IRIS Mariner 9 data revisited:: 2. Aerosol dust composition

Abstract

Most of the IRIS Mariner 9 data show strong absorption features due to dust in the Martian atmosphere. After averaging thousands of spectra, an instrumental effect has been identified and studied. Once the data are corrected for the instrumental effect, they allow the study of Martian dust composition. Spectra have been classified according to the depth of the main dust band, quantified here by a numerical parameter D. Measurements acquired in conditions of very low temperature have not been included in the study. Data inside each class have been averaged to get a better signal to noise ratio. In order to compute transmittance related to dust, a synthetic spectrum for a pure CO 2 atmosphere has been computed using the vertical temperature profile obtained by inversion of the 15-μm CO 2 band in each average spectrum. A simple numerical filtering has allowed us to locate the positions of minor bands (inside the main dust absorption feature); their depths are directly correlated to D values, and are therefore ascribed to dust. The locations of the transmissivity maxima and minima are indicative of a high silicon content in the suspended materials, while a direct comparison with transmittance spectra shows a strong similarity with albite, that is proposed here as the main dust component. Another feature around 1650 cm −1 is easily seen in average spectra and could be indicative of sulfate materials. A comparison between the elemental composition of albite (plus sulfate) and the one measured for loose soil at the Pathfinder site suggests the following dust composition (by mass): 50% albite, 10% sulfates, 20% pyroxenes, 20% olivines, although additional components are not excluded.