Detection of Kaolinite at Mawrth Vallis, Mars: Analysis of Laboratory Mixtures and Development of Remote Sensing Parameters

Introduction: Laboratory mixtures and spectral parameters were created to better characterize and detect kaolinite in the Mawrth Vallis region of Mars where Al-phyllosilicates, including kaolinite and montmorillonite, have been detected. We have prepared laboratory mixtures of the phyllosilicate minerals montmorillonite with kaolinite in seven intervals to better characterize spectra of Mars. Kaolinite and montmorillonite have been detected in hyperspectral visible/near-infrared (VNIR) Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) spectra of Mawrth Vallis [1] and continuing analyses of these spectra suggest that mixtures of kaolinite with montmorillonite and/or hydrated silica best replicate the spectra observed at Mawrth Vallis [2]. Spectra taken from the laboratory mixtures were used to gain information on detection limits for these phyllosilicate minerals in CRISM images. Laboratory spectra and CRISM spectra were used for development of an algorithm that detects kaolinite in CRISM images. Methods: The kaolinte (KGa-1b JB766) from Washington County, GA, and montmorillonite (SWy-2 JB784) from Crook County, WY, were dry sieved to <125 μm. The samples were mixed from pure montmorillonite to pure kaolinite at intervals of 15, 25, 30, 40, 50, 60, 75 wt.% kaolinite. The mixtures were then sieved again at <125 μm to homogenize the samples. Spectra were then measured using an ASD handheld spectrometer. Spectra from both the lab mixtures and Mawrth Vallis, Mars, were used to write an algorithm for detection of kaolinite in the CRISM images. Several CRISM images of Mawrth Vallis were examined and the purest kaolinite endmember pixels were identified and used as reference points for coding and testing the algorithm. Mineral detection parameters have been generated for use in analyzing CRISM images [3] and a parameter to measure band depth at 2.2 !m has been successful in detecting Al-phyllosilicates at Mawrth Vallis [1] and elsewhere on Mars [4]. Kaolinite spectra exhibit a characteristic doublet absorption feature with minima centered at 2.17 and 2.21 !m. Two separate parameters were written to describe and identify this feature in the images. One algorithm calculates the spectral slope of the maxima in between the doublet absorptions (slope from ~2.17-2.18 μm), while the other algorithm uses a spectral parameter to measure the band depth of the kaolinite absorption at 2.17 μm. Results: Mixture spectra are shown in Figures 1-2, a CRISM spectrum from Mawrth Vallis dominated by kaolinite is shown in Figure 3, and a map of kaolinitebearing regions in Mawrth Vallis image FRT848D is shown in Figure 4A,B. Kaolinite-montmorillonite Mixtures. The laboratory mixtures show that the spectrum of pure kaolinite has a doublet at 2.17 and 2.21 μm, while the pure montmorillonite spectrum has only a single absorption at 2.21 μm. The mixture spectra show that with decreasing kaolinite abundance, the 2.17 μm feature becomes shallower, as expected. At 60 wt.% kaolinite the 2.17 μm absorption feature becomes a shoulder and at 15 wt.% kaolinite the feature is no longer detectable.