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
