Comparison of the iron-manganese ratio dispersion in martian soils with the mars science laboratory apxs and

Introduction: The Alpha Particle X-ray Spectrometer (APXS) onboard the Mars Science Laboratory (MSL) rover Curiosity uses a combination of ParticleInduced X-ray Emission (PIXE) and X-ray Fluorescence (XRF) to determine the abundance of Fe and Mn with high precision and accuracy (<10%) [1]. Curiosity's payload also includes the ChemCam laser-induced breakdown spectroscopy (LIBS) instrument [2], where photons emitted by a pulsed plasma are detected wavelength dispersive with three spectrometers ranging from the ultraviolet to near infra-red, detecting element specific emissions for quantification of Fe and Mn. Both instruments detect and quantify Fe and Mn, two undoubtedly significant geochemical elements, but on a vastly different volume scale and with very different methods. Each instrument and method have their advantages and disadvantages. In this work we use ChemCam spectra and try to understand and partially reproduce the analysis presented by the ChemCam team in dedicated papers [3]. This work is done to get a better idea of the analysis and related uncertainties and the possible impact of matrix effects. We compare our preliminary results with published Mn peak areas and apply simple consistency checks comparing the wealth of ChemCam data available in the raw data set with trends we see in published APXS data using the Fe:Mn ratio. Methods: The APXS measures a sample of ~20mm diameter with element specific information depths ranging from 2-5 m for low z elements to ~50m for high z elements like Fe and Mn. A homogeneous sample is assumed to correct for XRF matrix effects, which limits the accuracy to ~10% for Fe and Mn. To date the MSL APXS has measured ~700 samples. ChemCam measured more than 2500 samples, each with typically 30 shots and 5-10 distinct spots, representing millions of spectra of ~0.5mm spot sized targets that ablate ~1-3m per shot. Analysis is done using multivariate methods for major elements [4] and typically univariate (peak area) approaches for minor or trace elements like Mn [3]. Motivation: Mn is undoubtedly a geochemically important element. The APXS with its large sample volume has significant limitations when the Mn minerals are distributed in millimeter sized grains and heterogeneous in depth. However, anomalous high APXS MnO concentrations like in Jones Marsh with ~4%, more than 10 times above the surrounding Murray bedrock, where the APXS homogeneous assumption works well, coincide with localized, very high Mn quantifications by ChemCam. Mitigations for this APXS limitation are rasters, covering different parts of the sample, like done on Stephen, where good correlations of Mn with Cu, Ni and Zn were used as evidence for certain Mn minerals [5]. The ultimate goal of this work is to try to use ChemCam point data to compare against the APXS large volume data for homogeneous samples and to possibly use ChemCam derived minerals for APXS modelling without the homogeneous assumption. Work like this will also be important in case MSL will ever encounter Mn enriched features big enough to drill ~5cm sample for in detail analysis with CheMin and SAM. Data Sets and Analysis: Peak areas of the characteristic X-ray lines in APXS spectra increase linearly with abundance, adjusted for an assumed homogeneous matrix accounting for matrix effects. Figure 1 (bottom) captures Fe and Mn peaks in Mars spectra. The Fe and Mn peaks correspond to a weighted-average contribution across the APXS field of view.