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A NASA space probe orbiting Mars has discovered deposits of opals in the mighty Valles Marineris canyon system* east of Tharsis. Opals aren't valuable enough to justify interplanetary trade, but the discovery is significant as it suggests that liquid water existed on Mars a billion years more recently than had been thought.
NASA orbital pic showing the Martian opal deposits
The opal mines of the Valles Marineris.
"This is an exciting discovery because it extends the time range for liquid water on Mars, and the places where it might have supported life," said Scott Murchie of Johns Hopkins University, in charge of the Mars Reconnaissance Orbiter's spectrometer scope.
"The identification of opaline silica tells us that water may have existed as recently as 2 billion years ago."
Until lately, Mars boffins had reckoned that the last liquid water disappeared from the Red Planet's surface at least three billion years ago. This was based on discoveries of "clay-like" phyllosilicates and hydrated sulphates. But the new opal finds have caused NASA to revise the date of the Great Drying forward by a billion years.