CheMin: A Definitive Mineralogy Instrument on the Mars Science Laboratory (MSL '09) Rover

CheMin is a miniature X-ray diffraction / X-ray fluorescence (XRD/XRF) instrument that has been chosen for the analytical laboratory of MSL. CheMin utilizes a microfocus source cobalt X-ray tube, a transmission sample cell and an energy-discriminating X-ray sensitive CCD to produce simultaneous 2-D X-ray diffraction patterns and X- ray fluorescence spectra from powdered or crushed samples. The X-ray source utilizes a conventional tungsten cathode and a reflection type cobalt anode. A focusing grid in the tube yields a 50 micron diameter photon source on the anode. The CCD detector is a 600X600 front illuminated frame transfer device having 40 micron square pixels and a deep depletion zone for high QE of 7 KeV X-rays (CoKa). Diffraction data are collected over the range 5-55 degrees 2-theta, with a resolution of 0.32 degrees 2-theta. During the mission, MSL's sample delivery system will provide up to 74 crushed and sieved samples to the CheMin instrument. Samples are delivered through a funnel to one of 26 reusable transmission sample cells on the CheMin sample wheel. Samples are loaded at the top, analyzed, then discarded in a sump by rotating the sample cell to the bottom of the wheel. Samples are analyzed for up to 10 hours during multiple sols. During analysis, sample cells are shaken at sonic frequencies by a piezoelectric actuator that causes convection and granular flow of the powdered material. The result is that all components of the sample are passed through CheMin's ~50 micron diameter beam in random orientations during the course of an analysis. In addition to the 26 reuseable sample cells, CheMin has 6 sealed cells containing XRF and XRD reference standards. These standards can be analyzed at any time to evaluate the health of the instrument or to generate calibrated XRD patterns. For each analysis, CheMin will produce energy-selected 2-D CoKa and CoKb patterns as well as a histogram of X-ray energy vs. number of photons. Diffraction data will be quantified using Rietveld refinement and Fluorescence data will be quantified using a fundamental parameters technique. Downlinked data from Mars will be validated in three ways: First, on-board standards will be used to calibrate unknowns and to control for non- optimal analysis conditions during the lifetime of the mission. Second, laboratory-based CheMin instruments will be used to analyze Mars analog materials. Third, a ray-tracing model has been developed that can simulate the CheMin geometry and corroborate diffraction results from specific sample mineralogies.