The Mars Reconnaissance Orbiter has found evidence of hydrated silica or opal, a form of mineral, over large areas in the Martian surface, including in the large martian canyon called Valles Marineris. The discovery was made by the Compact Reconnaissance Imaging Spectrometer (CRISM) instrument on the orbiter. The findings are published by Ralph Milliken of Jet Propulsion Laboratory and other scientists in the November issue of the journal Geology.
Opaline minerals were first found recently on Mars by Spirit rover in the Gusev crater.  The present find points to more widespread occurrence of the minerals in comparatively younger strata of Mars.  The find indicates that liquid water might have been present on the surface of Mars for a longer time than previously thought.  The previous view was that liquid water disappeared from the Martian surface three billion years ago. Now it is estimated that water could have been present as late as two billion years ago.
The presence of opal not only indicates water, but also that it was there long enough to alter some of the rocks. CRISM, which detected the mineral measures visible and infrared reflection spectra in 544 channels and has 20 times better resolution than previously deployed instruments. 
—Dr Scott Murchie, Johns Hopkins University Applied Physics Laboratory, MarylandHydrated minerals such as opal that indicate presence of water is yet another evidence of presence of liquid water on Mars. Opal found on Earth's surface usually contain 3 – 10% water, but can be as high as 20%.  Other water-bearing minerals found earlier are phyllosilicates and hydrated sulfates.  Presence of liquid water is also suggested by suspected water-sculpted land forms on Mars, such as gullies and river channels.  Currently water is present only as ice at both polar caps of Mars.
A number of outcrops of opaline minerals are found as thin layers over large distances, rimming the Valles Marineris canyon. It is expected that Martian explorations for past or present life will have to focus on similar younger terrains with hydrated minerals.
"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, from Johns Hopkins University Applied Physics Laboratory in Maryland, one of the co-authors. 

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Mars orbiter finds widespread evidence of water-bearing minerals