When the ice is exposed to the Martian atmosphere it slowly sublimes.
Snow was observed to fall from cirrus clouds. The clouds formed at a level in the atmosphere that was around −65 °C, so the clouds would have to be composed of water-ice, rather than carbon dioxide-ice (CO2 or dry ice) because the temperature for forming carbon dioxide ice is much lower than −120 °C. As a result of mission observations, it is now suspected that water ice (snow) would have accumulated later in the year at this location. The highest temperature measured during the mission, which took place during the Martian summer, was −19.6 °C, while the coldest was −97.7 °C. So, in this region the temperature remained far below the freezing point (0° C) of water.
Mars Exploration Rovers
The Mars Exploration Rovers, Spirit and Opportunity found a great deal of evidence for past water on Mars. Designed to last only three months, Opportunity continues to provide scientific discovery. The Spirit rover landed in what was thought to be a large lake bed. However, the lake bed had been covered over with lava flows, so evidence of past water was initially hard to detect. On March 5, 2004, NASA announced that Spirit had found hints of water history on Mars in a rock dubbed "Humphrey".
As Spirit traveled in reverse in December 2007, pulling a seized wheel behind, the wheel scraped off the upper layer of soil, uncovering a patch of white ground rich in silica. Scientists think that it must have been produced in one of two ways. One: hot spring deposits produced when water dissolved silica at one location and then carried it to another (i.e. a geyser). Two: acidic steam rising through cracks in rocks stripped them of their mineral components, leaving silica behind. The Spirit rover also found evidence for water in the Columbia Hills of Gusev crater. In the Clovis group of rocks the Mossbauer spectrometer (MB) detected goethite, that forms only in the presence of water. iron in the oxidized form Fe+++, carbonate-rich rocks, which means that regions of the planet once harbored water.
The Opportunity rover was directed to a site that had displayed large amounts of hematite from orbit. Hematite often forms from water. The rover indeed found layered rocks and marble-like hematite concretions. In its years of continuous operation, Opportunity is still sending back evidence that this area on Mars was soaked in liquid water in the past.
However, the MER rovers had been finding evidence for ancient wet environments that were very acidic. In fact, what Opportunity has mostly discovered, or found evidence for, was sulphuric acid, a harsh chemical for life. But in May 17, 2013, NASA announced that Opportunity found clay deposits that typically form in wet environments that are near neutral acidity. This find provides additional evidence about a wet ancient environment possibly favorable for life.
Mars Reconnaissance Orbiter
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. A major discovery was finding evidence of ancient hot springs. If they have hosted microbial life, they may contain biosignatures. Research published in January 2010, described strong evidence for sustained precipitation in the area around Valles Marineris. The types of minerals there are associated with water. Also, the high density of small branching channels indicates a great deal of precipitation.
Rocks on Mars have been found to frequently occur as layers, called strata, in many different places. Layers form by various ways, including volcanoes, wind, or water. Light-toned rocks on Mars have been associated with hydrated minerals like sulfates and clay.
The ice mantle under the shallow subsurface is thought to result from frequent, major climate changes. Changes in Mars' orbit and tilt cause significant changes in the distribution of water ice from polar regions down to latitudes equivalent to Texas. During certain climate periods water vapor leaves polar ice and enters the atmosphere. The water returns to the ground at lower latitudes as deposits of frost or snow mixed generously with dust. The atmosphere of Mars contains a great deal of fine dust particles. Water vapor condenses on the particles, then they fall down to the ground due to the additional weight of the water coating. When ice at the top of the mantling layer goes back into the atmosphere, it leaves behind dust, which insulates the remaining ice.
In 2008, research with the Shallow Radar on the Mars Reconnaissance Orbiter provided strong evidence that the lobate debris aprons (LDA) in Hellas Planitia and in mid northern latitudes are glaciers that are covered with a thin layer of rocks. Its radar also detected a strong reflection from the top and base of LDAs, meaning that pure water ice made up the bulk of the formation. The discovery of water ice in LDAs demonstrates that water is found at even lower latitudes.
Research published in September 2009, demonstrated that some new craters on Mars show exposed, pure water ice. After a time, the ice disappears, evaporating into the atmosphere. The ice is only a few feet deep. The ice was confirmed with the Compact Imaging Spectrometer (CRISM) on board the Mars Reconnaissance Orbiter.
On October 2012, the first X-ray diffraction analysis of Martian soil was performed. The results revealed the presence of several minerals, including feldspar, pyroxenes and olivine, and suggested that the Martian soil in the sample was similar to the weathered basaltic soils of Hawaiian volcanoes. The sample used is composed of dust distributed from global dust storms and local fine sand. So far, the materials Curiosity has analyzed are consistent with the initial ideas of deposits in Gale Crater recording a transition through time from a wet to dry environment.
On December 2012, NASA reported that Curiosity performed its first extensive soil analysis, revealing the presence of water molecules, sulfur and chlorine in the Martian soil. And on March 2013, NASA reported evidence of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock. Analysis using the rover's DAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of 60 cm (2.0 ft), in the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain.
- River valleys
Branched channels in Thaumasia quadrangle
The branched channels seen by Viking from orbit strongly suggested that it rained on Mars in the past. Location is Margaritifer Sinus quadrangle.
Channels near the rim of Ius Chasma. Their pattern and high density suggest precipitation as the source of the water. Location is Coprates quadrangle.
Tear-drop shaped islands caused by flood waters from Maja Vallis, as seen by Viking Orbiter. Location is Oxia Palus quadrangle
Flat terrain near the north pole of Mars showing polygonal patterns.
Crater wall inside Mariner Crater showing a large group of gullies
Gullies on one wall of Kaiser Crater. Gullies usually are found in only one wall of a crater. Location is Noachis quadrangle.
Gullies near Newton Crater
Glacier moving down valley, then spreading out on plain. Location is Ismenius Lacus quadrangle
This is the terminal moraine of a glacier. For scale, the box shows the approximate size of a football field. Location is Hellas quadrangle
Radar studies indicate that this glacier contains mostly pure water ice. It is moving from the right. Location is Ismenius Lacus quadrangle.
Tongue-Shaped Glacier. Location is Hellas quadrangle.
The white area is water ice that has been exposed by an impact. Location is Cebrenia quadrangle.
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