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CheMin A portable xrd/xrf instrument for planetary exploration
Curiosity Rover self portrait, Mars Science Laboratory, courtesy NASA/JPL
The Mars Science Laboratory (http://www.jpl.nasa.gov/missions/mars-science-laboratory-curiosity-rover-msl/) was launched on December, 2009, to arrive at Mars in October, 2010. Mars Science Lab (MSL) is twice as long and three times as heavy as the very successful Mars Exploration Rovers. MSL carries a variety of analytical and imaging instruments designed to collect martian soil samples and rock cores and analyze them for organic compounds and environmental conditions that could have supported microbial life now or in the past. Indiana University’s Department of Geological Sciences is participating on two of the instrument teams, MAHLI and CheMin. CheMin (for CHEmistry and MINeralogy) is a miniature X-ray diffraction/X-ray fluorescence (XRD/XRF) instrument designed to perform remote robotic mineralogical analyses of the surface of Mars. This instrument, a prototype of which is shown below, performs mineralogical analysis of crushed or powdered samples through the combined application of X-ray diffraction (mineral structure analysis) and X-ray fluorescence (elemental compositional analysis). With this instrument, the scientific community will, for the first time, be able to obtain truly definitive mineralogical information on Martian surface rocks and dusts. The key role that definitive mineralogy plays in understanding the Martian surface is a consequence of the fact that minerals are thermodynamic phases, having known and specific ranges of temperature, pressure and composition within which they are stable. More than simple compositional analysis, definitive mineralogical analysis can provide information about pressure/temperature conditions of formation, past climate, water activity and the like. Definitive mineralogical analyses are necessary to establish the origin or provenance of a sample. The search for evidence of extant or extinct life on Mars will initially be a search for evidence of present or past conditions supportive of life (e.g., evidence of water), not for life itself.
The CheMin collaboration includes scientists from Indiana University (D. Bish), NASA Ames Research Center (D. Blake, project leader), inXitu (P. Sarrazin, formerly a postdoctoral researcher with D. Blake), and Los Alamos National Laboratory (S. Chipera and D. Vaniman).
Figure 1. Geometry of the CheMin XRD/XRF instrument. a) (left) overall geometry of CheMin; b) (above right) XRD 2θ plot obtained by summing diffracted photons from the characteristic line of the X-ray source (colored magenta in figure 1a); c) (below right) X-ray fluorescence spectrum obtained by summing all of the X-ray photons detected by the CCD (XRF photons from the sample shown schematically in green and red in figure 1a).
CheMin (for CHEmistry and MINeralogy) is a miniature X-ray diffraction/X-ray fluorescence (XRD/XRF) instrument designed to perform remote robotic mineralogical analyses on solid bodies of the solar system such as Mars, Venus, Europa, the Moon, asteroids, and cometary nuclei. Several portable CheMin prototypes have been built, and the most recent prototype was successfully deployed at a variety of Mars analog sites in Death Valley, California, in May 2004. This instrument was recently chosen to be a part of the scientific payload on the next major Mars lander mission, Mars Science Laboratory, scheduled to launch in 2009.
Figure 2: CheMin III prototype in a laboratory setting with a 114mm Debye-Scherrer camera for scale.
The CheMin XRD/XRF instrument performs definitive mineralogical analysis of crushed or powdered samples through the combined application of X-ray diffraction (mineral structure analysis) and X-ray fluorescence (elemental compositional analysis). The two techniques are extraordinarily powerful and complementary; they constitute the preferred methods for mineralogical analysis of unknowns in terrestrial laboratories. Other techniques such as Mossbauer spectroscopy, infrared absorption spectroscopy (IR), Raman spectroscopy, differential thermal analysis or thermogravimetric analysis, while able to identify features attributable to specific minerals or to generally identify classes of minerals, are seldom definitive or quantitative and cannot unravel the complex mineral assemblages found in nature. Figure 1 shows how CheMin simultaneously accomplishes X-ray diffraction and X-ray fluorescence analyses of a single sample. The instrument uses a CCD (charge-coupled device) detector, whose pixels are sensitive to the energy of incoming photons. When operated in such a way that each pixel measures either one or no photons (so-called single photon counting mode), the instrument can obtain information on the energy of incoming photons (XRF) and also on the spatial distribution of photons of a particular energy (XRD). X-rays diffracted from a fine powder produce Laue rings on the CCD, shown below as magenta rings. Circumferential integration of these rings produces a conventional X-ray diffraction pattern, in which the angular distribution and intensity of diffraction peaks can be used to determine the types and amounts of minerals in a mixture. The CheMinIII prototype is shown in a laboratory setting in Figure 2, with an Oxford Instruments X-ray tube mounted to the instrument. A 114 mm diameter Debye-Scherrer camera is shown in the foreground for scale.
