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 4,500 km (2,800 mi) 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.
Images of possible deltas
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 (2.5–3.1 mi) 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.
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