Opaline Silica on Mars
Opaline silica (SiO2 · nH2O) occurs at several locations on Mars, although the definitive identification is contentious. The confirmed finding would indicate past aqueous activity. Aqueous free silica is a product of basalt weathering, when the interaction of water with mafic (i.e., Mg- and Fe-rich, silica-poor) rock rapidly dissolves olivine, pyroxene, and glass. Opaline silica could have been precipitated in hydrothermal or lacustrine (related to a lake) evaporitic environments on Mars. On Earth, it rarely persists more than a few million years, because it rapidly transforms into microcrystalline quartz during diagenesis. The persistence of opaline silica on Mars over much longer geologic timescales (billions of years) indicates that water was not present for extended periods of time.