1,720,998 research outputs found
Visible and Near-Infrared (VNIR) reflectance spectroscopy of glassy igneous material: Spectral variation, retrieving optical constants and particle sizes by Hapke model
Full text linkSilicate glasses with igneous compositions can be an important constituent of planetary surface material via effusive volcanism or impact cratering processes. Different planetary surfaces are mapped with hyper-spectrometers in the VNIR, and in this spectral range crystal field absorptions are useful in discriminating iron bearing silicate components. For these reasons studying glassy materials, and their optical constants, is an important effort to better document and understand spectral features of Solar System silicate crusts where glasses are present, but may be difficult to map. In our work we present a set of four different synthetic glasses, produced under terrestrial conditions, with variable composition and in particular an increasing amount of iron. The VNIR spectra show, for all the compositions, two absorptions are present near 1.1 and 1.9. μm but reflectance, slope and absorption shape varies with composition. We measured the reflectance of different particle sizes of the samples and used radiative transfer models to estimate the optical constants as a function of wavelength. We used the retrieved optical constants to estimate the particle size from the measured reflectances and the results fall within the known sieve range. We qualitatively discuss the effect of the shape and distribution of particles on the application of the model
Exponential Gaussian approach for spectral modeling: The EGO algorithmI. Band saturation
No full textCurve fitting techniques are a widespread approach to spectral modeling in the VNIR range [Burns, R.G., 1970. Am. Mineral. 55, 1608–1632; Singer, R.B., 1981. J. Geophys. Res. 86, 7967–7982; Roush, T.L., Singer, R.B., 1986. J. Geophys. Res. 91, 10301–10308; Sunshine, J.M., Pieters, C.M., Pratt, S.F., 1990. J. Geophys. Res. 95, 6955–6966]. They have been successfully used to model reflectance spectra of powdered minerals and mixtures, natural rock samples and meteorites, and unknown remote spectra of the Moon, Mars and asteroids. Here, we test a new decomposition algorithm to model VNIR reflectance spectra and call it Exponential Gaussian Optimization (EGO). The EGO algorithm is derived from and complementary to the MGM of Sunshine et al. [Sunshine, J.M., Pieters, C.M., Pratt, S.F., 1990. J. Geophys. Res. 95, 6955–6966]. The general EGO equation has been especially designed to account for absorption bands affected by saturation and asymmetry. Here we present a special case of EGO and address it to model saturated electronic transition bands. Our main goals are: (1) to recognize and model band saturation in reflectance spectra; (2) to develop a basic approach for decomposition of rock spectra, where effects due to saturation are most prevalent; (3) to reduce the uncertainty related to quantitative estimation when band saturation is occurring. In order to accomplish these objectives, we simulate flat bands starting from pure Gaussians and test the EGO algorithm on those simulated spectra first. Then we test the EGO algorithm on a number of measurements acquired on powdered pyroxenes having different compositions and average grain size and binary mixtures of orthopyroxenes with barium sulfate. The main results arising from this study are: (1) EGO model is able to numerically account for the occurrence of saturation effects on reflectance spectra of powdered minerals and mixtures; (2) the systematic dilution of a strong absorber using a bright neutral material is not responsible for band deformation. Further work is still required in order to analyze the behavior of the EGO algorithm with respect to the saturation phenomena using more complex band shapes than pyroxene band
The search for evidence of aqueous activity in putative paleolake basins on Mars using CRISM spectral data
No full textRevisiting the identification of methane on Mars using TES data
No full textThe presence and variability of methane in the Martian atmosphere has been investigated by several authors and spurred a lively discussion. In this context, we address our previous inference of spatial and temporal CH4 variability identified from Mars Global Surveyor Thermal Emission Spectrometer measurements which was used to suggest the possible existence of a martian methane cycle. The importance of the topic requires a clear assessment of such variability to correctly comprehend the possible production and destruction mechanisms of Martian methane. It is therefore important to carefully revisit previous results from a different perspective to confirm them before they are used for further investigations. We here describe in detail a new procedure used to validate these earlier Thermal Emission Spectrometer measurements and thoroughly analyze the results obtained with the revised procedure. In spite of our efforts of defining an efficient data analysis procedure, we have not been able to either confirm or refute the existence of the spatial and temporal variability of methane. Nevertheless, our work has produced new interesting tools, which, with the necessary adaptation, can be of some aid in processing and interpreting planetary spectra and, in general, for all the other cases requiring a preliminary selection of data included in very extensive datasets, which are difficult to be efficiently treated with traditional techniques
Spectral Evidence of Aqueous Activity in Two Putative Martian Paleolakes
Full text linkCRISM observations of putative paleolakes in Cankuzo and Luqa craters exhibit spectral features consistent with the activity of water. The spatial distributions suggest different formation scenarios for each site. In Cankuzo the distribution suggests postimpact alteration whereas in Luqa there are hints of possible formation of a layer of phyllosilicate materials
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