Matthiä K et al (2016) The Martian surface radiation environment – a comparison of models and MSL/RAD measurements. J Space Weather Space Clim 6:A13, DOI: 10.1051/swsc/2016008
Mitrofanov I et al (2012) Dynamic Albedo of Neutrons (DAN) experiment onboard NASA’s Mars Science Laboratory, Space Sci Rev 170:559–582, DOI: 10.1007/s11214-012-9924-y
Mitrofanov I et al (2014) Water and chlorine content in the Martian soil along the first 1900 m of the Curiosity rover traverse as estimated by the DAN instrument, J. Geophys. Res. Planets 119:1579–1596, DOI: 10.1002/2013JE004553
Pla-Garcia J et al (2016) The meteorology of Gale crater as determined from rover environmental monitoring station observations and numerical modeling. Part I: Comparison of model simulations with observations. Icarus 280:103–113, DOI: 10.1016/j.icarus.2016.03.013
Rafkin S C R et al (2014) Diurnal variations of energetic particle radiation at the surface of Mars as observed by the Mars Science Laboratory Radiation Assessment Detector, J. Geophys. Res. Planets, 119:1345–1358, DOI: 10.1002/2013JE004525
Rafkin S C R et al (2016) The meteorology of Gale Crater as determined from Rover Environmental Monitoring Station observations and numerical modeling. Part II: Interpretation. Icarus 180:114–138, DOI: 10.1016/j.icarus.2016.01.031
Smith M et al (2016) Aerosol optical depth as observed by the Mars Science Laboratory REMS UV photodiodes. Icarus 180:234–248, DOI: 10.1016/j.icarus.2016.07.012
Vasavada A et al (2017) Thermophysical properties along Curiosity’s traverse in Gale crater, Mars, derived from the REMS ground temperature sensor. Icarus 284:372–386, DOI: 10.1016/j.icarus.2016.11.035
Zeitlin C et al (2016) Calibration and Characterization of the Radiation Assessment Detector (RAD) on Curiosity. Space Sci Rev 201:201–233, DOI: 10.1007/s11214-016-0303-y
Footnotes
1RAD’s design and function is described in Hassler et al. (2012); two post-landing summaries of RAD performance and results are Matthiä et al. (2016) and Zeitlin et al. (2016)
2Betina Pavri, personal communication, September 22, 2017
3Hassler et al. (2013); Don Hassler, personal communication, email dated November 10, 2017
4Rafkin et al. (2014)
5Scot Rafkin, personal communication, email dated March 5, 2017
6The paper of record for DAN is Mitrofanov et al. (2012). A post-landing summary is in Mitrofanov et al. (2014).
7The main paper describing REMS is Gómez-Elvira et al. (2012). Post-landing articles summarizing REMS performance and results are Pla-Garcia et al. (2016), Smith et al. (2016), and Vasavada et al. (2017)
8Rafkin et al. (2016)
9Pla-Garcia et al. (2016)
10Gómez-Elvira et al. (2012)
11Martínez et al. (2016)
12Smith et al. (2016)
13Gómez-Elvira et al. (2012)
14Ashwin Vasavada, personal communication, email dated April 17, 2017
© Springer International Publishing AG, part of Springer Nature 2018
Emily LakdawallaThe Design and Engineering of CuriositySpringer Praxis Bookshttps://doi.org/10.1007/978-3-319-68146-7_9
9. Curiosity’s Chemistry Instruments
Emily Lakdawalla1
(1)The Planetary Society, Pasadena, CA, USA
9.1 INTRODUCTION
Curiosity has four instruments that study the chemistry of Martian materials. Two of them focus on elemental abundances. ChemCam is a remote sensing instrument, able to detect the elemental composition of a rock or soil from a distance of up to 7 meters by shooting it with a laser, a technique deployed in space for the first time on Curiosity. The Alpha Particle X-Ray Spectrometer (APXS) is a contact science instrument to examine the compositions of rocks and soils reached by the arm, and has a long Martian heritage.
