The seasonal polar ice caps of Mars are composed mainly of CO2ice1,2. A region of low (<30%) albedo has been observed within the south seasonal cap during early to mid-spring3,4. The low temperature of this 'cryptic region' has been attributed to a clear slab of nearly pure CO2ice, with the low albedo resulting from absorption by the underlying surface4. Here we report near-infrared imaging spectroscopy of the south seasonal cap. The deep and broad CO2absorption bands that are expected in the near-infrared with a thick transparent slab of CO2ice are not observed. Models of the observed spectra indicate that the low albedo results from extensive dust contamination close to the surface of a CO2ice layer, which could be linked to atmospheric circulation patterns5,6. The strength of the CO2absorption increases after mid-spring, so part of the dust is either carried away or buried more deeply in the ice layer during the CO2ice sublimation process.