Emily Lakdawalla

by The Design

  campaign to monitor sand ripple motion on the backs of the Bagnold dunes, to study the

  upper part of Peace Vallis where it enters the crater from the northwestern rim, and to get a look at possible future science locations on Mount Sharp (Figure 9.6). Long-distance imaging has dramatically better quality in season 3, after sol 981, now that the RMI can

  autofocus on distant targets.

  Dusting. The rapid expansion of air around the superheated plasma generated by

  ChemCam shots can very effectively remove very fine Martian dust from rock surfaces

  (Figure 9.7). A typical 30-shot observation blasts aside dust for a 6-to-9-millimeter diam-

  eter around the shot point.16 Although ChemCam hasn’t yet been used for this purpose deliberately its accidental use as a remote dust removal tool improves the value of Mastcam multispectral imaging on rocks, and the MAHLI team likes to image LIBS pits to see the

  color of dusted-off rocks.

  16 Maurice et al. (2016)

  9.2 ChemCam 307

  Figure 9.7. Two ChemCam raster targets at Windjana, “Stephen” (top) and “Neil” (bottom), were dusted off by ChemCam, revealing the dark color of the rock beneath the bright dust.

  MAHLI image 0627MH0001900010203555C00, NASA/JPL-Caltech/MSSS.

  9.2.2.3 Calibration

  Transforming ChemCam spectra into elemental abundances requires comparing the data

  to ChemCam measurements of samples of known composition. Initially, the ChemCam

  team’s calibration library contained 66 samples with compositions expected to be found

  on Mars that had been shot with the flight model of ChemCam under Earth ambient condi-

  tions. The original library performed well for common Mars materials like dust and basalt,

  but relatively poorly for more unusual materials like calcium sulfate veins and feldspars. 17

  Two laboratory copies of ChemCam (one each at Los Alamos and CNES) operate under

  Mars-like temperature and pressure conditions. The Los Alamos laboratory expanded the

  data set to 450 different types of rocks, covering a wider range of compositions than in the initial 66. The team began testing a new calibration model on ChemCam data during the

  mission’s second conjunction period, beginning on sol 1004, and delivered recalibrated

  versions of earlier major-element data to the Planetary Data System on December 3, 2015.

  17 Roger Wiens, personal communication, email dated December 17, 2015

  308 Curiosity’s Chemistry Instruments

  9.2.3 Anomalies

  The main anomaly encountered by ChemCam on Mars was the loss of the autofocus laser

  on sol 801 (section 9.2.1.1), which resulted in no autofocus capability until sol 983, when

  the new RMI-based autofocus algorithm was put into use. One drawback is that relying on

  the RMI means that autofocus doesn’t work at night or even, sometimes, in areas of deep

  shadow. ChemCam rarely performed observations at night before the anomaly because of

  the high energy cost of heating the mast actuators at night, so that restriction has had little effect, but the issue of rover shadowing has occasionally affected target selection.18 The new method also scans a shorter range of possible focal distances than the old method, so

  it requires a good-quality distance seed, meaning that shooting in the blind is no longer

  possible. On the plus side, laser autofocus didn’t work at distances longer than 18 meters, so the new autofocus method performs much better at imaging at infinity.

  9.3 APXS: ALPHA PARTICLE X-RAY SPECTROMETER

  APXS measures the elemental composition of rocks and soils by emitting alpha particles

  and X-rays at a surface and counting the X-rays that return. Curiosity’s APXS is the third

  in a line of similar instruments carried to Mars on Pathfinder and the Mars Exploration

  Rovers, with improvements that make it more sensitive, faster, and able to operate over a

  wider range of the Martian day. 19 The APXS team uses measured elemental abundances to group observed targets into classes of similar composition, often comparing rocks

  across rover landing sites. The principal investigator is Ralf Gellert of the University of Guelph, Canada, and APXS was provided to the mission by the Canadian Space Agency.

