Phyllosilicates in the Mawrth Vallis region of Mars

[1] OMEGA/Mars Express has discovered large outcrops rich in phyllosilicates in the region of Mawrth Vallis, Mars (around 20 W, 25 N). The region is located in Noachian highly cratered terrains, close to the limit of the Martian dichotomy, where the outflow channel Mawrth Vallis cuts the highlands. We have examined this region using OMEGA spectra of the surface from 0.9 mm to 2.6 mm, with spatial sampling from 500 m to 3 km, offering a full coverage of the region. OMEGA spectra show two broad bands centered at 1 mm and 2.2 mm, revealing the presence of clinopyroxene on dark surfaces. Phyllosilicates have been identified by absorption bands at 1.4 mm, 1.9 mm, and 2.2 or 2.3 mm. Comparison with laboratory spectra reveals similarities with Al-OH smectites such as montmorillonites, or Feor Mg-OH smectites such as nontronite. A precise location of the phyllosilicate-rich areas on visible HRSC images indicates that they are placed exclusively on bright outcrops, mostly on the plateaus, dated to the Noachian period. On HRSC and MOC images the phyllosilicate-rich outcrops reveal strong erosional features such as numerous residual buttes composed of layers a few meters thick. The phyllosilicate-rich unit corresponds to a geological unit more than 100 m thick, over a horizontal extension approximately of 300 km 400 km. This unit implies a large volume of altered rocks, either in situ or after transport and deposition, in Noachian terrains, revealing a different climatic and geologic environment from the present one.