Fitting the curve in Excel®:Systematic curve fitting of laboratory and remotely sensed planetary spectra

Spectroscopy in planetary science often provides the only information regarding the compositional and mineralogical make up of planetary surfaces. The methods employed when curve fitting and modelling spectra can be confusing and difficult to visualize and comprehend. Researchers who are new to working with spectra may find inadequate help or documentation in the scientific literature or in the software packages available for curve fitting. This problem also extends to the parameterization of spectra and the dissemination of derived metrics. Often, when derived metrics are reported, such as band centres, the discussion of exactly how the metrics were derived, or if there was any systematic curve fitting performed, is not included. Herein we provide both recommendations and methods for curve fitting and explanations of the terms and methods used. Techniques to curve fit spectral data of various types are demonstrated using simple-to-understand mathematics and equations written to be used in Microsoft Excel® software, free of macros, in a cut-and-paste fashion that allows one to curve fit spectra in a reasonably user-friendly manner. The procedures use empirical curve fitting, include visualizations, and ameliorates many of the unknowns one may encounter when using black-box commercial software. The provided framework is a comprehensive record of the curve fitting parameters used, the derived metrics, and is intended to be an example of a format for dissemination when curve fitting data

Reflectance and Emission Spectroscopy: Curve Fitting Methods with Application to Impact Glasses and the Varying Grain Size of Planetary Analogue Minerals

Spectroscopy, i.e., the measurement of electromagnetic radiation as a function of wavelength, is arguably the technique most responsible for the majority of what is collectively known about the composition of stars, the distances to galaxies, the age of the universe and so on. Spectroscopy is also the tool most used to discern the mineralogy of planetary bodies remotely. Measuring the speed at which a star is receding and its composition, or the composition of an interstellar cloud of gas are well understood uses of spectroscopy. When it comes to spectroscopies use to discern mineralogy, the scientific literature on the subject of the application of spectroscopy to the solid surfaces of asteroids and the nearby planets would lead one to conclude it too is as robust a measure as that of stellar composition or Doppler shift, although it is not. A number of properties of the target under investigation, namely, mineralogy, grain size, packing (i.e., loose grains versus consolidated rock), phase angle and temperature strongly affect the reflectance and emission spectrum of the common minerals encountered when interrogating planetary surfaces. These effects can be profound and significantly complicate our ability to robustly identify mineralogy when the properties of the surface are not known. The works herein address some of these issues, by firstly, providing a set of methods/functions and a set of guidelines for empirically curve fitting spectra in a robust and repeatable manner. Chapter 2 and its appendices were conceived in an effort to provide the spectroscopic community with a set of curve fitting tools, to be put freely in the hands of spectroscopists in the hopes that the community can see its way to providing fit metrics of spectra presented in the literature with transparency so the metrics can be widely understood and applied. Secondly, the methods presented in Chapter 2 were applied in Chapters 3 and 4 to the spectra of impact glasses and hydrothermal silicate evaporates to aid in their robust identification, and to the effects of significant grain size variation on the most common planetary surficial analogue materials pyroxene, olivine and basalt

Using Fluid-Mobile Elements to Decipher an Aqueous History Preserved in the Sedimentary Rocks of Gale Crater, Mars

Fluid-mobile elements preserve a record of the aqueous history of Mars in the sedimentary rocks of Gale Crater, and they are traceable with the rover Curiosity’s Alpha Particle X-ray Spectrometer (APXS). This is the fundamental principle behind the scientific questions addressed in the four main chapters of this dissertation. First, we conducted a field study at Hawai’i, an established Mars analogue, to investigate sedimentary processes and alteration in rocks comparable to those in Gale Crater. We conclude that open system, circumneutral weathering is very limited in Gale rocks; acid sulfate alteration and sedimentary mixing of diverse rocks is likely. Second, the Hawaiian samples were analyzed by Particle Induced X-ray Emission spectroscopy (PIXE) to evaluate the technique as an analogue for APXS. Third, we developed a novel, non-standard APXS calibration to determine the composition of a very thin layer of airfall dust on the rover. The global dust is enriched in the fluid-mobile elements sulfur and chlorine, but at a ratio found to be constant around the planet. This enables deconvolution of modern dust from underlying rock in APXS measurements to determine inherent sulfur and chlorine content in ancient rocks. Fourth, we used fortuitous zinc and germanium enrichments in Gale Crater rocks to establish a geochemical tracer pair for fluid processes in Gale. We propose a model in which the two trace elements were enriched together by hydrothermal fluids in the sediment source region, transported into the crater, and then fractionated by low temperature diagenetic fluids. The work presented here constrains fluid events affecting Gale sediments and can aid in unraveling fluid histories as Curiosity’s traverse continues

Calibration Of Calcic Pyroxenes For Interpreting Meteorite And Asteroid Spectra

Calcic pyroxenes are a group of silicate minerals that are found in meteorite and asteroid spectra. I investigated the relationship of high-calcium clinopyroxenes in comparison with orthopyroxenes, pigeonites, mixtures, and the meteorite group of angrites to try to find any trends for spectral calibrations. I used spectra from HOSERLab, RELAB, and the USGS Spectral Library. The main band minima of pyroxenes in spectra are around 0.9 or 1 µm for band I and 1.15 or 2 µm for band II, with the former minima for type A clinopyroxenes and the latter for type B. Using sample minima found from both lowest reflectances and polynomial fittings, I graphed the different pyroxene features to view possible trends from band positions and calcium, iron, and magnesium abundances. The type B clinopyroxenes continued on previous trends found in the literature, having a stronger band position relationship for the iron and calcium contents than the type A clinopyroxene samples. However, their band position relationships with iron and calcium were much less linear than the orthopyroxenes, which has also been seen previously in the literature. The two main band minima found from the different methods were similar in band position for band I, while due to noise and slope, the band II features were more diverse. I also examined how calcic clinopyroxenes interacted in comparison to other minerals in mixtures. There did not appear to be a trend for the type A clinopyroxenes, although it was also complicated by the larger error in band positions from reddened slopes and faintness. When in mixtures with orthopyroxene and/or olivine there needed to be a large amount, usually around ≥ 70 % calcic pyroxene, before the minima could be found. Angrite spectra were also looked at because they typically have a large amount of calcic clinopyroxene, including type A, although usually with larger amounts of titanium and aluminum. There were limited samples so I was unable to find any trends from them, but I investigated a set of mixtures from Cloutis et al. (2006a) and a set made for this study from HOSERLab that were made to resemble possible angrite and angrite parent-body spectra. From these, more data were available to show how a combined 1 µm feature from type A high-calcium pyroxene and olivine changes with differing amounts of both