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(1,2). A region of low (< 30%) albedo has been observed within the south seasonal cap during early to mid-spring(3,4). 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(4). 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(5,6). 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.