One of the most encouraging signs for those searching for evidence of early water on Mars has been the existence of clays. But a French-US team has now rained on that particular parade.
Discovered in 2005, the iron- and magnesium-rich clays were believed to have originated between 4.5 and 4 billion years ago as liquid water decomposed rock.
However, new research on rocks on Earth indicates that the Martian clays are probably of magmatic origin.
The team, headed by researchers from the Institut de Chimie des Milieux et Matériaux, studied basalts from the Mururoa Atoll in French Polynesia.
These basalts are composed of crystals, with the spaces between filled with a finely-crystallized material called mesostasis. This contains ferro-magnesian clays similar to those detected on Mars.
And, say the researchers, these clays were 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 out, forming various minerals including clays.
Martian magma has a high enough water and chlorine content for this process to have produced clays there too. And it's already known that in the planet's earliest days it was covered by a magma ocean, just like the early Earth.
On top of this, the team has also discovered 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 is bad news for those hoping for signs of ancient life. There's still plenty of evidence for the existence of liquid water around three billion years ago - traces of rivers, lakes and alluvial fans. But, says the team, there's nothing to suggest that it was there as far back as four billion years ago, as was believed until now.
In other words, the conditions neccessary for life didn't exist on Mars for nearly as long as previously thought, they say.
The Curiosity mission may soon supply more information. It's set to explore part of the Gale crater, for example, where sedimentary formations are believed to be evidence of more recent liquid water.