Emily Lakdawalla
Page 44
its sample chamber. Then SAM goes on to analyze the krypton that’s left in the sample
held in TLS: it evacuates the QMS and then performs an experiment in semi-static mode
with the gas from the TLS. Unfortunately, because xenon and krypton analysis happens
sequentially, SAM can’t directly measure isotopic ratios of krypton to xenon.52 SAM performed static experiments on sols 915 and 976.
49 Charles Malespin, personal communication, email dated April 12, 2017
50 Atreya et al. (2013)
51 ibid.
52 Conrad et al. (2016)
340 Curiosity’s Chemistry Instruments
Figure 9.22. Example pathway for noble gas enrichment (purple line) and methane enrichment (red line) experiments. In a dynamic mode noble gas experiment, the pump moves atmospheric gas past the scrubber to remove water and carbon dioxide and then sends it into the QMS. The same scrubber can also prepare enriched gas to be sent to the TLS for methane
measurement.
9.5.2.3 Methane enrichment
The atmospheric abundance of methane is usually too low for it to be measured precisely
in a direct atmospheric experiment. To detect methane, SAM uses the same scrubber as for
the noble gas experiment to remove carbon dioxide and water from the atmospheric gas,
then employs the adjacent getter to remove the nitrogen from the atmospheric gas, leaving
behind argon with trace amounts of methane. After two hours of scrubbing and getting,
SAM sends what’s left in the manifold to TLS (the red line in Figure 9.22). After the TLS
measurement is complete, SAM empties TLS and performs an empty-cell measurement.
Finally, SAM introduces atmosphere directly into TLS to perform a direct, non-enriched
atmosphere experiment. Methane enrichment experiments were performed on sols 572,
683, 865, 964, 1086, 1168, 1321, 1450, 1527, 1578, and 1708.
9.5 SAM: Sample Analysis at Mars 341
9.5.2.4 Atmospheric enrichment
To study the deuterium-to-hydrogen ratio in TLS with more precision, or to search for
trace higher-weight atmospheric molecules using QMS, SAM can use its scrubber to col-
lect those gases from the atmosphere, evacuate the instrument, and then release the col-
lected gas. Although SAM has attempted this experiment, there is not enough water in the
atmosphere for it to measure isotopes. It has done better at measuring atmospheric water
from solid samples.
9.5.2.5 Solid sample pyrolysis with evolved gas analysis (EGA)
Because of the MTBSTFA contamination (see section 9.5.1.7), the procedure now used for sample analysis on Mars is different from the one described in papers published before
landing.53 First, SAM performs a blank analysis run, going through the motions of a complete evolved gas experiment with no sample in the cup, to provide a measurement of the
experimental background. When it’s ready to receive a sample, SAM “conditions” (bakes)
one of the 59 quartz cells in the analysis oven in order to remove any hydrocarbon material that may have stuck to its walls, then allows it to cool. SAM works the pumps hard to
remove as much gas from the system as possible. The rover delivers three portions of
sample material that passed through the 150-micron sieve, each portion about 75 cubic
millimeters in volume. By keeping the pre-baked cup in a clean protected environment in
the oven, pumping out the system, and using a triple portion, the SAM team dramatically
reduces MTBSTFA contamination. SAM ramps the temperature up to 900–1100°C
(depending on which oven is being used) at a rate of about 35°C per minute, although the
rate of heating slows above 800°C. SAM directs helium gas through the sample as it is
being heated, sending any evolved gases to QMS for monitoring the chemical composition
as the temperature changes (the purple line in Figure 9.23).
9.5.2.6 Gas chromatograph mass spectrometry (GCMS)
While conducting an evolved gas experiment, SAM can send the evolved gas past the
hydrocarbon trap on its way to the QMS. After the evolved gas analysis is over and the
manifolds are evacuated, SAM purges a GC column, pressurizes the manifold to 100 kilo-
pascals with stored helium, flash-heats the hydrocarbon trap, and then directs the gas
through the GC column (the red line in Figure 9.23). SAM can only use one GC column
at a time, but the team has developed a mode where gas can be trapped on the injection trap of a second column for subsequent analysis later in the experiment. 54
53 Lakdawalla (2013)
54 Charles Malespin, personal communication, email dated April 12, 2017
342 Curiosity’s Chemistry Instruments
Figure 9.23. Example pathway for a solid sample evolved gas analyses (EGA). The simplest EGA pathway is shown in purple: helium gas runs through the sample cell in the oven and then through the QMS. In this diagram, the sample gas is also being directed past the hydrocarbon trap. In an evolved gas experiment, some of the gas can be sent to the TLS (red line).
