Water on Mars is an expression for all the water present on the planet Mars. In comparison to Earth, water is much less abundant on Mars in all three states of matter. Most of the water is locked in the cryosphere (permafrost and polar caps), so there are no bodies of liquid water, which could create a hydrosphere. Only a small amount of water vapour is in the atmosphere.^[1]

Current conditions on the planet surface do not support the long-term existence of liquid water. The average 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] creating large areas similar to Earth's oceans. However, the question remains as to where the water has gone.^[3]

There are a number^[4] of direct and indirect proofs of water's presence either on or under the surface, e.g. stream beds, polar caps, spectroscopic measurement, eroded craters or minerals directly connected to the existence of liquid water (such as Goethite). In an article in the Journal of Geophysical Research, scientists studied Lake Vostok in Antarctica and discovered that it may have implications for liquid water still being on Mars. Through their research, scientists came to the conclusion that if Lake Vostok existed before the perennial glaciation began, that it is likely that the lake did not freeze all the way to the bottom. Due to this hypothesis, scientists say that if water had existed before the polar ice caps on Mars, it is likely that there is still liquid water below the ice caps that may even contain evidence of life.^[5]

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 2,500 miles (4,020 km) long), evidence of water erosion and deposition, weather fronts, fogs, and more.^[6] 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.

• 
  Meander in Scamander Vallis, as seen by Mariner 9. Such images implied that large amounts of water once flowed on the surface of Mars.
• 
  Warrego Valles, as seen by Mariner 9. This image suggests that rain/snow was necessary to form this kind of branched network of channels.

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. Large areas in the southern hemisphere contained branched stream networks, suggesting that rain once fell. The flanks of some volcanoes are believed to have been exposed to rainfall because they resemble those caused on Hawaiian volcanoes. 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 flowed across the surface. 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. ^[7][8][9] Regions, called "Chaotic Terrain,"seemed to have quickly lost great volumes of water, causing large channels to be formed. The amount of water involved was almost unthinkable--estimates for some channel flows run to ten thousand times the flow of the Mississippi River. ^[10]Undergound 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. Click on the images for more detail. Some of the pictures are labeled with place names.

• 
  Streamlined Islands seen by Viking showed that large floods occured on Mars. Image is located in Lunae Palus quadrangle.
• 
  Tear-drop shaped islands caused by flood waters from Maja Vallis, as seen by Viking Orbiter. Image is located in Oxia Palus quadrangle. The islands are formed in the ejecta of Lod Crater, Bok Crater, and Gold Crater.
• 
  Scour Patterns, located in Lunae Palus quadrangle, were produced by flowing water from Maja Vallis, which lies just to the left of this mosaic. Detail of flow around Dromore Crater is shown on the next image.
• 
  Great amounts of water were required to carry out the erosion shown in this Viking image. Image is located in Lunae Palus quadrangle. The erosion shaped the ejecta around Dromore Crater.
• 
  Waters from Vedra Vallis, Maumee Vallis, and Maja Valles went from Lunae Planum on the left, to Chryse Planitia on the right. Image is located in Lunae Palus quadrangle and was taken by Viking Orbiter.
• 
  The ejecta from Arandas Crater acts like mud. It moves around small craters (indicated by arrows), instead of just falling down on them. Craters like this suggest that large amounts of frozen water were melted when the impact crater was produced. Image is located in Mare Acidalium quadrangle and was taken by Viking Orbiter.
• 
  This view of the flank of Alba Patera shows several chnnels/troughs. Some channels are associated with lava flows; others are probably caused by running water. A large trough or graben turns into a line of collapse pits. Image is located in Arcadia quadrangle and was taken by Viking Orbiter.
• 
  Branched channels in Thaumasia quadrangle, as seen by Viking Orbiter. Networks of channels like this are strong evidence for rain on Mars in the past.
• 
  The branched channels seen by Viking from orbit strongly suggested that it rained on Mars in the past. Image is located in Margaritifer Sinus quadrangle.
• 
  Ravi Vallis, as seen by Viking Orbiter. Ravi Vallis was probably formed when catastrophic floods came out of the ground to the right (chaotic terrain). Image located in Margaritifer Sinus quadrangle.

