by The Design
28
0
8
0
0
14
6
0
0
0
3
36
formation 1060 1061 1062 14b Buckskin
0
0
21
4
70
3
0
0
1
0
0
0
0
0
908
922
923
5b Telegraph Peak
1
7
25
5
23
8
0
1
0
0
1
2
0
27
882
884
885
6a
Mojave2
0
7
23
0
1
3
0
3
0
0
3
2
9
49
759
765
766
12a Confidence Hills
1
12
18
5
1
3
0
6
0
0
1
1
13
39
621
623
624
13a Windjana
4
34
4
15
0
10
1
1
1
2
0
0
8
20
Bradbury
group
279
282
283
12b Cumberland
1
16
22
2
0
4
1
1
2
2
0
0
18
31
182
195
196
13b John Klein
4
18
32
2
1
5
2
1
5
2
0
0
0
28
Gray shading denotes the second uses of those CheMin cells. Orange shading is darker when the mineral is more abundant.
* Greenhorn is an alteration halo within Big Sky type rock.
** Lubango is an alteration halo within Okoruso type rock.
The experiment has been highly successful, although as of sol 1800 it had not received
a sample since the sol 1536 drill feed anomaly (see Section 5.3.4.3). Minerals detected by CheMin are mostly those associated with basalt. The first sample CheMin ingested, at the
Rocknest sand shadow, has the mineralogy of basalt: plagioclase feldspar, pyroxene, and
olivine, with minor amounts of other minerals, like anhydrite (a calcium sulfate) and
quartz. But the rocks that Curiosity has drilled have much less olivine. Instead, there are minerals like clays, which speak of alteration by water. There is also a surprising amount
of potassium feldspar in Windjana and of tridymite in Buckskin, which suggest source
volcanic rocks with unexpected compositions for Mars.42 Some minerals form under particular conditions of temperature, pressure, humidity, acidity, and so on, so the CheMin
42 See, for example, Treiman et al. (2016) and Morris R et al (2016)
9.5 SAM: Sample Analysis at Mars 327
team use the mineral assemblages to interpret the history of the environments that the rock has experienced since its source rock formed. The CheMin analyses that have been delivered to NASA’s Planetary Data System to date are summarized in Table 9.4.
9.4.4 Anomalies and issues
No serious problems have occurred with the CheMin instrument, only minor issues with
individual experiments. The Buckskin sample appeared to suffer from poor grain motion
and particle clumping, resulting in on-ring and off-ring diffraction spots. 43 The subsequent sample, Big Sky, appeared to be slightly contaminated by the presence of Buckskin material in the system (either from CHIMRA or the CheMin inlet), diagnosed by the presence
of tridymite. Tridymite, an unusual high-temperature, low-pressure polymorph of quartz,
was abundant in the Buckskin sample but not observed in previous or later samples.
9.5 SAM: SAMPLE ANALYSIS AT MARS
Like CheMin, SAM is a sophisticated laboratory analysis instrument that has been minia-
turized to fit inside the belly of the rover. It is multiple instruments in one. Its two spectrometers analyze the chemical and isotopic composition of gases. SAM can either ingest
those gases directly from the atmosphere, or create gases by slowly heating solid samples
in an oven to drive off volatile materials. SAM doesn’t directly measure mineral composi-
tion, but the SAM team can deduce the presence of specific minerals by observing the
temperature at which gases are driven off from the sample. SAM also has a gas chromato-
graph that can help it identify organic compounds preserved in rocks. SAM performs
atmospheric analyses frequently (typically once every two weeks) and bakes solid samples
more rarely. Although SAM is not the first gas chromatograph mass spectrometer (GCMS)
sent to Mars, it is the first successful one since the Viking landers.44 (The failed Beagle 2
lander carried a GCMS.)
On Mars, SAM’s work has been complicated by two problems, both of which happened
prior to landing. A leaky wet chemistry cup contaminated the interior of SAM’s solid
sample manipulation system with an organic solvent, making it challenging to detect
Martian organic materials. And “Florida air” leaked into one chamber of the instrument
designed to measure atmospheric methane abundance, swamping the methane signal.
Over time, the SAM team has developed workarounds for both problems, increasing their
confidence in their results.
SAM was supplied to the mission by NASA’s Goddard Space Flight Center, but com-
ponents were built at several different institutions. The tunable laser spectrometer was
built at JPL. The gas chromatograph was provided by the University of Paris and
CNRS. Honeybee Robotics built the sample manipulation system. The mass spectrometer
and the gas processing system were developed at Goddard. It all came together at Goddard
before being delivered to JPL. The principal investigator for SAM is Paul Mahaffy.
