Mars Science Laboratory Alpha Particle X-ray spectrometer trace elements: Situational sensitivity to Co, Ni, Cu, Zn, Ga, Ge, and Br

Alongside major and minor elements, trace elements in Martian samples quantified through geochemical analyses by the Mars Science Laboratory (MSL, Curiosity) Alpha Particle X-ray Spectrometer (APXS) provide insight into ancient environments on Mars. APXS spectrum simulations facilitated the characterization of trace elements measured by the APXS under experimental situations commonly encountered on Mars. The precise quantification limits (PQL) obtained under ideal conditions are consistent with limits of detection in the literature and increase with degrading spectral quality. Major trace element species, such nickel, zinc, and bromine, with typical concentrations in the 100s of μg/g (or less), are quantifiable even with short (e.g., 20 min) early morning measurements between drives given the typical concentrations at the MSL site, Gale Crater. Target compositions currently in the public domain with major trace element species below the derived PQL specific to each measurement are noted. Quantification of copper, gallium, and germanium at 30 ppm and <10% uncertainty can be achieved with measurement durations of 6 h, 3 h, and 4 h respectively. In areas of local enrichment, such as the Murray formation, germanium abundance can be assessed with <10% uncertainty in 30 min. The detection and quantification of cobalt is hindered by the presence of overlapping peaks associated with the ubiquitous iron and nickel present in Mars samples. Only a single sample interrogated by Curiosity's APXS up to sol 2003 contains cobalt at detectable levels, coinciding with a composition similar to terrestrial ferromanganese crusts and nodules formed under inorganic aqueous conditions. The work presented herein emphasizes the need for acquiring quality APXS measurements where reliable trace element quantification is desired and offers guidance to improve the tactical operation of the MSL APXS instrument.