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A history of water on Mars

Analysing the make-up of hydrated silica on Mars with NIR spectroscopy and thermal-infrared spectroscopy can provide insights into the length of time water persisted on the surface of the planet, say US and Canadian researchers.

Hydrated silica encompasses a family of minerals composed almost entirely of silica (SiO2) and water, which can either be trapped as individual molecules within the silica structure or form films on the surface of the silica and within pores. On Earth, hydrated silica is readily created when silica-based rocks are exposed to water and so the discovery of hydrated silica on the surface of Mars provides convincing evidence that water once flowed over this now-barren planet.

Hydrated silica minerals can adopt a variety of different crystalline forms, from hydrated silicate glass with a poor crystalline structure to microcrystalline quartz. Whereas the water content of hydrated silica decreases with increasing crystallinity, the degree of crystallinity is actually associated with longer exposure to water, because the crystalline forms are created by the reaction between silica-based rock and water. The longer the reaction time, the more crystalline the resultant hydrated silica.

Using NIR spectroscopy to determine the crystallinity of hydrated silica on the surface of Mars can therefore provide an indication of how long liquid water existed on the surface. This is what researchers led by Joshua Bandfield at the University of Washington in Seattle have now done with the NIR spectrometer on board NASA's Mars Reconnaissance orbiter.

To make matters more complicated, however, the degree of crystallinity of hydrated silica is also influenced by various other factors, including temperature, pressure and pH, which can all speed up the crystal-forming process. So Bandfield and his team also used the thermal imaging system on board NASA's Mars Odyssey orbiter to analyse the SiO2 abundance of the hydrated silica. They did this because these other factors should cause greater weathering of the rock, increasing the bulk SiO2 abundance.

As the researchers report in Icarus, what they discovered was that the degree of crystallinity of hydrated silica varies widely over the surface of Mars, whereas it all has a broadly similar SiO2 abundance. This indicates that the degree of crystallinity of hydrated silica has mainly been determined by the length of time the silica-based rocks were exposed to water, with the highly crystalline forms exposed to water for longer than the poorly crystalline forms. This fits in with the theory that Mars gradually lost its surface water between three and four billion years ago.

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