Water on Mars

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An artist's impression of what ancient Mars may have looked like, based on geological data

Water on Mars exists almost exclusively as water ice. The Martian polar ice caps consist primarily of water ice, and further ice is contained in Martian surface rocks at more temperate latitudes (permafrost). A small amount of water vapor is present in the atmosphere.[1] There are no bodies of liquid water on the Martian surface.

Current conditions on the planet surface do not support the long-term existence of liquid water. The average atmospheric pressure and temperature are far too low, leading to immediate freezing and resulting sublimation. Despite this, research suggests that in the past there was liquid water flowing on the surface,[2][3] creating large areas similar to Earth's oceans.[4][5][6][7]

There are a number[8] of direct and indirect proofs of water's presence either on or under the surface, e.g. stream beds,[9][10][11] polar caps, spectroscopic measurement,[12] eroded craters or minerals directly connected to the existence of liquid water (such as goethite), grey, crystalline hematite, phyllosilicates, opal,[13] and sulfate.[14][15] With the improved cameras on advanced Mars orbiters such as Viking, Mars Odyssey, Mars Global Surveyor, Mars Express, and the Mars Reconnaissance Orbiter pictures of ancient lakes,[16][17][18][19][20][20][21][22] ancient river valleys,[9][23] and widespread glaciation[24][25][26][27][28] have accumulated. Besides the visual confirmation of water from a huge collection of images, an orbiting Gamma Ray Spectrometer found ice just under the surface of much of the planet.[29][30] Also, radar studies discovered pure ice in formations that were thought to be glaciers.[31][32][33][34][35][36] The Phoenix lander exposed ice as it landed, watched chunks of ice disappear,[37][38][39] detected snow falling,[40] and even saw drops of liquid water.[41][42][43]

Today, it is generally believed that Mars had abundant water very early in its history[44] during which snow and rain fell on the planet and created rivers, lakes, and possibly oceans.[45][46][47] Large clay deposits were produced. Life may even have come into existence. Large areas of liquid water have disappeared, but climate changes have frequently deposited large amounts of water-rich materials in mid-latitudes.[48][49][50][51] From these materials, glaciers and other forms of frozen ground came to be. Small amounts of water probably melt on steep slopes from time to time and produce gullies.[52][53] Recent images have also detected yearly changes on some slopes that may have been caused by liquid water.[54][55] Although Mars is very cold at present, water could exist as a liquid if it contains salts.[56] Salt is expected to be on the Martian surface.[57]

Details of how water has been discovered can be found in the sections that follow on the various orbiting and landing robots that have been sent to Mars. In addition, many bits and pieces of indirect evidence are listed here. Since several missions (Mars Odyssey, Mars Global Surveyor, Mars Reconnaissance Orbiter, Mars Express, Mars Opportunity Rover and Mars Curiosity Rover) are still sending back data from the Red Planet, discoveries continue to be made. One recent discovery, announced by NASA scientists on September 27, 2012, is that the Curiosity Rover found evidence for an ancient streambed suggesting a "vigorous flow" of water on Mars.[58][59][60]

Contents

Findings from probes

Mariner 9

Meander in Scamander Vallis, as seen by Mariner 9. Such images implied that large amounts of water once flowed on the surface of Mars.

Mariner 9 imaging revealed the first direct evidence of water in the form of 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.[61] 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. Launched in 1971, its mission ended the following year.

Viking program

By discovering many geological forms that are typically formed from large amounts of water, 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.[9] Large areas in the southern hemisphere contained branched valley networks, suggesting that rain once fell. The flanks of some volcanoes are believed to have been exposed to rainfall because they resemble those occurring on Hawaiian volcanoes.[62] 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.[63] Normally, material from an impact goes up, then down. It does not flow across the surface, going around obstacles, as it does on some Martian craters.[12][64][65] Regions, called "Chaotic Terrain,"seemed to have quickly lost great volumes of water which caused large channels to form downstream. The amount of water involved was almost unthinkable—estimates for some channel flows run to ten thousand times the flow of the Mississippi River.[23] Underground volcanism may have melted frozen ice; the water then flowed away and the ground just collapsed to leave chaotic terrain.

The images below, some of the best from the Viking orbiters, are mosaics of many small, high resolution images.

Frost at the Viking 2 landing site in Utopia Planitia

Results from Viking lander experiments strongly suggest the presence of water in the present and in the past of Mars. All samples heated in the gas chromatograph-mass spectrometer (GSMS) gave off water. However, the way the samples were handled prohibited an exact measurement of the amount of water. But, it was around 1%.[66] General chemical analysis suggested the surface had been exposed to water in the past. Some chemicals in the soil contained sulfur and chlorine that were like those remaining after sea water evaporates. Sulfur was more concentrated in the crust on top of the soil, than in the bulk soil beneath. So it was concluded that the upper crust was cemented together with sulfates that were transported to the surface dissoved in water. This process is common on Earth's deserts. The sulfur may be present as sulfates of sodium, magnesium, calcium, or iron. A sulfide of iron is also possible.[67]

Using results from the chemical measurements, mineral models suggest that the soil could be a mixture of about 80% iron-rich clay, about 10% magnesium sulfate (kieserite?), about 5% carbonate (calcite), and about 5% iron oxides (hematite, magnetite, goethite?). These minerals are typical weathering products of mafic igneous rocks. The presence of clay, magnesium sulfate, kieserite, calcite, hematite, and goethite strongly suggest that water was once in the area.[68] Sulfate contains chemically bound water, hence its presence suggests water was around in the past. Viking 2 found similar group of minerals. Because Viking 2 was much farther north, pictures it took in the winter showed frost.

Mars Global Surveyor

Map showing distribution of hematite in Sinus Meridiani, as seen by TES. This data was used to target the landing of Opportunity Rover. Hematite is usually formed in the presence of water. Opportunity landed here and found definite evidence for water.