The Viking landers carried out several experiments which strongly suggested the presence of water in the present and in the past. All samples heated in the gas chromatograph-mass spectrometer (GSMS0] gave off water. However, the way the samples were handled prohibited an exact measurement of the amount of water. But, it was around 1%.^[11] 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.^[12] Using results from the chemical measurements, mineral models suggest that the soil could be a mixture of about 90% 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.^[13] Sulfate contains chemically bound water, hence its presence suggests water was around in the past. Viking 2 found similar group of minerals. However, Viking 2 was much farther north and pictures taken in the winter showed frost.

• 
  Frost on Mars
• 
  Photo of the Viking 2 lander taken by the Mars Reconnaissance Orbiter in December 2006
• 
  Frost at the landing site (image slightly enhanced to bring out frost detail)

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.^[14] Olivine has been found in the SNC (shergottite, nakhlite, and chassigny) meteorites that are generally accepted to have come from Mars.^[15] Later studies have found that olivine-rich rocks to cover over 113,000 square kilometers of the Martian surface. That is 11 times larger than the five volcanoes on the Big Island of Hawaii.^[16]

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.^[17][18]

Hundreds of gullies were discovered that were formed from liquid water, possible in recent times. These gullies occur on steep slopes and mostly in certain bands of latitude.^{[19][20][21][22][23]} Below are some examples of gullies that were photographed by Mars Global Surveyor.

• 
  Group of gullies on north wall of crater that lies west of the crater Newton (41.3047 degrees south latitude, 192.89 east longitide). Image is located in the Phaethontis quadrangle.
• 
  Gullies in a crater in Eridania quadrangle, north of the large crater Kepler. Also, features that may be remains of old glaciers are present. One, to the right, has the shape of a tongue.
• 
  Gullies on one wall of Kaiser Crater. Gullies usually are found in only one wall of a crater. Location is Noachis quadrangle.
• 
  Full color image of gullies on wall of Gorgonum Chaos. Image is located in the Phaethontis quadrangle.

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.^[19]

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.^[24] 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,^[25] or even the growth of organisms.^[26][27] 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.^[28]

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

• 
  Layers in Tikonravev Crater in Arabia. Layers may form from volcanoes, the wind, or by deposition under water. The craters on the left are pedestal craters. Dark Slope Streaks are seen to originate from certain layers (you may need to click on image to see the streaks).
• 
  Tikonravev Crater floor in Arabia quadrangle. Click on image to see dark slope streaks and layers.
• 
  Dark streaks in Diacria quadrangle.
• 
  Dark Streaks in Arabia quadrangle. Crater is about the size of Earth's Meteor Crater in Arizona.

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 is too cold for liquid water to exist. However. water could exist as a liquid if it were mixed with various salts.

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.^[29]

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.^[30]

There was evidence of clouds and maybe fog.^[30]

In July 2003, at a conference in California, it was announced that the Gramma Ray Spectrometer (GRS) onboard 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.^[31] In both hemispheres, from 55 degrees latitude to the poles, Mars has a high density of ice just under the surface; one pound of soil contains a half of a pound of water ice. But, close to the equator, there is only 2 to 10% of water in the soil.^[32] Scientists believe that much of this water is locked up in the chemical structure of minerals, such asclay and sulfates. Previous studies with infrared spectroscopes have provided evidence of small amounts of chemically or physically bound water. ^[33][34] The Viking landers detected low levels of chemically bound water in the Martian soil. 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.^[32][35] Analysis of the data suggest that the southern hemisphere may have a layered structure.^[36] 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.^[31] There may be much more water because the instruments aboard the Mars Odyssey are only able to study the top meter or so of soil. Still, the amount of water discovered by this mission is huge. Estimates of the total amount of water that may be buried in the soil range up to a global layer 0.5 to 1.5 km deep.^[37]

The Phoenix lander confirmed the initial findings of the Mars Odyssey. 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.^[38]

Thousands of images returned from Odyssey support the idea that Mars once had great amounts of water flowing across its surface. Some pictures show stream patterns. Others show layers that may have formed under lakes. Deltas have been identified.^[39]

For many years researchers believed that glaciers existed under a layer of insulating rocks.^{[40][41][42][43][44]} Lineated deposits are one example of these probable 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. ^[43][44][45]

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.^[46]

