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

Abstract

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.

A global Mars dust composition refined by the Alpha‐Particle X‐ray Spectrometer in Gale Crater

“…Navigation camera (Navcam) images [Maki et al, 2012] confirmed that these fines were subsequently cleaned off of the center of the o-tray by the vibration and movement of fines scooped from the Rocknest sand shadow and portioned to the o-tray for analysis (Sols 91-95) [Berger et al, 2014]. Upon landing on 6 August 2012, the MSL sky crane retrorockets lofted surface fines from a depth of several centimeters, and a thin layer settled onto the o-tray.…”

“…To do this, we used the computer code APX-Yield [Schmidt et al, 2014;Campbell et al, 2014;Berger et al, 2014], which employs GUAPX subroutines to calculate the yield of characteristic X-rays for elements from layered samples using fundamental PIXE and XRF principles. To do this, we used the computer code APX-Yield [Schmidt et al, 2014;Campbell et al, 2014;Berger et al, 2014], which employs GUAPX subroutines to calculate the yield of characteristic X-rays for elements from layered samples using fundamental PIXE and XRF principles.…”

“…Although the dust sample likely has only partial areal coverage, previous work shows that the APXS signal varies across the spectrum with changes in areal coverage and is approximately proportional to the percent areal coverage of the particulate according to the areal coverage correction factor f A [Berger et al, 2014]. The code assumes that the layers have uniform thickness and homogeneous composition at the micrometer scale, which is reasonable for the air fall dust.…”

“…The Martian dust probably formed submillimeter scale porous aggregates as it was deposited [e.g., Johnson et al, 2002], which we assumed had a uniform mean thickness across the larger-scale~1.7 cm diameter APXS FOV. Increased yields due to secondary fluorescence of Ca (6-7% increase) and K (3-4% increase) in the thin dust sample by the Ti substrate [e.g., Berger et al, 2014] were modeled and corrected using APX-Yield software. APXS measurements of the o-tray have additional considerations.…”

2015 Atomic Spectrometry Update – a review of advances in X-ray fluorescence spectrometry and their applications

“…The authors presented abundances of the rock-forming elements, Al, Ca, Mg and Si (as well as Na) and provided a detailed description of analytical techniques, including the derivation of XRF line uxes and the conversion to elemental abundances. Berger et al 24 undertook experiments in their laboratory with the ight equivalent instrumentation to evaluate limitations in the analysis of thin particulate samples deposited on this tray and reported that apart from a drop in Fe and Mn signals, no signicant compositional differences were observed compared with surface measurements, except for a slight increase in Cl and S levels. Turning now to the Mars Science Laboratory Curiosity Rover, Campbell et al 23 reported changes to the basalt calibration target own on the mission and used to monitor the performance of the alpha particle excited XRF spectrometer.…”