Determination of the chemical composition of Martian soil and rocks: The alpha proton X ray spectrometer

Abstract

Abstract. The alpha proton X ray spectrometer (APXS) for the Mars Pathfinder mission is designed to provide a complete and detailed analysis of chemical elements in Martian soil and rocks near the landing site. The APXS instrument is carried on the Pathfinder Microrover, which will provide transportation to places of interest on the Martian surface. It consists of a complex sensor head, mounted on a simple but sophisticated APXS deployment mechanism (ADM) outside the warm electronics box (WEB) of the Microrover, and the instrument electronics, mounted inside the WEB. The ADM permits the instrument sensor head to be placed against soil and rock samples in arbitrary positions, ranging from horizontal to vertical, in order to perform in situ analysis. The possibility to transport the APXS to an arbitrary location, preselected on Earth, and to perform in situ analysis there, constitutes one of the most exciting aspects of the Pathfinder mission. The principle of the APXS technique is based on three interactions of alpha particles from a radioisotope source with matter: simple Rutherford backscattering, production of protons from (c•, p) reactions on light elements, and generation of characteristic X rays upon recombination of atomic shell vacancies created by c• bombardment. Measurement of the intensities and energy distributions of these three components yields information on the abundance of chemical elements in the sample. In terms of sensitivity and selectivity, data are partly redundant and partly complementary: alpha backscattering is superior for light elements (C, O), while proton emission is mainly sensitive to Na, Mg, A1, Si, S, and X ray emission is more sensitive to heavier elements (Na to Fe and beyond). A combination of all three measurements enables determination of all elements (with the exception of H and He) present at concentration levels above typically a fraction of 1%.

Mineralogic and compositional properties of Martian soil and dust: Results from Mars Pathfinder

“…The Mars Pathfinder mission exploited these opportunities by focusing on the morphological, geochemical, and mineralogical characterization of a geologically interesting and climatically diagnostic region of the Martian surface [Golombek, 1997]. Specifically, the mission carried a lander with enhanced multispectral imaging capabilities relative to Viking and an optimized magnetics properties experiment [Smith et al, 1997a], as well as the ability to measure the elemental chemistry of soils and rocks [Rieder et al, 1997a] from a highly mobile rover that could also perform soil mechanics experiments (digging, scraping) and provide up-close imaging of rock and soil textures [Rover Team, 1997a].…”

History of Applications of Radioactive Sources in Analytical Instruments for Planetary Exploration

“…Besides the 242 Cm and 244 Cm that were used in the APXS, the Mössbauer experiment on the MER mission used about 400 millicuries of 57 Co at the start of the mission, to obtain the mineralogy of the iron bearing rocks [10]. Similarly, 55 Fe has been used in an XRF instrument on Viking mission in 1976 [11] and on the Beagle 2 mission in 2003 [12].…”

“…The Alpha Proton X-ray Spectrometer (APXS) for the Mars Pathfinder mission in 1996 [12] was based on the design of the "Mini-Alpha" APXS [8] and used about 45 millicuries of 244 Cm alpha source instead the previously used 242 Cm isotope. Fig.7 is a photograph of the Pathfinder APXS flight instrument.…”

“…Therefore, an additional mode has been added to the original ASI instrument to detect the resulting characteristic x-rays that are produced when a sample is bombarded with a beam of alpha particles and x-rays from the same radioactive source. It is the combination of all three modes, the Rutherford alpha backscattering, the x-ray fluorescence (XRF) and the particle PIXE techniques that resulted in one integrated low power, low volume, but very powerful Alpha Proton X-ray Spectrometer (APXS) analytical instrument [8] that has been used in so many space missions [9][10][11][12][13][14][15][16][17][18][19][20]. A refined and miniaturized instrument with alpha, proton and x-ray modes (the "AlphaProton X-ray Spectrometer by Economou et al, 1976[8]) was proposed for analysis of Martian surface material during preparation for the Viking missions, but it was not selected for that mission.…”

Elemental surface analysis at ambient pressure by electron-induced x-ray fluorescence

“…This system has a spectral resolution between 165 eV ͑24 s dead time͒ and 200 eV ͑12 s dead time͒ full width at half maximum, which is somewhat better than the XR-100T system that was flown on Mars Pathfinder. 9 The detector was calibrated by using an Am x-ray fluorescing source. X-ray spectra from this target were collected with a commercial Kevex EDX system, and compared with the AEXS spectra.…”