• 
  Drainage features in Reull Vallis. Click on image to see relationship of Reull Vallis to other features. Location is Hellas quadrangle.
• Reull Vallis with lineated floor deposits. Click on image to see relationship to other features. Floor deposits are believed to be formed from ice movement. Location is Hellas quadrangle.
  Reull Vallis with lineated floor deposits. Click on image to see relationship to other features. Floor deposits are believed to be formed from ice movement. Location is Hellas quadrangle.
• 
  Auquakuh Valles. At one time a dark layer covered the whole area, now only a few pieces remain as buttes. Click on image to see layers. Layers may have formed from deposition on the bottom of lakes.
• 
  Huo Hsing Vallis in Syrtis Major quadrangle. Straight ridges may be dikes in which liquid rock once flowed.
• 
  Nirgal Vallis that runs in two quadrangles has features looking like those caused by sapping. Nirgal Vallis is one of many ancient river valleys studied by THEMIS.
• 
  The long channel Nirgal Vallis is showed where it connects to Uzboi Vallis. The crater Luki is 21 km in diameter.
• 
  Nirgal Vallis.
• 
  Nirgal Vallis Close-up.
• 
  Channels near Warrego Valles. These branched channels are strong evidence for flowing water on Mars, perhaps during a much warmer period.
• 
  Semeykin Crater Drainage. Click on image to see details of beautiful drainage system. Location is Ismenius Lacus quadrangle.
• 
  Delta in Ismenius Lacus quadrangle.
• 
  Delta in Lunae Palus quadrangle.
• 
  Delta in Margaritifer Sinus quadrangle.
• 
  Kasei Vallis.
• 
  Athabasca Valles showing source of its water, Cerberus Fossae. Note streamined islands which show direction of flow to south. Athabasca Valles is in the Elysium quadrangle.
• 
  Close-up of Padus Vallis. Padus Vallis is in the Memnonia quadrangle.
• 
  Channels West of Echus Chasma. The fine pattern of branching stream beds were probably fromed from water moving across the surface. Image is in Coprates quadrangle.
• 
  Dendritic channels on mesa of Echus Chasma. Image is 20 miles wide. Image is in Coprates quadrangle.
• 
  Branching channels on floor of Melas Chasma. Image is in Coprates quadrangle.

Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust. 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. The low density of craters on the mantle means it is relatively young.

Changes in Mars's orbit and tilt cause significant changes in the distribution of water ice. 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 returns to the atmosphere, it leaves behind dust, which insulates the remaining ice.^[47]

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 below suggests that groundwater sapping also contributed water.^[48]

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.

• 
  Blocks in Aram showing possible source of water. The ground collapsed when large amounts of water were released. The large blocks probably still contain some water ice. Location is Oxia Palus quadrangle.
• 
  Huge canyons in Aureum Chaos. Click on image to see the gullies which may have formed from recent flows of water. Gullies are rare at this latitude. Location is Margaritifer Sinus quadrangle.

The Phoenix lander confirmed the existence of large amounts of water ice in the northern regions of Mars. This finding was predicted by theory and was measured from orbit by the Mars Odyssey instruments.

On June 19, 2008, NASA announced that die-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.^[49][50][51]

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.^[52] Liquid water cannot exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods.^[53][54]

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.^[55]

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'.^[56] Due to the lack of consensus within the Phoenix science project, the issue had not been raised in any NASA news conferences.^[56] 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.^[57][58]

• 
  Die-sized clumps of bright material in the enlarged "Dodo-Goldilocks" trench vanished over the course of four days, implying that they were composed of ice which sublimated following exposure.^[49]
• 
  Color versions of the photos showing ice sublimation, with the lower left corner of the trench enlarged in the insets in the upper right of the images.

|}

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.

• 
  Comparison between polygons photographed by Phoenix on Mars...
• 
   ... and as photographed (in false color) from Mars orbit...
• 
   ... with patterned ground on Devon Island in the Canadian Arctic, on Earth.

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 esistance of past water.^[59] 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.^[60]

Snow was observed to fall from cirrus clouds. The clouds formed at a level in the atmosphere that was around -65 degrees 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 degrees 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.^[61] 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.^[62]

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.^[63] 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.^[64] Another paper claims that the previously detected snow could lead to a buildup of water ice.

The Mars Rovers Spirit and Opportunity found a great deal of evidence for past water on Mars. Although designed to last only three months, both are still operating after more than five years. So, discoveries continue to be recorded.

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, Ph.D., 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.^[65]

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. 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. ^[66][67]

Opportunity rover was directed to a site that had displayed large amounts of hematite from orbit. Hematite often forms from water. Sure enough, when Opportunity landed layered rocks and round hematite marbles 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).^[68] 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.