The other two composition instruments form Curiosity’s analytical laboratory, ingesting samples directly. CheMin is an X-ray fluorescence/X-ray diffraction instrument for determining crystalline mineralogy, the first instrument of its kind sent beyond Earth. Sample Analysis for Mars (SAM) is a fiendishly complex machine with an oven for heating samples to drive off gases. SAM’s manifolds and pumps can direct gas from oven or atmosphere to a gas chromatograph mass spectrometer and a tunable laser spectrometer for measuring molecular and isotopic gas composition.
9.2 CHEMCAM
ChemCam employs a process called laser-induced breakdown spectroscopy (LIBS) to measure the elemental composition of the targets it zaps (Figure 9.1). When it fires its laser, it converts some of the target into plasma. A telescope gathers the light emitted by the plasma and sends it to a spectrometer. The wavelengths of the emitted light are diagnostic for some elements. ChemCam also uses its telescope to capture high-resolution context images of the LIBS targets using its camera, the Remote Micro-Imager (RMI). Nothing like ChemCam has been sent to Mars (or any other planet) before. Table 9.1 lists facts about the ChemCam instrument.1
Figure 9.1. In fanciful artwork created for the ChemCam instrument proposal in 2004, ChemCam zaps a rock. By Jean-Luc Lacour for the ChemCam team.
Table 9.1. ChemCam facts.
Mast unit mass
5778 g
Mast unit dimensions
384 x 219 x 166 mm
Body unit mass
4789 g (of which 2344 g is thermo-electric cooler)
Body unit dimensions
197 x 238 x 154 mm
Fiber optic cable mass
63 g
Fiber optic cable dimensions
5753 mm x 1.4 mm diameter
Calibration target mass
161 g
Calibration target dimensions
146 x 51 x 16 mm; 1.56 m from ChemCam window
RMI CCD
1024 x 1024 pixels
RMI FOV
22.5 mrad
RMI IFOV
78 to 85 μrad vertical, 87 to 105 μrad horizontal
Autofocus laser wavelength
785 nm
LIBS laser wavelength
1067 nm
Like APXS, ChemCam can only sense elemental composition; it isn’t able to tell how the elements are arranged into minerals. Many rocks on Mars have essentially the same elemental composition (the same as basalt) but completely different mineralogy and geologic histories. Still, ChemCam is valuable for searching for targets for follow-up contact science. Also, ChemCam is able to detect lighter elements that aren’t accessible to APXS. Table 9.2 lists the major and minor elements detectable by ChemCam and APXS.Table 9.2. Major and minor elements detected by ChemCam and APXS.
Major elements
non-metallic elements
halogens
minor and trace elements
ChemCam only (lighter elements)
oxygen
hydrogen
carbon
fluorine
lithium
rubidium
strontium
barium
ChemCam and APXS
sodium
magnesium
aluminum
silicon
potassium
calcium
titanium
iron
phosphorus
sulfur
chlorine
chromium
manganese
nickel
zinc
APXS only (heavier elements)
bromine
copper
ChemCam is one of Curiosity’s international instruments. The Principal Investigator for ChemCam is Roger Wiens of the Los Alamos National Laboratory. The Deputy Principal Investigator is Sylvestre Maurice at the Institute de Recherche en Astrophysique et Planétologie. Part of it (the mast unit) was built at the Centre National d’Etudes Spatiales (CNES) in Tolouse, France, while Los Alamos built the body unit. ChemCam is operated out of Los Alamos and CNES, alternating every other week, and holding a hand-over phone meeting every Monday. The French ChemCam team must work very late into their local evening, on clocks usually 9 hours
ahead of those in Curiosity mission operations at JPL.