The Mars Global Surveyor's Thermal Emission Spectrometer (TES) is an instrument able to detect mineral composition on Mars. Mineral composition gives information on the presence or absence of water in ancient times. TES identified a large (30,000 square-kilometer) area (in the Nili Fossae formation) that contained the mineral olivine. It is thought that the ancient impact that created the Isidis basin resulted in faults that exposed the olivine. Olivine is present in many mafic volcanic rocks; in the presence of water it weathers into minerals such as goethite, chlorite, smectite, maghemite, and hematite. 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.[69] Olivine has been found in the SNC (shergottite, nakhlite, and chassigny) meteorites that are generally accepted to have come from Mars.[70] Later studies have found that olivine-rich rocks cover more than 113,000 square kilometers of the Martian surface. That is 11 times larger than the five volcanoes on the Big Island of Hawaii.[71]

On December 6, 2006 NASA released photos of two craters called Terra Sirenum and Centauri Montes which appear to show the presence of liquid water on Mars at some point between 1999 and 2001.[72][73]

Hundreds of gullies have been discovered that were formed from liquid water, possible in recent times. These gullies occur on steep slopes and mostly in certain bands of latitude.[74][75][76][77][78]

Below are some examples of gullies that were photographed by Mars Global Surveyor.

A few channels on Mars displayed inner channels that suggest sustained fluid flows. The most well-known is the one in Nanedi Valles. Another was found in Nirgal Vallis.[74]

Inner channel (near top of the image) on floor of Nanedi Valles that suggests that water flowed for a fairly long period. Image from Lunae Palus quadrangle.

Many places on Mars show dark streaks on steep slopes, such as crater walls. Dark slope streaks have been studied since the Mariner and Viking missions.[79] It seems that streaks start out being dark, then they become lighter with age. Often they originate with a small narrow spot, then widen and extend downhill for hundreds of meters. Streaks do not seem to be associated with any particular layer of material because they do not always start at a common level along a slope. Although many of the streaks appear very dark, they are only 10% or less darker than the surrounding surface. Mars Global Surveyor found that new streaks have formed in less than one year on Mars.

Several ideas have been advanced to explain the streaks. Some involve water,[80] or even the growth of organisms.[81][82] The generally accepted explanation for the streaks is that they are formed from the avalanching of a thin layer of bright dust that is covering a darker surface. Bright dust settles on all Martian surfaces after a period of time.[74]

Dark streaks can be seen in the images below, as seen from Mars Global Surveyor.

Some parts of Mars show inverted relief. This occurs when materials are deposited on the floor of a stream then become resistant to erosion, perhaps by cementation. Later the area may be buried. Eventually erosion removes the covering layer. The former streams become visible since they are resistant to erosion. Mars Global Surveyor found several examples of this process.[83] Many inverted streams have been discovered in various regions of mars, especially in the Medusae Fossae Formation,[84] Miyamoto Crater,[85] and the Juventae Plateau.[86][87]

Inverted Streams near Juventae Chasma, as seen by Mars Global Surveyor. These streams begin at the top of a ridge then run together.

Mars Pathfinder

Pathfinder found temperatures varied on a diurnal 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 fastest. At this location, the highest temperature never reached the freezing point of water (0 °C), so Mars Pathfinder confirmed that where it landed it is too cold for liquid water to exist. However, water could exist as a liquid if it were mixed with various salts.[88]

Surface pressures varied diurnally over a 0.2 millibar range, but showed 2 daily minimums and two daily maximums. 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 pressure on the Earth is generally close to 1000 millibars, so the pressure on Mars is very low. The pressures measured by Pathfinder would not permit water or ice to exist on the surface. But, if ice were insulated with a layer of soil, it could last a long time.[89]

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 believed 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.[90]

There was evidence of clouds and maybe fog.[90]

Mars Odyssey

Mars Odyssey found much evidence for water on Mars in the form of pictures and with a spectrometer it proved that much of the ground is loaded with ice. In July 2003, at a conference in California, it was announced that the Gamma Ray Spectrometer (GRS) on board the Mars Odyssey had discovered huge amounts of water over vast areas of Mars. Mars has enough ice just beneath the surface to fill Lake Michigan twice.[29] In both hemispheres, from 55 degrees 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.[30] Scientists believe that much of this water is locked up in the chemical structure of minerals, such as clay and sulfates. Previous studies with infrared spectroscopes have provided evidence of small amounts of chemically or physically bound water.[91][92] The Viking landers detected low levels of chemically bound water in the Martian soil.[66] It is believed that although the upper surface only contains a percent or so of water, ice may lie just a few feet deeper. Some areas, Arabia Terra, Amazonis quadrangle, and Elysium quadrangle contain large amounts of water.[93] Analysis of the data suggest that the southern hemisphere may have a layered structure.[94] Both of the poles showed buried ice, but the north pole had none close to it because it was covered over by seasonal carbon dioxide (dry ice). When the measurements were gathered, it was winter at the north pole so carbon dioxide had frozen on top of the water ice.[29] There may be much more water further below the surface; the instruments aboard the Mars Odyssey are only able to study the top meter or so of soil. If all holes in the soil were filled by water, this would correspond to a global layer of water 0.5 to 1.5 km deep.[95] The Phoenix lander confirmed the initial findings of the Mars Odyssey.[96] It found ice a few inches below the surface and the ice is at least 8 inches deep. When the ice is exposed to the Martian atmosphere it slowly sublimates. In fact, some of the ice was exposed by the landing rockets of the craft.[97]

View underneath Phoenix lander towards south foot pad, showing patchy exposures of a bright surface that was later proven to be water ice, as predicted by theory and detected by Mars Odyssey.

Thousands of images returned from Odyssey support the idea that Mars once had great amounts of water flowing across its surface. Some pictures show patterns of branching valleys. Others show layers that may have formed under lakes. Deltas have been identified.[16] For many years researchers believed that glaciers existed under a layer of insulating rocks.[98][99][100][101][102] 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. Some glaciers on the Earth show such features. Lineated floor deposits may be related to Lobate debris aprons, which have been proven to contain large amounts of ice by orbiting radar.[101][102][103] The pictures below, taken with the THEMIS instrument on board the Mars Odyssey, show examples of features that are associated with water present in the present or past.[104]

Dao Vallis begins near a large volcano, called Hadriaca Patera, so it is thought to have received water when hot magma melted huge amounts of ice in the frozen ground. The partially circular depressions on the left side of the channel in the image above suggests that groundwater sapping also contributed water.[105] In some areas large river valleys begin with a landscape feature called "Chaos" or Chaotic Terrain." It is thought that the ground collapsed, as huge amounts of water were suddenly released. Examples of Chaotic terrain, as imaged by THEMIS, are shown below.

