Chemistry & Camera (ChemCam)
PI: Roger C. Wiens, Los Alamos National Laboratory
Deputy PI: Sylvestre Maurice, CESR, France
The ChemCam instrument package consists of two remote sensing instruments: the first planetary science Laser-Induced Breakdown Spectrometer (LIBS) and a Remote Micro-Imager (RMI). The LIBS provides elemental compositions, while the RMI places the LIBS analyses in their geomorphologic context. Both instruments will help determine which rock and soil targets within the vicinity of the rover are of sufficient interest to use the contact and analytical laboratory instruments for further characterization. ChemCam also can analyze a much larger number of samples than can be studied with the contact and analytical laboratory instruments. For example, the ChemCam team anticipates making daily analyses of the soil at the rover location to understand variations within the soils both locally and regionally. Furthermore, it can provide valuable analyses of samples that are inaccessible to other instruments, such as vertical outcrops where LIBS can target individual strata using its submillimeter beam diameter. ChemCam has the capability, but is not required, to provide passive spectroscopy data of rocks and soils on Mars. The spectral range covered by LIBS is not typical of passive spectroscopy instruments, making it more difficult to know what information can be useful from the spectra. However, the passive spectroscopy does not have the distance limitation that LIBS does. Additional information about ChemCam can be found in the publications listed in the “Publications” link on the web site http://libs.lanl.gov.
The LIBS instrument uses powerful laser pulses, focused on a small spot on target rock and soil samples within 7 m of the rover, to ablate atoms and ions in electronically excited states from which they decay, producing light-emitting plasma. The power density needed for LIBS is > 10 MW/mm^2, which is produced on a spot in the range of 0.3 to 0.6 mm diameter using focused, ~14 mJ laser pulses of 5 nanoseconds duration. The plasma light is collected by a 110 mm diameter telescope and focused onto the end of a fiber optic cable. The fiber carries the light to three dispersive spectrometers which record the spectra over a range of 240 - 850 nm at resolutions from 0.09 to 0.30 nm in 6144 channels. The spectra consist of emission lines of elements present in the samples. Typical rock and soil analyses yield detectable quantities of Na, Mg, Al, Si, Ca, K, Ti, Mn, Fe, H, C, O, Li, Sr, and Ba. Other elements often seen in soils and rocks on Earth include S, N, P, Be, Ni, Zr, Zn, Cu, Rb, and Cs. It is anticipated that 50-75 laser pulses will be required achieve the desired 10% accuracy for major elements at 7 m distance.
The advantages of the LIBS instrument are:
- Remote elemental analysis with no sample preparation
- Ability to remove dust and weathering layers with repeated laser pulses trained on the same spot
- Simultaneous analysis of many elements
- Low detection limits for a number of minor and trace elements, including Li, Sr, and Ba
- Rapid analysis; one laser shot can constitute an analysis, though many spectra are often averaged for better statistics, still only taking a few seconds
- Small analysis spot size of < 0.6 mm diameter
- Ability to identify water and/or hydrated minerals
- Low power consumption resulting from very short analysis times
LIBS spectrum of basalt standard GBW07105 taken with ChemCam engineering model at 5 m distance in 7 Torr (9.3 mbar) of atmosphere. The major emission lines are labeled.
The Remote Micro-Imager (RMI) is intended as a context imager for the LIBS, though unlike LIBS, it has no restrictions on the distance to the targets it images. It images through the same telescope as the LIBS, with the camera wavelength response shown in the figure at right. The detector is a 1024 x 1024 pixel CCD. The RMI has a field of view of 19 milliradians. Due to optimization of the telescope for LIBS, the RMI resolution is not pixel-limited, and is approximately 100 microradians. The RMI can clearly distinguish the submillimeter LIBS spot on a metal plate at any distance within range of the LIBS. LIBS spots on rocks are more difficult to distinguish, but will be known from the pixel mapping, so the context of the LIBS spot can be determined.
Types of Investigations
The ChemCam instrument suite will be used to pursue the following investigations:
- Rapid remote rock identification, which will be the main method of rapidly determining whether a given sample is similar to or different from rocks already encountered during the mission, and if the latter, whether the sample warrants investigation by the analytical laboratory instruments.
- Quantitative elemental analyses, including trace elements, to support the MSL science objectives. Whole-rock analyses will require a number of (< 1 mm diameter) analysis spots on the same rock. Quantitative analyses will rely on using both the onboard calibration standards as well as comparison with LIBS analyses of standards in terrestrial laboratories.
