Current conditions on Mars do not support the long-term existence of large masses of liquid water on the surface of Mars. In the modern age of astronomy but before Mariner, it was once thought that if there were any water on Mars, it could only ever exist as a thin layer of frost at the poles, co-existing with frozen carbon dioxide.
With the latest pictures, sampling, and analysis from Mars, that thinking has completely changed. Water ice has been directly sampled by the Phoenix lander in 2008. Every single sample from the Viking lander experiments revealed the presence of a minimum of 1% water. Spectroscopic measurements, geological examination of mineral composition and even physical features: all tell us that at one time, liquid water and possibly even oceans of liquid water used to flow on Mars.
New photographs and new analysis now shows large-scale liquid water flow patterns on the surface of Mars. Where once early observations and science fiction writers speculated about a canal system, we now see a former river system on the scale of the Mississippi river, complete with large areas which could have been oceans. We also see huge river valleys, glacial erosion, evidence of actual rainfall, large floods which broke through ancient dams. Alas, nothing about them suggests the hand of intelligent crafting, simply the hand of nature and the water cycle.
The pattern of sulphur and chlorine found in much of the Martian soil suggest the way those elements would likely be left behind if oceans were to evaporate: a common pattern in parts of earth’s own deserts. Clay, magnesium sulphate, calcite, and water-loving goethite all argue in favour of a history of large areas of water.
ESA’s Mars Express has confirmed the existence of large amounts of water ice at Mars’ south pole as well as at the north: enough of it even to create steep cliffs of water ice. The existence of water ice at the south pole was previously thought impossible.
Meteorites believed to have originated on Mars also show evidence of having been touched by water. Some even show evidence that they may have been touched by bacterial life.
Even the mineral composition of Mars now suggests that there had been water in the past, large masses of water, at least some of which continues to persist in unusual forms. Crystals seem to abound on Mars: crystals which would have needed pressurised liquid water to carry the crystallising minerals from the deep into the open air.
Perhaps most unexpected of all, the discovery of opal by the Mars Reconnaissance Orbiter confirms a previously unsuspected place where at least some of the missing water might be stored. Hydrated silica, better known as an opal, is a gel which contains as much as 21% molecular water by weight. Essentially, opal is quartz, with water trapped in its crystal structure: and it can only be created when basaltic rock is dissolved in water, which then evaporates.
The discovery of opal moves the likely timeline when water still flowed openly on Mars forward, from three billion years ago to within a few hundred million years. Even that may be erring on the conservative side. Some geological evidence seems to suggest that there may have been open geysers within the past decade.
One caveat: We don’t yet know whether past water was truly persistent or the result of odd flooding events. Just because there was flowing water on the surface of Mars at some point in the past does not mean it remained on the surface of Mars throughout.
An indirect piece of evidence in favour of there still being a large liquid water reservoir under the poles comes to us from a study of Lake Vostok in Antarctica. The comparable heat flow patterns suggest that any lake deeper than 63 metres could have survived subsequent surface glaciation without itself freezing to the bottom: which further implies that organisms living in that lake could have survived to the present day.