MSL-APXS titanium observation tray measurements: Laboratory experiments and results for the Rocknest fines at the Curiosity field site in Gale Crater, Mars

The Mars Science Laboratory (MSL) rover, Curiosity, has a titanium science observation tray (o-tray), upon which portions from drilled and scooped Martian samples can be delivered for analyses by the Alpha-Particle X-ray Spectrometer (APXS). The standard APXS calibration approach to derive elemental concentrations cannot be applied to samples on the o-tray because they (1) have a nonuniform three-dimensional distribution within the APXS field of view and (2) are thin (< 50 μm) compared to the APXS information depth for heavy elements (> 90 μm). To develop techniques for interpreting MSL-APXS o-tray measurements, we conducted laboratory measurements of thin particulate basalt samples on Ti metal with the Flight Equivalent APXS Unit. The experiments demonstrate that, relative to an "infinitely thick" sample, increasing areal coverage of particulates on a Ti metal substrate results in a proportional decrease in the Ti signal and increase in the sample signal. Count rates for heavier elements (Mn and Fe) drop with decreasing sample thickness because the mean thickness is smaller than the APXS information depth. Similar effects were seen in the MSL-APXS o-tray measurement of Rocknest fines on Martian solar day 95, an aliquot of material delivered to Sample Analysis at Mars and Chemistry and Mineralogy. The thin layer effect caused a drop in Mn and Fe signals, which cannot be quantitatively compared to the in situ Rocknest target "Portage" because sample thickness was unknown. Otherwise, Rocknest fines on the o-tray had no significant compositional differences from Portage, except for slight increases in S and Cl. Key Points APXS response for samples on the MSL Ti science observation tray is investigated Results from MSL-APXS measurement of scooped Rocknest drift sample are presented Techniques for modeling and evaluating APXS-thin samples are presented