Emily Lakdawalla

by The Design

  3 millimeters to 90 millimeters in diameter. The beam bounces it off of a secondary mirror

  and then off of a curved primary mirror and out the instrument’s front window. The win-

  dow is 3 millimeters thick and made of silica glass to protect the optics from dust and

  temperature changes. The 90-millimeter-wide beam converges at the target distance,

  vaporizing rock into plasma. The same primary mirror collects the light from the plasma

  and bounces it off of the secondary mirror. But the collected light doesn’t go back from the secondary mirror toward the laser, because of a special “dichroic” lens in the optical path that reflects the 1067-nanometer laser light but is transparent to all the shorter wavelengths.

  Then most of the light gathered from the plasma bounces off a second dichroic mirror; the

  dichroic passes about 20% of the light to a different optical path for taking context images with the Remote Micro-Imager, sending the rest toward the fiber optic cable, to be used for spectroscopy.

  9.2.1.2 The ChemCam Body Unit

  The body unit is located inside the rover, on its right side (see Figure 9.3). Light from the

  mast unit travels down a fiber optic cable 5.743 meters long, wrapping three times around

  a mandrel connected to the mast’s elevation actuator, and another three times around a

  spool connected to the azimuth actuator. Then it runs down the mast, where it winds once

  around the mast deployment joint. It splits off from the rest of the bundle of cables from

  mast-mounted instruments, traveling across the top of the deck to a point close to the interior location of the body unit; it drops over the top edge to the side of the rover and then plugs in to the body unit (Figure 9.3).

  ChemCam spectroscopy. Once it reaches the interior of the rover, the light transmitted down the fiber optic cable enters a demultiplexer (a device that splits the light into different-wavelength portions). The demultiplexer splits off first the ultraviolet and then the violet range with dichroic lenses and finally employs an ordinary mirror to deliver the rest of the light to the longest-wavelength spectrometer. The three spectrometers are called ultraviolet (UV, from 240.1–342.2 nanometers), violet (VIO, 382.1–469.3), and visible and near infrared (VNIR, 474.0–906.5). The light passes through a slit to enter a spectrometer and then

  bounces off a collimating mirror and a grating that spreads the light out by wavelength.

  Then another collimating mirror delivers the rainbow of light to a CCD that is 2048 pixels

  wide. Because the ultraviolet and violet spectrometers cover narrow wavelength ranges,

  there is higher spectral resolution at shorter wavelengths: 20 pixels per nanometer in the

  ultraviolet and 23 pixels per nanometer in the violet. The wider wavelength range of the

  VNIR spectrometer produces a lower spectral resolution of 4 pixels per nanometer.

  ChemCam thermal management. The body unit resides inside the warm body of the

  rover, but its detectors have to be actively cooled. This was not the original plan; the

  ChemCam spectrometers were developed based on earlier information from the mission

  that specified a cooler rover interior (see section 1.5.2 for more on the unpleasant discovery of this issue).

  300 Curiosity’s Chemistry Instruments

  Figure 9.3. The ChemCam body unit. After Wiens et al. ( 2012 ).

  Solving this problem required a redesign of the ChemCam body unit. They thermally

  isolated the CCDs from the spectrometers, connecting them with copper thermal straps to

  three thermo-electric coolers located next to the rover exterior wall. The coolers radiate

  heat away from the CCDs across two paths: through the wall of the rover to the outside,

  and via a base plate to the rover avionics mounting panel to the rover’s heat rejection sys-

  tem (see section 4.4). Because of the rover’s selected equatorial landing site, summer heating is not as extreme as in the worst-case scenario, and the thermo-electric coolers

  permit ChemCam to be operated nearly the entire day, year-round. 3

  3 Roger Wiens, personal communication, email dated March 14, 2016

  9.2 ChemCam 301

  9.2.1.3 The ChemCam Calibration Target

  A variety of factors can affect LIBS calibration, so ChemCam includes a calibration target

  using ten samples of well-studied composition (Figure 9.4). Four of the samples are basaltic glass of different types (macusanite, norite, picrite, and shergottite), and four are

  ceramic mixtures of basalt, anhydrite (a sulfate mineral), and clay minerals in different

  proportions. Glass and ceramics have grain sizes that are very small, so are homogeneous

  even at the fine scale of the ChemCam LIBS laser shot. One sample is graphite, serving as

  a reference for carbon, and there is a titanium metal plate for wavelength calibration. The edge of the titanium plate is painted black, making a high-contrast edge against the white-painted calibration target plate, useful for checking the focus and resolution of the

