Carbonate minerals are important indicators of past liquid water on Mars. They form when water containing dissolved carbon dioxide interacts with rock, and their chemical makeup reflects the conditions of the water and the rock involved. However, the amount of carbonate found on Mars is much lower than expected based on climate models, which suggest the planet once had a thick atmosphere rich in carbon dioxide over 3 billion years ago. Scientists are puzzled by the missing carbon dioxide and the fact that the carbonates found on Mars fall into two distinct chemical groups: those rich in calcium and iron, and those rich in magnesium. This mystery has led researchers to re-examine the types of rocks that water might have interacted with in Mars' ancient past. Most previous studies assumed that water reacted primarily with mafic rocks, which are rich in iron and magnesium and are the most common type of rock on Mars. However, recent observations from orbiters and rovers have revealed the presence of feldspar-rich rocks, which contain higher amounts of calcium, sodium, and aluminum. These rocks may have once been more widespread on the Martian surface than previously thought. A recent study led by Chang-Chin Wang, a doctoral student at the University of Tokyo, explored whether the composition of the original rock—called the protolith—could explain the presence of calcium/iron-rich carbonates on Mars. Using a geochemical model, the researchers simulated water moving through columns of either mafic or feldspar-rich rock under conditions similar to those on ancient Mars, which were cold and rich in carbon dioxide. The simulations tracked how water chemistry changed as it dissolved minerals and formed new ones, considering both short and long-term interactions. The results showed that feldspar-rich rock tends to produce calcium/iron-rich carbonates under most conditions, while mafic rock only does so during brief interactions before the chemistry shifts toward magnesium-rich carbonates. This finding provides a new way to interpret surface samples: if calcium/iron-rich carbonates are found without traces of olivine—a magnesium-rich mineral that only survives limited water-rock interaction—then feldspar-rich rock is likely the source. The study also found that groundwater flowing through rock was more effective at forming carbonates than standing water. However, groundwater tended to dissolve carbonates near the surface and reprecipitate them deeper underground. This could explain why Mars might have a large reservoir of carbonates and the carbon dioxide they contain hidden beneath the surface, despite the current lack of visible carbonate deposits. The findings suggest that the chemical differences in Martian carbonates are not only a record of past climate and water activity but also reflect the types of rocks the water interacted with. This insight points to feldspar-rich areas and subsurface drilling as valuable targets for future missions aiming to uncover evidence of Mars' wetter past.