Phoenix

The Phoenix lander confirmed the existence of large amounts of water ice in the northern regions of Mars.[96] This finding was predicted by theory.[106] and was measured from orbit by the Mars Odyssey instruments.[30] 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 sublimated following exposure. Even though dry ice also sublimates under the conditions present, it would do so at a rate much faster than observed.[37][38][39]

On July 31, 2008, NASA announced that Phoenix confirmed the presence of water ice on Mars. During the initial heating cycle of a new sample, the Thermal and Evolved-Gas Analyzer's (TEGA) mass spectrometer detected water vapor when the sample temperature reached 0 °C.[107] Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods.[108][109]

Results published in the journal Science after the mission ended reported that chloride, bicarbonate, magnesium, sodium potassium, calcium, and possibly sulfate were detected in the samples. Perchlorate (ClO4), a strong oxidizer, was confirmed to be in the soil. The chemical when mixed with water can greatly lower freezing points, in a manner similar to how salt is applied to roads to melt ice. Perchlorate may be allowing small amounts of liquid water to form on Mars today. Gullies, which are common in certain areas of Mars, may have formed from perchlorate melting ice and causing water to erode soil on steep slopes.[110]

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'.[111] Due to the lack of consensus within the Phoenix science project, the issue had not been raised in any NASA news conferences.[111] One scientist's view poised that the lander's thrusters splashed a pocket of brine from just below the Martian surface onto the landing strut during the vehicle's landing. The salts would then have absorbed water vapor from the air, which would have explained how they appeared to grow in size during the first 44 Martian days before slowly evaporating as Mars temperature dropped.[111][112] Some images even suggest that some of the droplets darkened, then moved and merged; this is strong physical evidence that they were liquid.[41][42][43][113]

For about as far as the camera can see, the land is flat, but shaped into polygons between 2–3 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 flat particles (probably a type of clay) and rounded particles. Clay is a mineral that forms from other minerals when water is available. So, finding clay proves the existence of past water.[114] 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 sublimates.[115]

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 (dry ice) because the temperature for forming carbon dioxide ice is much lower—less than −120 °C. As a result of mission observations, it is now believed that water ice (snow) would have accumulated later in the year at this location.[40] The highest temperature measured during the mission was −19.6 °C, while the coldest was −97.7 °C. So, in this region the temperature remained far below the freezing point (0°) of water. Bear in mind that the mission took place in the heat of the Martian summer.[116]

Interpretation of the data transmitted from the craft was published in the journal Science. As per the peer reviewed data the site had a wetter and warmer climate in the recent past. Finding calcium carbonate in the Martian soil leads scientists to believe that the site had been wet or damp in the geological past. During seasonal or longer period diurnal cycles water may have been present as thin films. The tilt or obliquity of Mars changes far more than the Earth; hence times of higher humidity are probable.[117] The data also confirms the presence of the chemical perchlorate. Perchlorate makes up a few tenths of a percent of the soil samples. Perchlorate is used as food by some bacteria on Earth.[118] Another paper claims that the previously detected snow could lead to a buildup of water ice.

Mars Rovers

The Mars Rovers Spirit and Opportunity found a great deal of evidence for past water on Mars. Designed to last only three months, both were still operating after more than six years. Although Spirit got trapped in a sand pit, Opportunity continues to provide scientific discovery.

The Spirit rover landed in what was thought to be a huge lake bed. However, the lake bed had been covered over with lava flows, so evidence of past water was initially hard to detect. As the mission progressed and the Rover continued to move along the surface more and more clues to past water were found.

On March 5, 2004, NASA announced that Spirit had found hints of water history on Mars in a rock dubbed "Humphrey". Dr. Raymond Arvidson, PhD, the McDonnell University Professor and chair of Earth and planetary sciences at Washington University in St. Louis, reported during a NASA press conference: "If we found this rock on Earth, we would say it is a volcanic rock that had a little fluid moving through it." In contrast to the rocks found by the twin rover Opportunity, this one was formed from magma and then acquired bright material in small crevices, which look like crystallized minerals. If this interpretation holds true, the minerals were most likely dissolved in water, which was either carried inside the rock or interacted with it at a later stage, after it formed.[119]

By Sol 390 (Mid-February 2005), as Spirit was advancing towards "Larry's Lookout", by driving up the hill in reverse, it investigated some targets along the way, including the soil target, "Paso Robles", which contained the highest amount of salt found on the red planet. The soil also contained a high amount of phosphorus in its composition, however not nearly as high as another rock sampled by Spirit, "Wishstone". Squyres said of the discovery, "We're still trying to work out what this means, but clearly, with this much salt around, water had a hand here".

As Spirit traveled with a dead wheel in December 2007, pulling the dead wheel behind, the wheel scraped off the upper layer of the martian soil, uncovering a patch of ground that scientists say shows evidence of a past environment that would have been perfect for microbial life. It is similar to areas on Earth where water or steam from hot springs came into contact with volcanic rocks. On Earth, these are locations that tend to teem with bacteria, said rover chief scientist Steve Squyres. "We're really excited about this," he told a meeting of the American Geophysical Union (AGU). The area is extremely rich in silica – the main ingredient of window glass. The researchers have now concluded that the bright material must have been produced in one of two ways.[120] 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 important thing is that whether it is one hypothesis or the other, the implications for the former habitability of Mars are pretty much the same," Squyres explained to BBC News. Hot water provides an environment in which microbes can thrive and the precipitation of that silica entombs and preserves them. Squyres added, "You can go to hot springs and you can go to fumaroles and at either place on Earth it is teeming with life – microbial life.[121][122]

Opportunity rover was directed to a site that had displayed large amounts of hematite from orbit. Hematite often forms from water. When Opportunity landed, layered rocks and marble-like hematite concretions ("blueberries") were easily visible. In its years of continuous operation, Opportunity sent back much evidence that a wide area on Mars was soaked in liquid water.

During a press conference in March 2006, mission scientists discussed their conclusions about the bedrock, and the evidence for the presence of liquid water during their formation. They presented the following reasoning to explain the small, elongated voids in the rock visible on the surface and after grinding into it (see last two images below).[123] These voids are consistent with features known to geologists as "vugs". These are formed when crystals form inside a rock matrix and are later removed through erosive processes, leaving behind voids. Some of the features in this picture are "disk-like", which is consistent with certain types of crystals, notably sulfate minerals. Additionally, mission members presented first data from the Mössbauer spectrometer taken at the bedrock site. The iron spectrum obtained from the rock El Capitan shows strong evidence for the mineral jarosite. This mineral contains hydroxide ions, which indicates the presence of water when the minerals were formed. Mini-TES data from the same rock showed that it consists of a considerable amount of sulfates. Sulfates also contain water.

