by The Design
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 deliv-
ered 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 millime-
ters, 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 require-
ment of <150 micrometer powder is defined as the maximum crystal size that provides
good diffraction.
9.4 CheMin: Chemistry and Mineralogy 321
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 (Blak e et al. 2012 ). Bottom right: Sample inlet with inlet cover opened, showing 1 mm mesh (NASA/
JPL-Caltech/MSSS release PIA16163).
322 Curiosity’s Chemistry Instruments
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 boil-ing 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 peri-
odically shakes the cells at very high intensity for a few seconds, breaking up the agglom-
erations 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 rela-
tive 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 radia-
tion to exit the source and pass through the center of the sample cell. The narrow beam
9.4 CheMin: Chemistry and Mineralogy 323
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 cap-
tured 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 an
gle of diffraction, the sum of the theta angle of inci-
dence 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 rov-
er’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 pres-
ent, but with larger analytical error than for crystalline minerals.
324 Curiosity’s Chemistry Instruments
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 com-
puter merges information from many CheMin CCD frames to produce histograms before
transmitting the results to Earth. While mineral abundances from diffraction are quantita-
tive (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 shorten-
ing 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 degrada-
tion 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
40 David Vaniman, personal communication, email dated April 5, 2017
9.4 CheMin: Chemistry and Mineralogy 325
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.
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
adjacent “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.
41 David Vaniman, personal communication, email dated March 8, 2017
326 Curiosity’s Chemistry Instruments
Table 9.4. Summary of CheMin sample analyses, organized approximately in order of stratigraphy, from stratigraphically highest to lowest (as in a stratigraphic column). Data are from the CheMin releases to NASA’s Planetary Data System.
t
y
e
l
e
e
lfat
lfid
te
p
le so
e
etit
m su
e
iu
mp
me
a
agioclase feldspar
n su
mati
lc
ay
Grou
Sa
CheMin deliver
CheMin analysis star
CheMin cell
Na
olivin
pyroxene
pl
alkali feldspar
silic
magn
iro
he
ca
akaganeite
jarosite
apatit
cl
amorphous
Modern
74
77
79
1a Rocknest (scoop 4)
13
22
33
0
1
2
0
0
2
0
0
0
0
27
basaltic
93
94
95
7a Rocknest (scoop 5)
16
20
29
1
1
1
0
1
1
0
0
0
0
29
sand
1224
1226
1236
7a Gobabeb (scoop 1)
21
24
0
0
2
0
0
1
0
0
0
0
34
1321
1323
1325
8a
Lubango**
0
4
11
0
1
3
0
1
6
0
0
0
0
75
Stimson
1332
1334
1335
7b
Okoruso
0
21
27
2
1
11
0
1
1
0
0
1
0
35
formation 1137 1139 1140 8a Greenhorn*
0
6
21
0
51
9
0
3
10
0
0
0
0
0
1119
1121
1122
7b
Big Sky
0
26
37
0
3
10
0
2
1
0
0
0
0
20
1495
1496
1496
4b
Sebina
1
2
12
2
0
0
0
6
7
0
1
0
51
19
1464
1466
1470
5a
Quela
2
2
14
2
0
0
0
6
5
0
0
0
52
16
1422
1425
1425
8b
Marimba
2
1
17
3
0
0
0
6
7
0
1
0
40
23
Murray
1361
1362
1363
12a Oudam
0
5