9.5.1.7 Sample cups
There are three types of sample cups: 59 quartz cups, 9 foil-capped wet chemistry cups, and 6 foil-capped metal cups containing calibration samples. Inside each quartz cup is a “frit,” a porous disk made of powdered quartz, on which the sample rests. The quartz disk is positioned to hold the sample in the hottest part of the oven, and its pores permit helium gas to flow through the sample, quickly carrying evolved gases away, to minimize chemical reactions between evolved gases and the remaining sample.
SAM carries wet chemistry cups because the Martian samples could potentially contain organic molecules that would decompose into simpler molecules instead of entering a gaseous state, meaning that they would not be easy to detect by high-temperature pyrolysis and gas chromatograph mass spectrometry. When samples containing perchlorate are heated, the perchlorate rapidly oxidizes the organics to carbon dioxide, water, and other simple compounds. The organic substances can potentially be kept whole if they are first “derivatized” – made more volatile by reactions with other chemicals. These reactions have to happen in a solvent, so nine of the sample cups are wet chemistry cells. Seven of them contain a mixture of two solvents called N-methyl-N-(tert-butyldimethylsilyl) trifluoroacetamide (MTBSTFA) and dimethylformamide (DMF), designed to be used at relatively low temperatures (75–300°C). When organic molecules react with MTBSTFA, they turn into compounds that are more volatile – making them amenable to gas chromatography – and also more stable, making them less likely to decompose when heated in the SAM oven in the presence of perchlorate. The remaining two wet cells are targeted at the most complex types of organic molecules: they contain a mixture of tetramethylammonium hydroxide (TMAH) and methanol, which can break up complex organics like lipids and proteins at moderate oven temperatures of more than 340°C.
9.5.1.8 SAM oven
There are two ovens, one each for the outer and inner rings of the carousel. One can heat samples to 900°C, the other to 1100°C. SAM heats the oven with a heater wire, only half a millimeter thick, made of a platinum-zirconium alloy. At lower temperatures, volatile materials in rocks (such as bound water) are driven off. More volatile organics are released at temperatures between 300°C and 600°C; these may have existed in the rock as smaller organic molecules, or may come from larger organic molecules that break down upon heating. At temperatures above 500°C, carbonates may release carbon dioxide and sulfates may release sulfur dioxide. The temperature at which these gases appear can be diagnostic of the minerals originally present in the sample.
As the oven temperature slowly ramps up, QMS can detect how the composition of evolved gases changes with temperature, helping to identify the minerals that are decomposing to give off the gas. Because QMS only measures one mass-to-charge ratio at a time, the SAM team usually selects a narrow range to focus on in these ramping experiments. This narrow range can be changed mid-experiment.
9.5.1.9 Thermal considerations
Many of SAM’s components require heating to relatively high temperatures for operation, not only the ovens. The tubes and manifolds operate at a temperature of 135°C in order to prevent organic molecules from sticking to them. When it’s time to release hydrocarbons from the trap, the trap has to be flash-heated to 350°C. The traps in front of the GC columns must be flash-heated to 100–250°C. These temperatures are all quite high, yet other components (like the electronics, pumps, and trap coolers) need to operate at more typical spacecraft temperatures. SAM manages all these temperatures with the help of heat pipes internal to SAM as well as the rover heat rejection system for cooling, and a large number of tiny heaters wrapped around tubes, manifolds, and traps for heating. SAM has more than 60 temperature sensors.
9.5.1.10 SAM testbeds
A vital component of the SAM experiment are several duplicates of the instrument or its components in different laboratories. The most elaborate one of these is a duplicate of the SAM instrument in a laboratory at Goddard Space Flight Center. The duplicate lives inside a chamber that is designed to simulate the temperature and pressure conditions of Mars and the interior of the rover. There is even a high-fidelity duplicate of the part of the rover avionics mounting plate and all of its heat rejection system tubing that keeps SAM cool as it operates. The SAM team uses the testbed to develop new scripts and procedures.
In order to understand the results of QMS and GCMS experiments conducted on Mars, the SAM team attempts to reproduce the results in the testbed. Based on results from Mars, SAM scientists hypothesize what materials may have been present in the original sample before heating, combustion, and/or derivatization. They prepare a sample of the hypothesized composition, and run it through the SAM testbed using the same settings as the experiment on Mars to see if they can reproduce the results. In this way, laboratory experimentation is a critical element of the SAM solid sample experiments; obtaining the data from Mars is only the beginning of the experiment.
9.5.1.11 Electronics
SAM has 8 electronics modules. It can operate autonomously while the rover is otherwise asleep, powering itself off after finishing an experiment. Total data volume for a single SAM experiment is about 30 megabytes. SAM stores data in a 64 megabyte flash memory array. SAM is programmed in a BASIC-like language that allows the science team to construct new experiments, manipulating the many pathways through SAM and the many combinations of possible actions, with simple combinations of commands.
9.5.1.12 Organic Check Material
Five pucks of silica glass doped with fluorinated hydrocarbons are located inside hermetically sealed cans on a stand mounted to the front of the rover and can be used by the SAM team as an end-to-end calibration test, to make sure that the sample acquisition process does not alter samples acquired on Mars. Curiosity can test a sample of organic check material by positioning the drill on one of the pucks and acquiring a sample of it just as it would acquire a sample of Martian rock. None of them has yet been drilled on Mars. The loss of the drill feed mechanism means they cannot be drilled in the future unless a new process is developed that does not rely on drill stabilizers to align the drill. See section 5.6 for more about the organic check material.
