Abstract

The recent identification of large deposits of sulphates by remote sensing and in situ observations has been considered evidence of the past presence of liquid water on Mars. Here we report the unambiguous detection of diverse phyllosilicates, a family of aqueous alteration products, on the basis of observations by the OMEGA imaging spectrometer on board the Mars Express spacecraft. These minerals are mainly associated with Noachian outcrops, which is consistent with an early active hydrological system, sustaining the long-term contact of igneous minerals with liquid water. We infer that the two main families of hydrated alteration products detected—phyllosilicates and sulphates—result from different formation processes. These occurred during two distinct climatic episodes: an early Noachian Mars, resulting in the formation of hydrated silicates, followed by a more acidic environment, in which sulphates formed.

1. Institut d'Astrophysique Spatiale, Université Paris-Sud and CNRS (UMR 8617) F-91405 Orsay, France
2. Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
3. Interactions et Dynamique des Environnements de Surface, F-91405 Orsay, France
4. Earth and Planetary Sciences, Washington University, St Louis, Missouri 63130, USA
5. IAS, Université Paris-Sud, F-91405, France
6. LESIA, Observatoire de Paris/Meudon, 92195 Meudon, France
7. IFSI-INAF, Via del Fosso del Cavaliere, 00133 Rome, Italy
8. Space Research Institute (IKI) 117997, 84/32 Profsoyuznaya Str, Moscow, Russia
9. IDES, Université Paris-Sud, F-91405 Orsay, France
10. Observatoire Midi-Pyrénées, 31000 Toulouse, France
11. Laboratoire de Planétologie, 38400 Toulouse, France
12. Planétologie, Université de Nantes, 44322 Nantes, France
13. DLR, Berlin, 12489, Germany
14. MPAE, Lindau, Germany
15. Earth and Planetary Sciences, Washington University, Saint-Louis, Missouri 63130, USA
16. Geological Sciences, Brown University, Providence, Rhode Island 02912, USA
17. LMD, Université de Paris 6, 75252, Paris, France.

Correspondence to: F. Poulet¹ Correspondence and requests for materials should be addressed to F.P. (Email: francois.poulet@ias.fr).

Received 28 July 2005; Accepted 27 September 2005

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