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.

Some places on Mars show Inverted Relief. In these locations, a stream bed appears as a raised feature, instead of a valley. 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.^[69]

• 
  Inverted Stream Channels in Antoniadi Crater. Location is Syrtis Major quadrangle.
• 
  Inverted Channels near Juventae Chasma. Channels were once regular stream channels. Scale bar is 500 meters long. Location is Coprates quadrangle.
• 
  Inverted Channel in Miyamoto Crater. Image is located in Margaritifer Sinus quadrangle. The scale bar is 500 meters long.
• 
  Inverted Channel with many branches in Syrtis Major quadrangle.

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 found by the Mars Rovers on the surface. Places with chloride minerals may have once held various life forms. Furthermore, such areas should preserve traces of ancient life.^[70]

Rocks on Mars have been found to frequently occur as layers, called strata, in many different places. Columbus Crater is one of many craters that contain layers. Rock can form layers in a variety of ways. Volcanoes, wind, or water can produce layers.^[71] 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 hydraded minerals like sulfates. The Mars Rover Opportunity examined such layers close-up with several instruments. Some layers are probably made up of fine particles because they seem to break up into fine dust. In contrast, other layers break up into large boulders so they are probably much harder. Basalt, a volcanic rock, is thought to form layers composed of boulders. Basalt has been identified all over Mars. Instruments on orbiting spacecraft have detected clay (also called phyllosilicates) in some layers.^[72][73] Scientists are excited about finding hydrated minerals such as sulfates and clays on Mars because they are usually formed in the presence of water.^[74] Places that contain clays and/or other hydrated minerals would be good places to look for evidence of life.^[75]

Below are a few of the many examples of layers that have been studied with HiRISE.

• 
  Becquerel Crater layers. Click on image to see fault. Location is Oxia Palus quadrangle.
• 
  Light colored layers in Eos Chaos. Location is Coprates quadrangle.
• 
  Columbus Crater Layers. This false-color image is about 800 feet across. Some of the layers contain hydrated minerals. Location is Memnonia quadrangle.
• 
  Layers in west slope of Asimov Crater. Location is Noachis quadrangle.
• 
  Close-up of layers in west slope of Asimov Crater. Shadows show the overhang. Some of the layers are much more resistant to erosion, so they stick out. Location is Noachis quadrangle.
• 
  Ophir Chasma Wall. Location is Coprates quadrangle.
• 
  Tithonium Chasma. Location is Coprates quadrangle.
• 
  Layers west of Juventae Chasma. Scale bar is 500 meters long. Location is Coprates quadrangle.

Much of the surface of Mars is covered by a thick smooth mantle that is thought to be a mixture of ice and dust. 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.

• 
  Niger Vallis with features typical of this latitude. Chevon pattern results from movement of ice-rich material. Click on image to see chevron pattern and mantle. Location is Hellas quadrangle.
• 
  Ptolemaeus Crater Rim. Click on image to see excellent view of mantle deposit. Location is Phaethontis quadrangle.
• 
  Atlantis Chaos. Click on image to see mantle covering and possible gullies. The two images are different parts of the original image. They have different scales. Location is Phaethontis quadrangle.
• 
  Dissected Mantle with layers. Location is Noachis quadrangle.

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. 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.^[47]

HiRISE has carried out many observations of gullies that are assumed to have been caused by recent flows of liquid water. These were first discovered by the Mars Global Surveyor. Below are some of the many hundreds of gullies that have been studied with HiRISE. Many gullies are imaged over and over to see if any changes occur.

• 
  Crater wall inside Mariner Crater showing a large group of gullies.
• 
  Charitum Montes Gullies. Image located in Argyre quadrangle.
• 
  Jezza Crater,as seen by HiRISE. North wall (at top) has gullies. Dark lines are dust devil tracks. Scale bar is 500 meters long. Image located in Argyre quadrangle.
• 
  Lohse Crater Gullies on Central Peak. Image located in Argyre quadrangle.
• 
  Gullies in Green Crater.
• 
  Close-up of gullies in Green Crater. Image located in Argyre quadrangle.
• 
  Scalloped Terrain at Peneus Patera. Scalloped terrain is quite common in some areas of Mars.
• 
  Maunder Crater. The overhang is part of the degraded south (toward bottom) wall of crater. The scale bar is 500 meters long.
• 
  Asimov Crater. Bottom of picture shows southeastern wall of crater. Top of picture is edge of mound that fills most of the crater.
• 
  Gullies on mound in Asimov Crater. Location is Noachis quadrangle.
• 
  Gullies in Phaethontis quadrangle. Notice how channels curve around obstacles.
• 
  Gullies with branches in Phaethontis quadrangle.

