A Reinterpretation of Ism Data: Quantitative Analysis of Pyroxene

Introduction: Near-infrared spectra of pyroxenes have distinct absorption features at approximately 1 and 2 µm. The positions of these features are dependent on the composition of the pyroxene [1,2]. Pyrox-ene spectra have been observed on the Martian surface [e.g., 3], and at high spectral resolution by the ISM imaging spectrometer [4] on board the Phobos-2 spacecraft in 1989. Previous researchers [5] have de-convolved ISM spectra to identify the components of the spectra which comprise the pyroxene signal and determined the 1 and 2 µm band center locations. We have developed a quantitative model for determining pyroxene Ca/Fe/Mg composition from the center position of the 1 and 2 µm features in laboratory pyroxene spectra. We have applied our model to the identified band centers of the ISM pyroxene spectra to more precisely estimate the compositions of pyroxenes on the Martian surface. Methods: Spectra from 24 pyroxene samples [1] were used to determine a model relationship between Ca/Fe/Mg content and 1 and 2 µm absorption feature centers. Four spectra were not used because visual inspection of the spectra indicated contamination by other materials. The remaining 20 spectra were de-convolved into one linear wavelength continuum and five Gaussian functions, two of which indicated the 1 and 2 µm features. We used a multiple linear regression analysis to model Ca and Fe content as a function of 1 and 2 µm band center position. Mg content is a constrained dependent parameter; the selection of Mg as the dependent parameter is arbitrary. Separate models were developed for low-calcium pyroxenes (LCP) and high-calcium pyroxenes (HCP). Results: The linear regression analysis yielded equations for calculating the Ca and Fe content of both LCP and HCP. The model uncertainty (error) in Ca was 2.2% and 2.5% for LCP and HCP, respectively. For Fe content, the model uncertainty was 2.8% and 1.8% for LCP and HCP, respectively. Mustard et al. [5] applied the Modified Gaussian Model [6] to determine band centers for the 1 and 2 µm features observed in ISM data of Mars. We applied our composition model to these data and calculated the compositions of the pyroxenes evident in the ISM data (Figure 1). Discussion: The ISM spectra show a mixture of HCP and LCP. Based on the strength of the 1 and 2 µm absorption features, the relative abundance of