The Distribution and Composition of Olivine on Mars

Introduction: The global distribution and composition of olivine is an important clue in understanding the internal and external evolution of Mars. Olivine exhibits a range of Mg:Fe in the solid solution from forsterite to fayalite; these compositional changes lead to distinguishable spectral variations in laboratory data [1, 2, 3]. Olivine is a significant component of several Martian meteorites and has been identified on the surface of Mars using thermal infrared (TIR) remote sensing techniques [4]. Previous remote sensing studies in TIR data have been limited to a particular geographic region [5, 6] or have not included a wide range of olivine compositions [7, 8]. Approach: We acquired emission spectra [9] of granular samples of olivine with compositions Fo68, Fo60, Fo53, Fo39, Fo35, Fo29, Fo18, and Fo10 [10]. The TES spectral library [11] contains spectra of Fo91 and Fo1, and together these data provide a broad range of olivine chemistries. These spectra were convolved to the spectral resolution of the MGS Thermal Emission Spectrometer (TES) to compare with the Martian global dataset. We used two complimentary techniques, spectral indices and deconvolution, to map the occurrence and abundance of olivine on Mars from these data. Spectral index results are presented here; global deconvolutions are in progress. TES spectra are collected from ~1650 to 200 cm -1 (~6 – 50 m) at 10 cm -1 sampling and have spatial footprints of 3 ~6 km on the surface of Mars [12]. We analyzed data meeting specific quality constraints, e.g. temperature > 250 K, albedo < 0.2, low risk of phase inversions, etc. Spectral Indices: Spectral indices are simple mathematical tools that calculate the strength of a specific spectral feature associated with a particular phase. They are computationally fast and can utilize a portion of the spectrum relatively unaffected by microphonic noise present in TES data collected during later orbits [7, 8]. They can, therefore, be run over the entire TES dataset (~9 million spectra). We created spectral indices to identify TES spectra that contain a strong emission minimum from 20 – 40 m characteristic of olivine. In olivines, spectral features in the TIR shift to longer wavelengths as Mg:Fe decreases [1, 3], and we created an index for each olivine composition(s) distinguishable by this feature. The general form of the band indices is: I fo# = 1 + 3 ( ) /2 2 where each is either a single emissivity value, or the average of adjacent emissivity values, on the spectrum. In our indices, 2 is at the feature minimum and 1 and 3 are values on the feature’s flanks; index values greater than 1.0 indicate the presence of an emission minimum. Index values for each olivine composition or range were calculated for the TES spectral library to identify any ambiguous detections from alternate minerals; however, high index values for this particular spectral feature were only received for a few carbonates, phases not observed in the dark regions of Mars [13]. Unfortunately, the Martian hematite spectrum derived from target transformation [14] is slightly shifted compared to the TES library spectrum, and its minima overlie the index bands for Fo1-10 obscuring index mapping of fayalite.