THERE is indirect spectroscopic evidence for the presence of sulphates and carbonates on the martian surface(1-5), and such minerals are also found in SNC meteorites(6), which are thought to be of martian origin. But although carbonates are expected to be abundant in the martian regolith(7-9), attempts to detect them directly have been unsuccessful(10,11). Here we report laboratory studies of the decomposition of calcium carbonate and magnesium sulphate under ultraviolet irradiation, which mimic the conditions under which photodecomposition of surface minerals by solar ultraviolet light might occur on Mars. We find that, even for a low abundance(6) of carbonate minerals in the martian regolith, the rate of CO2 release due to photodecomposition is higher than the rate of CO2 loss from the atmosphere by solar-wind-induced sputtering processes(12-15), making this profess a potential net source of atmospheric CO2 over time. SO2 is also released from the sulphate, albeit more slowly. The rate of carbonate degradation is high enough to explain the apparent absence of these compounds at the martian surface.