No signature of clear CO₂ ice from the 'cryptic' regions in Mars' south seasonal polar cap

The seasonal polar ice caps of Mars are composed mainly of CO₂ ice. A region of low (< 30%) albedo has been observed within the south seasonal cap during early to mid-spring. The low temperature of this 'cryptic region' has been attributed to a clear slab of nearly pure CO₂ ice, with the low albedo resulting from absorption by the underlying surface. Here we report near-infrared imaging spectroscopy of the south seasonal cap. The deep and broad CO₂ absorption bands that are expected in the near-infrared with a thick transparent slab of CO₂ ice are not observed. Models of the observed spectra indicate that the low albedo results from extensive dust contamination close to the surface of a CO₂ ice layer, which could be linked to atmospheric circulation patterns. The strength of the CO₂ absorption increases after mid-spring, so part of the dust is either carried away or buried more deeply in the ice layer during the CO₂ ice sublimation process.