43 Morris et al. (2016)
44 The SAM instrument description paper is Mahaffy et al. (2012); useful summaries of post-landing performance are in Millan et al. (2016) and Franz et al. (2017)
328 Curiosity’s Chemistry Instruments
9.5.1 How SAM works
SAM is a large gold-plated aluminum box occupying a substantial portion of Curiosity’s
interior (Figure 9.17; see Figure 4.3 for its location inside the rover). SAM’s two detectors are a Quadrupole Mass Spectrometer (QMS) and a Tunable Laser Spectrometer (TLS). It
can also use a Gas Chromatograph (GC) in concert with its mass spectrometer to perform
GC-MS experiments. Other subsystems include two solid sample inlet tubes; two atmo-
spheric inlets; a Sample Manipulation Syste
m with 74 sample cups on a carousel; a Gas
Processing System that collects and moves gases around the interior of SAM; and an elec-
tronics box. Analyzing atmospheric samples requires use of only the gas processing system,
TLS, and QMS. Analyzing solid samples requires those as well as the sample inlets, carou-
sel, and ovens. Use of the GC is an option for both solid and atmospheric samples.
Figure 9.17. Components of the SAM instrument. Top and lower left images courtesy Paul Mahaffy. Bottom right NASA/JPL-Caltech release PIA13463.
9.5 SAM: Sample Analysis at Mars 329
9.5.1.1 Gas Processing System
The Gas Processing System is a spaghetti of tubing, valves, manifolds, heaters, pumps,
and gas reservoirs. It includes:
• 2 helium reservoirs: These contain a nonrenewable supply of helium, which is used
as a carrier gas, adding pressure to move sample gases through the gas chromato-
graph. The reservoirs have a volume of 180 cubic centimeters each; when launched,
the helium they contained was at a pressure of 14,000 kilopascals. The helium
launched with SAM would fill about four large (30-centimeter) party balloons.
• 1 low-pressure oxygen gas reservoir, used for combustion experiments.
• 1 low-pressure calibration gas reservoir.
• 2 turbomolecular vacuum pumps (“wide-range pumps”), to move gases through the
system.
• 14 manifolds with 1 to 10 valves each (manifolds are chambers, junctions of many
tubes; by selectively opening valves at manifolds, the SAM team can steer gases in
myriad ways through the system).
• 2 high-conductance valves (that is, large valves).
• 52 micro-valves (that is, tiny valves; these are welded directly to the manifolds to
save mass).
• Many transfer tubes, a lot of them wrapped with heaters.
• A hydrocarbon trap, which can be cooled to collect organic compounds and heated
to release them to the gas chromatograph; it also separates heavier from lighter
noble gases.
• A scrubber system that can remove carbon dioxide, which is the most abundant gas
in Mars’ atmosphere, therefore enriching other gases; the scrubber can also trap
water, which can later be released by heating it.
• 2 getters that can remove all except some noble gases.
Figure 9.18 is a diagram of the gas processing system. In section 9.5.2 we’ll see how gases move through this system.
9.5.1.2 Quadrupole Mass Spectrometer (QMS)
A mass spectrometer uses electric and magnetic fields to separate charged particles accord-
ing to their mass-to-charge ratio ( m/z). Curiosity’s mass spectrometer (Figure 9.19) is sensitive to molecular masses from 1.5 to 535.5 daltons, with a resolution of 0.1 dalton.
The quadrupole design is very similar to the one that was on the Galileo Jupiter atmo-
spheric probe; its ion source and detector are based on ones used by the ill-fated comet
mission CONTOUR.
The QMS can operate continuously, taking readings of the number of ions that reach
the detector every 0.02 seconds. It measures a single mass-to-charge ratio at a time. To use the QMS, the SAM team chooses a mass range of interest and sweeps through the range
by changing the voltage of the power supplied to the spectrometer. If they want to focus on a very small mass range to tease apart interesting compounds with slightly different
masses, they can take small voltage steps each time to achieve their maximum resolution.
330 Curiosity’s Chemistry Instruments
Figure 9.18. Schematic diagram of the SAM gas processing system. Tiny gray numbered squares are the micro-valves. HC = High-capacity valve. After Mahaffy et al. ( 2012 ).
Alternatively, they can sweep across their entire mass range using larger steps. Sweeping
through the range in smaller steps takes longer; the choice of range and steps depends on
what the SAM team is looking for in their experiment.