To perform GCMS on the trapped hydrocarbons, after the EGA run is complete, SAM purges
the instrument and then uses helium carrier gas to move hydrocarbons from the flash-heated trap through a GC column into the QMS (blue line).
9.5.2.7 Tunable laser spectrometry (TLS)
To measure isotopic ratios of methane, carbon dioxide, and/or water evolved from solid
samples, SAM scientists select a particular “cut” of the temperature ramp that they want
to send to TLS. As the oven passes through that temperature range, all the evolved gas is
sent to TLS. TLS then performs its experiments on the collected gas. Because there might
be enough of any given gas to saturate the detector, TLS pumps out some of the collected
gas at intervals and repeats the observation several times.
9.5 SAM: Sample Analysis at Mars 343
9.5.2.8 Combustion
Some carbon compounds are not volatile even at the highest temperatures that the SAM
ovens can achieve. To search for these compounds, SAM can add oxygen from an
onboard reservoir to a solid sample and then heat it to a high temperature. Any carbon
compounds in the sample will combust, turning into carbon dioxide. After the combus-
tion has had some time to take place, SAM sends the evolved gas to TLS to measure the
isotopic ratio of the carbon and hydrogen in the sample. SAM has performed this experi-
ment once, on the Cumberland sample. The experiment took three days, on sols 555,
556, and 558.
9.5.2.9 Wet chemistry and opportunistic derivatization
SAM can detect of amino acids and other organic compounds with one of the nine
derivatization cups, which would be punctured before delivery of a solid sample and
evolved gas analysis. The leak of MTBSTFA into the interior of SAM provided an oppor-
tunity to do a long-term “opportunistic derivatization” experiment on one sample (from
the Cumberland drill hole) without employing any of the cups. Sample dropoffs can be
made to a cup that is then not analyzed for a long time, a procedure the team calls “doggy
bagging”. Over time, organics in the sample react with the MTBSTFA vapor inside SAM,
and GCMS analysis can reveal derivatization products. Curiosity analyzed Cumberland
samples in this way on sols 822, 823, 837, and 839, and a full Mars year later on sols 1543, 1546, 1591, and 1593.
9.5.3 SAM on Mars
The SAM experiment has been highly successful. SAM has measured the composition of
past and present Martian air and monitored it over time. It has successful
ly measured iso-
topes of even the rarest noble gas, xenon. It has detected low-molecular-weight organic
compounds of Martian origin. It has detected methane and observed rapid changes in
atmospheric methane abundance. SAM has been used for an experiment never before
conducted in space, to perform potassium-argon dating to measure the ages of drilled
rocks. Even the problem of the MTBSTFA contamination has been turned into a benefit,
with the performance of long-term derivatization experiments.
The complexity of SAM experiments and the amount of power they require drive a lot
of the supratactical planning on Curiosity. A typical evolved-gas analysis can take three
sols (one for preconditioning, one for sample preparation and delivery, and one for heating and evolved gas analysis). The evolved gas analysis usually takes 4 to 6 hours to run to
completion, depending on the selected options, and it can leave the rover with a relatively low state of battery charge. For that reason it is common to conduct SAM experiments
over weekends and use one weekend sol to recharge batteries.
344 Curiosity’s Chemistry Instruments
Figure 9.24. Schematic diagram of the SAM sample carousel, showing the locations of the numbered cups of different types, and which ones have been used as of sol 1800. For the identities of samples in each used cup, refer to Table 9.6 . Data courtesy Charles Malespin.
As of the time of Curiosity’s second extended mission proposal in January 2016, SAM
had 75% of its helium supply left, but the pumps were nearing their design lifetime.
Duplicates of the pumps on Earth have been tested to survive twice their design lifetime.
So, barring any unforeseen events, SAM should have considerable life left. The SAM team
is carefully rationing use of the pumps to ensure that they will still be working when the
rover reaches the clay-rich layers beyond Vera Rubin Ridge. Usage of the cups and experi-
ments run to date are summarized in Figure 9.24 and Table 9.6, respectively.
9.5 SAM: Sample Analysis at Mars 345
Table 9.6. Summary of the SAM solid sample experiments.
