by The Design
The APXS team classifies rocks along different compositional trends, and then uses those
classifications to look for correlations between compositional variance and changes
in location, elevation, and terrain types.33 They also look for correlations with rock types and compositional trends observed on Spirit and Opportunity traverses.
Scientists who work with the APXS instruments on Curiosity and previous rovers have
devised their own method of classifying rocks. The APXS team looks for clusters in major-
element composition, and also for “co-variations,” patterns of groups of major elements
occurring together in rocks (Figure 9.13). 34 Each time the APXS team encounters a new rock that is compositionally distinct from previous observations, they name a new class for the first, best-described target. Sometimes, the APXS team identifies subclasses within
larger groupings. Figure 9.13 shows two ways in which APXS-defined rock classes cluster in composition along different axes. The top chart shows relative abundance of iron and
silicon in the rocks; the bottom chart shows sodium and potassium.
Because Mars dust is rich in sulfur and chlorine, the APXS team subtracts those elements
from their analyses and renormalizes the remaining elements to make up 100% of the rock
before comparing one rock to another (Table 9.3). The APXS rock classifications are not, 30 Ralf Gellert, personal communication, email dated May 10, 2016
31 Berger et al. (2014)
32 Berger et al. (2016)
33 Thompson et al. (2016)
34 Schmidt et al. (2016)
316 Curiosity’s Chemistry Instruments
Figure 9.13. Two examples of charts used to identify clusters in APXS rock classifications.
Top: rocks analyzed with APXS generally trend from silicon-poor and iron-rich (mafic rocks, like basalt) to silica-rich and iron-poor (like the very silica-rich Buckskin site). Gale soils (red triangles) are similar to the average Mars crust composition. Bottom: The association of potassium to sodium abundance in Gale rocks is more complex. Again, Gale soils are similar to average Mars crust composition, but most rocks that APXS has examined are richer in
potassium; some of those are very sodium-rich, and some are very sodium-poor. Note that rock classes that overlap in one of these two diagrams may be entirely separate in the other.
Graphs courtesy Mariek Schmidt.
Precision error
0.64
0.05
0.19
0.01
0.13
0.01
0.08
0.07
0.14
0.02
0.09
0.1
0.02
10
10
20
56.2
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0.34
9.4
0.1
4.77
5.98
2.5
0.44
1.2
10.8
1.54
99.76
0.17
64.3
1.14
6.3
0.39
10.8
0.1
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6.84
2.9
0.50
1.4
Greenhorn
Greenhorn
1130
160
107
336
y
43.4
0.93
9.7
0.42
17.4
0.4
8.52
6.87
2.8
0.47
0.9
6.9
1.27
99.94
0.17
47.3
1.01
10.6
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0.4
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7.49
3.1
0.51
1.0
1114
411
273
205
).
Ronan
Big Sk
2016
68.1
1.51
6.1
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4.4
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3.45
3.87
2.2
0.82
1.3
7.2
0.71
99.84
58
0.38
74.1
1.64
6.6
0.11
4.8
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3.75
4.21
2.4
0.89
1.4
al. (
Buckskin
Buckskin
1057
110
206
Thompson et
ve
1.07
0.39
0.3
4.47
4.29
2.8
0.65
1.4
5.8
0.52
0.23
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0.42
0.4
4.77
4.58
3.0
0.69
1.5
rom
51.8
12.4
13.5
99.07
85
55.4
13.3
14.4
Confidence Hills
Moja
809
839
1737
45.8
0.94
10.8
0.19
13.2
0.3
7.17
7.46
3.3
0.97
0.9
7.8
0.97
98.87
10
0.29
50.2
1.03
11.9
0.21
14.5
0.3
7.86
8.18
3.6
1.06
1.0
Mount Bastion
Heimdall
399
214
782
42.0
0.85
8.8
0.65
20.6
0.4
8.26
6.54
3.0
0.78
0.8
6.1
1.15
98.68
0.26
45.3
0.92
9.4
0.70
22.2
0.4
8.90
7.05
3.2
0.84
0.9
Bell Island
Eqalulik
323
276
963
167
e
ate and chlorine
erneck
0.91
8.9
0.41
0.3
9.80
5.40
3.0
0.62
1.0
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1.13
0.20
0.93
9.1
0.42
0.3
5.51
3.1
0.63
1.0
John Klein
W
169
46.9
20.5
98.63
694
794
401
47.8
20.9
10.00
APXS team-defined compositional classes. F
ve sulf
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0.67
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3.67
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1.53
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Et Then
Secure
560
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194
232
183
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28.3
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5.38
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2.22
0.7
om sites of dif
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Rocknest 3
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680
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0.89
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ve, renormalized to remo
0.93
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2.8
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472
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176
44.9
23.3
APXS r
e M
werre
0.54
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Jak
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97
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33
Same as abo
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11.8
Example
get
T
T
iO2
iO2
Table 9.3.
Class
Tar
Sol
SiO2
T
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
SO3
Cl
Total
Ni ppm
Zn ppm
Br ppm
K2O/Na2O
SiO2
T
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
318 Curiosity’s Chemistry Instruments
directly, identifications of rock type; instead, they help the science team identify trends in the data and group rocks of similar composition together. Combined with ChemCam elemental
compositions, CheMin mineralogy, and observations of rock textures from Mastcam and
MAHLI, the APXS rock classifications can help tell the stories of Gale’s rocks.
9.3.4 Anomalies
In general, APXS has been a healthy and productive instrument with one minor technical
issue. Since launch, Curiosity’s APXS has exhibited an unusual behavior that has never
been seen in any other instrument from previous generations. Occasionally, in the middle
of acquiring data, it stops counting real X-ray events, instead counting only spurious X-ray counts at the lowest detectable energy. The APXS team calls this behavior “lockup.” The
ultimate cause of the behavior is unknown; it happens randomly. To prevent loss of data,
the team splits long APXS integrations into two parts and reboots the instrument in
between them, reducing the risk that early lockup would cause the loss of all the data from a single integration. 35 This mitigation strategy has prevented any loss of observations. If it happens early in an integration, lockup can affect the signal-to-noise ratio of observations, but these effects have been minimal in practice. 36
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.
35 Slavney (2013)
36 Ralf Gellert, personal communication, email dated May 10, 2016
37 The main reference for the description of the CheMin instrument is Blake et al. (2012); a useful summary of how it has worked on Mars is in Downs (2015)
9.4 CheMin: Chemistry and Mineralogy 319
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 environ-
ment 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<
br />
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 alumi-
num with variable amounts of sodium, potassium, and calcium. Feldspars are com-
mon 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 feld-
spars, 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 oxidiz-
ing 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
38 Vaniman et al. (2014)
39 Léveillé et al. (2015)
320 Curiosity’s Chemistry Instruments
• 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.
9.4.2.1 The CheMin Sample Handling System