Spirit Rover found evidence for water in the Columbia Hills of Gusev crater. In the Clovis group of rocks the Mossbauer spectrometer(MB) detected goethite.[124] Goethite forms only in the presence of water, so its discovery is the first direct evidence of past water in the Columbia Hills's rocks. In addition, the MB spectra of rocks and outcrops displayed a strong decline in olivine presence,[125] although the rocks probably once contained much olivine.[126] Olivine is a marker for the lack of water because it easily decomposes in the presence of water. Sulfate was found, and it needs water to form. Other rock groups also contained sulfates. One type of soil, called Paso Robles, from the Columbia Hills, may be an evaporate deposit because it contains large amounts of sulfur, phosphorus, calcium, and iron.[127] In addition, the MB found that much of the iron in Paso Robles soil was of the oxidized, Fe+++ form, which would happen if water had been present.[128]

After Spirit stopped working scientists studied old data from the Miniature Thermal Emission Spectrometer, or Mini-TES and confirmed the presence of large amounts of carbonate-rich rocks, which means that regions of the planet may have once harbored water. The carbonates were discovered in an outcrop of rocks called "Comanche."[129][130]

Evidence of water on Mars[58][59][60]

Peace Vallis and related alluvial fan near the Curiosity rover landing ellipse and landing site (noted by +).
"Hottah" rock outcrop on Mars - an ancient streambed viewed by the Curiosity rover (September 14, 2012) (close-up) (3-D version).
"Link" rock outcrop on Mars - compared with a terrestrial fluvial conglomerate - suggesting water "vigorously" flowing in a stream.
Curiosity rover on the way to Glenelg (September 26, 2012).

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 hot springs. These may have contained life and may now contain well-preserved fossils of life. Research, in the January 2010 issue of Icarus, described strong evidence for sustained precipitation in the area around Valles Marineris.[86][87] The types of minerals there are associated with water. Also, the high density of small branching channels indicates a great deal of precipitation because they are similar to stream channels on the Earth.

Springs in Vernal Crater, as seen by HIRISE. These springs may be good places to look for evidence of past life because hot springs can preserve evidence of life forms for a long time. Location is Oxia Palus quadrangle.

Some places on Mars show inverted relief. In these locations, a stream bed appears as a raised feature, instead of a depression. The inverted former stream channels may be caused by the deposition of large rocks or due to cementation of loose materials. In either case erosion would erode the surrounding land and consequently leave the old channel as a raised ridge because the ridge will be more resistant to erosion. Images below, taken with HiRISE show sinuous ridges that are old channels that have become inverted.[131]

Using data from Mars Global Surveyor, Mars Odyssey and the Mars Reconnaissance Orbiter, scientists have found widespread deposits of chloride minerals. Usually chlorides are the last minerals to come out of solution. A picture below shows some deposits within the Phaethontis quadrangle. Evidence suggests that the deposits were formed from the evaporation of mineral-enriched waters. Lakes may have been scattered over large areas of the Martian surface. Carbonates, sulfates, and silica should precipitate out ahead of them. Sulfates and silica have been discovered by the Mars Rovers. Places with chloride minerals may have once held various life forms. Furthermore, such areas should preserve traces of ancient life.[57]

Rocks on Mars have been found to frequently occur as layers, called strata, in many different places. Layers form by various ways. Volcanoes, wind, or water can produce layers.[132] Many places on Mars show rocks arranged in layers. Scientists are happy about finding layers on Mars since layers may have formed under large bodies of water. Sometimes the layers display different colors. Light-toned rocks on Mars have been associated with hydrated minerals like sulfates. Instruments on orbiting spacecraft have detected clay (also called phyllosilicates) in some layers.[14] Scientists are excited about finding hydrated minerals such as sulfates and clays on Mars because they are usually formed in the presence of water.[133] Below are a few of the many examples of layers that have been studied with HiRISE.

Evidence of water from chloride deposits in Phaethontis quadrangle. Picture from HiRISE.

Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust.[134] This ice-rich mantle, a few yards thick, smoothes the land. But in places it displays a bumpy texture, resembling the surface of a basketball. Because there are few craters on this mantle, the mantle is relatively young. The images below, all taken with HiRISE, show a variety of views of this smooth mantle.

The mantle is thought to result from frequent, major climate changes. Changes in Mars's 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.[135] 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.[136] HiRISE has carried out many observations of gullies that are assumed to have been caused by recent flows of liquid water. Many gullies are imaged over and over to see if any changes occur. Some repeat observations of gullies have displayed changes that some scientists argue were caused by liquid water over the period of just a few years.[137] Others say the flows were merely dry flows.[138] These were first discovered by the Mars Global Surveyor. Below are some of the many hundreds of gullies that have been studied with HiRISE.

Of interest from the days of the Viking Orbiters are piles of material surrounding cliffs. These deposits of rock debris are called lobate debris aprons (LDAs). These features have a convex topography and a gentle slope from cliffs or escarpments; this suggests flow away from the steep source cliff. In addition, lobate debris aprons can show surface lineations just as rock glaciers on the Earth.[12] In 2008, research with the Shallow Radar on the Mars Reconnaissance Orbiter provided strong evidence that the LDAs in Hellas Planitia and in mid northern latitudes are glaciers that are covered with a thin layer of rocks. Radar from the Mars Reconnaissance Orbiter gave a strong reflection from the top and base of LDAs, meaning that pure water ice made up the bulk of the formation (between the two reflections).[102][103]). The discovery of water ice in LDAs demonstrates that water is found at even lower latitudes. Future colonists on Mars will be able to tap into these ice deposits, instead of having to travel to much higher latitudes. Another major advantage of LDAs over other sources of Martian water is that they can easily detected and mapped from orbit. Below are examples of lobate debris aprons that were studied with HiRISE.

Bright part is water ice that has been exposed by impact. The ice was identified using CRISM on the MRO. Location is Cebrenia quadrangle.

Research, reported in the journal Science in September 2009,[139] 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 (MRO). The ice was found in five locations. Three of the locations are in the Cebrenia quadrangle. These locations are 55.57° N, 150.62° E; 43.28° N, 176.9° E; and 45° N, 164.5° E. Two others are in the Diacria quadrangle: 46.7° N, 176.8° E and 46.33° N, 176.9° E.[140][141][142] This discovery proves that future colonists on Mars will be able to obtain water from a wide variety of locations. The ice can be dug up, melted, then taken apart to provide fresh oxygen and hydrogen for rocket fuel. Hydrogen is the powerful fuel used by the space shuttle main engines.

