The discovery would provide strong evidence that liquid water once flowed on the Red Planet. Such carbonates might also harbor the fossils of ancient Martian bacteria. It would be just a zinger of a finding. Nanxu arch in the Guilin karst region, China. Note the white color of the carbonate cliff face.
Carbonate rock can also be pink or other colors, depending on the impurities present.
Image courtesy of Peter L. Smart, University of Bristol. Both means 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 such as the white cliff faces pictured above.