The Planetary Instrument for X-ray Lithochemistry (PIXL) is a micro-focus X-ray fluorescence (XRF) spectrometer on NASA’s Perseverance rover. PIXL is capable of performing fine-scale measurements of the elemental distribution in rocks and soils on the Martian surface [1]. PIXL utilises a Rh target X-ray tube operated at 28 kV and 20 μA, which emits a continues spectrum of X-ray energies that are focused onto the sample surface using a polycapillary optic. PIXL has two silicon drift detectors on either side of the optic that collect the fluorescence signal. Many minerals also produce diffraction peaks that are observable with the PIXL geometry in a similar energy range to the XRF peaks of interest. This creates an interesting problem where diffraction peaks overlap with fluorescence peaks, at varying energy positions and intensities over large numbers (thousands) of spectra. A Machine Learning (ML) algorithm is currently used to identify diffraction peaks when they result from single crystal diffraction and have distinctively different signatures in the two detectors [2]. However, in the case of powder diffraction where the diffraction peaks are equal in both detectors, they are often mistaken for fluorescence peaks or Rh L lines from the X-ray tube target. We have identified powder diffraction peaks in XRF spectra by first fitting the spectra with a comprehensive physics-based model to describe the various expected components of an XRF spectrum, and then analysing the residuals from the fit to identify diffraction peaks present equally in both detectors. The process has been automated to explore the ‘Dourbes’ abraded patch in the Séítah formation of Jezero crater, Mars, and match potential mineral groups with the identified diffraction peaks.
[1] A. C. Allwood, L. A. Wade, M. C. Foote, et al., PIXL: Planetary Instrument for X-ray Lithochemistry (vol 216, 134, 2020), Space Sci. Rev. 217, 2021, 28. doi:10.1007/s11214-021-00801-2
[2] M. M. Tice et al., “Alteration history of Séítah formation rocks inferred by PIXL x-ray fluorescence, x-ray diffraction, and multispectral imaging on Mars,” Sci. Adv., vol. 8, no. 47, p. eabp9084, Nov. 2022, doi: 10.1126/SCIADV.ABP9084.