The finding of water on Mars, even if was mostly subsurface, was perhaps the single biggest boon to exploration possible. mars appears to have been wetter in the distant past.
Evidence of a wet Mars in the past
Ancient rivers - Modern gullies
There are many giant flood channels and associated tree-like network of tributaries found on Mars. One theory about the formation of the ancient river valleys is that rather than floods, they were created by the slow seeping out of groundwater. This observation is supported by the sudden ending of the river networks in theatre shaped heads, rather than tapering ones. Also valleys are often discontinuous, small sections of uneroded land separating the parts of the river. Evidence in favor of heavy or even catastrophic flooding is found in the giant ripples in the Athabasca Vallis.
Equatorial frozen sea
Surface features consistent with pack ice have been discovered in the southern Elysium Planitia. What appear to be plates of broken ice, ranging in size from 30 m to 30 km, are found in channels leading to a flooded area of approximately the same depth and width as the North Sea. The plates show signs of break up and rotation that clearly distinguish them from lava plates elswhere 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.
An Ancient coastline
The Mars Ocean Hypothesis conjectures that the Vastitas Borealis basin was the site of a primordial ocean of liquid water 3.8 billion years ago.
Impact crater morphology
Impact crater morphology provides information about the physical structure and composition of the surface. Degraded impact structures clearly record variations in volcanic, fluvial, and eolian activity.
Surface Water
Liquid water
Prior to the arrival of humans liquid water cuold not exist on the surface of Mars with its present low atmospheric pressure, except at the lowest elevations for short periods. Recently the rise in mean AP fro 6 mm to 17 mm has resulted in water exsisting as a liquid when the temperature is above zero.
Ice patches
Robot probes in the 21C identidied broad sheets of ice in the bottom of craters located on Vastitas Borealis, a broad plain that covers much of Mars' far northern latitudes, at approximately 70.5° North and 103° East. Now we find ice patches are common and in the summer, in the warmer climes, they unfreeze.
Polar ice caps
Mars has polar ice caps that contain 85% highly carbon dioxide (CO2) ice and 15% water ice that change with the Martian seasons.Each cap has surface deposits of carbon dioxide ice that form a polar "hood" during Martian winter, and then sublimate during the summer uncovering the underlying cap surface of layered water ice and dust. The southern polar cap (Planum Australe) differs from the northern polar cap (Planum Boreum) in that it appears to contain at least some permanent deposits of CO2, which are changing on the time scale of years.
The southern polar cap has a 3 kilometres thick slab of about 80% water ice. There is a small sheet of liquid water between the ice and Mars' crust.
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. Additionally, an ice permafrost mantle stretches from the poles to latitudes of about 60°.
Mars Subsurface Water
Cosiderable liquid water exists below the Mars subsurface and makes our civilization possible. Ground radar analysis indicates that the Martian subsurface is a reservoir for huge amounts of ice, and there will usually be liquid water at depths of from 2-10 km. The total amount of water in the Martian aquifer, should it be distributed liquid surface water, has been estimated to be equivalent to a global layer 400 m deep.
The shallowest liquid water is found at sites that are at both low elevation and low latitude. These include: the northwest interior of Hellas Basin, Amazonis Planitia, Chryse Planitia, Utopia Planitia, Isidis Planitia, East of Elysium Planitia, and the confluence of Ganges, Capri, and Eos Chasmas.
An aquifer is an underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, silt, or clay) from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology. Various techniques have been used to drill down and tap into these aquifers.
Related terms include: an aquitard, which is an impermeable layer along an aquifer, and an aquiclude (or aquifuge), which is a solid, impermeable area beneath an aquifer. The surface of saturated material in an aquifer is known as the water table.
