Seeking phyllosilicates in thermal infrared data: A laboratory and Martian data case study

Previous analyses of Thermal Emission Spectrometer (TES) data produce results that could suggest a widespread distribution of phyllosilicate minerals over the surface of Mars, whereas studies of visible to near-infrared (VNIR) data indicate a more limited distribution. We use VNIR detections of phyllosilicates in the vicinity of the Nili Fossae to determine the spectral characteristics of phyllosilicate-bearing material in the thermal infrared (TIR). By investigating areas of VNIR phyllosilicate detection in more detail, we find that the phyllosilicate-bearing material corresponds to spectral variation in Thermal Emission Imaging System decorrelation-stretched TIR images and differences in infrared spectral shape that are consistent with, but not uniquely attributable to, mixtures of phyllosilicates and basalt. Phyllosilicate phases are modeled from TES data at abundances that average 5% over the region and at abundances near the 10-15% detection limit in our specific regions of interest. Deconvolution of numerical mixtures of phyllosilicate and basalt spectra indicates that these low abundances of phyllosilicates likely are not influenced by uncertainties greater than the 10-15% uncertainty of the method. TES spectra and modeled abundances vary between the phyllosilicate-bearing material and the surrounding region, but this difference in composition cannot be attributed solely to the presence of phyllosilicates. We believe the inconsistencies in phyllosilicate occurrence between TES and VNIR analyses may be explained by the inclusion of phyllosilicates in the models of TES data as substitutes for poorly crystalline phases (e. g., allophane) not currently available in public infrared spectral libraries.