9.5.1.3 Tunable Laser Spectrometer
The tunable laser spectrometer was designed to measure only three specific gases: meth-
ane, carbon dioxide, and water. It can make very precise measurements of the ratios of
deuterium to hydrogen; carbon-12 to carbon-13; and ratios among oxygen-16, -17, and
-18. It can directly measure the atmospheric abundance of methane and water to a preci-
sion of 2 parts per billion by volume. On Mars, the TLS has been able to detect a few other compounds with absorption features in its wavelength range, including hydrogen fluoride
and “a mystery chlorine compound that we are working to identify. ”45
45 Paul Mahaffy, personal communication, email dated April 8, 2017
9.5 SAM: Sample Analysis at Mars 331
Figure 9.19. Top left: the quadrupole mass spectrometer. Top right: the tunable laser spectrometer. Bottom: diagram of the tunable laser spectrometer. The red lines show the light path as it is bounced repeatedly between two mirrors to make the laser light take a long path through the sample gas. Images courtesy Paul Mahaffy.
TLS has three components: a foreoptics chamber containing lasers and mirrors; a sam-
ple cell; and a detector chamber. The three chambers are separated by windows. TLS
works by filling a cylindrical sample cell with gas, and then shooting an infrared laser into the sample cell (Figure 9.19). The sample cell is 20 centimeters long and 5 centimeters wide. Mirrors at both ends of the cylinder bounce the laser back and forth through the cell multiple times. A 2.78-micron laser, used for carbon dioxide and water, bounces through
the cell 43 times, for a path length of 8.93 meters; a 3.27-micron laser, for methane,
bounces 81 times, for a path length of 16.8 meters. Gases in the sample chamber absorb
some of the infrared light at wavelengths specific to each gas. Bouncing it back and forth
so many times gives trace gases more opportunity to absorb light before it reaches the
detector. Cooling the 2.78-micron laser can tune its wavelength to 2.785 microns to access
a carbon dioxide absorption line, and 2.783 microns for water. By measuring the intensity
of the laser light after it exits the cell, TLS can measure the abundance of gases to part-per-billion sensitivity.
332 Curiosity’s Chemistry Instruments
The tunable laser spectrometer doesn’t operate continuously, unlike the QMS. SAM
waits until the TLS chamber is filled to some desired pressure, and then performs a TLS
reading. Then it usually will pump out some of the gas and perform another reading at a
lower pressure. It usually repeats this a few times. Reading at multiple different pressures makes sure that there will be at least one reading for each gas at which the detector is not saturated.
At some time prior to launch, the tunable laser spectrometer’s foreoptics chamber
leaked, introducing Earth atmosphere into it, to a pressure of 76 millibars. 46 The SAM
team refers to this as “Florida air” although it probably contains air from Maryland and
California as well. The Florida air contained 10 parts per million of methane, way above
the part-per-billion levels that exist in the Martian atmosphere. Because the lasers pass
through the foreoptics chamber on their way to the sample cell, the methane in the foreop-
tics chamber swamps the signal from methane in the Martian sample. When they discov-
ered the problem, shortly after landing, the SAM team changed how they operated the
TLS. They now pump the foreoptics chamber down to a pressure of 11 millibars before
running a methane experiment. Then they make measurements both before and
after intro-
ducing the Martian sample into the chamber, and a third time after venting the Martian
sample. They subtract the empty-cell spectrum from the full-cell spectrum, leaving them
with a contribution only from the full cell.
Used in concert with the GC and QMS, the tunable laser spectrometer can help deter-
mine the abundances of gases that can be hard to pull out of mass spectrometer measure-
ments, such as methane and the different isotopes of oxygen present in water.
9.5.1.4 Gas Chromatograph (GC)
Gas chromatography helps a mass spectrometer distinguish among different compounds
by separating a gas mixture into its individual constituents, spreading out their arrivals at the mass spectrometer over time. It is particularly focused on compounds made of light
elements: hydrocarbons and atmospheric gases. There are actually six different gas chro-
matograph “columns.” Originally, only one of the columns could be used at a time. Each
of the six columns is a tube 30 meters long but only a quarter of a millimeter in diameter.
The tubes are wound into coils to pack them inside the instrument. The coils are visible on the side of the instrument in Figure 9.17.
Each of the columns is designed to work on different ranges of molecular weights
(Table 9.5). The inside surface of each column is coated with a different substance. The substances grab hold of gas molecules and release them. Different gas molecules have
more or less affinity for each column’s coating. The less sticky molecules exit the column
first, the stickiest last. In addition, large molecules tend to move more slowly down the
column than small molecules. One of the columns (column 4) can compare abundances of
46 Webster et al. (2014)
9.5 SAM: Sample Analysis at Mars 333
Table 9.5. SAM’s gas chromatograph columns. After Mahaffy et al. (2012 ).
Column Injection trap? Gases targeted
1
no
Medium molecular weight organics (organics having 5 to 15 carbon atoms)
2
no
High molecular weight volatile organic compounds, including derivatives
of organics having more than 15 carbons
3
no
Volatile gases, including low-weight organics (1–2 carbons)
4
yes
Medium molecular weight organics and enantiomers (left- versus right-
handed) of specific classes of organics
5
yes
Medium molecular weight organics