Columnar jointing

Columnar jointing in basalt, Marte Vallis

In 2009, HiRISE discovered columnar jointing in rocks on Mars.[143] Such jointing is accepted as having involved water. To make the parallel cracks of columnar jointing, more cooling is necessary, and water is the most logical choice. Scientists calculate that the water was present intermittently for a few months to a few years.[144]

Light-toned layered deposits

HiRISE has sent back many images of large surface areas that are termed "light-toned layered deposits." These 30–80 meter thick deposits are believed to have been formed from the action of water. They contain evidence of stream channel systems.[145] Furthermore, chemical data from the Compact Reconnaissance Imaging Spectrometer orbiting the planet have shown water related mineral forms: opal (hydrated silica) and iron sulfates.[146] These can be formed from the action of low temperature acid solutions reacting with basaltic rocks. These features of light-toned layered deposits strongly suggest that there was long lasting precipitation and surface runoff during the Hesperian epoch of Martian history.[86][147]

Sources of Martian water

Map of the Tharsis region on Mars, which contains many volcanoes.

Volcanoes give off great amounts of gas when they erupt. The gases are usually water vapor and carbon dioxide. Estimates put the amount of gas released into the atmosphere as enough to make the Martian atmosphere thicker than the Earth's. The water vapor from the volcanoes could have made enough water to place all of Mars under 120 meters of water.[citation needed] In addition, all the carbon dioxide released would have raised the temperature of the planet due to the greenhouse effect, by trapping heat in the form of infrared radiation. So the eruption of lava on Tharsis could have made Mars Earth-like in the past. With a thicker atmosphere, oceans and/or lakes may have been present.[148]

Groundwater on Mars

One group of researchers proposed that some of the layers on Mars were caused by groundwater rising to the surface in many places, especially inside of craters. According to the hypothesis, groundwater with dissolved minerals came to the surface, in and later around craters, and helped to form layers by adding minerals (especially sulfate) and cementing sediments. This hypothesis is supported by a groundwater model and by sulfates discovered in a wide area.[149][150] At first, by examining surface materials with Opportunity Rover, scientists discovered that groundwater had repeatedly risen and deposited sulfates.[123][151][152][153][154] Later studies with instruments on board the Mars Reconnaissance Orbiter showed that the same kinds of materials exist in a large area that included Arabia.[155]

Evidence of frozen water

Ice patches

On July 28, 2005, the European Space Agency announced the existence of a crater partially filled with frozen water;[156] some then interpreted the discovery as an "ice lake".[157] Images of the crater, taken by the High Resolution Stereo Camera on board the European Space Agency's Mars Express spacecraft, 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.

Equatorial frozen sea

Surface features consistent with pack ice have been discovered in the southern Elysium Planitia.[158] 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.[159] Not all scientists agree with these conclusions.[20][160][161]

Glaciers

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 believed to still contain enormous amounts of water ice.[12][162] Recent evidence has led many planetary scientists to believe that water ice still exists as glaciers with a thin covering of insulating rock.[98][99][100][101][102][163] 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.[164] Glaciers are believed to be associated with Fretted terrain, many volcanoes,[20][165] 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.[166] Other pictures below show various features that appear to be connected with the existence of glaciers.

Polar ice caps

The Mars Global Surveyor acquired this image of the Martian north polar ice cap in early northern summer.

Both the northern polar cap (Planum Boreum) and the southern polar cap (Planum Australe) are believed 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 an average of 3 kilometres (1.9 mi)[citation needed]thick slab of ice[167] with varying contents of frozen water, depending on its latitude; the polar cap is a mixture of 85% CO2 ice and 15% water ice.[168] 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.[168] The third part encompasses the vast permafrost fields that stretch for tens of kilometres away from the scarps.[168][169] 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.[167]

Results, published in 2009, of shallow radar measurements of the North Polar ice cap determined that the volume of water ice in the cap is 821,000 cubic kilometers (197,000 cubic miles). That's 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 (dividing the ice cap volume by the surface area of Mars is how this number is found). The radar instrument is on board the Mars Reconnaissance Orbiter.[170]

Ground ice

For many years, various scientists have suggested that some Martian surfaces look like periglacial regions on Earth.[12] 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.[171][172] 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.[173] 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.[174]

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 above, the larger cones were made when the steam went through the thicker layers of lava. The difference between highest elevation (red) to lowest (dark blue) is 170 metres (560 ft).[175]

Scalloped topography

Stages in scallop formation in Hellas quadrangle

Certain regions of Mars display scalloped-shaped depressions. The depression are believed 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 air as ice formed on dust when the climate was different due to changes in the tilt of the Mars pole.[134][176] 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.[87][177]

Possible evidence of flowing water

In August 2011, NASA announced the discovery of seasonal changes in gullies near crater rims on the Southern hemisphere. This suggests salty water flowing and then evaporating, possibly leaving some sort of residue.[178]

On September 27, 2012, NASA scientists announced that the Curiosity rover found evidence for an ancient streambed suggesting a "vigorous flow" of water on Mars.[58][59][60]. In particular, analysis of an ancient streambed indicated that the water ran quickly, possibly at hip depth. The discovery marks an important achievement for Curiosity, and supports the notion that Mars was once capable of harboring life.

Remnants of the now dried-up stream were found inside the Gale Crater within which Curiosity is working. Proof of running water came in the form of rounded pebbles and gravel fragments that could have only been weathered by strong 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. Because there are many channels like this, NASA scientists believe the flows were continuous or repeated for long durations, and not intermittent.

"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," noted Curiosity scientist William Dietrich speaking through NASA's official release. "Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it."

Mars meteorites

Mars meteorite ALH84001

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 injection into space.[179]

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.[180]

In 1996, a group of scientists reported on chemical fossils in Allan Hills 84001, a meteorite from Mars.[181] Many studies disputed the validity of the fossils.[182][183] It was found that most of the organic matter in the meteorite was of terrestrial origin.[184]

Lakes

A variety of lake basins have been discovered on Mars.[185] Some are comparable in size to the largest lakes on Earth, such as the Caspian Sea, Black Sea, and Lake Baikal. Lakes that are 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, which was explored by the Mars Exploration Rover Spirit. Another is near Parana Valles and Loire Vallis.[186] Some lakes are believed to have formed by precipitation, while others were formed from groundwater.[16][17] Lakes are believed to have existed in the Argyre basin,[18][19] the Hellas basin,[20][21] and maybe in Valles Marineris.[20][22][187][188]

Research, published in January 2010, suggests that Mars had lakes, each around 20 km wide, 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 new 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.[189][190] These lakes would be another place to look for evidence of present or past life.

