Mineralogical Capabilities of the CheMin XRD/XRF instrument on Mars Science Laboratory (MSL ’11)

A principal goal of the Mars Science Laboratory (MSL ‘11) mission is to identify and characterize present or past habitable environments on Mars. By determining the mineralogical composition of rocks or soil, one can often deduce the conditions under which they formed, or their subsequent diagenetic or metamorphic history. The CheMin mineralogical instrument [1-3] will return quantitative X-ray diffraction data (XRD) and qualitative X-ray fluorescence data (XRF; 14 < Z < 92) from scooped soils and drilled rock powders collected from the Mars surface. Small amounts (45-65 mm3) of sample material sieved to <150 µm will be delivered through a funnel to one of 27 reuseable sample cells located on a sample wheel. Sample cells are 8-mm diameter discs bounded by 7-µm thick Mylar or Kapton windows spaced 170 µm apart. Within this volume, the sample is shaken by piezoelectric vibration at sonic frequencies, causing the powder to flow past a narrow, collimated X-ray beam in random orientations during the course of an analysis. CheMin is designed to have a Minimum Detection Limit (MDL) of <3% by mass, accuracy better than 15% and precision better than 10% for phases present in concentrations >4X MDL (12%). CheMin uses a Co X-ray tube so that absorption in iron-rich samples is minimized. The resolution of the diffraction patterns is <0.35° 2θ, and the angular measurement range is 4-55° 2θ. The capabilities of the FM instrument were tested during ThermoVac using mineral and ceramic standards contained on the FM sample wheel. Standards include 88:12 and 97:3 mixtures of beryl:quartz for assessment of the accuracy and precision of quantitative analyses, miminum detection limits, 2θ range and 2θ resolution; a compositionally diverse ceramic material for XRF evaluation; arcanite (K2SO4); and an amphibole. Analyses were performed under Mars atmospheric pressure at a range of Rover Avionics Mounting Platform (RAMP) temperatures from -40C to +26C. Within the predicted Mars RAMP operating temperatures of 0C to +20C, peak-to-background ratios in the XRD pattern and FWHM of elemental peaks in XRF spectra were within proscribed limits. The beryl:quartz mixtures show some anomalous phase segregation during sample shaking that is being investigated. Mineral detection and energy resolution capabilites meet or exceed requirements. References: [1] http://msl-scicorner.jpl.nasa.gov/Instruments/CheMin/. [2] LPSC40 #1484 (2009). [3] Martian Phyllosilicates: Recorders of Aqueous Processes #7006 (2008).