Why Look for Carbonates on Mars?
The motivation to search for carbonates on Mars is the mineral’s relationship to water. Carbonates form when carbon dioxide gas dissolves in water releasing negatively charged carbonate ions (anions, CO32-) that bind to a variety of positively charged ions (cations) such as calcium or magnesium. This means that carbonate minerals precipitate out of carbon dioxide-rich solutions; they form readily in the presence of water and a carbon dioxide atmosphere. If the hypotheses for a ancient thick carbon dioxide atmosphere and water on Mars are true, including an ancient northern ocean, widespread smaller standing bodies of water, and outflow channels (as depicted in the graphic on the right), then one line of evidence would be the presence of carbonate rocks.