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.^[76] Recently, research with the Shallow Radar on the Mars Reconnaissance Orbiter has provided strong evidence that the LDAs in Hellas Planitia 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 balk of the formation (between the two reflections).^[77] Based on the experiemnts of the Phoenix lander and the studies of the Mars Odyssey from orbit, frozen water is now know to exist a just under the surface of Mars in the far north and south (high latitudes). The discovery of water ice in LDA's 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 LDA's over other sources of Martian water is that they can easily detected and mapped from orbit. Lobate Debris Aprons are shown below from the Phlegra Montes which are at a latitude of 38.2 degrees north. The Phoenix lander set down at about 68 degrees north latitude, so the discovery of water ice in LDA's greatly expands the range of easily available on Mars.^[78] It is far easier to land a spaceship near the equator of Mars, so the closer water is available to the equator the better it will be for future colonists.

Below are examples of Lobate Debris Aprons that were studied with HiRISE.

• 
  Lobate Debris Apron in Phlegra Montes, Cebrenia quadrangle. The debris apron is probably mostly ice with a thin covering of rock debris, so it could be a source of water for future Martian colonists. Scale bar is 500 meters long.
• 
  Close-up of surface of a Lobate Debris Apron. Note the lines that are common in rock glaciers on the Earth. Image located in Hellas quadrangle.
• 
  View of Lobate Debris Apron along a slope. Image located in Arcadia quadrangle.
• 
  Place where a lobate debris apron begins. Note stripes which indicates movement. Image located in Ismenius Lacus quadrangle.

Research, reported in the journal Science in September 2009,^[79] demonstrated that some new craters on Mars show exposed, pure, water ice. After a time, the ice disappears, evaporating into the atomsphere. The ice is only a few feet deep. The ice was confirmed with the Compact Imaging Spectrometer (CRISM)] onboard the Mars Reconnaissance Orbiter (MRO). The ice was found in a total of 5 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.^[80][81][82] 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.

Studies of impact crater densities on the Martian surface allow us to identify three broad epochs in the planet's geological timescale, as older surfaces have many craters and younger ones have few.^[83] The epochs were named after places on Mars that belong to those time periods. The precise timing of these periods is not known because there are several competing models describing the rate of meteor fall on Mars, so the dates given here are approximate. From oldest to youngest, the time periods are:

• Noachian epoch (named after Noachis Terra): Formation of the oldest extant surfaces of Mars between 4.6 and 3.5 billion years ago. Noachian age surfaces are scarred by many large impact craters. The Tharsis bulge is thought to have formed during this period, with extensive flooding by liquid water late in the epoch.
• Hesperian epoch (named after Hesperia Planum): 3.5 to 1.8 Ga BP. The Hesperian epoch is marked by the formation of extensive lava plains. Formation of Olympus Mons likely began during this epoch^[84].
• Amazonian epoch (named after Amazonis Planitia): 1.8 Ga BP to present. Amazonian regions have few meteorite impact craters but are otherwise quite varied. Lava flows on Mars continued during this period.

The studying of craters is based upon the assumption that crater-forming impactors have hit the planet all throughout history at regular intervals, and there is no way to exactly date an area just based upon the number of impacts, only to guess that areas with more impacts must be older than areas with fewer impacts. For example this logic breaks down if a large number of asteroids had hit at once, or if there were long periods where few asteroids hit.

Based on recent observations made by the OMEGA Visible and Infrared Mineralogical Mapping Spectrometer on board the Mars Express orbiter, the principal investigator of the OMEGA spectrometer has proposed an alternative timeline based upon the correlation between the mineralogy and geology of the planet. This proposed timeline divides the history of the planet into 3 epochs; the Phyllocian, Theiikian and Siderikan.^[85][86]

• Phyllocian (named after the clay-rich phyllosilicate minerals that characterize the epoch) lasted from the formation of the planet until around 4000 million years ago. In order for the phyllosilicates to form an alkaline water environment would have been present. It is thought that deposits from this era are the best candidates to search for evidence of past life on the planet. The equivalent on Earth is much of the hadean eon.

