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Determining Mineralogy on Mars with the CheMin X-Ray Diffractometer The CheMin team logo illustrating the diffraction of minerals on Mars. Robert T. Downs1 and the MSL Science Team 1811-5209/15/0011-0045$2.50 DOI: 10.2113/gselements.11.1.45 he rover Curiosity is conducting X-ray diffraction experiments on the The mineralogy of the Martian surface of Mars using the CheMin instrument. The analyses enable surface is dominated by the phases found in basalt and its ubiquitous Tidentifi cation of the major and minor minerals, providing insight into weathering products. To date, the the conditions under which the samples were formed or altered and, in turn, major basaltic minerals identi- into past habitable environments on Mars. The CheMin instrument was devel- fied by CheMin include Mg– Fe-olivines, Mg–Fe–Ca-pyroxenes, oped over a twenty-year period, mainly through the efforts of scientists and and Na–Ca–K-feldspars, while engineers from NASA and DOE. Results from the fi rst four experiments, at the minor primary minerals include Rocknest, John Klein, Cumberland, and Windjana sites, have been received magnetite and ilmenite. CheMin and interpreted. The observed mineral assemblages are consistent with an also identifi ed secondary minerals formed during alteration of the environment hospitable to Earth-like life, if it existed on Mars. basalts, such as calcium sulfates KEYWORDS: X-ray diffraction, Mars, Gale Crater, habitable environment, CheMin, (anhydrite and bassanite), iron Curiosity rover oxides (hematite and akaganeite), pyrrhotite, clays, and quartz. These secondary minerals form and INTRODUCTION persist only in limited ranges of temperature, pressure, and The Mars rover Curiosity landed in Gale Crater on August ambient chemical conditions (i.e. humidity, water activity, 6, 2012, and is making its way up the fl anks of Mt. Sharp. Eh, pH, etc.), and provide clues about the habitability of One of the principal goals of the mission is to identify Mars. A source rock, such as basalt, may be transformed and characterize past habitable environments on Mars into a range of different mineral assemblages depending on (Grotzinger et al. 2012; Meyer 2012). Determining the the physical and chemical conditions that it experiences. mineralogy and chemical compositions of Martian rocks Thus, knowledge of the minerals in Martian environments and soils provides constraints on formation and altera- provides insight about the conditions under which the tion pathways, and consequently provides information on samples were formed or altered and, in turn, about past, climate and habitability through time. Sample composi- potentially habitable environments on Mars. tions are measured with the Mars Science Laboratory (MSL) in three ways: (1) ChemCam, which uses laser-induced In addition to crystalline minerals, signifi cant amounts breakdown spectroscopy to measure the composition of a of amorphous and poorly crystalline compounds occur spot smaller than ~550 µm from a distance of 7 m (Maurice on the Martian surface (Blake et al. 2013; Bish et al. 2013; et al. 2012); (2) Sample Analysis at Mars (SAM), which Vaniman et al. 2014). These amorphous / poorly crystalline digests sieved or drilled powders in a furnace and measures materials are not well understood, but they likely occur their compositions by mass spectrometry (Mahaffy et both as primary phases, such as volcanic and impact glasses, al. 2012); and (3) the alpha particle X-ray spectrometer and as secondary phases, through the alteration of primary (APXS), which uses a combination of X-ray fl uorescence minerals. Their identifi cation and characterization can lead and alpha particle induced X-ray emission (Gellert et al. to an understanding of how they formed. For instance, the 2015 this issue). X-ray diffraction on MSL is conducted with presence of volcanic glass indicates that the glass has not the chemistry and mineralogy instrument CheMin (Blake undergone aqueous alteration since its formation because et al. 2012), which provides diffraction patterns of particles such glass weathers readily in the presence of water. In smaller than 150 µm and results in accurate and time- addition, the presence of poorly crystalline phases such as tested mineralogical identifi cation and quantitative phase allophane and hisingerite indicates low-temperature altera- abundances. Minerals are natural crystalline materials tion of volcanic glass and olivine, respectively. Orbital and with limited ranges of chemical composition. The century- in situ measurements of the amorphous / poorly crystal- old technique of X-ray diffraction is exquisitely sensitive line material found globally in Martian soils suggest that to the crystal structure and secondarily sensitive to the it consists of at least three phases: nanophase iron oxide chemical composition of minerals; as such, the CheMin (Morris et al. 2008), an amorphous silicate consistent with instrument is used to identify the minerals and, through altered volcanic glass, and amorphous sulfate (Vaniman et