9.4 CHEMIN: CHEMISTRY AND MINERALOGY
CheMin brings two powerful analytical laboratory techniques to Mars: X-ray diffraction and X-ray fluorescence, allowing direct measurement of mineral composition on Mars for the first time. On Earth, X-ray diffraction and X-ray fluorescence require refrigerator-sized pieces of equipment. Developing Curiosity’s miniaturized version was the result of more than two decades of work. The development of the CheMin instrument for Curiosity enabled the development of a portable X-ray diffraction/X-ray fluorescence CheMin-like instrument for use in the field on Earth.37 At the start of the mission, the principal investigator of CheMin was David Blake of NASA Ames Research Center; Tom Bristow (also of Ames) has taken over the role in the second extended mission.
CheMin agitates a finely powdered sample of rock or loose sediment in front of a beam of X-rays. The X-rays diffract through the lattice structures of the minute crystals, generating a diffraction pattern of concentric rings that is recorded by a detector – actually a charge-coupled device, the same kind of detector that is at the heart of a digital camera. The angles at which the X-rays diffract are diagnostic of the minerals present. The impinging X-rays can also produce X-ray fluorescence in the sample, allowing measurement of the elemental composition of the sample, complementing APXS measurements. It is the CheMin instrument that drives the requirement for CHIMRA to prepare small samples of powder of less than 150 micrometers in diameter. CheMin has 27 sample cells, and cells can be reused for up to a total of 74 sample measurements over the course of the mission.
9.4.1 Scientific background
While ChemCam and APXS identify the elements present in a target, CheMin identifies how those elements are assembled into minerals. Minerals can give clues to the environment in which a rock formed. Here is a brief summary of the minerals that CheMin has found in Mars rocks:Olivine. A magnesium-iron silicate common in basaltic rocks that is generally dark or green in color. The magnesium-rich endmember is called forsterite (Mg2SiO4), the iron-rich endmember fayalite (Fe2SiO4).
Pyroxene. A large group of magnesium-iron-calcium silicates, with lesser amounts of other elements such as sodium, aluminum, chromium, manganese and titanium; common in basaltic rocks. Pyroxenes come in different crystal structures called clinopyroxenes and orthopyroxenes, and are usually dark or green in color. CheMin has detected several specific pyroxenes including the orthopyroxene enstatite and the clinopyroxenes augite and pigeonite.
Feldspar. Another large group of silicate minerals which contain silicon and aluminum with variable amounts of sodium, potassium, and calcium. Feldspars are common in all rock types. Feldspars are further divided into plagioclase feldspars (sodium to calcium mixtures) and alkali feldspars (sodium to potassium mixtures). Feldspar tends to be lighter in color than olivine and pyroxene and can be gray to white to pink depending on composition. CheMin has detected plagioclase feldspars, and the alkali feldspar sanidine.
Quartz. This is the most common crystalline form of pure silicon dioxide on Earth. There are other crystal forms of silicon dioxide, and in addition to quartz, CheMin has detected tridymite, which forms under conditions of high temperature and low pressure on Earth, as well as cristobalite, which can form at a range of conditions. All have a white color when powdered.
Magnetite. An iron oxide in which some of the iron is reduced (Fe2+Fe3+ 2O4). It has a black color when powdered. Magnetite is a common trace or minor component of basaltic rocks.
Hematite. An iron oxide in which all of the iron is oxidized. It indicates an oxidizing environment and has a red color when powdered.
Iron sulfides, including pyrite and pyrrhotite, which indicate reduced environments and have a black color when powdered.
Akaganeite. An iron oxyhydroxide with chloride that has a rusty yellow color when powdered. It may represent altered pyrrhotite.38
Jarosite. An iron sulfate with potassium, sodium, and/or hydronium. It has a yellow color when powdered. On Earth, it commonly forms when iron sulfide (pyrite) is altered by water.39
Calcium sulfates. These can occur with or without water molecules incorporated into their crystal structure. Anhydrite is pure calcium sulfate, without water. Gypsum has significant water in its crystal structure. Bassanite is intermediate between the two. All have a white color. When rocks containing calcium sulfates are drilled and introduced into the warm CheMin instrument, gypsum or bassanite can dehydrate into anhydrite within a few sols.
