Advantages of first-derivative reflectance spectroscopy in the VNIR-SWIR for the quantification of olivine and hematite
Introduction
Spectroscopic observations of the planet Mars are essential for the study of its surface mineralogy. On Earth, fine-grained deposits are considered for estimating the chemical composition of the bulk crust, and equally Martian soils have similarly been adopted to represent homogeneous samples of the Mars surface mineralogy (Yen et al., 2005). The determination of the composition and the texture of the Martian surface mineralogy can be used as a record of aqueous chemical weathering, active biological processes, planet interior composition and former habitability of Mars. The determination of surface composition by ordinary laboratory means is a long and costly process, which likewise requires the utilization of hazardous chemical reagents, for instance, total surface heavy metal concentrations are decided by wet digestion utilizing strong acids like HNO3, HCl, HF or HClO4. In contrast, Visible Near Infrared Reflectance Spectroscopy (VNIRS) combined with chemometrics is progressively being utilized as a part of surface science as a quick, cost-productive, non-dangerous and ecologically friendly substitute to conventional research facility examinations (Bellon-Maurel et al., 2010; Stenberg et al., 2010).
Even though several landers (Viking 1 and 2, Phoenix, Mars Pathfinder, Sojourner, Mars Global Surveyor, Mars Odyssey, Spirit, Opportunity and Curiosity) carried instruments to measure in situ the chemical compositions of Martian soils, the mineral compositions of these soils have not been well determined. However, in situ measurements done by Mars Exploration Rovers (MER) at both Spirit and Opportunity landing sites, Gusev crater, and Meridiani Palnum respectively, and by Curiosity at Gale crater, gave clear information about the nature of minerals in the soils near these two sites. The carried instruments of interest for the identification and quantification of minerals were the Miniature Thermal Emission Spectrometers (MiniTES) (Christensen et al., 2003), Mössbauer spectrometers (MB) (Christensen et al., 2003), Alpha Particle X-ray Spectrometers (APXS) (Berger et al., 2014; Rieder et al., 2003; Thompson et al., 2016), Sample Analysis at Mars (SAM) (Millan et al., 2019, 2016), and the Chemical and Mineralogy (CheMin) (Downs, 2015). The MiniTES developed remote measurements of emitted thermal infrared energy of several soils in many samples at the Spirit landing site in Gusev crater and the Opportunity landing site in Meridiani Planum, these two sites are on opposite sides of the planet. The results identified the presence of a variety of minerals, including olivine, pyroxene, feldspar, sulfates, silica and clays, with the olivine, pyroxenes and feldspars dominating the MiniTES data(Harry Y. McSween, McGlynn and Rogers, 2010). To achieve the mineral composition of Meridiani surface more precisely, Rogers and Aharonson(Rogers and Aharonson, 2008) used a linear least square fitting routine to model the MiniTES basaltic sand end-member spectrum of Glotch and Bandfield (Glotch and Bandfield, 2006). The Mössbauer spectrometer was used to identify and quantify the iron-bearing minerals by measuring the oxidation state of Fe at both sites. Eight Fe-bearing phases were recognized: olivine, pyroxene, ilmenite, magnetite, nanophase ferric oxide (npOx), hematite, goethite, and a Fe3+ sulfate(Morris et al., 2006). The quantification of these minerals was however being valued by using a combination of the MB data, the modeling of the MiniTES data, and the APXS data(H. Y. McSween et al., 2008). The APXS measures the abundances of elements by direct contact with the rock or the surface by measuring the backscattered alpha particles and X-Rays from the sample. According to the results of this combination and using different assumptions about the mineralogy of sulfur and chlorine, the authors separated the minerals into alteration and igneous materials and they explained the presence of alteration minerals with the igneous minerals by the limited water on the planet. The SAM instrument is a gas chromatograph-mass spectrometer, it is used to analyze the organic composition of Mars surface to support the past habitability of Mars studies. The results of SAM analysis provided evidence for the presence of organic compounds including sulfates and perchlorates in the near Martian surface(Glavin et al., 2013). The CheMin is a powder X-ray Diffraction (XRD) instrument that also has X-ray Fluorescence (XRF) capabilities. The main minerals identified by CheMin include olivines, pyroxenes and feldspars. Secondary minerals formed during alteration of the basaltic minerals such as Anhydrite, basanite, hematite, clays and quartz were also identified by CheMin. These secondary minerals provide information on climate and past habitability through time on Mars (Downs, 2015).