crystal-chemical calculations, to determine constraints on al. 2004; Chipera and Vaniman 2007). Along with these their empirical compositions. globally distributed amorphous components, there are local hydrated silica deposits in various places on Mars, such as Valles Marineris, western Hellas Basin, and the Nili Fossae region; their geologic settings suggest that they 1 Department of Geosciences, University of Arizona formed by aqueous alteration of preexisting crystalline or Tucson, AZ 85721-0077, USA amorphous phases (Ehlmann et al. 2009). E-mail: [email protected] ELEMENTS, VOL. 11, PP. 45–50 45 FEBRUARY 2015 THE DIFFRACTOMETER power source (illustrated in Fig. 12 of Blake et al. 2012). CheMin’s angular range of 5° to 50° 2θ with <0.35° 2θ X-ray diffraction techniques are the “gold standard” for resolution is suffi cient to identify and quantify virtually identifying minerals, but the use of diffractometers on all minerals. planetary missions has been problematic mainly because they are bulky and massive. They range from lab diffractom- This setup is not too different from the original diffraction eters that are generally the size of a double-wide refrigerator geometry of Bragg and von Laue, except that they used to the huge and powerful synchrotrons located in national photographic fi lm instead of a CCD detector. What truly laboratories that are from several hundred meters to several differs, however, is the way the sample is processed. Powder kilometers in circumference. In addition to their large size, diffraction studies on Earth typically utilize samples that the power requirements of X-ray sources are severe. The are ground into a fi ne powder ideally consisting of ~5 µm diffraction instruments in my lab operate at about 50,000 diameter grains, and the grains are carefully placed into volts, and, before MSL, it seems that NASA had never sent the diffraction position on the instrument. On Mars, we do a high-voltage power supply into space. Around 1990, in not have the luxury of grinding samples into fi ne powders. spite of these signifi cant obstacles, Dave Blake, from NASA– Instead, we sieve the unconsolidated dust, sand, or drill Ames, and Dave Bish, Steve Chipera, and Dave Vaniman, tailings with Curiosity’s CHIMRA processing system to from DOE–Los Alamos, independently came up with ideas <150 µm (Anderson et al. 2012) and drop the sieved sample on how to put a diffractometer into space. They met at into an automated shaker, which consists of a small sample a Lunar and Planetary Science Conference, where their chamber with transparent, plastic windows attached to posters were located next to each other, and decided to piezoelectric actuators that vibrate the sample like a tuning pool their respective talents to solve the problem. Materials fork at ~2000 Hz. This shaker sends the grains into convec- scientist and engineer Philippe Sarrazin joined the team as tive motions (as shown in Figs. 3 and 6-8 of Blake et al. a postdoc in 1994 to solve diffi cult practical problems and 2012). The constant movement of grains allows the X-ray condense the instrument to the size of a breadbox (FIG. 1). beam to sample crystals in an endless stream of random orientations that, over time, provide a two-dimensional powder diffraction pattern collected as bitmaps from the CCD (FIG. 3). The bitmaps are downloaded from Mars when transmission time and bandwidth are available, and, after 20–25 hours of collecting data, the diffraction pattern is ready to be analyzed. It is fi rst converted into the typical one-dimensional pattern familiar to most scientists, as shown in FIGURE 3. In the event that the instrument gets clogged, there is a “chaos” mode of larger-amplitude vibra- tions. We actually found that our nominal vibration during data collection was too strong, and we were losing sample from the top of the cell. To decrease the vibration, we invented the “kumbayatic” mode, where a neighboring cell vibrates strongly enough to induce grain circulation but the sample cell itself is not being vibrated. For most samples, however, we now vibrate the cell directly, but at the lowest-possible energy. The CheMin instrument being loaded into the rover FIGURE 1 body during rover assembly. Note that for extraterres- trial use the powder diffractometer has been reduced from the size of a refrigerator to the size of a breadbox. PHOTO COURTESY OF NASA/ JPL-CALTECH The instrument, described in detail by Blake et al. (2012), has a cobalt X-ray source that operates at a nominal voltage of 28 keV and a current of 100 µA. During an analysis, a collimated X-ray beam is directed through powdered or crushed sample material. An X-ray-sensitive CCD (charge- coupled device) detector is positioned on the opposite side of the sample from the source and directly detects X-rays diffracted or fl uoresced by the sample (FIG. 2). The CCD detector is operated in single-photon counting mode (the detector is read out suffi ciently often that most pixels contain either no charge or charge derived from a single photon).

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