Figure 3: low-2θ detection limit of the CheMin III prototype; left: XRD pattern of non-purified silver behenate CH3(CH2)20COO-Ag with the first ring at d001=58.38Å, 1.75° 2θ Co Kα, right: XRD pattern of a smectite clay (SWa-1) with trace of quartz.
Hydrous minerals such as clays are important indicators of water activity and habitability. The ability to identify and characterize the large d-values (low 2θ angles) characteristic of clay minerals and zeolites is essential in a mineralogical instrument intended for Mars exploration. CheMin has been tested using long-spacing standards and performs well to at least as low as 2° 2θ (Figure 3.). The characterization of complex mineral assemblages having several minerals in varying amounts will require longer periods of integration. The rock type (e.g., basalt, andesite, sedimentary) and the type of mineralogic information required will dictate the duration of analysis. For example, in a basalt, one might want to know the identity and amount of all major and minor minerals, as well as refined unit-cell parameters for some of the phases, information that can provide clues to the chemical composition and perhaps degree of cation order of individual phases. These data, in addition to the overall XRF analysis of the sample, can then be used to fully characterize the structure and composition of each phase. Figure 4 shows results obtained from a mineralogically complex rock (an andesite) containing seven major minerals. The results of a Rietveld refinement using the data are shown as an inset to the figure.
Figure 4: CheMin III data collected from a crushed sample of andesite (LANL #P52, sieved to >150 µm); left: 2D XRD image; right: 1D pattern obtained by integration of the diffraction image and results of the Rietveld refinement.
Mars Science Laboratory
CheMin in Badwater, Death Valley, California in May, 2005. Rock hammer is on the left for scale. The entire CheMin instrument, including all electronics, cooling, sample manipulation device, and batteries, is on the unit at left and a laptop computer sits on a plastic box on the right.
CheMin has been proposed as an analytical laboratory instrument for the Mars ’09 Mars Science Laboratory. One of the critical goals of this mission is definitive mineralogy of at least three different geological sites within the landing ellipse. Although the Mars ’09 landing site will not be chosen until additional reconnaissance imaging has been completed, three scenarios are proposed, based on results from the on-going Mars Exploration Rover (MER) mission.
Mars Exploration Program
The principal goals of NASA’s Mars exploration program are to determine whether conditions existed on the planet that could have supported life, and if so, if life developed. The search for evidence of life, prebiotic chemistry or volatiles supportive of life on Mars will require the identification of rock types that could have preserved these. Anything older than a few million years (>99.9% of Mars history) will either be a rock, or will only be interpretable in the context of the rocks that contain it. The key role that definitive mineralogy plays in this search is a consequence of the fact that minerals are thermodynamic phases, having known and specific ranges of temperature, pressure and composition within which they are stable. More than simple compositional analysis, definitive mineralogical analysis can provide information about pressure/temperature conditions of formation, past climate, water activity and the like. Definitive mineralogical analyses are necessary to establish the origin or provenance of a sample. The search for evidence of extant or extinct life on Mars will initially be a search for evidence of present or past conditions supportive of life (e.g., evidence of water), not for life itself.
