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

Abstract

The seasonal polar ice caps of Mars are composed mainly of CO2 ice1,2. 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 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 patterns5,6. 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.

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Figure 1: Reflectance spectrum of a region at 344° E, 59° S observed by OMEGA at L s 142°.
Figure 2: Albedo of the southern seasonal cap in mid-spring at 1.08 µm (continuum).
Figure 3: Reflectance spectra of representative regions within the southern seasonal cap.
Figure 4: Spectral evolution of a representative area in the cryptic region at 80° E, 82.5° S.

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Acknowledgements

We acknowledge the support of CNES, which funded the development of OMEGA and its scientific exploitation. Author Contributions Y.L., J.-P.B. and B.G. were responsible for data reduction and observation planning; S.D., F.P. and B.S. contributed to the spectral modelling from optical constants; M.V. performed the modelling of the aerosol contribution; and F.F. provided input on the link with general circulation models.

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Correspondence to Yves Langevin.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

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Supplementary Discussion

This file provides details on the procedures implemented for evaluating the contribution of aerosols, correcting for atmospheric absorption features and modelling spectra. (PDF 71 kb)

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Langevin, Y., Douté, S., Vincendon, M. et al. No signature of clear CO2 ice from the ‘cryptic’ regions in Mars' south seasonal polar cap. Nature 442, 790–792 (2006). https://doi.org/10.1038/nature05012

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