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 surface⁴. 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.

1. Institut d'Astrophysique Spatiale, CNRS/Université Paris XI, Orsay Campus, 91405, France
2. Laboratoire de Planétologie de Grenoble, CNRS/Université Joseph-Fourier, Grenoble, 38400, France
3. Laboratoire de Météorologie Dynamique, CNRS/Université Paris VI, Paris, 75005, France

Correspondence to: Yves Langevin¹ Correspondence and requests for materials should be addressed to Y.L. (Email: yves.langevin@ias.u-psud.fr).

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