Carbonate rocks on Earth can either be formed through a purely chemical process or by the action of living things. Both require liquid water. The chemical pathway involves carbon dioxide gases that dissolve in surface waters. CO2 molecules combine with water to form carbonate ions, which in turn join with calcium or magnesium to create a solid that settles onto the sea floor. Geologic changes can later expose such deposits, revealing beautiful features.
Marine organisms can also produce carbonates for shells or other hard parts.
When these organisms die, their shells sink to the bottom. They then accumulate and eventually form a carbonate deposit at the bottom of the ocean. One example of this type of carbonate is blackboard chalk, which comprises the majority of carbonates in our planet's crust. When viewed through an IR spectrometer, carbonate compounds have a distinctive infrared signature. Scientists had hoped this sensor would find regions of exposed carbonate among the Martian landscape.