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 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.Table 9.4. Summary of CheMin sample analyses, organized approximately in order of stratigraphy, from stratigraphically highest to lowest (as in a stratigraphic column). Data are from the CheMin releases to NASA’s Planetary Data System.
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 miniaturized 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 composition, 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 chromatograph 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 components 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.
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 atmospheric inlets; a Sample Manipulation System with 74 sample cups on a carousel; a Gas Processing System that collects and moves gases around the interior of SAM; and an electronics 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, carousel, 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.
9.5.1.1 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 chromatograph. 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.
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 ).
9.5.1.2 Quadrupole Mass Spectrometer (QMS)
A mass spectrometer uses electric and magnetic fields to separate charged particles according 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 atmospheric probe; its ion source and detector are based on ones used by the ill-fated comet mission CONTOUR.
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.
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. Alternatively, they can sweep across their entire mass range using larger steps. Sweeping thro
ugh 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: methane, 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 precision 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
TLS has three components: a foreoptics chamber containing lasers and mirrors; a sample 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.
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 foreoptics chamber swamps the signal from methane in the Martian sample. When they discovered 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 introducing 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 determine the abundances of gases that can be hard to pull out of mass spectrometer measurements, 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 chromatograph “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 left- versus right-handed enantiomers of organic compounds – an experiment that’s actually related to astrobiology – but SAM has not yet identified any enantiomers.47 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
6
yes
Volatile organic compounds having 1 to 4 carbons, ammonia, and sulfur-containing compounds
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 chromatograph’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 species 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.
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.
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 cat
ch 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.
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.
The Design and Engineering of Curiosity Page 38
