Detection of Serpentine on Mars by Mro-crism and Possible Relationship with Olivine and Magnesium Carbonate in Nili Fossae

Introduction: Serpentines are 1:1 trioctahedral phyllosilicates with general formula (Fe,Mg)6Si4O10(OH)8 and form during metamorphism, hydrothermal activity, or weathering of ultramafic rocks. Serpentine is an expected aqueous alteration product on Mars, given the mafic to ultramafic crust and prevalence of olivine. Serpentinziation reactions produce H2, which can serve as an important energy source for chemosynthetic organisms [1] or react abiotically with CO2 to produce methane [2]. The detection of methane, with heterogeneous distribution and seasonal variation in abundance, has been reported in data from the Planetary Fourier Spectrometer [3] and from ground-based observations [4]. We report here the detection of serpentine by the CRISM spectrometer on the Mars Reconnaissance Orbiter in the Thaumasia and Nili Fossae regions of Mars [5]. A possible relationship with the olivine-Mg carbonate rocks in Nili Fossae [6] is discussed. Identification and occurrences of serpentine: In three CRISM images to date, a spectral class occurs which is distinguished by a sharp absorption at 2.32 μm and additional absorptions at 2.51 and 2.11 μm (Fig. 1). There is a 1.9 μm band indicating the presence of H2O, and which varies in strength and breadth in the three spectra. There is an additional sharp vibrational band at 1.39 μm. All of these features are consistent with the presence of an Mg serpentine e.g. antigorite, lizardite, or chrysotile. Serpentines have numerous combination overtones of metal-OH bends and stretches from 2.3-2.7 μm [7-9]. The strongest of these is centered at 2.32 μm for Mg-OH in Mg serpentines. In Fe serpentines, this band is at longer wavelengths and sometimes accompanied by aluminum bands at shorter wavelengths [10]. The 2.50-2.51 μm band is the next strongest of these features. Serpentines also have a diagnostic band at 2.10-2.12 μm. Near 1.4 μm, serpentines have an OH stretch overtone at 1.39 for Mg serpentines which shifts to somewhat longer wavelengths (1.40-1.41 μm) for Fe serpentines [7]. Serpentines can be distinguished from the Mg smectite, saponite, which also has a band at 2.32 μm, by the sharpness and position of the 1.39 band and the additional band at 2.10 μm (Fig. 1). Stoichiometric serpentine does not have molecular water in its structure which would create the 1.9 μm band so we are likely observing serpentine in mixtures. In all three images, the area of the surface with this spectral signature is less than 7x7 CRISM pixels (18m/pixel).