Evidence for weathering on early Mars from a comparison with terrestrial weathering profiles

Phyllosilicates on Mars are widespread in the ancient crust suggesting the presence of liquid water at the martian surface and therefore warmer conditions during its early history. However, the role of the ancient climate in the alteration process, which produced these phyllosilicates, remains under debate, because similar mineral assemblages can be produced by hydrothermal alteration at depth. This paper focuses on the origin of coincident outcrops of Fe/Mg bearing phyllosilicates and Al-bearing phyllosilicates, which are observed in several regions of Mars. We performed a detailed mineralogical comparison between a section in Nili Fossae, Mars, and a weathering profile located at Murrin Murrin, Western Australia. The Murrin Murrin profile is developed in Archaean serpentinized peridotite massifs over a ∼40 m thick sequence. It has three alteration zones: the serpentine mineral saprolite is found at the bottom, immediately overlain by Fe/Mg-bearing smectites and then Al-bearing phyllosilicates (kaolinite) mixed with iron hydroxides. This example illustrates how Al-dominated minerals can derive from the alteration of initially Al-poor ultramafic rocks by the intense leaching of Mg2+. This mineralogical sequence is very similar to that detected locally in Nili Fossae by orbital spectroscopy. By analogy, we propose that the mineral assemblage detected on Mars is the result of long-term weathering, and thus could be the best evidence of past weathering as a direct result of a climate significantly warmer than at present.

► The role of climate in the formation of phyllosilicates observed at the surface of Mars remains at present time under debate.
► We compare a weathering profile developed in archean serpentinites in Australia and a section located in Nili Fossae in Mars.
► We discuss on the alteration processes and the related past climatic conditions which occurred at the surface of Mars.
► By analogy, we propose that the phyllosilicate assemblage detected on Mars is the result of long-term weathering.
► This could be the best evidence of past weathering as a direct result of a climate significantly warmer than the present one.