Lake deltas

Researchers have found a number of examples of deltas that formed in Martian lakes.[191] Finding deltas is a major sign that Mars once had a lot of 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 and several are pictured below.[16]

Mars Ocean Hypothesis

The Mars Ocean Hypothesis states that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet’s geologic history.[192] [193] This primordial ocean, dubbed Oceanus Borealis,[194] 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 require a warmer climate and thicker atmosphere to allow liquid water to remain at the surface.[195]

Observational evidence

There are several physical features in the present geography of Mars that suggest the existence of an ocean. Networks of valleys that merge into larger channels imply erosion by a liquid agent, and resemble ancient riverbeds on Earth. Enormous channels, 25 km wide and several hundred meters deep, appear to direct flow from underground aquifers in the Southern uplands into the Northern plains.[195]

Research published in the Journal of Geophysical Research – Planets, shows a much higher density of flow paths than formerly believed (more than twice as many). Regions on Mars with the most valleys are comparable to what is found on our Earth. In the research, the team developed a computer program to identify valleys by searching for U-shaped structures in topographical data.[196] The large amount of valley networks strongly supports rain on the planet in the past. The global pattern of the martian valleys could be explained with a big northern ocean. A large ocean in the northern hemisphere would explain why there is a southern limit to valley networks; the southernmost regions of Mars, located farthest from the water reservoir, would get little rainfall and would not develop valleys. In a similar fashion the lack of rainfall would explain why Martian valleys become shallower as you go from north to south.[197]

Much of the northern hemisphere of Mars is located at a significantly lower elevation than the rest of the planet (the Martian dichotomy), and is unusually flat. Along the margins of this region are physical features indicative of ancient shorelines.[194] Sea level must follow a line of constant gravitational potential. After adjustment for polar wander caused by mass redistributions from volcanism, the Martian paleo-shorelines meet this criterion.[198] The Mars Orbiter Laser Altimeter (MOLA), which accurately determined the altitude of all parts of Mars, found that the watershed for an ocean on Mars covers three-quarters of the planet.[199]

A study, published in Nature, in June 2010 concluded that an Ocean covered 36% of Mars. The study was based on dozens of deltas that were at the same elevation.[191][200]

Although phyllosilicates, i.e. clay, have been observed on Mars, the northern lowlands are known to show only few such abundances in early geological layers, a fact that has so far contradicted the theory of a northern ocean on Mars. A 2011 numerical simulation study found though that an ocean of water on the northern hemisphere would have had a temperature near the freezing point "which would have hindered the formation of phyllosilicate minerals in the ocean basin" and would therefore explain the relative absence of clay minerals on the northern hemisphere.[201]

Theoretical issues

The existence of liquid water on the surface of Mars requires both a warmer and thicker atmosphere. Atmospheric pressure on the present day Martian surface only exceeds that of the triple point of water (6.11 hPa) in the lowest elevations; at higher elevations water can exist only in solid or vapor form. Annual mean temperatures at the surface are currently less than 210 K, significantly less than what is needed to sustain liquid water. However, early in its history Mars may have had conditions more conducive to retaining liquid water at the surface.

Calculations of the volume of one of the supposed oceans yielded a number that would mean that Mars was covered with as much water as the Earth.

The water that was in this ocean may have escaped into space, been deposited in the ice caps, or have been trapped in the soil.[7]

Alternative ideas

The existence of a primordial Martian ocean remains controversial among scientists.[202] The Mars Reconnaissance Orbiter's High Resolution Imaging Science Experiment has discovered large boulders on the site of the ancient seabed, which should contain only fine sediment.[203] The interpretations of some features as ancient shorelines has been challenged. Some have been shown to be of volcanic origin.[7]

Possibility of Mars having enough water to support life

Life is generally understood to require liquid water. Some evidence suggests that Mars had enough water to form lakes and to carve huge river valleys.[204][205] Vast quantities of water have been discovered frozen beneath much of the Martian surface. Nevertheless, many significant issues remain.

Benton Clark III, a member of the Mars Exploration Rover (MER) team, surmises that Martian organisms could be adapted to a sort of suspended animation for millions of years.[220] Indeed, some organisms can endure extreme environments for a time. Measurements performed on Earth under 50 meters of permafrost, showed that half of the microorganisms would accumulate enough radiation from radioactive decay in rocks to die in 10 million years, but if organisms come back to life every few million years they could repair themselves and reset any damaged systems, especially DNA.[221][222] Other scientists are in agreement.

The discovery of organisms living in extreme conditions on Earth has brought renewed hope that life exists, or once existed on Mars.[223][224][225] Colonies of microbes have been found beneath almost 3 kilometers of glaciers in the Canadian Arctic and in Antarctica.[226] Could microbes live under the ice caps of Mars? In the 1980s, it was thought that microorganisms might live up to a depth of a few meters under ground.[227] Today, we know that a wide variety of organisms grow to a depth of over a mile. Some live on gases like methane, hydrogen, and hydrogen sulfide that originate from volcanic activity. Mars has had widespread volcanic activity.[228] It is entirely possible that life exists near volcanoes or underground reservoirs of hot magma.[229] Some organisms live inside of basalt (the most common rock on Mars) and produce methane. Methane has been tracked on Mars.[230] Some[who?] believe there must be some (possibly biological) mechanism that is producing methane since it will not last long in the present atmosphere of Mars.[231] Other organisms eat sulfur compounds; the same chemicals that have been found in many regions of Mars. Scientists have suggested that whole communities of organisms could thrive near areas heated by volcanic activity. Studies have shown that certain forms of life have adapted to extremely high temperatures (80° to 110 °C).[232] With all the volcanic activity on Mars, one would suppose that certain places have not yet cooled down.[233] An underground magma chamber might melt ice, then circulate water through the ground. Remains of hot springs like the ones in Yellowstone National Park have actually been spotted by the Mars Reconnaissance Orbiter.[234][235] Minerals accociated with hot springs, such as opal and silica have been studied on the ground by Spirit Rover and mapped from orbit by the Mars Reconnaissance Orbiter.[210] Some volcanoes, like Olympus Mons, seem relatively young to the eyes of a geologist. However, no warm areas have ever been found on the surface. The Mars Global Surveyor scanned most of the surface in infrared with its TES instrument. The Mars Odyssey's THEMIS, also imaged the surface in wavelengths that measure temperature.