• Theiikian (named, in Greek, after the sulfate minerals that were formed), lasting until about 3500 million years ago, was a period of volcanic activity. In addition to lava, gasses - and in particular sulfur dioxide - were released, combining with water to create sulfates and an acidic environment. The equivalent on Earth is the eoarchean era and the beginning of the paleoarchean era.

• Siderikan, from 3500 million years ago until the present. With the end of volcanism and the absence of liquid water, the most notable geological process has been the oxidation of the iron-rich rocks by atmospheric peroxides, leading to the red iron oxides that give the planet its familiar color. The equivalent on Earth is most of the archean all of the proterozoic and up to now.

Crater morphology provides information about the physical structure and composition of the surface. Impact craters allow us to look deep below the surface and into Mars geological past. Lobate ejecta blankets (pictured left) and central pit craters are common on Mars but uncommon on the Moon, which may indicate the presence of near-surface volatiles (ice and water) on Mars. Degraded impact structures record variations in volcanic, fluvial, and eolian activity.^[87]

The Yuty crater is an example of a Rampart crater so called because of the rampart-like edge of the ejecta. In the Yuty crater the ejecta competely covers an older crater at its side, showing that the ejected material is just a thin layer.^[88]

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. In addition, all the carbon dioxide released would have raised the temperature of 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.^[89]

Below are pictures of volcanic features as seen by various spacecraft.

• 
  Map of Tharsis quadrangle with major features indicated. Tharsis contains many volcanoes, including Olympus Mons, the tallest known volcano in the solar system. Notice Ceraunius Tholus, although it looks small, it is about as high as Earth's Mount Everest.
• 
  Lower volcano is Ceraunius Tholus and upper volcano is Uranius Tholus as seen by Mars Global Surveyor Mars Orbiter Camera. Ceraunius Tholus is about as high as Earth's Mount Everest.
• 
  Albor Tholus as seen by THEMIS. The elongated features represent collapse of material into empty lava tubes. Albor Tholus is in the Elysium quadrangle.
• 
  Elysium Mons, as seen by Mariner 9
• 
  MOLA image of Elysium Mons. Click on image to see relative positions of Hecates Tholus (top), Elysium Mons (middle) and Albor Tholus (bottom).

On July 28 2005, the European Space Agency announced the existence of a crater partially filled with frozen water;^[90] some then interpreted the discovery as an "ice lake".^[91] 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.

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 aged some 2 to 10 million years.^[92] Not all scientists agree with these conclusions.

Glaciers formed much of the observable surface in large area of Mars. Much of the area in high latitudes, especially the Ismenius Lacus quadrangle, is believed to still contain enormous amounts of water ice.^[76][93] Recent evidence has led many planetary scientists to believe that water ice still exists as glaciers with a thin covering of insulating rock.^{[40][41][42][43][44]}Fretted terrain, many volcanoes, even some craters are believed to have many glaciers. 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.^[94] Glaciers are not pure ice; they contain dirt and rocks. At times, they will dump their load of materials 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 and more or less round hole remains.^[95] 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.

Other pictures below show various features that appear to be connected with the existance of glaciers.

• 
  Moreux Crater moraines and kettle holes, as seen by HIRISE. Location is Ismenius Lacus quadrangle.
• 
  Clanis and Hypsas Valles, as seen by HiRISE. Ridges are probably due to glacial flow. So water ice is under a thin layer of rocks. Location is Ismenius Lacus quadrangle.
• 
  Gullies and possible remains of old glaciers in a crater in Eridania quadrangle, north of the large crater Kepler. One suspected glacier, to the right, has the shape of a tongue. Image taken with Mars Global Surveyor.
• 
  Tributary Glacier, as seen by HiRISE. Location is Ismenius Lacus quadrangle.
• 
  Coloe Fossae Lineated Valley Fill, as seen by HiRISE. Scale bar is 500 meters long. Location is Ismenius Lacus quadrangle.
• 
  Tongue-Shaped Glacier, as seen by Mars Global Surveyor. Location is Hellas quadrangle.