9.4.2 How CheMin works
CheMin is located inside the body of the rover, occupying the front center. An articulated inlet cover, part of the Sample Acquisition, Processing, and Handling (SA/SPaH) system (see section 5.8) protects its inlet from infalling dust. Samples pass through a 1-millimeter sieve over a funnel and into one of 27 sample cells located on a wheel. Once analysis is complete, the wheel is rotated 180° to dump the samples into a sump at the bottom of the instrument. An X-ray source generates the X-rays that CheMin directs through the sample, and a CCD detects the diffracted and fluoresced X-rays. Components of the CheMin instrument are shown in Figure 9.14.
Figure 9.14. Components of the CheMin instrument. Top: installation of CheMin, June 15, 2010 (NASA/JPL-Caltech release PIA13231). The rover body and CheMin are upside down. Middle: dual-cell assembly, otherwise known as a “tuning fork,” showing both Kapton and Mylar window materials (from Blake et al. 2012 ). Bottom left: sample wheel assembly (Blake et al. 2012 ). Bottom right: Sample inlet with inlet cover opened, showing 1 mm mesh (NASA/JPL-Caltech/MSSS release PIA16163).
9.4.2.1 The CheMin Sample Handling System
CheMin’s funnel is equipped with a mesh screen with wires spaced 1 millimeter apart, to protect the instrument from too-large particles. When CHIMRA delivers a sample, CheMin vibrates three piezoelectric actuators around the funnel to encourage all of the sample to pass through without sticking. The sample cells are mounted in pairs on a wheel. The wheel holds 27 sample cells and 5 reference standards. Fourteen of the sample cells have Kapton windows, and 13 have Mylar windows. The Mylar-windowed cells provide cleaner results, having little observable effect on the diffraction patterns produced by CheMin. But Mylar scratches easily and can be damaged by acidic samples. The Kapton-windowed cells will last longer, but Kapton produces a peak in the diffraction patterns that could interfere with the detection of some clay minerals.
MAHLI routinely images the CheMin inlet and mesh screen to make sure that all delivered material has passed through. When MAHLI observes any material clumping on the screen, CheMin can be commanded to run the piezoelectric actuators on its funnel to encourage any sticking material to drop. See Table 5.4 for a summary of MAHLI CheMin inlet imaging.
CHIMRA delivers a maximum of 76 cubic millimeters of sample to a sample cell. Each sample cell has a miniature funnel-shaped reservoir with a volume of 400 cubic millimeters, directing sample into a 10-cubic-millimeter active cell. The space within the active part of the cell is a very thin disk 8 millimeters in diameter and 175 micrometers deep. That depth is optimal for the 150-micrometer limit of sample powder size. The requirement of <150 micrometer powder is defined as the maximum crystal size that provides good diffraction.
Each pair of sample cells has a piezoelectric actuator, referred to as a “piezo.” To get the grains inside a sample cell moving, CheMin uses the piezos to vibrate them at a range of frequencies, with the goal of getting the sample cell to vibrate at its resonant frequency, like a tuning fork. The vibrations of the two tines of the fork – the two sample cells in each pair – balance each other out so that little of the vibratory energy gets transmitted to the rest of the instrument. The intense vibration makes the mineral grains circulate like a boiling liquid, tumbling them randomly to present all orientations of all minerals to the X-ray beam. The dynamics of the vibration are extremely important. If the vibration happens at too low an intensity, grains could separate by density. It’s also possible for grains to agglomerate into immobile masses wedged between the two sides of the cell;
once formed, such agglomerations tend to grow. To prevent both of these from occurring, CheMin periodically shakes the cells at very high intensity for a few seconds, breaking up the agglomerations and mixing up the grains that may have separated by density. Vibration of the powdered sample in the cells had to be tested out at Mars gravity, tests that the CheMin team performed, 3 to 4 seconds at a time under microgravity created in a Piper Cherokee aircraft performing parabolic flights over the Pacific Ocean. Occasionally, the vibration turns out to be too intense, encouraging sample to bounce up and out of the cell, so CheMin is sometimes commanded to vibrate one of the cells next to the one that actually contains the sample in order to move the sample around more gently, a method that the CheMin team calls “kumbayatic” mode. (Seriously.)
