CHEMIN: A MINERALOGICAL INSTRUMENT FOR MARS EXPLORATION

David F. Blake

The identification of the types of rocks on Mars that may harbor evidence of present or past life (i.e., biomarkers) will require in situ mineralogical analysis. In order to establish the conditions under which a rock formed, the identity of each mineral present and its amount must be determined. In terrestrial laboratories, X-ray Diffraction and X-ray Fluorescence (XRD/XRF) are the techniques of choice for such characterizations.

Recent progress in X-ray technology allows the consideration of simultaneous X-ray diffraction (XRD: mineralogic analysis) and high-precision X-ray fluorescence (XRF: chemical analysis) in systems scaled down in size and power to the point where they can be mounted on landers or small robotic rovers. The CHEMIN XRD/XRF instrument, which simultaneously collects XRD and XRF data, has been proposed in the past for a variety of solar system missions and is presently proposed for three separate Mars scout missions, including a precision lander, a penetrator and a lander equipped with a drill.

NASA was awarded a patent in 1996 (US Patent No. 5,491,738) for the CHEMIN concept. The instrument received a commercial "R&D 100 award" as one of the top 100 innovative technologies of 1998. A SBIR (Small Business, Innovative Research) phase II proposal has been awarded to Moxtek, Inc. to build and commercialize a laboratory version of CHEMIN.

CHEMIN is a CCD-based simultaneous X-ray diffraction / X-ray fluorescence instrument. The device is designed to characterize the elemental composition and mineralogy of small fine-grained or powder samples. The name CHEMIN refers to the instrument’s combined CHEmical and MINeralogic capability.

Both diffraction and fluorescence data are obtained simultaneously by operating the CCD in single-photon counting mode. Energy discrimination is used to distinguish between diffracted primary beam photons and fluorescence photons. Diffraction data are obtained in transmission mode, and resolution is presently sufficient on the prototype instrument to allow application of the Rietveld refinement method to the diffraction data. X-ray fluorescence data will be obtained for all elements, 4<Z<92.

A diagram of the proposed CHEMIN flight instrument is shown in figure 1. In operation, the carousel of the instrument (which is the only moving part) is rotated to place one of 40 collection grids in a position to receive a soil sample or a sample of drill cuttings from a rock. The carousel is then rotated to place the grid in the analysis position between the X-ray source and CCD. A combination of carousel rotation and 1-2 mm motion along the x-axis allows the entire substrate to be sampled sequentially by the X-ray beam. An intelligent systems program determines the location of sample material suitable for analysis and supervises data collection.

A prototype of the CHEMIN instrument has been operable since July, 1996. After optimization of the X-ray source collimation, diffraction data were obtained in the Fall of 1996 of sufficient quality to be used with advanced diffraction data analysis methods such as Rietveld refinement. Various sample handling systems are presently being pursued, and designs have been proposed for terrestrial use in commercial laboratories, in the International Space Station, and in the proposed Mars Sample Return Handling Facility.

Figure 1. Cross-sectional diagram of the proposed CHEMIN flight instrument.