(Updated on 28 September 2015). Since now we were thinking that liquid water cannot exist on the surface of Mars due to its low atmospheric pressure, except at the lowest elevations for short periods. But the newest discovery changes this theory from its basis. New findings from NASA’s Mars Reconnaissance Orbiter (MRO) provide the strongest evidence yet that liquid water flows intermittently on present-day Mars. Using an imaging spectrometer on MRO, researchers detected signatures of hydrated minerals on slopes where mysterious streaks are seen on the Red Planet. These darkish streaks appear to ebb and flow over time. They darken and appear to flow down steep slopes during warm seasons, and then fade in cooler seasons. They appear in several locations on Mars when temperatures are above minus 10 degrees Fahrenheit (minus 23 Celsius), and disappear at colder times. Liquid water on Mars can exist even today.
Water ice is in no short supply however, with two polar ice caps that appear to be made largely of water. In March 2007, NASA announced 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 m. Additionally, a permafrost mantle stretches from the pole to latitudes of about 60°.
Large quantities of water ice are thought to be trapped underneath Mars’s thick cryosphere. Radar data from Mars Express and the Mars Reconnaissance Orbiter have revealed the presence of large quantities of water ice both at the poles (July 2005) and at mid-latitudes (November 2008). The Phoenix Mars Lander directly sampled water ice in shallow martian soil on July 31, 2008.
Liquid water in the past
A large release of liquid water is thought to have occurred when the Valles Marineris formed early in Mars’s history, forming massive outflow channels. A smaller but more recent outflow may have occurred when the Cerberus Fossae chasm opened about 5 million years ago, leaving a supposed sea of frozen ice still visible today on the Elysium Planitia, centered at Cerberus Palus. However, the morphology of this region may correspond to the pooling of lava flows, causing a superficial morphology similar to ice flows, which probably draped the terrain established by earlier massive floods of Athabasca Valles. The rough surface texture at decimeter (dm) scales, a thermal inertia comparable to that of the Gusev plains, and the presence of hydrovolcanic cones, are consistent with the lava flow hypothesis. Furthermore, the stoichiometric mass fraction of water in this area, to tens of centimeter depths, is only about 4%, which is easily attributable to hydrated minerals, and inconsistent with the presence of near-surface ice.
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. Some of the evidence believed to indicate ancient water basins and flows has been negated by higher resolution studies taken at resolution about 30 cm by the Mars Reconnaissance Orbiter. However, in 2004, Opportunity detected the presence of the mineral jarosite on “El Capitan”, a rock on the outcrop of Opportunity Ledge. Jarosite forms only in the presence of acidic water, and the presence of jarosite is seen as proof that water once existed on Mars.
Hydrology of Mars – water equivalent displayed according the presence of hydrogen:
Source of diagrams and credits belongs to Los Alamos Labs/USA.
And how Mars looked like in the past when there was large amount of water on Martian surface? For example like this (credit of images):