Samples are usually analyzed for 20 to 40 hours over many sols, although usable data can be obtained in about 2 to 3 hours. Longer analysis times improve detection of minor minerals and improve the quality of the data. Once analysis is complete, the wheel rotates 180° to dump the sample into an internal sump, vibrating the cell to encourage the sample to fall out. The reusable sample cells have no covers, so dumping happens automatically as a result of rotating the wheel. The reference standard sample cells are covered with high-efficiency particulate air filters to prevent their powdered contents falling out with wheel rotation.
There are five standards. One is amphibole, an iron-rich silicate mineral sourced from Gore Mountain, New York, selected because it has a range of intense peaks in its X-ray diffraction pattern, as well as several closely-spaced peaks that allow the CheMin team to monitor how well they are resolving close peaks. Two other standards are mixtures of synthetic beryl and quartz in different proportions, selected because beryl has clearly defined widely-spaced peaks, allowing the team to monitor the crispness of individual peak profiles; the intermixed quartz helps them check how well they can determine relative mineral abundances, especially for minor minerals. One standard is synthetic arcanite, a potassium sulfate that also has well-defined diffraction and is strongly X-ray fluorescent, with precisely known sulfur and potassium content. Finally, a doped ceramic standard was made from a mix of the clay mineral nontronite with numerous salts, providing a standard with a wide variety of fluorescence peaks.
9.4.2.2 The CheMin X-Ray Source
CheMin’s X-ray tube emits a cone of X-radiation using a cobalt anode. The X-rays have an energy of 6.925 keV. A pinhole plate allows only a 70-micrometer-wide beam of radiation to exit the source and pass through the center of the sample cell. The narrow beam illuminates just 0.02% of the volume of material in the sample cell, but by vibrating the sample cell, different grains move in and out of the analysis area at different, random orientations. After passing through the cell, the direct beam falls onto a beam trap on the detector. The X-ray source has a power supply that is pressurized with a mixture of sulfur hexafluoride and nitrogen gas.
9.4.2.3 The CheMin Detector, X-Ray Diffraction, and X-Ray Fluorescence
The detector is a 600-by-1182-pixel CCD, of which 600-by-582 pixels is an active area. The pixels are unusually large for a CCD (40 micrometers square). An impinging X-ray photon deposits thousands of electrons in the CCD in a cloud that is tens of microns in diameter; the large size of the pixels means that almost all of this charge is usually captured in one pixel, rather than being spread across several pixels, so that each time an X-ray photon hits the detector, the detector records not only the position but also the energy of the photon. A filter in front of the detector prevents it from being exposed to visible-wavelength photons. A cryocooler brings the temperature of the detector down to at least 48°C below the interior of the rover, thereby minimizing dark current on the CCD.
X-ray diffraction happens when X-rays interact with crystalline lattices of atoms and constructively interfere, producing diffraction spikes at certain angles away from the direction of the X-ray beam. The angle of diffraction, the sum of the theta angle of incidence and the equal theta angle of reflection (hence 2-theta), depends on the wavelength of the X-ray and the spacing between atoms in the crystal lattice. The CheMin detector can record diffracted X-rays at 2-theta angles ranging from 5–50°, with an angular resolution of 0.30–0.35°.