  APXS is located on the turret and has to be deployed to close proximity or in contact

  with a rock or soil (Figure 9.8). Because it requires the arm and several hours for a high-quality measurement, it gets used less frequently than the remote sensing instruments. It

  sees heavy use at sample sites and significantly less frequent use at stops during traverses.

  APXS measurements also assist the team in selecting drill targets. By analyzing drill tail-

  ings, APXS can help the CheMin team constrain the composition of the component of the

  drilled rock that is amorphous and therefore not accessible to CheMin mineralogical anal-

  ysis. APXS measurements of potassium in drill tailings combined with SAM measure-

  ments of argon gas evolved from a sample have been used to measure the exposure ages of

  outcrops through potassium-argon dating. 20 Elements that APXS can detect are listed in Table 9.2.

  18 William Rapin, personal communication, email dated April 5, 2016

  19 There is no peer-reviewed publication describing the APXS as there is for most other instruments; there is only an LPSC abstract: Gellert et al. (2009); two other good sources of information on the instrument and its performance on Mars are Gellert et al. (2015) and Campbell et al. (2012)

  20 Farley et al. (2014)

  9.3 APXS: Alpha Particle X-Ray Spectrometer 309

  Figure 9.8. APXS deployed onto the John Klein drill target. Mosaic of four Mastcam images taken on sol 168. Credit: NASA/JPL-Caltech/MSSS.

  9.3.1 How APXS works

  APXS has three main elements: a sensor head located on the end of the robotic arm; an

  electronics unit located in the front left corner of the body; and a calibration target that is mounted below the MAHLI calibration target attached to the shoulder azimuth actuator.

  Just like MAHLI, the APXS sensor head is separated from the turret by a set of three

  springy wire rope assemblies to isolate the instrument from vibrations caused by drilling

  and CHIMRA sample processing. Parts of APXS are shown in Figure 9.9.

  310 Curiosity’s Chemistry Instruments

  Figure 9.9. Parts of APXS. Top: flight hardware (NASA/University of Guelph). Middle: photo taken during initial turret checkout (MAHLI image 0032ML0000620000100855E01,

  NASA/JPL-Caltech/MSSS). Lower left: photo showing location of calibration target (NASA/

  JPL-Caltech release PIA14255). Lower right: MAHLI photo of the APXS calibration target

  taken during initial checkout on sol 34 (0034MH0000480010100038E01, NASA/JPL-

  Caltech/MSSS).

  9.3 APXS: Alpha Particle X-Ray Spectrometer 311

  The sensor head contains six curium-244 sources. Three of them are covered in a tita-

  nium foil and emit both alpha particles and X-rays, while the other three are more thor-

  oughly sealed and emit only X-rays. A cutaway drawing of the internal workings of the

  sensor head is shown in Figure 9.10. When the alpha particles impinge on atoms in the upper tens of micrometers of the target, they cause the atoms to eject inner-shell electrons, which emit X-ray photons as they fall back to their ground state, a process called particle-induced X-ray emission (PIXE). Impinging X-rays can have the same effect, in X-ray-

  induced fluorescence (XRF). Particle-induced X-ray emission is a more efficient process

  for small-mass atoms (sodium to titanium), while X-ray-induced fluorescence is more

  effective for larger atoms (chromium to strontium). APXS uses a silicon drift detector to

&
nbsp; detect and count these emitted X-rays. The half-life of curium-244 is 18.1 years, more than the anticipated lifetime of Curiosity’s power supply, so degradation of the curium sources

  should not have a significant impact on APXS use over the course of the mission.

  Figure 9.10. Cross-section of the APXS sensor head. University of Guelph.

  APXS “sees” deeper into a target for heavy elements (to depths of 50 microns or

  greater) than for light elements (for which APXS may only be measuring the topmost 5

  microns). The energy of X-ray emission depends on the element, so by measuring the

  amount of X-ray emission with respect to wavelength, APXS can detect the elements that

  are present (see Figure 9.11). 21 Employing an analysis derived from observations of samples of known composition on Earth, the APXS team can convert these X-ray spectra into

  21 Dickinson et al. (2012)

  312 Curiosity’s Chemistry Instruments

  Figure 9.11. Example APXS data. Analyzing the height of peaks in the spectrum (left) allows the APXS team to measure the composition of the rock, expressed as oxides (right).

