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MapX

2D XRF for Planetary Exploration

The search for evidence of life or its processes takes on two major themes:
  1. the identification of environments that have or once had the potential to harbor life (habitability); and
  2. the detection of morphological or chemical features suggestive of extinct or extant life (biosignatures).
Compositional heterogeneity at the cm-to-100µm scale can reveal geological processes indicative of past or present habitability, and morphological and compositional heterogeneity on a similar length scale can provide evidence of life’s processes. The Mapping X-ray Fluorescence Spectrometer (MapX) is an arm-based in-situ instrument designed to identify these features on planetary surfaces[1].

Instrument Description

MapX is a first-of-its-kind imaging Alpha Particle X-ray Spectrometer (iAPXS), a full-frame elemental imager capable of analyzing surface regolith in situ without sample preparation. MapX has no moving parts and will utilize 244Cm radioisotope sources, eliminating the complexity and risk of High Voltage Power Supplies and X-ray tubes. Figure 1 shows a schematic of the instrument, which consists of X-ray / γ-ray / α-particle sources, a Micro-Pore Optic (MPO) focusing lens and CCD imager.

The MPO lens derives from “lobster-eye” multichannel optics used for X-ray astronomy[2]. It is implemented here in a flat geometry for 1:1 focusing. This lens provides a much improved aperture when compared to pin-hole camera optics having similar spatial resolution, and true focusing when compared to polycapillary collimating optics also used for X-ray mapping. The MPO lens has a depth of field of ~1cm allowing rough unprepared surfaces to be imaged with negligible loss of resolution. Images are collected by placing the instrument’s contact plate directly onto the regolith surface to be analyzed. The CCD is exposed and read rapidly (several frames per second) such that each pixel records either a single photon from the sample or background. The number of electron hole pairs generated in a single pixel is directly proportional to the energy of the X-ray photon, and after summing a large number of individual frames, an XRF spectrum is recorded for each pixel of the CCD. Each individual 0.3 sec. frame is a complete image; however, many frames must be summed to produce low-noise elemental images and quantifiable XRF spectra. “Touch and Go” measurements can be made in as little as 10 minutes. Instrument control, image collection, data processing and communication with the rover are accomplished in a second unit located in the instrument bay of the rover (“RAMP unit”).
Walter et al (2019)
Figure 1. Schematic diagram of the MapX instrument.

Data Analysis

MapX nominally collects thousands of images that are assembled into an HDF5 data cube. An unsupervised machine learning algorithm resident on the instrument computer produces “Regions Of Interest” (ROI) on the sample, comprised of common elemental compositions. Quantifiable XRF spectra are generated from each ROI. Downlinked data products include: Elemental maps 11<Z<40, instrument-selected[3] Regions of Interest (ROI) having common compositions, and quantifiable XRF spectra from each ROI. Because the entire imaging experiment is preserved in an HDF5 file, additional data products such as line scans or alternative ROI can be generated after the fact. ROI compositions are used in conjunction with the RRUFF database[4] to determine putative mineralogy.

Example MapX datasets

Samples were imaged first on an EDAX commercial laboratory instrument (~50 µm resolution), then on MapX-III, a third-generation prototype of MapX (~150 µm resolution). Note: in all figures, X-ray tube sources are used in place of 244Cm. Figure 2 shows a partial MapX-III dataset collected from a quartz sandstone decorated with hematite crystals. Figure 3 shows a partial MapX-III dataset collected from a polished stub of 1.9 Gyr old Gunflint Chert Stromatolite. ROI and XRF spectra returned by MapX-III are sufficient to identify this rock as a chert with preserved carbonates displaying stromatolitic features.

Figure 2. Hematite crystals on quartz sandstone. a). image of sandstone fragment, scale bar = 1 cm; b). RGB elemental image from EDAX commercial instrument, Fe=Red, Si=Green, K=Blue; c). RGB elemental image from MapX prototype, same color scheme as b); d). instrument selected ROI, H = hematite, QS = quartz sandstone; e). XRF spectra from yellow (hematite) and purple (quartz sandstone) ROI.

Figure 3. Precambrian Gunflint Stromatolitic Chert. a). image of thin section, scale bar = 1 cm; b). RGB elemental image from EDAX commercial instrument, Fe=Red, Ca=Green, Si=Blue; c). RGB elemental image from MapX prototype, same color scheme as b); d). instrument selected ROI, C = Chert, FC = Ferroan Carbonate; e). XRF spectra from ROI.

MapX Flight Instrument

Figure 4 shows cut-away and solid 3D views of the MapX camera head, containing the sources, imaging optics, CCD and camera electronics (“Arm Unit”). A second processing unit (“RAMP Unit”) is located in the body of the rover and contains the computer processor, communications and instrument control software. Table 1 shows the proposed mass, volume and power requirements of the instrument as well as survival and operating temperatures.

Figure 4. 3D models of the MapX flight instrument. MapX. Right: Rendering of the arm mounted instrument in a flight like configuration.
Table 1. MapX Environmental Conditions and Operating Parameters (244Cm sources)

References

  • [1]. Walroth, R.C. et al. (2019). “MapX: A Full-field X-ray Fluorescence Imager for In-Situ Habitability and Biosignature Investigations.” 9th Intl. Conf. on Mars, abstr. #6329. 
  • [2]. Fraser, G., et al. (2010). “The mercury imaging X-ray spectrometer (MIXS) on bepicolombo.” Planet. Space Sci. 58 (1-2), 79–95, 2010. https://doi.org/10.1016/j.pss.2009.05.004 
  • [3]. Walroth, R.C. et al. (2019). “Machine Learning Approaches to Data Reduction from the MapX X-ray Fluorescence Instrument for Detection of Biosignatures and Habitable Planetary Environments.” AbSciCon abstr. #142-177. 
  • [4]. Lafuente B, et al. (2015). “The power of databases: the RRUFF project.” In: Highlights in Mineralogical Crystallography, T Armbruster and R M Danisi, eds. Berlin, Germany, W. De Gruyter, pp 1-30  
     https://rruff.info/about/downloads/HMC1-30.pdf