Determination of the Light Element Fraction in MSL APXS Spectra
Abstract
Additional light invisible components (ALICs), measured using the alpha particle X-ray spectrometer (APXS), represent all light elements (e.g. CO3, OH, H2O) present in a sample below Na, excluding bound oxygen. The method for quantifying ALICs was originally developed for the Mars Exploration Rover (MER) APXS (Mallet et al, 2006; Campbell et al, 2008). This method has been applied to data collected by the Mars Science Laboratory (MSL) APXS up to sol 269 using a new terrestrial calibration. ALICs are investigated using the intensity ratio of Pu L-alpha Compton and Rayleigh scatter peaks (C/R). Peak areas of the scattered X-rays are determined by the GUAPX fitting program. This experimental C/R is compared to a Monte Carlo simulated C/R. The ratio of simulated and experimental C/R values is called the K-value. ALIC concentrations are calculated by comparing the K-value to the fraction of all invisibles present; the invisible fraction is produced from the spectrum fit by GUAPX. This method is applied to MSL spectra with long integration duration (greater than 3 hours) and with energy resolution less than 180 eV at 5.9 keV. These overnight spectra encompass a variety of geologic materials examined by the Curiosity Rover, including volcanic and sedimentary lithologies. Transfer of the K-value calibration produced in the lab to the flight APXS has been completed and temperature, geometry and spectrum duration effects have been thoroughly examined. A typical limit of detection of ALICs is around 5 wt% with uncertainties of approximately 5 wt%. Accurate elemental concentrations are required as input to the Monte Carlo program (Mallet et al, 2006; Lee, 2010). Elemental concentrations are obtained from the GUAPX code using the same long duration, good resolution spectra used for determining the experimental C/R ratios (Campbell et al. 2012). Special attention was given to the assessment of Rb, Sr, and Y as these element peaks overlap the scatter peaks. Mineral effects, supported by CheMin results (Blake et al, 2013), as well as accurate geometry and environmental conditions are also considered for producing the best bulk chemistry, despite complications inherent to surface dust on unbrushed rock surfaces. The calculation of ALICs by the MSL APXS is a useful tool for producing in-situ values for volatile elements and provides an intermediate connection between the sub millimeter cubed scale of possible hydrogen concentrations found by ChemCam and the meter cubed hydrogen signature detected by DAN. These values also provide useful constraints on the absolute volatile concentrations found by SAM. MSL ALIC results are compared to previous results obtained for the 'dry' soils collected by the APXS on the MER rovers and provide a unique comparison of Martian samples from distinct locations on Mars. Results of MSL APXS data, including comparison with previous rover APXS analyses and complementary data from Curiosity's other instruments, will be presented.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.P23C1799P
- Keywords:
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- 6225 PLANETARY SCIENCES: SOLAR SYSTEM OBJECTS Mars;
- 5410 PLANETARY SCIENCES: SOLID SURFACE PLANETS Composition