Mars' oldest rocks are found in its southern hemisphere: the crust of this region formed between 4.5 and 4 billion years ago. It is there that iron- and magnesium-rich clays were discovered in 2005. The presence of this type of mineral, considered to have originated from the decomposition of rocks through the action of liquid water, was interpreted as indicating that water was present on the Martian surface in the distant past. However, a team of researchers has shown that these clays are probably of magmatic origin.
To lend weight to their hypothesis, the researchers studied basalts from the Mururoa Atoll (French Polynesia). These basalts are composed of well-formed crystals between which there are small spaces filled with a finely-crystallized material called mesostasis. This contains ferro-magnesian clays similar to those detected on Mars. The researchers have shown that these clays formed from residual water-rich magmatic liquids trapped in the empty spaces between the crystals. When the magma finally cooled, the constituents of these residual fluids precipitated, forming various minerals including clays. In this case, no aqueous alteration took place.
The scientists noticed that the Martian magma fulfilled all the conditions, in particular a high water and chlorine content, for this process to have been able to produce abundant clays on the basaltic surface of Mars. In addition, it is known that shortly after Mars' formation it was covered by a magma ocean, just like the early Earth. During this period clays could have formed. On top of this, they also showed that the infrared spectrum of the Martian clays measured by the orbiters Mars Express and Mars Reconnaissance Orbiter is identical to that of the Mururoa clays.
This work may have repercussions on the search for indicators of life on Mars. Although the presence of liquid water around 3 billion years ago is attested to by traces of rivers, lakes and alluvial fans, there is nothing to suggest that it existed at periods as remote as 4.5 to 4 billion years ago, as believed until now. The period of time favorable to the emergence of life on Mars may have been much shorter than thought. The Curiosity mission, which will explore part of the Gale crater on Mars, whose sedimentary formations are evidence of the presence of liquid water at a much more recent period, should make it possible to clear up a certain number of doubts.
Magmatic precipitation as a possible origin of Noachian clays on Mars. Alain Meunier, et al., Nature Geosciences, on line 9 September 2012.
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