Since the 1970's with the Viking and Mariner-9 missions, liquid water traces were discovered at the Martian surface. Let make clear that because of the low pressure (6 to 10 mbar) and low temperatures, the surface conditions on Mars do not allow water to maintain its liquid state.
These first observations put in evidence two types of phenomena: the dry rivers, or branching out valleys, and the breaking up valleys. These observations also disclosed that some regions of Mars were ravined by important water flows which eroded craters of various kilometers in diameter. These breaking ups could result from the continued emergence of liquid water coming from the underground. For example, on Athabasca Valleys, it was estimated that the breaking up represented a 100 m high waterfront over several kilometers wide. Since then, the existence of underground water has been confirmed by Mars Odyssey's orbit and Phoenix lander module's in situ measurements.
Since these first discoveries, the observations focused on proof seeking for long-length liquid water presences. In fact, a breaking up lasts a few days when a river can run during thousands years but not much more. However, life is known to had taken a few hundreds of million years to emerge. Looking at these constants, mineralogical deteriorations of volcanic and magmatic rocks resulting from the slow action of water have to be searched.
The first confirmation of this deterioration took place in March 2004. Indeed, the US Opportunity rover which landed on the Meridiani Planum site did analyse the outcropping rocks located at a few meters from its landing point. These rocks proved to be rocks of sedimentary origin hosting haematite spherules (blue-green hard balls). The presence of these sulphates had been interpreted as resulting from the episodic interactions of acid liquid water interrupted by long-length dry periods. But these sulphates are not enough to prove that Mars experienced a warm and damp period in its early history.
Clays: the most important discovery of the decade on Mars
At the same time, but from orbit, the European Mars Express orbiter started its observations. The orbiter embarked the OMEGA infrared and visible spectrometer built up by the Institute of Space Astrophysics of Orsay (France) and LESIA. OMEGA was to find hydrated sulphates on various Martian sites but its main discovery was released in late 2005: phyllosilicates, or clays, were localised in more than some twenty sites. Unlike sulphates, the clay formation required a lot of not so much acid water during geological durations
This discovery was probably the most important for more than ten years and that success is the result of a French instrument under the responsibility of Professor Bibring and financially supported by CNES. It proved that Mars did experience a warm and damp period in its first hundreds of million years, some 4.6 billion years ago. Beyond, a global climatic alteration would have occurred which could correspond to the loss of the dynamo effect in the planet's depths and the disappearance of the dipolar magnetic field. In the absence of this protective field, the planet's atmosphere was in direct contact with the solar wind ions. These progressively triggered the escape of the atmosphere towards the interplanetary medium and the progressive disappearance of the greenhouse effect. Since the pressure and temperature were decreasing, Mars became the currently known frozen desert.
But what about tomorrow?
After the water seeking, the new priority for the current and future missions and especially the US Mars Science Laboratory and its 900 kg rover which took off in past November are the traces of life and carbon seeking.
The site selection was surrounded by numerous debates among the scientists. More than 50 sites were proposed by scientists and based on both scientific interest and landing safety criteria, only 4 sites were shortlisted: the Eberswalde, Holden, Gale and Mawrth Vallis craters. All four presented the characteristics of hosting clays observed by OMEGA. This characteristics revealed the exceptional interest of clays in the life emergence proof seeking in the Mars' early history.
The rover is scheduled to land on Mars in August 2012 and explore the clay soils looking forward to amazing discoveries.