The messages we receive from Mars about the presence of carbonates are quite contradictory. On the one hand, images of the planet clearly show the signatures of past bodies of standing water, where the accumulation of sedimentary deposits should have occurred. On the other hand, the apparent absence of carbonates in the homogeneous fines covering the Martian surface raises the question: where could carbonates hide now? In this paper we discuss the subject and demonstrate that a proposed destruction mechanism, the photodecomposition caused by UV radiation, cannot account alone for the missing carbonates. By means of simulations we show that the difficulties in detecting the carbonates can be ascribed to the low temperatures of the emitting zones, observed up to now. Finally, we derive the minimum amount of such minerals that could possibly be detected in future observations of Mars, and we compare the results of simulations with observed spectra. ¿ 2001 American Geophysical Union