  This sample data comes from the John Klein drill target, the location APXS was measuring in Figure 9.8.

  elemental abundances. Using the dust removal tool to brush off a measurement site

  improves APXS’s ability to sense the abundances of lighter elements in the rock rather

  than the dust. For a list of all spots brushed for APXS analysis, see Table 5.2. The brush can still leave as much as 30% of the original dust covering behind.22 The most dust-free targets that APXS examines are dumped drill fines.

  Curiosity’s APXS has two main improvements over the ones on Spirit and Opportunity.

  It has a cooler for its detector that can reduce its temperature by 30°C, which allows APXS

  to operate at ambient temperatures up to –5°C. For comparison, the Mars Exploration

  Rover APXS works only at temperatures below –40°C, which means it mostly has to be

  used at night. The cooler allows Curiosity’s APXS to be used during the daytime, although

  it is more frequently used overnight because data quality is better, especially during the

  summer. (The APXS team performed regular characterizations of instrument performance

  at different times of day during the summer after sol 1600 to better characterize its performance over a range of temperatures.) The cooler also improves the Curiosity APXS reso-

  lution over that of the Spirit and Opportunity APXS.23

  22 Perrett et al. (2017)

  23 Slavney (2013)

  9.3 APXS: Alpha Particle X-Ray Spectrometer 313

  There is also no alpha channel on Curiosity’s APXS (that is, unlike Spirit and

  Opportunity, Curiosity does not detect backscattered alpha particles). Throwing out the

  alpha channel allowed the instrument to be designed with the X-ray detector much closer

  to the surface; the closest range of 19 millimeters compares to 30 for the Spirit and

  Opportunity APXS. That, in turn, increases the sensitivity of the Curiosity APXS by a fac-

  tor of 3; reduces the spot size of an APXS measurement; and speeds up data acquisition by

  a factor of 5. Curiosity’s APXS can get a “quick look” measurement of the major elements

  in only 20 minutes, and high-quality results in only 2 hours.24

  APXS and ChemCam both measure elemental compositions. Initially, APXS and

  ChemCam measurements of target compositions did not match very well, but the match

  has improved over time, especially as the ChemCam team has improved its calibration

  (see section 9.2.2.3).

  The APXS calibration target, like the MAHLI calibration target (see section 7.4.2), was covered with dust kicked up during the landing. That complicated the use of the calibration target for its intended purpose, but the APXS team adjusted their calibration about 6

  months after landing to account for the presence of the dust. 25 Another calibration adjustment around sol 1200 improved estimation of manganese abundances.26

  9.3.2 Using APXS

  APXS was first deployed on the rock Jake Matijevic on sol 46. It can be particularly dif-

  ficult to find time for APXS observations when Curiosity’s goal is long-distance driving,

  because arm activities can’t take place at a new location until post-drive data are received on Earth. Initially, contact science days and driving days were mostly distinct. But on sol 102, the rover drivers performed the first “touch-and-go” observation, where the rover

  deploys the APXS on a target for a short integration in the morning before stowing the arm

  and driving away. Touch-and-goes allow the APXS team to track changes in major- element

  rock composition during long traverses without major impact on drive durations. One

  advantage to APXS over other instruments is that it consumes negligible data volume,

  using only 32 kilobytes of memory to store several spectra. It is also cheap in terms of

  power, especially if used overnight, when its detector doesn’t need to be cooled.