When CheMin is operating, the CCD typically records 30-second exposures, called “frames.” In that short period, any given pixel on the detector usually records at most one incident X-ray; therefore, a single CheMin frame records not only the position but also the intensity of individual events. CheMin can store up to 2730 such images in memory (or about 22 hours of data). But that would be too much data to transmit to Earth, so the rover’s main computer can stack 10 to 200 individual exposures into a single image, called a “minor frame.” Only pixels whose values correspond to the 6.925-keV energy of the X-ray source are selected for inclusion in the minor frame. On Earth, some or all of the minor frames from a single sample can be stacked into a “major frame.” Then the pixel values can be summed around the concentric circles of the 2-D diffractogram, producing a 1-D diffractogram (see figure 9.15 for examples). The CheMin team uses these 1-D patterns for analysis of mineral composition, but they also downlink the 2-D diffractograms to diagnose problems, such as blobs caused by mineral grains that became stuck during vibration. Individual peaks in the diffractogram can be diagnostic of specific minerals, but each mineral has a set of peak positions and intensities that reveal the crystal structure. The team uses all the peaks to determine mineral abundances. If there are non-crystalline (amorphous) materials present in the sample, they show up in the diffractogram as a very broad hump upon which the narrower diffraction spikes are superimposed. The team can use the shape and height of the hump to quantify the amount of amorphous materials present, but with larger analytical error than for crystalline minerals.
Figure 9.15. Example CheMin data from the Windjana sample. Left: A CheMin 2-D diffractogram. Black semicircle at center of the left edge is the CheMin beam stop. Light circles are from diffraction in lattice planes in minerals. Right: CheMin 1-D diffractogram produced by summing all the values of the pixels with diffracted cobalt radiation at a given distance (angle) from the X-ray beam. Sharp peaks identify specific crystalline minerals. Broad hump (dashed red line) indicates the presence of amorphous (noncrystalline) material. After Treiman et al. ( 2016 ).
The CheMin CCD can also function as an X-ray fluorescence detector. The pixel energy values in each frame serve as a measure of the energy of the X-rays that hit the detector. Many of these X-rays are diffracted ones with the same energy as the cobalt source, but other, lower-energy X-rays come from X-ray fluorescence. The energy of the fluoresced X-rays depends on the elements present. CheMin is not sensitive to elements lower in mass than potassium. CheMin reports this data in the form of a histogram (counts of pixel energy values). As with the diffractograms, to conserve downlink volume, the rover computer merges information from many CheMin CCD frames to produce histograms before transmitting the results to Earth. While mineral abundances from diffraction are quantitative (with mineral detection limits of 1 to 2% by weight) chemical composition from X-ray fluorescence is qualitative.
Over time, the CCD will accumulate damage from cosmic rays as well as from the neutrons emitted by the MMRTG. The effects of this damage can be reduced by shortening exposure times or by binning the data before processing it. CCD health is checked periodically (about every 18 months), and as of 2017 there has been no detectable degradation in CCD performance.40
9.4.3 Using CheMin
CheMin analysis is usually performed overnight, at a time when the rover operates at cooler temperatures. A typical overnight analysis takes 10 hours. The analysis is usually repeated over multiple nights to improve counting statistics, so most samples are analyzed for a total of 20 to 40 hours. CheMin can continue to analyze a sample in this way until a new sample is ready for delivery, at which point the instrument usually dumps the old sample in order to receive the new one. Occasionally, CheMin accepts two samples in adj
acent “tuning forks,” permitting it to continue analyses on both. CheMin kept both John Klein and Cumberland in adjacent cells for about 200 sols, and also stored the Mojave2/Telegraph Peak, Greenhorn/Big Sky, and Lubango/Okoruso sample pairs in this way. As the mission has proceeded, the team has begun to reuse Mylar-windowed cells, keeping unused cells in reserve against a future date when the rover will encounter different rock types (e.g. Vera Rubin Ridge and the clay- and sulfate-rich rocks beyond it).41 Sol 640 was the first test of re-using a sample cell. Figure 9.16 is a schematic of the CheMin sample wheel showing sample cell use.
Figure 9.16. Schematic diagram of the CheMin sample wheel showing which cells have been used as of sol 1800. Information courtesy David Vaniman and Elizabeth Rampe.
The Design and Engineering of Curiosity Page 37