Both the 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^[96] with varying contents of frozen water, depending on its latitude; the polar cap is a mixture of 85% CO₂ ice and 15% water ice.^[97] 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.^[97] The third part encompasses the vast permafrost fields that stretch for tens of kilometres away from the scarps.^[97][98] 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.^[99]

Results, published in 2009, of 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 this volume by the surface area of Mars is how this number is found). The radar instrument is onboard the Mars Reconnaissance Orbiter.^[100]

For many years, various scientists have suggested that some Martian surfaces look like Periglacial regions on Earth.^[101] 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.^[102][103] 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.^[104]

• 
  Polygons on the ground
• 
  Stone rings on Spitsbergen
• 
  Ice wedges seen from top
• 
  Phoenix landing-day image near north pole of Mars showing flat terrain, containing what appears to be a polygonal pattern, stretching from the foreground to the horizon.
• 
  This image was taken from a helicopter in Canadian Arctic
• 
  Patterned ground in the form of polygonal features is associated with ground ice. It is rare to be found this from the pole. (45 degrees north latitude). Picture taken by Mars Global Surveyor.

It is now accepted that over thirty meteorites have been found that came from Mars. These Mars meteorites have provided scientists with a wonderful opportunity to analyze the rocks of Mars. Some of them contain evidence that these rocks were exposed to water when on Mars.

In 1983 it was suggested by Smith et al. ^[105] that meteorites in the so called SNC group (Shergottites, Nakhlites, Chassignites) originated from Mars, from evidence from an instrumental and radiochemical neutron activation analysis of the meteorites. They found that the SNC meteorites possess chemical, isotopic, and petrologic features consistent with data available from Mars at the time, findings further confirmed by Treiman et al. ^[106] a few years later, by similar methods. Then in late 1983, Bogard et al. ^[107] showed that the isotopic concentrations of various noble gases of some of the shergottites were consistent with the observations of the atmosphere of Mars made by the Viking spacecraft in the mid-to-late 1970s.

In 2000, an article by Treiman, Gleason and Bogard ^[108] gave a survey of all the arguments used to conclude the SNC meteorites (of which 14 had been found at the time) were from Mars. They wrote, "There seems little likelihood that the SNCs are not from Mars. If they were from another planetary body, it would have to be substantially identical to Mars as it now is understood."

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. That is they were in water when sitting on Mars. The first shergottite, the Shergotty meteorite, fell at Sherghati, India in 1865.^[109]

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.^[110] There are 7 known nakhlites, the first of which, the Nakhla meteorite, fell in El-Nakhla, Alexandria, Egypt in 1911 and had an estimated weight of 10 kg. The most recent nakhlite was found in Antarctica on December 15, 2003.^[110]

Evidence of life on Mars may already been found. In 1996, a group of scientists reported on chemical fossils in Alan Hills 84001, a meteorite from Mars.^[111] Many studies disputed the validity of the fossils.^{[112] [113]} For example, it was found that most of the organic matter in the meteorite was of terrestrial origin. ^[114] But, a recent study shows that magnetite in the meteorite was produced by Martian microbes. The study, published in the journal of the Geochemical and Meteoritic Society, used more advanced high resolution electron microscopy than was possible 13 years ago. ^[115]

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 water. Deltas often 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. Below, are pictures of a few.^[39]

• 
  Delta in Ismenius Lacus quadrangle, as seen by THEMIS.
• 
  Delta in Lunae Palus quadrangle, as seen by THEMIS.
• 
  Delta in Margaritifer Sinus quadrangle as seen by THEMIS.
• 
  Probable delta in a crater to the NE of Holden Crater, as seen by Mars Global Surveyor. Image in Margaritifer Sinus quadrangle.

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.^[116] This primordial ocean, dubbed Oceanus Borealis,^[117] 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.^[118]

There are several physical features in the present geography of Mars that suggest the existence of an ocean. Networks of gullies 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.^[118]

Research published in the Journal of Geophysical Research — Planets, shows a much higher density of stream channels then formerly believed (more than twice as much). 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. ^[119] 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 develop no valleys. In a similar fashion the lack of rainfall would explain why Martian valleys become shallower as you go from north to south. ^[120]

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.^[117] 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 criteria.^[121] The Mars Orbiter Laser Altimeter (MOLA), which accuratly determined the altitiude of all parts of Mars, found that the watershed for an ocean on Mars covers three-quarters of the planet.^[122]

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.^[123]

The existence of a primordial Martian ocean remains controversial among scientists.^[124] 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.^[125] The interpretations of some features as ancient shorelines has been challenged. Some have been shown to be of volcanic origin.^[126]

