This image shows the western side of an elongated pit depression in eastern Noctis Labyrinthus. Along the pit’s upper wall is a light-toned layered deposit.
CRISM spectra extracted from the light-toned deposit are consistent with the mineral jarosite, which is a potassium and iron hydrous sulfate. On Earth, jarosite can form in ore deposits or from alteration near volcanic vents, and indicates an oxidizing and acidic environment. The Opportunity rover discovered jarosite at the Meridiani Planum landing site, and jarosite has been found at several other locations on Mars, indicating that it is a common mineral on the Red Planet. [More at link]