The possibility of liquid water on Mars has been examined. Although water would quickly boil or evaporate away, lake-sized bodies of water would quickly be covered with an ice layer which would greatly reduce evaporation. With a cover of dust and other debris, water under ice might last for some time and could even flow to significant distances as ice-covered rivers.[236] Lake Vostok in Antarctica may have implications for liquid water still being on Mars because if the lake existed before the perennial glaciation began, is likely that the lake did not freeze all the way to the bottom. Accordingly if water existed before the polar ice caps on Mars, it is likely that there is still liquid water below the ice caps.[237] Large quantities of water could be released, even today, by an asteroid impact. It has been suggested that life has survived over millions of years by periodic impacts which melted ice and allowed organisms to come out of dormancy and live for a few thousands of years.[238][239] But if impacts brought the water, maybe liquid water did not exist on the surface very long. Large river valleys could have been made in short periods of time (maybe just days) when impacts caused water to flow as a giant flood.[240] We suppose that Mars had great amounts of water because of the existence of so many large river valleys.[10][11] Maybe, valleys did not take thousands to millions of years to form as on the Earth.[241] It is accepted that a vast network of channels, resembling many Martian channels, were formed in a very short time period in eastern Washington State when floods were caused by a breakout of an ice-dammed lake. So, perhaps not that much water was involved and maybe it did not last long enough for life to develop.

Studies have shown that various salts present in the Martian soil could act as a kind of antifreeze—keeping water liquid well below its normal freezing point.[88][242] Some calculations suggest that tiny amounts of liquid water may be present for short periods of time (hours) in some locations.[243][244] Some researchers have calculated that when taking into consideration insolation and pressure factors that liquid water could exist in some areas for about 10% of the Martian year;[245] others estimate that water could be a liquid for only 2% of the year.[246] Either way, that may be enough liquid water to support some forms of hardy organisms. It may not take much liquid water for life; organisms have been found on Earth living on extremely thin layers of unfrozen water in below-freezing locations.[247] Research described in December 2009, showed that liquid water could form in the daytime inside of snow on Mars. As light heats ice, it may be warming up dust grains located inside. These grains would then store heat and form water by melting some of the ice. The process has been already been observed in Antarctica. Enough water may be produced for physical, chemical, and biological processes.[248][249]

Valleys and channels

History of water on Mars.jpeg

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.[64][65][250]

The images below, some of the best from the Viking Orbiters, are mosaics of many small, high resolution images. Click on the images for more detail. Some of the pictures are labeled with place names.

The high resolution Mars Orbiter Camera on the Mars Global Surveyor has taken pictures which give much more detail about the history of liquid water on the surface of Mars. 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 weathering processes have denuded these, indicating the river valleys are old features. Another theory about the formation of the ancient river valleys is that rather than floods, they were created by the slow seeping out of groundwater. This observation is supported by the sudden ending of the river networks in theatre shaped heads, rather than tapering ones. Additionally, valleys are often discontinuous, small sections of uneroded land separating the parts of the river.[251]

On the other hand, evidence in favor of heavy or even catastrophic flooding is found in the giant ripples in the Athabasca Vallis.[252]

Research, published in the Journal of Geophysical Research 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 by scientists.[253]

Many Mars researchers now agree that the Martian water worn features can be divided into two distinct classes: 1. dendritic (branched), terrestrial-scale, widely distributed, Noachian-age "valley networks" and 2. exceptionally large, long, single-thread, isolated, uncommon, Hesperian-age "outflow channels". Consensus seems to be emerging that the latter formed in single, catastrophic ruptures of subsurface water reservoirs, possibly sealed by ice, discharging colossal quantities of water across an otherwise ultra-arid Mars surface.[254] The former, however, probably indicate prolonged "wet" (though still arid by terrestrial standards) conditions on Noachian-era Mars, with an active ongoing hydrological cycle.[255]

Higher resolution observations from spacecraft like Mars Global Surveyor also revealed at least a few hundred features along crater and canyon walls that appear similar to terrestrial seepage gullies.[256] The gullies tended to be Equator facing and in the highlands of the southern hemisphere, and all greater than 30° north or south latitude.[257] The researchers found no partially degraded (i.e. weathered) gullies and no superimposed impact craters, indicating that these are very young features.

Liquid water

Mosaic shows some spherules partly embedded.
Photo of Microscopic rock forms indicating past signs of water, taken by Opportunity

Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods.[258][259] Recently, the discovery of gully deposits that were not seen ten years ago provided evidence to support the popular belief that liquid water flowed on the surface in the recent past.[260][261] There is some disagreement in the scientific community as to whether or not the new gully deposits were formed from liquid water. A paper, published in the January 2010 issue of Icarus, concluded that the observed deposits were probably dry flows that were started by a rockfall in steep regions.[85][262]

Among the findings from the Opportunity rover is the presence of hematite on Mars in the form of small spheres on the Meridiani Planum. The spheres are only a few millimetres in diameter and are believed to have formed as rock deposits under watery conditions billions of years ago. Other minerals have also been found containing forms of sulfur, iron, or bromine such as jarosite. This and other evidence led a group of 50 scientists to conclude in the December 9, 2004 edition of the journal Science that "Liquid water was once intermittently present at the Martian surface at Meridiani, and at times it saturated the subsurface. Because liquid water is a key prerequisite for life, we infer conditions at Meridiani may have been habitable for some period of time in Martian history." Later studies suggested that this liquid water was actually acid because of the types of minerals found at the location.[263][264] On the opposite side of the planet, the mineral goethite, which (unlike hematite) forms only in the presence of water, along with other evidence of water, has also been found by the Spirit rover in the "Columbia Hills".

On July 31, 2008, NASA announced that the Phoenix lander confirmed the presence of water ice on Mars,[265] as predicted on 2002 by the Mars Odyssey orbiter.

