PIXE and XRF on Mars

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Each of the Mars Exploration Rovers which attracted worldwide attention for their exploits and discoveries in 2004/9 carries an Alpha-particle X-Ray Spectrometer (APXS).

The APXS contains a 244Cm source which excites X-ray emission in situ from Martian rock and soil samples, together with a small silicon drift detector which produces the X-ray spectra. There are two modes of excitation: the alpha particles from the 244Cm decay cause a variant of PIXE: and the L X-rays from the plutonium daughter cause X-ray fluorescence (XRF). Fortunately, this variant of PIXE has its greatest sensitivity for the lightest elements (Na - Ca) whereas XRF is best for the heavier elements (Ca - Zr). As a result, the APXS has excellent sensitivity all the way across the range of elements that occur in most minerals and rocks.


The Martian spectra have up till now been analyzed by Dr. Ralf Gellert (co-developer of the APXS) with fitting software developed at the Max Planck Institut in Mainz. We have now developed a variant of GUPIX to do this task, taking full advantage of the databases and procedures that have been tested and refined over many years. This new code GUAPX is now complete, and it has been used to re-calibrate the MER laboratory APXS, using a suite of spectra from simple chemical standards and from geological standard reference materials recorded much earlier by Dr. Gellert. Our results show excellent consistency for those standards that are perfectly homogeneous, and prove our assertion that the APXS can be described by a single instrumental constant (H-value). But they also reveal interesting new issues such as a dependence of the calibration on the actual rock type; for igneous rocks - basalts, andesites and rhyolites, we observe (Campbell et al., 2008) an influence of the mineral assemblage within the rock upon the calibration.

The most exciting outcome of the MER mission was perhaps the observation of large amounts of Cl, Br and S in the APXS spectra. These are thought to be from salts that were the residue of evaporation of large bodies of water in the past. This raises the question of whether the APXS spectra can provide any indication of bound water within the actual rocks being analyzed. Obviously the x-rays of hydrogen and oxygen cannot be detected, and so any approach has to be an indirect one. We have developed an approach as follows:

Such an approach has been used in the past to determine the presence or absence of light elements (although not on Mars!). A rigorous Monte Carlo simulation of elastic and inelastic scattering intensities has been developed by C.L. Mallett, J.M. O'Meara and J.L. Campbell to predict the R/C ratio, which is then compared to the value obtained by fitting the spectrum with GUAPX. This approach has been calibrated using measurements on recognized geochemical reference standards (see publications below). We find that Martian basaltic soils and rocks give results that agree very closely with our calibration, thus providing support for the method. However the bright high-sulfate subsurface "Paso Robles" soils churned up in several places by the Spirit rover's tires are in strong disagreement, and from this deviation we are able to derive their water content. This is the first in-situ measurement of mineralogically bound water on the Martian surface, and the results fit well with the reconstructed mineralogy.

In further development of GUAPX, we have used the fundamental parameters calibration described above to analyse the data from individual geo-standards, some of which were measured by Gellert but not included in his own calibration. We regard these individual geostandards as unknowns, and we have to use a fully iterative approach with GUAPX, in which all elements are converted to oxides and a 100% oxide total is enforced (the "closure rule") . By "tailoring" our calibration, i.e. by developing sub-calibrations for basalts, andesites and rhyolites, we are able to get excellent results for element concentrations in individual examples of these rock types, including, for example, the Zagami martian meteorite. In cases where the oxides do not sum to 100%, we are able to measure the bound water content: here the phyllo-silicate standard UB-N is a good example; our determination of 14% agrees well with the actual (CO2 +H2O) content of 11%. we have thus developed a second approach for measuring bound water, but it only works if the measurements of standards and unknowns are all done within a fixed geometry. On the MER mission, the geometry varied from sample to sample, and unfortunately there was no instrument attached to the APXS to provide the sample-detector distance; if that had been the case, this method could have given a direct measurement of water content.

We are now applying GUAPX to the calibration of the laboratory APXS for the Mars Science Laboratory mission.




A GUPIX-based approach to interpretation of the PIXE+XRF spectra from the Mars Exploration Rovers: I Homogeneous standards. J. L. Campbell, J. A. Maxwell, S. M. Andrushenko, S. M. Taylor, B. N. Jones, W. Brown-Bury. Nucl. Instr. Meth, Phys. Research B269 (2011) 59-68.

A GUPIX-based approach to interpretation of the PIXE+XRF spectra from the Mars Exploration Rovers: II Geochemical standards. J. L. Campbell, A. M. McDonald, G. M. Perrett, S. M. Taylor. Nucl. Instr. Meth, Phys. Research B269 (2011) 69 - 81.

A fundamental parameters approach to calibration of the Mars Exploration Rover Alpha Particle X-ray Spectrometer: 2. Analysis of unknown samples. J.L. Campbell, S.M. Andrushenko, S.M. Taylor, and J.A. Maxwell J. Geophys. Res., 115, E04009, doi:10.1029/2009JE003481, 2010.

A Fundamental parameter approach to calibration of the Mars exploration rover alpha particle x-ray spectrometer. J.L. Campbell, M. Lee, B.N. Jones, S.M. Andrushenko, N.G. Holmes, J.A. Maxwell. J. Geophys. Research: Planets, 114, E04006, doi: 101.1029/2008JE003272, 2009.

Quantitative in-situ determination of hydration of bright high-sulfate Martian soils. J.L. Campbell, R. Gellert, M. Lee, C.L. Mallett, J.A. Maxwell and J.M. O'Meara. J. Geophys. Research: Planets 113, E06S11, JE002959 (2008)

Calibration of the MER alpha-particle x-ray spectrometer for detection of "invisible" OH and H2O possibly present in Martian rocks and soil
C.L. Mallett,  J. M. O'Meara,  J.A. Maxwell and J. L. Campbell.  X-Ray Spectrometry 35 (2006) 329-337 

Simulation of the relationship between element concentrations and X-ray yields in the Mars Exploration Rover's X-ray spectrometer
M. Omand, J.L. Campbell, J.A. Maxwell. Nucl. Instrum Meth. in Phys. Research B229 (2005) 123-136