Alternate theories for the creation of surface gullies and channels include wind erosion^[127], liquid carbon dioxide^[118], and liquid methane.^[128]

Life as we know it depends on liquid water. We have come to believe that Mars had enough water to form lakes and to carve huge river valleys.^{[129] [130]} Vast quantities of water have been discovered frozen beneath much of the Martian surface. Nevertheless, many significant isssues remain. When did the water once flow on Mars? ^{[131][132][133]} Does liquid water ever exist on Mars today? Are there special environments that could support life? Could life on Mars stay viable for millions of years when conditions turned hostile for life? Could life have adapted for high salt, high acid conditions? Did water hang around long enough for life to develop and evolve?

Mars areas have been extremely dry for long periods, as marked by the presence of olivine that would be decomposed by water^[134]. On the other hand, many areas contain clay and/or sulfates, which indicate the presence of liquid water on the surface.^[135] Now, the sulfates introduce a special problem: sulfates form under acid conditions.^[136] Can life develop in acid? On Earth some organisms live in acid.^{[137] [138]} However, could life originate in acid? ^[139] The saltiness of the soil could be a major obstacle for life.^[140] Salt has been used by the human race as a major perservative since most organisms can not live in highly salted water (halophile bacteria being an exception).^[141] The Phoenix mission discovered perchlorate, a highly oxidizing chemical in the soil. Although some organisms use perchlorate, the chemical could be hostile to life. Other research from different sources show that some areas of Mars may not be that hostile to life. Carbonates, which do not form in acid solutions have been found in Martian meteorites, by the Phoenix lander, and by the Compact Reconnaissance Imaging Spectrometer, an instrument aboard the NASA Mars Reconnaissance Orbiter.^{[142] [143]}

Benton Clark III, a member of the Mars Exploration Rover (MER) team, imagine that Martian organisms could be adapted to a sort of suspended animation for millions of years.^[144] Indeed, some organisms can endure extreme environments for a time. Measurements performed 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.^[145]

The discovery of organisms living in extreme conditions on Earth has brought renewed hope that life exists, or once existed on Mars.^{[146][147][148]} Colonies of microbes have been found beneath almost 3 kilometers of glaciers in the Canadian Arctic and in Antarctica.^[149] 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.^[150] 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.^[151] It is entirely possible that life exists near volcanoes or underground resorviors of hot magma.^[152] Some organisms live inside of basalt (the most common rock on Mars) and produce methane. Methane has been tracked on Mars.^[153] 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. Other organisms eat sulfur compounds; the same chemicals that have been found in many regions of Mars. Some 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 degrees C).^[154] With all the volcanic activity on Mars, one would suppose that certain places have not yet cooled down.^[155] An underground magam 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 Reconnaisance Orbiter.^[156][157] 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 Reconnaisance Orbiter.^[135] 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 possiblity 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.^[158] 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.^[159][160] 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.^[161] We suppose that Mars had great amounts of water because of the existance of so many large river valleys. Maybe, valleys did not take thousands to millions of years to form as on the Earth.^[162] 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 way below its normal freezing point.^{[163] [164]} Some calculations suggest that tiny amounts of liquid water may be present for short periods of time (hours) in some locations.^[165] 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.^[166]

On April 1 2005 the official NASA website reported having the first pictures of water on Mars; this turned out to be just a picture of a glass of water resting on two Mars Bars.^[167]

• Evolution of water on Mars and Earth
• Life on Mars#Liquid_water
• Mars#Hydrology
• Atmosphere of Mars#Water
• Geology of Mars#Hydrology
• Mars_Express#Scientific_discoveries_and_important_events
• Mars Global Surveyor#Discovery of water on Mars
• 2001 Mars Odyssey#Water on Mars
• Phoenix (spacecraft)#Presence of shallow subsurface water ice
• Extraterrestrial liquid water
• Mars Ocean Hypothesis
• Colonization of Mars
• Climate of Mars
• Water vapor#Extraterrestrial water vapor
• Extraterrestrial life
• Scientific information from the Mars Exploration Rover mission
• Lobate Debris Apron
• Vallis

• High resolution images of Mars — detailed observation of water signs
• The Case of the Missing Mars Water
• Wikinews articles about water on Mars
• [2] - for Mars animation of how Tharsis evolved and affected the amount of water on the planet.

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