by The Design
6
yes
Volatile organic compounds having 1 to 4 carbons, ammonia, and sulfur-
containing compounds
left- versus right-handed enantiomers of organic compounds – an experiment that’s actu-
ally related to astrobiology – but SAM has not yet identified any enantiomers. 47
Three of the six columns have injection traps that adsorb interesting gas species as
they arrive at the start of the column, in order to concentrate them. Flash heating of the
traps releases the gas molecules within a span of 4 to 10 seconds, boosting the chromato-
graph’s ability to cleanly separate them into different constituents. Later in the mission, the SAM team has experimented with splitting the gas collected on the hydrocarbon trap
into two columns by using one column that lacks a trap and one that has one. After the
experiment with the trapless column is complete, SAM can heat the trap and analyze the
same gas with the other column. For columns 2 through 6, as gas molecules exit the tube,
they pass across a thermal conductivity detector (TCD), which can detect the major spe-
cies in the gas down to the part-per-million level. From the GC, the separated gases go
into the QMS.
9.5.1.5 Solid Sample Inlet Tubes
SAM has two solid sample inlet funnels and tubes, one each to access the outer and
inner rings of the sample manipulation system carousel. The tubes are 4.1 millimeters in
diameter, amply large to accommodate any particle that may have passed through the
1- millimeter sieve in CHIMRA. From the top of the funnel to the bottom of the inlet
tube, the sample falls a distance of 25.4 centimeters. When SAM is ready to accept a
solid sample, it positions a sample cup under an inlet tube. The rover opens an inlet
cover (part of the Sample Acquisition, Processing, and Handling (SA/SPaH) system, see
section 5.8) and vibrates the solid sample inlet tubes using piezoelectric actuators to prevent the incoming sample from sticking to the tubes. The system is designed to
ensure that at least 98% of the delivered sample makes it into the waiting sample cup.
The inlet tubes have heaters that allow them to be warmed to up to 120°C in case
hydrated minerals build up on the inlet funnel; heating and vigorous shaking would
hopefully loosen the material.
47 Paul Mahaffy, personal communication, email dated April 8, 2017
334 Curiosity’s Chemistry Instruments
9.5.1.6 Sample Manipulation System (SMS)
The Sample Manipulation System is a double-ringed carousel with 74 sample cups
(Figure 9.20). It has two motors. A carousel actuator can both rotate the carousel and move the elevator into position. An elevator actuator can raise a sample cup to pierce a foil seal, accept a new sample, or place the sample cup in an oven.
When SAM is not in use, both carousel and elevator are locked. A component called the
“trash can” is parked under the solid sample inlets. Any sample material temporarily stuck
to an inlet tube will fall into the trash can during driving and thereby not contaminate the rest of the instrument. To accept a sample, the carousel rotates to place a sample cup under an inlet tube. After the sample falls in, the carousel rotates to place the cup underneath one of two pyrolysis ovens. While this is happening, the elevator is still locked to the carousel, so it rotates with the carousel. Once the sample cup is in place, a latch releases the elevator from its locked position, and the carousel actuator rotates the elevator around the ring until it is positioned underneath the sample cup. Operating the elevator disengages the sample
cup from the carousel and then slowly raises the sample cup into the pyrolysis oven. The
elevator pushes a copper ring fused to the stem of the sample cup hard against a knife-
sharp titanium ring at the bottom opening of the oven, creating a hermetic seal. SAM cups
can be reused multiple times; each time a cup is reused, the elevator has to push the cup
harder against the titanium knife-edge to maintain the seal.
Figure 9.20. The SAM sample manipulation system with its 74 sample cups. MTBSTFA cups are for wet chemistry; quartz cups are for evolved gas analysis; and trash can is placed under the sample inlet tubes to catch any wayward sample material when SAM is not in use. After this photo was taken, two of the quartz cups were replaced with wet-chemistry TMAH cups (see text for explanation of the acronyms). Image courtesy Paul Mahaffy.
9.5 SAM: Sample Analysis at Mars 335
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) tri-
fluoroacetamide (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 chroma-
tography – 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 tetramethylammo-
nium 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 decom-
posing to give off the gas. Because QMS only measures one mass-to-charge ratio at a time,
the SAM team usually s
elects a narrow range to focus on in these ramping experiments.
This narrow range can be changed mid-experiment.
336 Curiosity’s Chemistry Instruments
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 col-
umns 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 con-
struct new experiments, manipulating the many pathways through SAM and the many
combinations of possible actions, with simple combinations of commands.
9.5 SAM: Sample Analysis at Mars 337
9.5.1.12 Organic Check Material
Five pucks of silica glass doped with fluorinated hydrocarbons are located inside hermeti-
cally 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 mea-
sure carbon and oxygen isotopes in carbon dioxide.
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 measure-
ment, 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
48 Mahaffy et al. (2013)
338 Curiosity’s Chemistry Instruments
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.
9.5.2.2 Noble gas enrichment
Noble gases (argon, krypton, and xenon) make up a very small fraction of the Martian atmo-
sphere. 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.
9.5 SAM: Sample Analysis at Mars 339
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 dif-
ferently to maintain higher pressure in the QMS to im
prove 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.
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 iso-
topes, 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 vacu-
ums out the tubing and manifolds. Finally, SAM opens the valves between the hydrocar-
bon 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
