Former Bodies of Water on Mars 
In summer 1965, the first close-up images from Mars showed a cratered desert with no signs of water. However, over the decades, as more parts of the planet were imaged with better cameras on more sophisticated satellites, Mars showed evidence of past river valleys, lakes and present ice in glaciers and in the ground. It was discovered that the climate of Mars displays huge changes over geologic time because its axis is not stabilized by a large moon, as Earth's is. Also, some researchers maintain that surface liquid water could have existed for periods of time due to geothermal effects, chemical composition, or asteroid impacts. This article describes some of the places that could have held large lakes.
Overview.
Besides seeing features that were signs of past surface water, researchers found other types of evidence for past water. Minerals detected in many locations needed water to form. An instrument in "2001 Mars Odyssey" orbiter mapped the distribution of water in the shallow surface. When the "Phoenix" lander fired its retrorockets to land in the far north, ice was exposed.
When water enters a large body of water, such as a lake, a delta may form. Many craters and other depressions on Mars show deltas that resemble those on Earth. In addition, if a lake lies in a depression, channels entering it will all stop at the same altitude. Such an arrangement is visible around places on Mars that are supposed to have contained large bodies of water, including around a possible ocean in the north.
Lake formation in the past has been suspected by various researchers for quite some time. One study found 205 possible closed-basin lakes in craters on Mars. The basins have an inlet valley that cuts the crater rim and flows into the basin, but they have no visible outlet valley. The total volume of the basins is equivalent to a depth of 1.2 meters spread evenly over the Martian surface. However, this amount is a small fraction of the current water ice stores on Mars. Another study found 210 open-basin lakes. These were lakes with both an inlet and an outlet; hence water must have entered the basin, and reached the height of the outlet. Some of these lakes had volumes similar to Earth's Caspian Sea, Black Sea, and Lake Baikal. A study presented at the 2018 Lunar and Planetary Science Conference found 64 paleolakes in the northwest Hellas region. The team suggested that these lakes formed from an ocean that occupied the Hellas basin and southeast lowland. CRISM data for the region showed aqueous minerals such as Fe/Mg smectites, anhydrous chloride, and probably carbonates. Such an ocean was suggested by a team of researchers in 2016. Forty-eight possible extinct lakes were found in Arabia Terra. Some were classified as open-basin systems because they showed evidence for an outlet channel. These lakes ranged from tens of meters to tens of kilometers in size. Many of these lakes were discovered by looking for inverted reliefs.
Some lakes in craters in Terra Sabaea are believed to have formed from the melting of glaciers on their rims. Inverted streams are found on the floors of some craters. Water from glaciers carried debris in channels and consequently that debris was left behind after the surrounding ground eroded.
In a study released in 2018, researchers found 34 paleolakes and associated channels in the northeastern Hellas Basin. Some were close to the Hadriacus volcano. Dikes from the volcano could have created hydrothermal systems, thereby allowing ice to melt. Some appeared to have formed from precipitation, others from groundwater.
Moreover, some basins on Mars form part of long chains of lakes. The Naktong/Scamander/Mamers Valles lake-chain system is about 4500 km (2800 miles) long, with a drainage area similar to that of the Missouri-Mississippi rivers. Another, the Samara/Himera Vallis system, is 1800 km long. Many of the long chains of lakes are found in the Margaritifer Sinus quadrangle.
Some of the lakes appear to have had a high volume as compared to their drainage area; hence, it is thought that some of the water was groundwater. Further evidence is the existence of knobby material on the basin floors. These knobs could have been formed when large amounts of water left the ground.
In February 2019, a group of European scientists published geological evidence of an ancient planet-wide groundwater system that was probably connected to a Martian ocean. The study was of 24 craters that did not display an inlet or outlet; hence, water for the lake would have come from the ground. All craters were located in the northern hemisphere of Mars. These craters had floors lying roughly 4000 m below Martian 'sea level' (a level that, given the planet's lack of seas, is defined based on elevation and atmospheric pressure). Features on the floors of these craters could only have formed in the presence of water. Many craters contain multiple features showing that the water level in the craters rose and fell over time. Deltas and terraces were present in some craters. Minerals such as various clays and light-toned minerals that form in water are found on some of the crater floors. Also, layers are found in some of these craters. Taken together, these observations strongly suggest that water was present in these places. Some of the craters studied were Pettit, Sagan, Nicholson, Mclaughlin, du Martheray, Tombaugh, Mojave, Curie, Oyama, and Wahoo. It seems that if a crater was deep enough, water came out of the ground and produced a lake.
Mars ocean.
The Mars ocean hypothesis postulates that nearly a third of the surface of Mars was covered by an ocean of liquid water early in the planet's geologic history. This primordial ocean, dubbed paleo-ocean and Oceanus Borealis, would have filled the Vastitas Borealis basin in the northern hemisphere, a region which lies 4–5 km (2.5–3 miles) below the mean planetary elevation, about 3.8 billion years ago. Evidence for this ocean includes geographic features resembling ancient shorelines, and the chemical properties of the Martian soil and atmosphere. However, for such an ocean to have existed, early Mars would have required a magnetosphere, a denser atmosphere, and warmer climate to allow liquid water to remain at the surface.
