Aqueous minerals are minerals that form in water, either by chemical alteration of pre-existing rock or by precipitation out of solution. The minerals indicate where liquid water existed long enough to react chemically with rock. Which minerals form depends on temperature, salinity, pH, and composition of the parent rock. Which aqueous minerals are present on Mars therefore provides important clues to understanding past environments. The OMEGA spectrometer on the "Mars Express" orbiter and the MER rovers both uncovered evidence for aqueous minerals. OMEGA revealed two distinct kinds of past aqueous deposits. The first, containing sulfates such as gypsum and kieserite, is found in layered deposits of Hesperian age (Martian middle age, roughly from 3.7 to 3 billion years ago). The second, rich in several different kinds of phyllosilicates, instead occurs rocks of Noachian age (older than about 3.7 billion years). The different ages and mineral chemistries suggest an early water-rich environment in which phyllosilicates formed, followed by a dryer, more saline and acidic environment in which sulfates formed. The MER Opportunity rover spent years exploring sedimentary rocks formed in the latter environment, full of sulfates, salts, and oxidized iron minerals.