9.5.2 Types of SAM experiments
There are nine basic types of SAM sequences: four for atmospheric gases, and five for solid samples.
9.5.2.1 Direct atmospheric measurement
SAM performs a direct atmospheric experiment about once every two weeks. See Figure 9.21 for a diagram. With a direct atmospheric measurement, SAM QMS can measure the mixing ratios of carbon dioxide, argon, nitrogen, oxygen, and carbon monoxide, the ratio of argon-40 to argon-36, and isotopes of carbon in carbon dioxide. TLS can measure carbon and oxygen isotopes in carbon dioxide.
Figure 9.21. Example sample pathways through the gas processing system for atmospheric measurements. Purple line shows a path from atmospheric inlet 1 through the QMS. The ingested atmosphere fills several manifolds (3, 4, 5, and 7) while only a small amount leaks through valve 10 into the QMS. The red line shows a path from atmospheric inlet 2 through the TLS. The TLS is evacuated first (dashed red line), then air is introduced (solid line), a measurement taken, and some of the gas is vented (dashed red line), the measurement is repeated, and so on. Either spectrometer can ingest a sample from either inlet.
SAM begins by heating all the tubes and manifolds that will carry gas, and also heats the chosen atmospheric inlet, in order to drive off any water that may have adsorbed onto the cold surface. It pumps down the spectrometers and takes background spectra. Then it shuts the valves connecting the pumps to the manifolds, and opens the valve connected to the inlet. Gas enters the inlet and fills the interior of several manifolds until it’s stopped at valves 20 and 21 within manifold 7, where there is a pressure sensor. Finally, SAM opens one of the two valves connecting the manifold to the QMS (valves 11 or 12, see Figure 9.21). These are capillary valves that only allow a tiny amount of gas into the QMS at a time. Because the composition of the atmosphere won’t change
over the period of the measurement, the QMS can take tiny voltage steps to get the highest precision possible.
For TLS, SAM fills the sample chamber with Martian atmosphere and then shuts all the valves. TLS takes a reading at full pressure and then at lower pressure steps, with a pump removing sampled atmosphere at intervals, in order to make sure there will be at least one observation that doesn’t saturate the detector. SAM can do both QMS and TLS in a single sol, or one or the other. Initial experiments used both, but most experiments since have used only one or the other spectrometer in order to increase the integration time and therefore the sensitivity of the measurement.48
9.5.2.2 Noble gas enrichment
Noble gases (argon, krypton, and xenon) make up a very small fraction of the Martian atmosphere. Some of the noble gas atoms have been present since Mars formed, while others exist because of the radioactive decay of Martian materials. Depending on when Mars formed, when it was geologically active, and when it lost its atmosphere, there will be more or less of different noble gases in the atmosphere, so atmospheric isotopes can provide clues to the history of Martian geologic history. But the abundance of each isotope is tiny; except for the three isotopes of argon, SAM cannot detect them without concentrating them.
There are three different types of noble gas enrichment experiments: dynamic, semi-static, and static. The names refer to how quickly gas exits the QMS. Dynamic mode is used for more abundant gases. In semi-static and static modes, SAM’s pumps work differently to maintain higher pressure in the QMS to improve the detection of trace gases.49
The purple line in Figure 9.22 shows a dynamic mode noble gas enrichment experiment. It works like a direct atmospheric measurement, except that SAM passes the Mars air over a chemical scrubber that removes 95% of all carbon dioxide and water from the air, as well as some of the other non-noble gases. What is left behind in the manifold is mostly nitrogen and argon. After the scrubber has been allowed to work on the gas in the manifold for a while, valve 12 opens to let it flow into the QMS. QMS scans the mass range of the argon isotopes and can sensitively measure the argon-40/argon-36 and argon-38/argon-36 ratios.50 SAM has performed this experiment once, on sol 231.
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.
To measure argon-38, they operate SAM in semistatic mode. Semistatic mode works like dynamic mode, except that SAM also employs a getter attached to the QMS in order to remove methane, and the valve between the QMS and pump is partially shut in order to increase the pressure inside QMS. Semistatic experiments have allowed SAM to measure the abundance of argon-38 compared to the more common argon-40 and argon-36 isotopes, and also krypton isotope abundances.51 SAM performed semistatic experiments on sols 341, 364, and 976.
Finally, there is static mode (not shown in Figure 9.22). After passing the Mars air over the scrubber, SAM sends it past a hydrocarbon trap, which also traps xenon. What’s left (mostly nitrogen, argon, and krypton) passes into TLS, which acts as a reservoir for this experiment. There is so little xenon that SAM continues to ingest Mars air and trap xenon for 90 minutes. Then SAM closes the valves to TLS and the hydrocarbon trap, and vacuums out the tubing and manifolds. Finally, SAM opens the valves between the hydrocarbon trap and QMS, warms the hydrocarbon trap, and pumps gently to slowly fill QMS with xenon while letting as little of it escape as possible. In this experiment, QMS scans only that range relevant to the masses of xenon isotopes. When most of the xenon has been released, the pump stops, and QMS finishes the xenon analysis by scanning what’s left in 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.
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.
9.5.2.4 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 collect those gases from the atmosphere, evacuate the instrument, and then release the collected 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).
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).
9.5.2.6 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 kilopascals 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
9.5.2.7 Tunable laser spectrometry (TLS)
To measure isotopic ratios of methane, carbo
n 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.
9.5.2.8 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 combustion 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 experiment 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 opportunity 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.
The Design and Engineering of Curiosity Page 39