Exploration of an ancient brine deposit
Meridiani Planum, where the MER rover Opportunity landed, appears to be an ancient evaporative lake environment. The report of up to 40 wt% sulfate salts at that locality indicates that evaporite sediments have played an important role in the hydrogeologic history of Mars. Data available to date support the presence of the mineral jarosite (a hydrous Fe-sulfate), Mg-sulfate, and lesser amounts of salts containing Cl and Br. These data imply that several sulfates, mixed with halogen salts, combine to form a complex salt assemblage. One of the most exciting features of the Meridiani sediments is the possibility that the salts may be hydrated. Water abundances in hydrated salts on Earth can be considerably greater than water abundances in hydrous silicates such as clays and zeolites. Water storage in minerals may be a significant source of the elevated hydrogen abundances seen in some of the equatorial regions by the Odyssey spacecraft, with abundances up to 8-9 wt% water equivalent present in areas where water ice should not be stable.13 Salt hydrates in evaporite sediments might account for some equatorial water. Could such a water-rich system harbor life at depth, or at least preserve evidence of brine-dwelling organisms? The ability to quantify hydrated mineral assemblages such as those found at Meridiani Planum will be important for reconstructing brine evolution and for determining the nature of interactions between brine minerals and detrital mineralogy. CheMin XRD/XRF data will be highly useful in interpreting brine chemistry and the extent and nature of the ancient habitable zone that existed on early Mars.
Exploration of a basaltic terrain
The present-day surface environment of Mars appears to be inhospitable to life. The most likely habitable zones in the present or remote past would have existed in the sub-surface. Should life have evolved in the subsurface, chemosynthesis (the utilization of chemical energy from the environment by primitive organisms) must have been the principal energy-harvesting strategy. If one is searching for evidence of subsurface chemosynthetic life, exploration of a basaltic terrain is an important component of this search. Ferrous iron contained in the mineral olivine (as well as in basaltic glass, pyroxene and some oxide minerals) could provide a source of redox energy for chemosynthetic life). CheMin can determine the amount and composition of the major rock-forming minerals in basaltic rocks. In addition, refined unit-cell parameters of phases such as olivine, (Fe,Mg)2SiO4 allow the determination of the amounts of Fe++ available for reaction with water. Once primary mineralogy is determined, one must search for and quantify secondary mineral assemblages that can indicate water-rock interactions. Even the passage of small amounts of water through the rock system can leave signatures in the form of hydrated phases such as clays, zeolites, chlorite, serpentine and a host of other H2O- or OH-bearing minerals as well as non-hydrous alteration minerals. Such assemblages, if found on Mars, would suggest the presence of a habitable zone having both liquid water and an energy source.
Mineralogy and Chemistry of the Global Soil Component:
The mineralogic compositions of Martian soils, which include globally distributed components, are still poorly constrained. Initial modeling of Viking XRF results suggested a mixture of clays, kieserite, calcite and rutile. The minerals of detrital basalt plus a Cl-bearing phase must also be present. In a summary of the Pathfinder soils, Bell suggests that IR spectral data are consistent with the presence of poorly crystalline or nanophase ferric oxide(s), sometimes mixed with small but varying amounts of well-crystallized ferric and ferrous phases. An important objective of soil analysis will be to determine the mineralogy and hydration state of the Martian soil "duricrust" which contains a significant amount of sulfate. Widespread distribution of sulfate salts as cementing agents in soil might be due to recently active pedogenic processes, possibly associated with acidic weathering, that do not require surface water or groundwater. The extent to which water participated in the generation of the Mars soil(s) is important in determining the presence/absence of surface habitable zones over Mars history. With the possible exception of some of the nanophase materials, all of the minerals listed above (and many more) can be readily identified and quantified using CheMin. A definitive mineralogic analysis of Mars soil will be important in developing an understanding of the oxidizing nature of the Mars surface, the extent to which water participated in its generation, and the presence of habitable zones on or near the Mars surface over geologic time.
In addition to Indiana University participation (D. Bish), this collaboration includes the NASA Ames Research Center (D. Blake, project leader), inXitu (P. Sarrazin, formerly a postdoctoral researcher with D. Blake and now the primary instrument designer), and Los Alamos National Laboratory (S. Chipera and D. Vaniman).