The surface of Mars today, seen here by Mars Pathfinder, is desert-like and very dry.
WATER ON MARS
Viking Orbiter images revealed a variety of features on the Martian surface that look as though they were made by flowing water: tear-drop shaped islands in the middle of ancient rivers, canyons, tributaries, even whole networks of valleys. High resolution pictures returned by Mars Global Surveyor recently have provided far more detailed views of these intriguing landforms, and will enable geologists to say more about how they were formed.
Martian water is important; it has played a significant part in shaping the surface, but it is vital to the existence of Martian life (either past or present) and would be useful in any future human colonisation of the planet. Water is heavy (try carrying a bucketful) and it would be extremely costly to launch anything like the quantities needed to supply a long term human space mission. Therefore, if humans intend to stay for long periods of time on Mars, they will either have to take water with them, manufacture water, or find it locally when they arrive. This means that the current whereabouts of Martian water is an important logistical issue. The whole topic of Martian water boils down to two fundamental questions: Was Mars once wet ? Where is the water now?
Water in the past
Viking Orbiter images show outflow channels that are very similar to channels formed on Earth by water. (Note, however, that Valles Marineris is not a water feature, but a tectonic feature). The water erosion features on Mars indicate two different scenarios. Firstly, some features appear to have been formed by catastrophic flooding, possibly triggered by a meteoritic impact. Other features look as though they have been eroded gently by flowing water over a longer period of time.
Long grooves, and islands the shape of teardrops, within Maja Vallis.	The Ravi Vallis outflow channel emerging from chaotic terrain.
Ravi Vallis
Crater dating techniques show that the valley networks, formed slowly, are located in parts of Mars where the surface is over 3,000 million years old. So, if water did create the valley networks, it was way back in geological history. This in turn implies that Martian temperatures were higher than they are now, otherwise the water would merely freeze and not flow and erode. Higher temperatures in turn suggest that Mars had a greater greenhouse effect at work. At which point the calculations run into the Faint Young Sun Paradox.
The Faint Young Sun Paradox requires Martian temperatures in the past to be higher in order to support liquid water on the surface, but at the same time recognises that the Sun, a few thousand million years ago, was fainter and provided Mars with less heat. Now, the Sun is warmer, but Mars has got colder. There is a way out of the paradox, and that is to introduce volcanoes into the equation.
Volcanoes, which definitely exist on Mars, and definitely were active, would have produced carbon dioxide and sulphur dioxide, just as Earth's volcanoes do today. Carbon dioxide is a greenhouse gas, so that gets the temperature up high enough for water.
Sulphur dioxide, residing in the upper atmosphere, would have acted as a thermal blanket, absorbing ultraviolet radiation from the Sun and warming the atmosphere further. For this theory to work, all we need to do is to explain what has happened to the thick (5 bars) atmosphere of carbon dioxide. As carbon dioxide is soluble in water (rainfall is carbonic acid, not pure water), it would have slowly dissolved into the Martian lakes and rivers, forming carbonate rock. This happens today on Earth.
On Earth, the currently active volcanoes are still producing carbon dioxide as fast as the oceans are absorbing it, so the gas is recycled. On Mars, we know that the volcanoes shut down, as there are is no volcanic activity on Mars anymore. The result would be the slow loss of the carbon dioxide, a reduction in global temperatures, and the freezing of the water. This still leaves one problem - the absorption of a large atmosphere of carbon dioxide creates a deep layer of calcite. No such layer has been detected on Mars, but it may reside under the surface. Further exploration is needed before the Martian water question can be answered in full.
Evidence of water today
It is increasingly certain that there is a reservoir of water in the top layer of Martian soil and far more than previously thought. Only the top few centimetres are needed to hold ten times as much water as there is currently in the atmosphere and icecaps. It had been estimated that if all the Martian water in the atmosphere and in the polar caps suddenly fell as rain onto the Martian surface, the result would be a global ocean with a depth of just 0.01 centimetres. Revised calculations suggest that the water present on Mars equates to global ocean of about 10 centimetres deep. It has also been demonstrated that there are large amounts of water in both polar caps. It had been thought that only the north polar cap contained water, but investigations have shown that the surface features at the south polar cap are consistent with a cap composed largely of water, with just a skin of carbon dioxide.
Hubble Space Telescope images of the polar cap undergoing seasonal change.
These new insights into the quantity and whereabouts of Martian water have come from studying Mars Global Surveyor images and the multi-spectral images acquired between 2002 and 2004 by NASA's 2001 Mars Odyssey spacecraft which is equipped to detect hydrogen- most likely occurring in the form of water ice- just below the surface.
In images returned by Mars Global Surveyor relatively fresh looking gullies have been found. They have been located between latitudes 30� and 70� in both Martian hemispheres, on slopes facing away the direction of the mid-day sun. It appears that water accumulating below ground protected from direct sunlight builds up until it is able to escape in a brief catastrophic flood. Geologists think that the fresh appearance of the channels is an indicator that liquid water is present today in some areas, less than 500 metres below the surface.
Water could be present deeper within Mars' crust. With depth, temperatures rise because the core of the planet is hot. Water could be found at depth where it is warmer. The melting depth, called a melting isotherm, could be between 1 to 8 kilometres down, varying with latitude. However, it is unlikely that the water would be pure - it would have a variety of chemicals dissolved in it, lowering the freezing point. Just as adding salt to roads in winter lowers the freezing point of water- so they remain wet- the mechanism at work on Mars could mean that liquid water occurs relatively close to the surface.