The mystery of where all the water-altered minerals are hiding on Mars may have been solved by the Mars Express orbiter. The riddle arises due to abundant evidence which points to the activity of running and standing water across the face of Mars, but with very few signs of the minerals that the interaction of water and volcanic rocks should produce - such as carbonates and sulphates.One exception is the haematite - a water-altered iron mineral - that led NASA to land Opportunity on Meridiani Planum. The absence of other such minerals was particularly puzzling in these vast, flat plains, which otherwise show strong evidence of once having been the basins of ancient Martian oceans.But now, new data from the Omega visible and near-IR imaging spectrometer onboard Mars Express has found a large region - 60 kilometres by 200 km - that shows the clear spectral signature of calcium-rich sulphates, probably gypsum.

This means that at least a portion of that northern "ocean" area was indeed covered by standing water for a long time.The discovery is a major development in the ongoing quest for evidence of past Martian water. Writing this week in Science, a team led by Yves Poulin of the Institut d'Astrophysique Spatiale in Orsay, France, say that "these observations demonstrate that water alteration played a major role in the formation of the constituting minerals of northern circumpolar terrains".The spectral data, covering 352 wavelength bands, very closely match the spectrum of gypsum. So close, in fact, that the researchers have called it an "unambiguous detection of gypsum".

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