  RMI. The calibration target is mounted on the back of the rover, tilted 37.9° away from

  vertical. The mast points 28.5° downward in order to target the center of the target. The

  target is 1.56 meters from ChemCam. It is less dusty than the Mastcam calibration target,

  and has been imaged by both Mastcams (sols 14, 718, and 838) for cross-instrument cali-

  bration purposes. Unlike the Mastcam calibration target, it is far enough from the mast that the right Mastcam can view it in focus.

  Figure 9.4. The ChemCam calibration target as seen by the right Mastcam on sol 838. Many laser shot points are visible. Image 0838MR0036830000500777E01. Credit: NASA/JPL-Caltech/MSSS.

  302 Curiosity’s Chemistry Instruments

  9.2.2 Using ChemCam

  9.2.2.1 Sun-safety

  The mast points some of the instruments (Mastcams and Navcams) at the Sun regularly for

  navigation and science, but the ChemCam instrument is sensitive to the Sun, so there are

  many restrictions on mast motion in order to protect ChemCam. Depending on the focus

  position of ChemCam, it is either “sun-safe” (in which case it can tolerate the Sun passing through its field of view) or “sun-unsafe” (in which case the Sun passing through the field of view could seriously damage the instrument). During tactical planning, engineers check

  mast pointing to make sure that ChemCam will always be sun-safe before approving a

  sequence.4

  Even when ChemCam is sun-safe, the Sun shining into its window can heat the instru-

  ment, so engineers have to plan sequences to make sure that the Sun will not shine directly into the ChemCam window for longer than 3 minutes. They do this by defining a cone

  spanning plus or minus 16° around the ChemCam boresight, and modeling how long the

  Sun stays within it. Obviously, ChemCam observations will always be sun-safe if the

  instrument is pointing below the horizon. Adding 4° to the 16° cone to account for the

  curvature of Mars’ horizon yields 20° below the horizon as an always sun-safe pointing

  direction for the mast that requires no further checking. Also, slews of the mast to move to a new pointing position happen quickly enough that they don’t need to be checked for

  ChemCam warming as long as ChemCam is sun-safe.

  When ChemCam is being used and so is in a sun-unsafe focal range, the Sun must never

  be permitted to shine into its window. This has to hold true even if a rover anomaly hap-

  pens in the middle of a ChemCam observation, an anomaly that may take several sols to

  resolve. The rover planners define a “keepout cone” 17° away from the ChemCam bore-

  sight. Sweeping this cone across the sky along the path of the Sun during
a given sol pro-

  duces a “keepout band”, a region in which ChemCam must never be allowed to perform

  an observation.

  With onboard fault protection software, the rover checks to ensure that mast pointings

  will be sun-safe before performing them. The rover even checks sun-safety once per sec-

  ond as it drives, and will stop a drive if sun-safety will be violated. (This has never actually happened.)

  Targets located 2 meters away present a different kind of hazard to ChemCam. The

  problem is that the “antireflection” coating on the front window is good at allowing sunlight and LIBS-produced plasma illumination to pass through it, but is not quite as antireflective at the 1067-nanometer wavelength of the laser. Some laser light bounces back from the

  front window on every laser pulse. For two distance ranges, 1.20 to 1.36 meters and 1.942

  to 2.217 meters, the reflected laser light can be focused by the primary mirror onto the secondary mirror, possibly causing some damage to the secondary mirror at the spot of the

  laser hit. The ChemCam team has stated that the instrument can perform about 100,000

  shots in these ranges without risking serious damage. The shorter of the two distance ranges is never needed, because it is even closer to ChemCam than the calibration target; ChemCam

  4 Described in detail in Peters et al. (2016)

  9.2 ChemCam 303

  is likely never to be so close to a rock, because it would have to be next to a nearly vertical cliff. (The artwork in Figure 9.1 notwithstanding, it is an unlikely situation for Curiosity to be so close to such a steep cliff.) The longer of the two distances covers ranges extremely close to the rover, and only a few hundred such shots have been made.

