Mars Mysteries

Liquid water cannot exist on the surface of Mars due to low
atmospheric pressure, which is less than 1% that of Earth's,[25] except
at the lowest elevations for short periods.[26][27] The two polar ice
caps appear to be made largely of water.[28][29] 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 meters (36 ft).[30] A
permafrost mantle stretches from the pole to latitudes of about 60°.[28]
Large quantities of water ice are thought to be trapped within the
thick cryosphere of Mars. Radar data from Mars Express and the Mars
Reconnaissance Orbiter show large quantities of water ice at both poles
(July 2005)[66][67] and at middle latitudes (November 2008).[68] The
Phoenix lander directly sampled water ice in shallow Martian soil on
July 31, 2008.[69]

Landforms visible on Mars strongly suggest that liquid water has existed on the planet's surface. Huge linear swathes of scoured ground, known as outflow channels, cut across the surface in about 25 places. These are thought to be a record of erosion caused by the catastrophic release of water from subsurface aquifers, though some of these structures have been hypothesized to result from the action of glaciers or lava.[70][71] One of the larger examples, Ma'adim Vallis is 700 km (430 mi) long, much greater than the Grand Canyon, with a width of 20 km (12 mi) and a depth of 2 km (1.2 mi) in places. It is thought to have been carved by flowing water early in Mars's history.[72] The youngest of these channels are thought to have formed as recently as only a few million years ago.[73] Elsewhere, particularly on the oldest areas of the Martian surface, finer-scale, dendritic networks of valleys are spread across significant proportions of the landscape. Features of these valleys and their distribution strongly imply that they were carved by runoff resulting from precipitation in early Mars history. Subsurface water flow and groundwater sapping may play important subsidiary roles in some networks, but precipitation was probably the root cause of the incision in almost all cases.[74]

Along crater and canyon walls, there are thousands of features that appear similar to terrestrial gullies. The gullies tend to be in the highlands of the southern hemisphere and to face the Equator; all are poleward of 30° latitude. A number of authors have suggested that their formation process involves liquid water, probably from melting ice,[75][76] although others have argued for formation mechanisms involving carbon dioxide frost or the movement of dry dust.[77][78] No partially degraded gullies have formed by weathering and no superimposed impact craters have been observed, indicating that these are young features, possibly still active.[76] Other geological features, such as deltas and alluvial fans preserved in craters, are further evidence for warmer, wetter conditions at an interval or intervals in earlier Mars history.[79] Such conditions necessarily require the widespread presence of crater lakes across a large proportion of the surface, for which there is independent mineralogical, sedimentological and geomorphological evidence.[80]

Further evidence that liquid water once existed on the surface of Mars comes from the detection of specific minerals such as hematite and goethite, both of which sometimes form in the presence of water.[81] In 2004, Opportunity detected the mineral jarosite. This forms only in the presence of acidic water, which demonstrates that water once existed on Mars.[82] More recent evidence for liquid water comes from the finding of the mineral gypsum on the surface by NASA's Mars rover Opportunity in December 2011.[83][84] It is believed that the amount of water in the upper mantle of Mars, represented by hydroxyl ions contained within the minerals of Mars's geology, is equal to or greater than that of Earth at 50–300 parts per million of water, which is enough to cover the entire planet to a depth of 200–1,000 m (660–3,280 ft).[85]

In 2005, radar data revealed the presence of large quantities of water ice at the poles[66] and at mid-latitudes.[68][86] The Mars rover Spirit sampled chemical compounds containing water molecules in March 2007. The Phoenix lander directly sampled water ice in shallow Martian soil on July 31, 2008.[69]

On March 18, 2013, NASA reported evidence from instruments on the Curiosity rover of mineral hydration, likely hydrated calcium sulfate, in several rock samples including the broken fragments of "Tintina" rock and "Sutton Inlier" rock as well as in veins and nodules in other rocks like "Knorr" rock and "Wernicke" rock.[87][88][89] Analysis using the rover's DAN instrument provided evidence of subsurface water, amounting to as much as 4% water content, down to a depth of 60 cm (24 in), during the rover's traverse from the Bradbury Landing site to the Yellowknife Bay area in the Glenelg terrain.[87] In September 2015, NASA announced that they had found conclusive evidence of hydrated brine flows on recurring slope lineae, based on spectrometer readings of the darkened areas of slopes.[90][91][92] These observations provided confirmation of earlier hypotheses based on timing of formation and their rate of growth, that these dark streaks resulted from water flowing in the very shallow subsurface.[93] The streaks contain hydrated salts, perchlorates, which have water molecules in their crystal structure.[94] The streaks flow downhill in Martian summer, when the temperature is above −23 degrees Celsius, and freeze at lower temperatures.[95] On September 28, 2015, NASA announced the presence of briny flowing salt water on the Martian surface.[96]

Researchers believe that much of the low northern plains of the planet were covered with an ocean hundreds of meters deep, though this remains controversial.[97] In March 2015, scientists stated that such an ocean might have been the size of Earth's Arctic Ocean. This finding was derived from the ratio of water to deuterium in the modern Martian atmosphere compared to that ratio on Earth. The amount of Martian deuterium is eight times the amount that exists on Earth, suggesting that ancient Mars had significantly higher levels of water. Results from the Curiosity rover had previously found a high ratio of deuterium in Gale Crater, though not significantly high enough to suggest the former presence of an ocean. Other scientists caution that these results have not been confirmed, and point out that Martian climate models have not yet shown that the planet was warm enough in the past to support bodies of liquid water.[98]