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Planetary Instrument for X-ray Lithochemistry (PIXL)

The PIXL (Planetary Instrument for X-ray Lithochemistry) for the Mars-2020 rover is a microfocus X-ray fluorescence instrument that rapidly measures elemental chemistry at sub-millimeter scales by focusing an X-ray beam to a tiny spot on the target rock or soil and analyzing the induced X-ray fluorescence.

Figure 1. PIXL breadboard maps of a paleoarchean altered conglomerate with unconformity surface, revealing complex lithology including rounded pyrite, chromite, zircon, K-Al-Cr clay in the fuchsitic mudstone, Mn,Fe,Ca-carbonate in alteration vein, silicification of the conglomerate matrix. Map size is 20 mm by 10 mm with a step size of 0.15 mm. Credit: NASA/JPL-Caltech
Scanning the beam reveals spatial variations in chemistry in relation to fine-scale geologic features such as laminae, grains, cements, veins, and concretions (Figure 1).

The high X-ray flux enables high sensitivity and short integration times: most elements are detected at lower concentrations than possible on previous landed payloads to Mars, and several new elements can be detected that were not previously detectable on these missions. Fast acquisition allows rapid scanning so that PIXL reveals the associations between different elements and the observed textures and structures. The same spectra can be summed for bulk analysis, allowing comparison with bulk chemistry measurements at sites previously explored on Mars. With PIXL’s simple design comes operational efficiency and experimental flexibility—an instrument that can adapt to different scientific opportunities to produce a diverse set of scientifically powerful data products within the constraints of the mission.

The instrument consists of a main electronics unit in the rover’s body and a sensor head mounted on the robotic arm. The sensor head includes an x-ray source, X-ray optics, X-ray detectors, and high-voltage power supply (HVPS), as well as a micro-context camera (MCC) and light-emitting diode (LED). The PIXL can detect elements: Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Br, Rb, Sr, Y, Ga, Ge, As, and Zr, with important trace elements such as Rb, Sr, Y and Zr detectable at 10’s ppm level. An example is shown in Figure 2.

Figure 2. This figure demonstrates that the PIXL instrument detects important trace elements (red) at 10’s ppm. Insert: Rb, Sr, Y, Zr are clearly detected due to the lack of interfering excitation lines at 13–17 keV in PIXL (continuum subtracted using a modeled fit to Bremsstrahlung). Credit: NASA/JPL-Caltech

The advantages of the PIXL instrument are:

  • High spatial resolution with a 0.12-mm-diameter beam enabling sub-mm scale geochemistry correlated with texture
  • Measurement flexibility: point, line, or map analysis at variable step size as small as 0.1 mm
  • Spatial coverage, targeting and position knowledge: fine step size and micro-context camera will provide resolution and footprint at hand-lens scale
  • Fast spectral acquisition: measure most major and minor elements at 0.5 wt% in 5 seconds.
  • Large range of detectable elements (26+ elements)
  • High sensitivity: detect important trace elements at 10’s ppm level
  • Operational efficiency: The combined effect of PIXL’s speed, sensitivity, resolution, coverage/targeting, and measurement flexibility allows PIXL to search efficiently and locate the places of interest and to observe small patches of clean rock
  • Operational robustness: tolerant to surface roughness