The silicate component of Martian dust

Abstract

Absorption features in telescopic reflectance spectra of Mars during 1978 were detected and analyzed. Also detected and analyzed were absorption features in Mariner 7 Infrared Spectrometer and Mariner 9 Infrared Interferometer Spectrometer spectra. The Bands Data Analysis System described by R. L. Huguenin and J. L. Jones (1986, J. Geophys. Res. 91, 9585–9598) was used. Atmospheric CO 2 bands were all detected with an average error of 50 cm −1, providing a test of the sensitivity and accuracy of feature extraction from the Mars data. Absorption features that were attributed to H 2O ice were detected in the North Polar Cap region, as well as in regions to the north and east of Hellas basin, and near the Elysium Montes. Additional absorptions were assigned to structural hydroxyl within a strongly hydrogen-bonded acidic material. Features in the Mariner 9 spectra suggested that the material may be a silicate. Hydroxyl stretch fundamentals were deduced to occur at 2661 and 2824 cm −1, consistent with acidic material having strong hydrogen bonding. In-plane and out-of-plane structural hydroxyl deformation fundamentals were proposed to occur at 1498 and 909 cm −1, respectively, from which a hydrogen bridge length of ∼2.4 A ̊ was derived. SiO b (bridging oxygen) and SiO t (terminal oxygen) stretch fundamentals were deduced to occur at 1176 and 995 cm −1, respectively. SiO b and SiO t asymmetric bend fundamentals were deduced to occur near 482 and 397 cm −1, respectively, while a SiO bSi deformation fundamental was proposed to occur near 697 cm −1. These fundamentals suggested that the SiO bSi bond angle may be approximately linear (170°) and that the estimated SiO b bridge length may be ∼1.61 A ̊ . The totally symmetric silicate stretch fundamental was deduced to occur near 894 cm −1, from which silicate polymerization equivalent to O/Si = 3.8 ± 0.2 was derived. This is consistent with the derived SiO bSi bond angle and bridge length. An in-plane MOH libration fundamental near 678 cm −1 was derived, consistent with Ca 2+ and/or Mg 2+ being the dominant cations in the vicinity of OH −. An analog compound that has a very similar set of structural hydroxyl, silicate, and librational fundamentals is Ca 2(HSiO 4)OH, with principal differences proposed to be due to the 0.2 ± 0.2 lower O/Si ratio and the 0.08-Å shorter hydrogen bridge length in the Mars material. Viking compositional data suggest that Mg 2+, rather than Ca 2+, may be the dominant cation in the Mars material. The differences from the analog phase properties suggest that the material may be similar to H-forsterite (Fo 79). It is proposed that the silicate component of the dust may be an incipient alteration (hydrolysis) product of olivine-rich ultramafic or mafic material, involving a process that resulted in minimum loss of mobile cations and that perserved the high O/Si ratio of the starting material.