Observational evidence.
Features first shown by the "Viking" orbiters in 1976 revealed two possible ancient shorelines near the pole, Arabia and Deuteronilus, each thousands of kilometers long. Several physical features in the present geography of Mars suggest the past existence of a primordial ocean. Networks of gullies that merge into larger channels imply erosion by a liquid flow and resemble ancient riverbeds on Earth. Enormous channels, 25 km wide and several hundred meters deep, appear to have flowed directly from underground aquifers in the Southern uplands into the Northern plains. Much of the northern hemisphere of Mars is at a significantly lower elevation than the rest of the planet (the Martian dichotomy), and is unusually flat. The low elevation would cause water, if it existed, to gather there. An ocean would tend to level out the ground underneath it.
The acceptance of a vast northern ocean has waxed and waned over the decades. Beginning in 1998, scientists Michael Malin and Kenneth Edgett set out to investigate, using cameras on board the "Mars Global Surveyor" with a resolution five to ten times better than those of the "Viking" orbiter, in places that would test shorelines proposed by others in the scientific literature. Their analysis was inconclusive at best, and reported that the shoreline varies in elevation by several kilometers, rising and falling from one peak to the next for thousands of miles. This report cast doubt on whether the features truly mark a long-gone sea coast, and has been taken as an argument against the Martian shoreline (and ocean) hypothesis.
Research published in 2009 shows a much higher density of stream channels than formerly estimated. Regions on Mars with the most valleys are comparable to what is found on Earth. The research team developed a computer program to identify valleys by searching in topographical data for U-shaped structures. The large extent of valley networks found strongly supports rain on the planet in the past. The global pattern of the Martian valleys could be explained by an extensive 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, farthest from the water reservoir, would get little rainfall and would develop no valleys. Similarly, the lack of rainfall would explain why Martian valleys become shallower from north to south.
A 2010 study of river deltas on Mars revealed that seventeen of them are found at the altitude of a proposed shoreline for a Martian ocean. This is what would be expected if the deltas were all next to a large body of water.
Research published in 2012 using data from MARSIS, a radar on board the "Mars Express" orbiter, supports the hypothesis of a former large northern ocean. The instrument revealed a dielectric constant of the surface similar to those of low-density sedimentary deposits, massive deposits of ground-ice, or a combination of the two. The measurements were not like those of a lava-rich surface.
In March 2015, scientists stated that evidence exists for an ancient volume of water that could comprise an ocean, likely in the planet's northern hemisphere and about the size of Earth's Arctic Ocean. This finding was derived from the ratio of water and deuterium in the modern Martian atmosphere compared to the ratio found on Earth and derived from telescopic observations. Eight times as much deuterium was inferred at the polar deposits of Mars than exists on Earth (VSMOW), suggesting that ancient Mars had significantly higher levels of water. The representative atmospheric value obtained from the maps (7 VSMOW) is not affected by climatological effects as those measured by localized rovers, although the telescopic measurements are within range to the enrichment measured by the "Curiosity" rover in Gale Crater of 5–7 VSMOW.
Valles Marineris canyon system.
Valles Marineris is the largest canyon system in the Solar System, and much evidence suggests that all or parts of the canyon system contained lakes. It is located in the Coprates quadrangle. The walls of the canyons often contain many layers. The floors of some of the canyons contain large deposits of layered materials. Some researchers think that the layers were formed when water once filled the canyons.
Layered deposits, called interior layered deposits (ILDs), in various parts of Valles Marineris, especially Candor Chasma and Juventae Chasma, have led many researchers to suspect that they were formed when the whole area was a giant lake. However, many other ideas have been advanced to attempt to explain them. High-resolution structural and geologic mapping in west Candor Chasma, presented in March 2015, showed that the deposits on the floor of the Candor Chasma are basin-filling sediments that were deposited in a wet playa-like setting; hence water was involved in their formation.
Minerals that generally require water for their formation have been found in ILDs, thus supporting water in the system. The European Space Agency's "Mars Express" found possible evidence for the sulfates epsomite and kieserite, minerals that form in water. Ferric oxide in the form of crystalline grey hematite, which typically requires water for its formation, was also detected.
Although there is much controversy about a lake in the whole of Valles Marineris, a fairly strong case can be made for smaller lakes.
Melas Chasma is thought to have once contained a lake, as it is the deepest part of the Valles Marineris system at 11 km (7 miles) below the surrounding surface. From here to the outflow channels there is about a 0.03-degree slope upward to the northern plains, which means that if the canyon was filled with liquid, there would be a lake 1 km deep before the fluid would flow out onto the northern plains. Melas Chasma is the widest segment of the Valles Marineris canyon system, located east of Ius Chasma at 9.8°S, 283.6°E in Coprates quadrangle. It cuts through layered deposits that are thought to be sediments from an old lake that resulted from runoff of the valley networks to the west. Support for abundant past water in Melas Chasma comes from the discovery by "MRO" of hydrated sulfates that need water for their formation. Moreover, in a 2015 study of southwestern Melas Chasma, using high-resolution image, topographic and spectral datasets, eleven fan-shaped