Studies have shown that various salts present in the Martian soil could act as a kind of antifreeze—keeping water liquid at temperatures far below its normal freezing point.[266][267] Some calculations suggest that tiny amounts of liquid water may be present for short periods of time (hours) in some locations.[268] Some researchers have calculated that when taking into consideration insolation and pressure factors that liquid water could exist in some areas for about 10% of the Martian year;[269] others estimate that water could be a liquid for only 2% of the year.[270] Either way, that may be enough liquid water to support some forms of hardy organisms. It may not take much liquid water for life—organisms have been found on Earth living on extremely thin layers of unfrozen water in below-freezing locations.[271] Research in December 2009 showed that liquid water could form in the daytime inside of snow on Mars. As light heats ice, it may be warming up dust grains located inside. These grains would then store heat and form water by melting some of the ice. This process has already been observed in Antarctica. Enough water may be produced for physical, chemical, and biological processes.[272][273]

Polar ice caps

The Mars Global Surveyor acquired this image of the Martian north polar ice cap in early northern summer.
North polar layered deposits can be seen on the trough wall imaged here by HiRISE.

Both the northern polar cap (Planum Boreum) and the southern polar cap (Planum Australe) are believed 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 an average of 3 kilometres (1.9 mi) thick slab of CO2 ice[167] with varying contents of frozen water. Depending on its latitude, the polar cap can be a mixture of 85% CO2 ice and 15% water ice.[168] 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.[168] The third part encompasses the vast permafrost fields that stretch for tens of kilometres away from the scarps.[168][274] 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.[275]

Research, published in January 2010 using HiRISE images, says that understanding the layers is more complicated than was formerly believed. 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 affect 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). In addition, many times what one is seeing is not a real layer, but a fresh covering of frost. 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 did see more detail within layers.[276]

Ice patches

On July 28, 2005, the European Space Agency announced the existence of a crater partially filled with frozen water,[156] which some then interpreted as an "ice lake".[157] Images of the crater, taken by the High Resolution Stereo Camera on board the European Space Agency's Mars Express spacecraft, 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.

Equatorial frozen sea

Surface features consistent with 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 some 2 to 10 million years ago.[159]

Ancient coastline

A striking feature of the topography of Mars is the flat plains of the northern hemisphere. With the increasing amounts of data returning from the current set of orbiting probes, what seems to be an ancient shoreline several thousands of kilometres long has been discovered. Actually, 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. Some researchers do not agree that these formations are real shorelines.[277][278] One major problem with the conjectured 2 Ga old shoreline is that it is not flat — i.e. does not follow a line of constant gravitational potential. However, a 2007 Nature 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.[279] The Mars Ocean Hypothesis conjectures that the Vastitas Borealis basin was the site of a primordial ocean of liquid water 3.8 billion years ago.[194]

Glaciers and ice ages

Perspective view of a 5-km-wide, glacial-like lobe deposit sloping up into a box canyon along the crustal dichotomy boundary on Mars. The surface has moraines, deposits of rocks that show how the glacier advanced.

Many large areas of Mars have been shaped by glaciers. Much of the area in high latitudes, especially the Ismenius Lacus quadrangle, are believed to still contain enormous amounts of water ice.[280][281] Recent evidence has led many planetary scientists to believe that water ice still exists as glaciers with thin coverings of insulating rock.[98][99][100][101][102] 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.[282] Glaciers are believed to be associated with Fretted terrain, many volcanoes, and even some craters. Researchers have described glacial deposits on Hecates Tholus,[283] Arsia Mons,[191] Pavonis Mons,[284] and Olympus Mons.[285]

Ridges of debris on the surface of the glaciers indicate the direction of ice movement. The surface of some glaciers have rough textures 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.[286] Glaciers are not pure ice; they contain dirt and rocks. At times, they dump their loads of material 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, a more or less round hole remains.[287] On Earth we call these features kettles or kettle holes. Mendon Ponds Park in upstate NY has preserved several of these kettles. The picture from HiRISE below shows possible kettles in Moreux Crater.

Pictures below show various features that appear to be connected with the existence of glaciers.

Many mid-latitude craters contain straight and/or curved ridges of material that resemble glacial moraines on the Earth. 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 sublimates. As a result, the rock debris is just dropped, and melt water is not produced so the remains of these glaciers do not appear the same as on the Earth. Various names have been applied to these ridged features. Depending on the author, they may be called arcuate ridges,[288] viscous flow features,[289] Martian flow features, or moraine-like ridges. Many, but not all, seem to be associated with gullies on the walls of craters and mantling material.[290]

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, similar to some glaciers on the Earth. Lineated floor deposits may be related to Lobate Debris Aprons, which have been proven to contain large amounts of ice by orbiting radar.[101][102][103]

For many years, researchers believed that on Mars features called Lobate Debris Aprons looked like glacial flows. It was thought that ice existed under a layer of insulating rocks.[98][99][100] With new instrument readings, it has been confirmed that Lobate Debris Aprons contain almost pure ice that is covered with a layer of rocks.[101][102]

Ice ages on Mars are far different than the ones that our Earth experiences. Ice ages on Mars, that is when ice accumulates, occur during warmer periods.[291] During a Martian ice age, the poles get warmer. Water ice 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 generously with dust. The atmosphere of Mars contains a great deal of fine dust particles. 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.[136] 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 that is thought to be a mixture of ice and dust.[292][293][294] This ice-rich mantle, a few yards thick, smoothes the land. But in places it displays a bumpy texture, resembling the surface of a basketball. Because there are few craters on this mantle, the mantle is relatively young. It is believed that this mantle was put in place during a relatively recent ice age. The mantle covers areas to the equivalent latitude of Saudi Arabia and the southern United States.

The images below, all taken with HiRISE show a variety of views of this smooth mantle.

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. 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.[295] Its poles get much more direct sunlight 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.[296] A 2008 study provided evidence for multiple glacial phases during Late Amazonian glaciation at the dichotomy boundary on Mars.[297]

Glaciers on volcanoes

Using new MGS and Odyssey data, 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). The ice could have been deposited when the tilt of Mars changed the climate, thereby causing more moisture to be present in the atmosphere. Studies suggest the glaciation happened in the Late Amazonian period, the latest period in Mars history. Multiple stages of glaciations probably occurred.[298] The ice present today represents one more resource for the possible future colonization of the planet. Researchers have described glacial deposits on Hecates Tholus,[283] Arisia Mons,[191] Pavonis Mons,[284] and Olympus Mons.[285]

See also

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