Emily Lakdawalla

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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

  9.6 REFERENCES

  Atreya S et al (2013) Primordial argon isotope fractionation in the atmosphere of Mars

  measured by the SAM instrument on Curiosity and implications for atmospheric loss.

  Geophys. Res. Lett. 40:5605–5609, DOI: 10.1002/2013GL057763

  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

  Spectrometer in Gale Crater. Geophys Res Lett 43:67–75, DOI: 10.1002/2015GL066675

  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/

  s11214-012-9905-1

  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

  with Mars Science Laboratory. Earth Planet Sci Lett 454:1–9, DOI: 10.1016/j.

 

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