Calibration of the Mars Science Laboratory Alpha Particle X-Ray Spectrometer

We have used a suite of over 60 geochemical reference standards for the calibration of the Mars Science Laboratory (MSL) Alpha Particle X-ray Spectrometer (APXS). For the elements P, S, Cl and Br we have supplemented this suite by adding various amounts of relevant chemical compounds to a powdered basalt standard. Special attention has been paid to include phyllosilicates, sulphates and a broad selection of igneous basalts as these are predicted key deposits at the MSL landing site, Gale Crater. The calibration is performed from first principles using x-ray excitation cross sections for the alpha particle and x-ray radiation source and an assumed homogeneous sample matrix. Remaining deviations indicate significant influences of mineral phases especially for light elements in basalts, ultra-mafic rocks and trachytes. Supporting x-ray diffraction work has helped to derive empirical, iterative corrections for distinct rock types, based on the first APXS analysis, assuming a homogeneous sample. These corrections have the potential to significantly improve the accuracy of APXS analyses, especially when other MSL instrument results, such as x-ray diffraction data from ChemMin, are included in the overall analysis process.