  9.2.2.2 Types of Observations

  The ChemCam team divides their operational history into three seasons. Season 1 covers

  sols 0 to 800. Season 2, when ChemCam had no autofocus capability, lasted from sols 801

  to 980. Season 3, in which ChemCam uses its RMI for autofocus, is from 981 to the pres-

  ent.5 See section 9.2.1.1 above for a description of the different autofocus modes.

  LIBS observations. To perform a LIBS observation, the science team looks at Navcam

  and Mastcam images that were taken on a previous sol and selects a target. Navcam stereo

  images provide geometric information, while higher-resolution Mastcam images can be

  used to fine-tune target selection. The farthest target that ChemCam has attempted LIBS

  on was Mell, on sol 530, at a distance of 7.45 meters, but 90% of targets are much closer,

  within 4.5 meters; the team restricts quantitative analyses to targets within 5 meters. 6

  Before ChemCam can begin an observation, the autofocus and LIBS lasers may need pre-

  heating. The telescope autofocuses on the target and collects a “before” RMI image. The

  instrument collects a passive spectrum to be used later, for subtraction from the LIBS

  spectrum. Then the LIBS laser fires many laser pulses, usually 30 but occasionally some

  other number. It can collect a maximum of 150 spectra at 3 per second (referred to as a

  “burst”) before having to pause to transfer data. After the LIBS operation is complete,

  RMI takes an “after” image, then moves to the next target or returns the focus to the sun-

  safe position. The experimental result is a spectrum whose peaks imply the presence of

  different elements.

  Depth profiling. Most of the time, the ChemCam team averages the data from many LIBS

  shots at each point to improve the signal-to-noise ratio of LIBS data. However, by analyz-

  ing shots individually, it is possible to perform depth profiles for elements. Each LIBS shot ablates approximately 1 micrometer of material, and ChemCam has occasionally found

  composition to change at that scale. For example, in the rock Bathurst Inlet, lithium, rubidium, sodium, and potassium concentrations decrease with depth, possibly “due to aqueous

  alteration processes (i.e. frost deposition, followed by melt and evaporation or sublima-

  tion) that have preferentially mobilized the alkalis.” 7 Another rock had a thin layer of manganese on the surface, thin enough for ChemCam to penetrate through.8 ChemCam

  has performed depth profiles of up to 1000 shots. 9

  5 Maurice et al. (2016)

  6 Maurice et al. (2016)

  7 Ollila et al. (2014)

  8 Lanza et al. (2016)

  9 Maurice et al. (2016)

  304 Curiosity’s Chemistry Instruments

  Rasters. ChemCam commonly performs several observations in a “raster” or array.

  Rasters can be of any size, but 1-by-5, 1-by-10, and 3-by-3 arrays are the most common.

  It takes about 30 minutes to perform a 1-by-5 raster, 40 for a 3-by-3, and an hour for a

  1-by-20. The most common spacing between points is 2 milliradians (giving approxi-

  mately 1-millimeter point-to-point spacing at locations close to the rover) but the spacing may be wider or narrower.10 The pointing accuracy of the mast is such that ChemCam has been able to perform rasters within drill holes (Figure 9.5). RMI images are taken at the beginning and end of the raster observation. When necessary for large rasters, additional

  images are taken in the middle. During season 2 (sols 801 to 980), when ChemCam had

  no autofocus capability, there were few rasters (see section 9.2.1.1). Unfortunately, this coincided with virtually all of the time spent at the first major field site after arrival at Mount Sharp, Pahrump Hills.