  APXS can be used either in a contact mode (where the arm is commanded to move

  APXS toward a target until the contact sensor is triggered) or in a hover mode of 5 to 20

  millimeters from the target. When APXS is used in contact mode on unconsolidated mate-

  rials, it may leave a print of the contact sensor in the soil (Figure 9.12). For unconsolidated materials, it is only used in contact mode in locations where the rover wheel has driven

  over the soil, compacting it. Touching unconsolidated materials can make the contact plate

  of APXS dirty, so the rover cleans it after every time it is used on soil. Cleaning APXS

  involves holding it down, then turning it sideways, then rotating it 180°, using CHIMRA

  vibration to gently shake it for 20 seconds in each pose. 27

  24 Dickinson et al. (2012)

  25 Ralf Gellert, personal communication, email dated May 10, 2016

  26 Mariek Schmidt, personal communication, email dated April 17, 2017

  27 Ashwin Vasavada, personal communication, email dated March 28, 2017

  314 Curiosity’s Chemistry Instruments

  Figure 9.12. The APXS “nose print” in the fine sand at Gobabeb as seen from Mastcam. The inner, open circle of the APXS contact plate is 29 millimeters in diameter. Mastcam image 1234MR0057070010603445E01. Credit: NASA/JPL-Caltech/MSSS.

  When examining fluffy targets or irregular surfaces on which it can’t be used in contact

  mode, APXS is often deployed using a technique called proximity mode, “proxmode” for

  short. APXS takes very short (10-second) integrations as it is brought closer to a target,

  signaling the arm to stop when it senses that it has reached the optimal distance of about 5

  millimeters of standoff. 28 When the sensor head is in contact with the target, the sources are only about 19 mm from the surface, and the sampled area has a diameter of 17 millimeters – similar in size to the diameter of a drill hole. When it is 20 millimeters from the target, the sampled area is larger, with a diameter of at most 31 millimeters. For comparison, the brush clears an area 45 millimeters or more in diameter. MAHLI images with a

  toolframe distance of 2 centimeters (a standard close-up observation distance) cover an

  area 33 millimeters wide, comparable to the APXS field of view.

  APXS is used heavily at drill sites. Typically, they brush a site near the drill location

  and use APXS to measure the composition of the rock to be drilled. They also perform an

  APXS analysis on the drill tailings (which come from the top 15 millimeters of the drill

  hole) as well as on the dumped pre- and post-sieve fractions of the drill powder (which

  represent the fine and coar
se fractions of the drilled rock from 15 millimeters to the full drill depth). APXS integrations of dump sites taken before dumping can be compared to

  the post-dump analyses. In general, analysis of drill dump piles has yielded similar results to the pre-drill brushed spots. 29

  28 Ralf Gellert, personal communication, email dated May 10, 2016

  29 Gellert et al. (2015)

  9.3 APXS: Alpha Particle X-Ray Spectrometer 315

  Curiosity APXS’s rapid integration has allowed the team to regularly perform APXS

  raster observations. The APXS team most commonly uses rasters when examining a target

  that contains objects smaller than the field of view, like veins, concretions, and pebbles.

  Rasters are either a line of 3 observations in a row, or a 2-by-2 array with a central point for a total of 5 observations. Typically, APXS integrates for 20 minutes at each point, and then integrates overnight over the center point. The rasters help the APXS team to separate the signal of the small target of interest from the background signal.30

  The observation tray was specifically designed to be used for APXS analyses of sample

  material (see section 5.7). However, the observation tray has not seen heavy use. The amount of sample that CHIMRA drops on it is small relative to the diameter and depth of

  the APXS sampling region, so for heavy elements like iron and magnesium in in particu-

  lar, the sample being tested is thin and APXS calibrations do not apply, meaning that

  samples can’t be directly compared to each other. 31 An unforeseen problem was the behavior of particulate material after it is dropped to the observation tray; vibrations within

  CHIMRA get transferred to the rover and bounce the sample away from the center of the

  tray almost as soon as it is emplaced. The mission has found it more useful to measure

  samples dumped from CHIMRA onto the ground than to continue use of the observation

  tray. One unforeseen use of the observation tray was a convenient location for APXS to

  measure the composition of airfall dust.32

  9.3.3 APXS rock compositional classes