The importance of water searching on Mars is strongly linked to the interest about the knowledge of the evolution of the planet. As matter of fact, despite Mars is characterized by similar geomorphological and orbital properties to the Earth, its evolutionary history has been extremely different.
No less important are the exobiological implications. Observational evidences show signs of a past in which the alleged presence of liquid water on the surface, in an atmosphere hotter and denser, was the cause of the formation of structures that remind those produced by waterways on Earth. If the origin of these formations was due to water, the planet may have hosted biotic or prebiotic forms.
Today, low temperatures and extremely rarefied atmosphere make the planet incapable of retaining large amounts of water in the atmosphere or on the surface, making the presence of water in the liquid state extremely unlikely.
Despite this, the water vapor in the Martian atmosphere appears to be a key indicator of climate change, occurring on the planet during the seasonal cycle and for periods of time, even much longer. The periodic variation of its abundance in the Martian atmosphere is mainly due to the combined effects of the H2O exchange between the atmosphere and reservoirs on the surface (eg ice caps, regolith) and the mechanism of atmospheric transportation. In addition, it is more probably the presence of large amounts of water trapped below the surface.
In this context it is extremely important to understand the role of the different contributions of surface water vapor and mechanisms for exchange with the atmosphere.
What we now know about atmospheric water vapor has been inferred only from indirect measurements obtained with ground-based telescopes and (eg ‘s Hubble Space Telescope) and planetary exploration missions.
To date, however, no direct measurement of water vapor has been carried out and remains a series of questions to which it was not possible to answer by means of indirect measurements and observations from remote.
The atmosphere of Mars contains an average of ~ 1 to 2 × 1015 g of water vapor. Even when the vapor is in saturation (ie where it is close to condensation point), the amount of H2O that the atmosphere can hold is very small (only ~ 300 parts per million of the total volume). By comparison with our planet, where the volume of precipitable water is of the order of 104 km3 (ten thousand cubic kilometers) of Mars that amount is reduced to just 1 to 2 km3. In other words, if all the water in the atmosphere worsens, the entire planet will be coated with an uniform layer of the order of 1 micron (one millionth of a meter!).
Although water vapor is a minor constituent of the Martian atmosphere, it plays a fundamental role. The distribution, the abundance variation and the dynamics of the vapor in the atmosphere are closely related to the climatic evolution of the planet and act as markers of the presence of possible sources of H2O still present on the surface and below it. The presence of water on Mars is of fundamental importance in the perspective of future human missions to the planet. No less important are the exobiological implications related to the presence and abundance on the planet (actually and especially in the past).
Much of what we know about the circulation of water vapor in the Martian atmosphere derived from observations made from Earth (eg, Clancy et al., 1992), by the Mars Atmospheric Water Detection (MAWD) on board the Viking missions (Jakosky and Farmer, 1982) and the Thermal Emission Spectrometer (TES) on board the Mars Global Surveyor (Smith, 2002). The experiment MAWD gave an almost total spatial coverage for more than one Martian year (1977-1978), which allowed to reconstruct the seasonal variations of abundance of water vapor and the movement on planetary scale. What is amazing is the substantial existence of real seasonal cycles, just like on Earth. Furthermore, despite of the atmospheric H2O is present only in trace amounts, thanks to low characteristic temperatures of the Martian environment, water vapor often reaches the saturation point. We must remember that when, on Earth, atmospheric water reaches the saturation begin the processes that lead to the rains. Unfortunately on Mars, the atmospheric pressure is so low that the surface is very close to the so called triple point of water (6.1 mbar). The triple point of water is a condition in which water can exist simultaneously in its three fundamental states: solid, liquid and gaseous.
Precisely this makes highly unlikely the persistence of liquid water on the surface. The water cycle on Mars is, therefore, almost exclusively limited to the phenomena of exchange between the atmosphere and surface by means condensation and sublimation processes.
But there is an important point not yet considered … the past of Mars!
Thanks to the traces on the surface, It seems clear that some liquid elements was abundants in the past. There are still doubts if it were just water or carbon dioxide in the liquid state. Increasingly, however, the scientific community agrees on the assumption of a history of water.
The fundamental question that arises at this point is: persisted the water in the liquid phase for a time sufficient to support the evolution of some form of life? At this point, the assumptions are rather conflicting.
Geomorphological traces on the surface suggest, at a first observation, that the presence of liquid water in large quantities was been possible for a long time. Past abundances, however, could be due to phenomena of volcanism, capable of supporting temporary higher atmospheric pressures. So, volcanic eruptions would have allowed the renewal of the water vapor content in the atmosphere… for short periods interspersed with periods of stasis. A condition, therefore, of unstable equilibrium. The force of gravity on Mars is about one third of that of the Earth, therefore most of the molecules may have dispersed each time in space. There may be times of plenty followed by periods of drought comparable to those that exist today.
The beds of the rivers were dug at intervals very long and in such conditions life would never have had time to form. On the other hand, the presence of the largest known volcanos of the Solar System, lead us to believe in anything but sporadic phenomena. If we consider, for example, the Olympus Mons, with its titanic 25 km high, we can imagine a time when huge eruptions have added to the atmosphere a large amount of substances. As well as the volcanic chain in Tharsis Region.
It seems, moreover, that some recent observations of Curiosity, are proving the existence of geological phenomena still in place (see).
If this will be confirmed one would assume that in the remote future these volcanoes, still dormant, will be able to wake up to return the planet to a condition far from barren.
All these are questions for which we are still far from giving a definite answer. What is becoming increasingly clear is a picture of a remote past in which a hypothetical space voyager, coming to the orbits of the terrestrial planets would have observed a star system full of promise, with well over two planets whose surfaces were full of flowing rivers and seas …. two planets that in the future humans will call Earth and Mars.