  Figure 9.5. Example raster data within a drill hole, Telegraph Peak. Top left: MAHLI image 0911MH0004750000303057R00 of the drill hole under nighttime illumination. A tiny white

  vein is visible in the drill hole wall. Bottom left: ChemCam RMI image CRM_479251063_

  CCAM03921 of the drill hole; red square shows the region of a 4x4 raster targeting the vein and the area around it. Right: Zoom in on the vein and the raster of ChemCam measurements showing how hydrogen, calcium, and iron content vary among the different observations.

  Within the vein there is an increase in calcium and hydrogen and a decrease in iron, thought to indicate that the vein is composed of a hydrated calcium sulfate, likely bassanite. This set of observations was performed during ChemCam Season 2. Courtesy William Rapin.

  10 Roger Wiens, personal communication, email dated March 26, 2016

  9.2 ChemCam 305

  Passive mode. As a part of every observation, the spectrometers first gather spectral information using reflected sunlight without firing the LIBS laser. Such “passive” observations

  can also be gathered without taking any LIBS data, useful on distant targets. Passive spec-

  tra have helped the team identify iron oxidation states, diagnosing the mineralogical shift from less-oxidized magnetite to more-oxidized hematite in bedrock along the rover’s traverse after it reached the Bagnold dune field. The team also identified the presence of iron sulfates from their 430-nanometer absorption feature in passive spectra taken of rocks

  around Pahrump Hills. ChemCam passive sky observations investigate the abundance of

  water vapor and oxygen in the atmosphere, useful for comparison to REMS and SAM

  measurements of local humidity and oxygen abundance. 11

  Blind targeting. The rapid pace of drive campaigns means that there is often not time to receive the Navcam data needed for targeting ChemCam images before the rover drives

  away. Beginning on sol 318, to gather some ChemCam data during drives, ChemCam

  performed blind-targeted measurements of a patch of ground directly to the right of the

  rover at the end of drives, at a distance that would be 3 meters away if the ground were

  level. Initially, they performed only single-point analyses, but they added blind line scans with multiple shot points b
eginning on sol 386.12 Blind targeting was performed only during ChemCam season 1 (until sol 801), because the new autofocus algorithm requires a

  distance seed derived on Earth from analysis of Navcam images. 13 Blind targeting was eventually replaced by AEGIS targeting.

  AEGIS targeting. AEGIS stands for Automated Exploration for Gathering Increased

  Science. 14 It is a set of artificial-intelligence algorithms to enable a rover to autonomously select and/or refine observation targeting, first used on the Opportunity Mars Exploration

  Rover. For Curiosity, AEGIS permits ChemCam target selection without waiting for

  instructions from the ground. It runs on the rover’s main computer, acquiring Navcam

  images, identifying potential targets, filtering them based on criteria supplied by the

  ChemCam team, and then ranking the targets. The ChemCam team can adjust the target

  selection criteria each time an AEGIS sequence is planned, prioritizing outcrop, dark or

  light rocks, or other kinds of targets. On Mars, the whole process takes only 4 to 8 minutes and can be performed immediately after the end of a drive. AEGIS can also refine the

  pointing of ChemCam LIBS shots, running on RMI images to select bright veins or grains

  for targeting. Adding AEGIS capability to Curiosity began in summer 2015; uplink was

  spread out over many weeks. The code was installed into flight software on sol 1141.

  Checkouts were complete in February 2016, on sol 1237. AEGIS was used for routine

  operations for the first time on sol 1343. 15 The Navcam mode has been used more often than the RMI mode.

  11 Roger Wiens, personal communication, email dated March, 26, 2016

  12 Cousin et al. (2014)

  13 Roger Wiens, personal communication, email dated March 26, 2016

  14 Francis et al. (2016)

  15 Francis et al. (2017)

  306 Curiosity’s Chemistry Instruments

  Figure 9.6. ChemCam RMI long-distance observation of Mount Sharp, sol 1283. Credit: NASA/JPL-Caltech/LANL/MSSS/James Sorenson.

  Long-distance imaging. The RMI is Curiosity’s highest-resolution camera, and is often pointed at very distant targets. For instance, the team used the RMI in a long- distance