1. Institut d'Astrophysique Spatiale, Orsay Campus, 91405, France
2. LESIA, Observatoire de Paris/Meudon, 92195 Meudon, France
3. IFSI-INAF, Rome, Italy
4. IKI, Moscow, Russia
5. OrsayTerre, Orsay Campus, 91405, France
6. Laboratoire de Planétologie de Grenoble, 38400, France
7. A list of all the members of the OMEGA team and their affiliations appears at the end of the paper

Correspondence to: Jean-Pierre Bibring¹ Email: bibring@ias.u-psud.fr

Abstract

The inventory of water and carbon dioxide reservoirs on Mars are important clues for understanding the geological, climatic and potentially exobiological evolution of the planet¹. From the early mapping observation of the permanent ice caps on the martian poles^{2, 3}, the northern cap was believed to be mainly composed of water ice, whereas the southern cap was thought to be constituted of carbon dioxide ice. However, recent missions (NASA missions Mars Global Surveyor and Odyssey) have revealed surface structures⁴, altimetry profiles⁵, underlying buried hydrogen⁶, and temperatures of the south polar regions that are thermodynamically consistent with a mixture of surface water ice and carbon dioxide⁷. Here we present the first direct identification and mapping of both carbon dioxide and water ice in the martian high southern latitudes, at a resolution of 2 km, during the local summer, when the extent of the polar ice is at its minimum. We observe that this south polar cap contains perennial water ice in extended areas: as a small admixture to carbon dioxide in the bright regions; associated with dust, without carbon dioxide, at the edges of this bright cap; and, unexpectedly, in large areas tens of kilometres away from the bright cap.

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