9.2.1 How ChemCam works
ChemCam consists of four distinct pieces of hardware: the mast unit, body unit, the electrical and optical cables connecting them, and a calibration target. The mast unit contains the LIBS laser, telescope, and camera, as well as electronics. The body unit contains the spectrometer.
9.2.1.1 The ChemCam Mast Unit
Remote warm electronics box. Figure 9.2 shows the mast unit. The mast unit is located inside the remote warm electronics box, the large “head” of the mast. The mast unit must be kept to between –40°C and +35°C for instrument health. Thermostats inside the electronics box trigger survival heaters every night, when the temperature falls below –35°C. The instrument is designed to be used at temperatures between –20°C and +20°C, so when ambient temperatures are cold, it is warmed to –15°C in order to be used. Any warming is performed at a maximum rate of 5°C per minute.
Figure 9.2. ChemCam Mast Unit (front and back). Images courtesy Roger Wiens.
LIBS laser. The LIBS laser is based upon a commercial laser, but was redesigned to reduce its mass by a factor of 10 and to make it reliable under the rigors of spaceflight. Its wavelength is 1067 nanometers, in the near-infrared. It should be able to sustain at least 20 million shots over its lifetime. Individual pulses last 5 nanoseconds, and it can fire up to 10 times per second. The laser fires through a telescope that focuses the beam diameter to between 0.25 and 0.35 millimeters.
Remote Micro-Imager. The Remote Micro-Imager is a flight spare from the camera system developed for the Philae lander on ESA’s Rosetta mission. (Other flight spares of the same instrument were used on the ill-fated Phobos Grunt mission.) RMI has a 1024-pixel-square CCD and captures black-and-white images using wavelengths of light from 450 to 950 nanometers. It has an auto-exposure feature. It can repeatedly capture images as fast as 1 frame per second. The RMI is intended to provide context imaging for LIBS shot points at a resolution finer than is achievable with the right Mastcam. It has also turned out to be useful for long-distance imaging.
Autofocus laser. To focus the laser on a target, ChemCam employs a second laser, with a wavelength of 785 nanometers, for an autofocus operation. The autofocus laser is mounted to the back of the secondary mirror. To prepare for a ChemCam LIBS operation, the team would estimate the distance to the target using Navcam stereo ranging, passing that value to ChemCam as a part of the instrument’s commands. The autofocus laser illuminated the target at 638 different test focus steps around that estimated distance, and a photodiode recorded how the intensity of the reflected light varied with focus. The intensity followed a generally bell-shaped curve. These data were sometimes noisy and the top of the curve sometimes flat, so electronics determined the best-focus position for LIBS and imaging by finding the axis of symmetry of that intensity curve.
Autofocus using RMI. Unfortunately, the autofocus laser failed on sol 801. The ChemCam team implemented a quick workaround within 15 days. ChemCam was commanded to perform nine sets of LIBS observations for each observation point at a range of focal distances around the best-guess distance determined from Navcam images; only one of these would turn out to be in focus.2 This process generated results but was wasteful in terms of time and data, and the team worked quickly in parallel to develop a new autofocus method using the RMI. The new autofocus capability was uploaded to Mars on sol 980 and used for the first time on sol 983. The RMI takes nine to eleven images, and performs a simple algorithm to measure image contrast, selecting the highest-contrast image to determine the in-focus distance and then proceeding with LIBS analysis. The method is similar to that employed by the Mastcams and MAHLI, except that the color cameras measure image complexity rather than image contrast to find the best-focus position. Regardless of the method, it takes the RMI about two minutes to autofocus.
LIBS operation. Once the instrument has determined the best-focus position for LIBS, it is ready to fire. The LIBS laser beam passes through two sets of lenses, expanding it from 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 window 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).
Figure 9.3. The ChemCam body unit. After Wiens et al. ( 2012 ).
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).
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 system (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
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 calibration purposes. Unlike the Mastcam calibration target, it is far enough from the mast that the right Mastcam can view it in focus.
The Design and Engineering of Curiosity Page 34