Type of run
Sample
Sol # Cup # GC Hydrocarbon temp cut(°C)
GCMS
Blank (Rocknest)
88
15
145 - 529
GCMS
Rocknest
93
15
145 - 529
GCMS
Rocknest
96
13
97 - 422
GCMS
Rocknest
99
11
529 - 816
GCMS
Rocknest
171
7
242 - 383
GCMS
Blank (John Klein)
177
23
311 - 816
GCMS
John Klein
196
23
311 - 816
GCMS
John Klein
199
25
242 - 639
GCMS
John Klein
224
27
242 - 639
GCMS
John Klein
227
29
570 - 792
GCMS
Blank (Cumberland)
276
33
442 - 569
GCMS
Cumberland
281
33
442 - 569
GCMS
Cumberland
286
35
571 - 792
GCMS
Cumberland
290
39
226 - 347
NG geochronology Cumberland
353
41
n/a
GCMS
Cumberland
367
51
226 - 347
GCMS
Cumberland
381
45
226 - 347
GCMS
Cumberland
394
45
226 - 347
NG geochronology Blank (Cumberland)
408
47
n/a
GCMS
Cumberland
415
47
247 - 620
GCMS
Blank (Cumberland)
421
31
247 - 620
NG geochronology Blank (Cumberland)
428
53
n/a
GCMS
Blank (Windjana)
602
10
20 - max
GCMS
Windjana
624
10
20 - max
NG geochronology Windjana
653
12
n/a
NG geochronology Windjana reheat
685
12
n/a
NG geochronology Windjana doggy bag
763
14
n/a
GCMS
Blank (Confidence Hills)
769
60
386 - max
GCMS
Confidence Hills
773
60
386 - max
GCMS
Cumberland doggy bag
822
51
20 - max
(opportunistic derivatization)
GCMS
Cumberland doggy bag
823
51
20 - max
(opportunistic derivatization)
GCMS
GC clean
835
51
n/a
GCMS
Blank (Cumberland)
837
45
20 - 550
GCMS
Blank (Cumberland)
839
45
20 - max
GCMS
Mojave
887
62
200 - max
EGA
Telegraph Peak
928
66
n/a
GCMS
GC clean
981
n/a
n/a
GCMS
GC clean
998
n/a
n/a
GCMS
GC clean
1071 n/a
n/a
GCMS
Buckskin
1076 24
150 - 300, 450 - 550,
650 - max
GCMS
GC clean
1117 n/a
n/a
EGA
Big Sky
1130 26
n/a
EGA
Greenhorn
1147 26
n/a
GCMS
Blank (Greenhorn)
1171 68
560 - 713, 780 - 830
(continued)
346 Curiosity’s Chemistry Instruments
Table 9.6. (continued)
Type of run
Sample
Sol # Cup # GC Hydrocarbon temp cut(°C)
GCMS
Greenhorn
1178 68
376 - 562, 714 - 921
EGA
Gobabeb <150um
1224 28
n/a
EGA
Gobabeb >150um
1237 30
n/a
GCMS
GC clean
1246 n/a
n/a
GCMS
&nb
sp; Calibration cup
1286 20
n/a
EGA
Oudam
1382 22
n/a
NG geochronology Mojave stepped heating part 1
1402 64
n/a
NG geochronology Mojave stepped heating part 2
1403 64
n/a
NG geochronology Mojave doggy bag stepped
1429 62
n/a
heating part 1
NG geochronology Mojave doggy bag stepped
1430 62
n/a
heating part 2
EGA
Marimba
1443 32
n/a
GCMS
GC clean
1539 n/a
n/a
GCMS
Cumberland doggy bag
1543 39
20 - 500
(opportunistic derivatization)
GCMS
Cumberland doggy bag
1546 39
20 - max
(opportunistic derivatization)
GCMS
GC clean
1580 n/a
n/a
Doggy bag
Oudam
–
22
n/a
Doggy bag
Marimba
–
32
n/a
Doggy bag
Cumberland
–
33
n/a
Doggy bag
Quela
–
62
n/a
Doggy bag
Quela
–
64
n/a
Doggy bag
Telegraph Peak
–
66
n/a
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Berger J et al (2014) MSL-APXS titanium observation tray measurements: Laboratory
experiments and results for the Rocknest fines at the Curiosity field site in Gale Crater,
Mars. J Geophys Res 119:1046–1060, DOI: 10.1002/2013JE004519
Berger J et al (2016) A global Mars dust composition refined by the Alpha-Particle X-ray
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Blake D et al (2012) Characterization and calibration of the CheMin mineralogical
instrument on Mars Science Laboratory. Space Sci Rev 170:341–399, DOI: 10.1007/
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Campbell et al (2012) Calibration of the Mars Science Laboratory Alpha Particle X-ray
Spectrometer. Space Sci Rev 170:319–340, DOI 10.1007/s11214-012-9873-5
Conrad P et al (2016) In situ measurement of atmospheric krypton and xenon on Mars
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