On the other hand, reflectance spectroscopy is one the fastest growing areas in remote sensing, minerals can be identified through their absorption features. Specifically, the parameters of the reflectance or absorption, which are the band center, shape, strength and width, at given wavelengths help determine its species(Calvin et al., 2015; Sabins, 1999). According to Goetz(Goetz et al., 2009), information on the grain size, composition and abundance of the materials being studied can be inferred from the reflectance feature. The features are determined by the positioning of the absorbing species and its location in the crystal structure of the particle(van der Meer et al., 2012). The shape of an absorption feature is also a function of bond length and the symmetry of the site, resulting in prominent alterations due to change in cations such as iron and magnesium(Adams, 1974; Cloutis and Gaffey, 1991). These parameters are the information predominantly used in spectroscopy. Two major points to consider are the position of the maximum absorption wavelength, which indicates what the material is, and the depth of the absorption, which correlates to the relative abundance of that material.(Kodikara et al., 2012). VNIR data from the Mars Express OMEGA (Observatoire pour la Minéralogie, l’Eau, les Glaces et l’Activité) in combination with information from the Mars Exploration Rover, offers wide information on the aqueous past of Mars and the mineralogical evolution of its crust. OMEGA has recorded 90% of the surface at a spatial sampling of 1.5–5 Km and showed that the Fe-bearing pyroxene is the most commonly distributed mineral detected by OMEGA on dark terrain where Martian crust is exposed. Olivine was also found in some pyroxene rich areas. OMEGA data contribute to the identification of the altered phases on Mars and to the examination for the possible role that water played in that alteration: OMEGA allows discrimination among gas, frost, ice, water absorbed, and water bound in hydrated mineral (Bibring et al., 2006). The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) that is a hyperspectral imager on the Mars Reconnaissance Orbiter (MRO) spacecraft that can cover wavelengths from 362 to 3920 nm at 6.55 nm/channel to depict crustal mineralogy and to plot the mineralogy of significant parts at high spectral resolution, and to characterize seasonal variations in atmospheric dust and ice aerosols and water content of surface materials. CRISM data allowed the discovery of thousands of new minerals and new phyllosilicate exposures with a factor-of-ten increase in spatial resolution and modest increase in spectral resolution over previous IR spectrometers (Ehlmann and Edwards, 2014).
Derivative analysis is a powerful tool for enhancing the resolution of overlapping spectra of mixtures, the elimination of contributions from backgrounds absorbances and for the quantification of substances(O’Haver, 1979). The value of the first derivative equals zero at the wavelength where the reflectance value is maximum (peak) and the values of the first derivative increase with the increase of the slope (rate of change) of the reflectance with respect to the wavelength. Szalai et al. reported that iron oxide minerals can be identified by UV-VIS-NIR reflectance measurements, with the help of the first and second derivatives. The first derivative is mainly used in the NIR range and the second derivative is used in the VIS range to eliminate the baseline shift(Szalai et al., 2013).
In this work, we intended to assess the utility of the first order derivative analysis to estimate a specific mineral concentration in a mixture by finding features that correlate to the relative concentration of this specific mineral. The selected minerals for this work are the terrestrial analogues of some of the dominant minerals on Mars, this work will be applied to study Martian terrains that do not show hydration features and to investigate its feasibility to study the hydrated phases after 2000 nm in olivine bearing mixtures.
Section snippets
Samples preparation
In order to determine specific minerals composition in mineral mixtures from spectral features, a multilevel approach was designed and followed. In this context, we have first started by evaluating this approach in the laboratory using metal oxide mixtures and in a second phase by testing this quantification method on earth minerals mixtures. Iron oxide and manganese oxides were used for the first phase of the study, and later we applied the testing method on five minerals. The five minerals
Iron oxide-manganese oxide mixtures
Fig. 2a shows the spectral profiles of various concentrations of iron oxide. The broad absorption band in the region 750–1100 nm that is due to crystal field absorption of ferrous-iron (Fe2+) is therefore a promising region to quantify iron oxide. Manganese oxide seems to drastically influence the reflectance of the spectra, with a substantial diminution of the albedo in the visible-near infrared range (more specifically between 550 nm and 1010 nm). The wavelength at minimum reflectance of
Conclusions
For the critical issue of quantifying the minerals and monitoring the changes in their concentrations on Mars, imaging spectroscopy techniques can make a significant contribution. This paper demonstrates the high potential of applying the first-order derivative to the VNIR-SWIR reflectance spectra of mineral mixtures for the identification of absorption features and optimum wavelengths that are related to the amount of olivine or hematite in a mixture. The significance of this method is that
Declaration of competing interest
The authors declare that they have no competing interests.
Acknowledgements
The authors would like to thank the anonymous the UAE Space Agency for time funding grant number Z01-2016-0016 and Mr. Cijo Xavier from Zayed University for his help in conducting the tests on the mixtures.
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