With its solar panels their cleanest in years, NASA’s decade-old Mars Exploration Rover Opportunity is inspecting a section of crater-rim ridgeline chosen as a priority target due to evidence of a water-related mineral. Orbital observations of the site by another NASA spacecraft, Mars Reconnaissance Orbiter, found a spectrum with the signature of aluminum bound to oxygen and hydrogen.
Researchers regard that signature as a marker for a mineral called montmorillonite, which is in a class of clay minerals called smectites. Montmorillonite forms when basalt is altered under wet and slightly acidic conditions. The exposure of it extends about 800 feet (about 240 meters) north to south on the western rim of Endeavour Crater…. [More at link]