- Soil and pebble composition surveys. The ChemCam team plans to make a measurement of the soil near the rover each sol to document the range of soil compositions over which the rover traverses. These measurements may signal the presence of a new geological region, and will tell about the compositional similarity of the dust from place to place on Mars. The RMI can provide documentation on soil grain sizes without the need for placement of the contact instruments.
- Detection of hydrated minerals. LIBS sensitivity for hydrogen is unique and will be an important indicator of bound water in minerals.
- Depth profiles of rock weathering layers. LIBS can provide weathering profiles on a fine scale for small features, a unique capability.
- Rapid remote identification of surface ices/frosts. LIBS can unambiguously detect water ice.
- Geomorphology and imaging science. The high resolution imaging provided by RMI will enable detailed studies of the weathering processes of surfaces, and provide opportunities to image closeup many details with comparable or slightly higher resolution than Mastcam and without the need to drive up to a sample and deploy contact instruments.
- Complement other techniques for rock identification in cases of dust or weathering. ChemCam can use its laser to remove dust or weathering surfaces to aid other instruments in their investigations.
- Assist with Sample Acquisition, Processing, and Handling (SA/SPaH). ChemCam analyses can guide decisions on which samples within the robotic arm workspace should be sampled for in situ instrument analyses. ChemCam can provide imaging and compositional analyses on the samples being obtained for the in situ instruments. The small analysis spot size will be important in this regard. For example, without ChemCam the connection between a given XRD pattern taken by the analytical instruments and a distinctive mineral grain seen in the images might only be inferred at best.
A measurement with ChemCam can take many forms due to its versatility. However, for illustrative purposes, the figure at right shows a potential analysis sequence that acquires RMI images before and after the LIBS analysis. This type of sequence would be most often used for rapid rock identification. Each analysis of this type should take six minutes or less, excluding thermal delays. A thermoelectric cooler (TEC) is turned on to cool the detectors some minutes prior to instrument turn-on. The target is acquired by motion of the rover mast elevation and azimuth gimbals. The instrument is turned on, and the LIBS laser and the autofocus laser (continuous-wave, or CW laser) are warmed if needed. The telescope is focused on the target. RMI image acquisition and LIBS analyses are performed, and a background (laser-off) spectrum is taken. The mast can then acquire other targets and the focus and shoot sequence can be repeated. Other types of analyses will include:
- Depth profiles > 0.5 mm, requiring > 500 laser shots on the same spot.
- Soil surveys, probably utilizing much the same analysis sequence as shown.
- Quantitative analyses, which will require a number of analysis spots on a single rock. Quantitative analyses would be carried out in conjunction with:
- Calibration targets. These are mounted on the rover and are used to calibrate the LIBS spectra.
- Miscellaneous images independent of any LIBS analyses. These will be used to characterize samples for the analytical laboratory, for general geomorphology studies, and to provide the highest resolution images of distant features.
- Passive UV-visible spectra. The LIBS spectrometers provide the opportunity for passive spectra taken using ambient sunlight and without the laser plasma.
The figure and photos below show a functional block diagram of the ChemCam suite. The package consists of two separate units: “Body Unit” and “Mast Unit,” which are further broken down into modular components. The spectrometers and data processor are in the Body Unit, while the laser, imager, telescope, and focus laser are in the Mast Unit. The ChemCam Mast Unit is mounted on the rover mast just above Mastcam and Navcam. The boresight, at a height of 2.1 m above the ground, is coaligned with both Mastcam and Navcam. The Mast Unit is provided by CESR (funding from CNES), while LANL is responsible for the Body Unit. JPL is responsible for the fiber optic cable that transmits light from the telescope to the spectrometers. JPL also provided the thermoelectric cooler that cools the spectrometer CCDs. Parts and targets for the onboard calibration target were provided by C. Fabré (Nancy), V. Sautter (MNHN, Paris), D. Vaniman (LANL), and D. Dyar (Mount Holyoke College). The onboard rover calibration targets for LIBS consist of natural and synthetic volcanic glasses (Fabré et al., 2009, 2010) and ceramics consisting of mixtures of smectite and kaolinite with anhydrite and basalt to simulate Martian sedimentary samples (Vaniman et al., 2009). Also included is a graphite disk for carbon identification and a titanium plate for general use.
Calibrations, Data, and Operations
Calibration of the LIBS data involves preflight calibrations, postlanding calibrations using the onboard targets, and comparisons with spectra obtained on Mars analogs in terrestrial laboratories. Preflight calibration targets consist of approximately seventy standards, two-thirds of which are igneous standards covering a somewhat larger range of SiO2 abundances than basalts and andesites. The preflight standards also include a range of “dirty” sulfates, and a number of sedimentary materials. “Dirty” standards that are mixtures of materials are preferred over pure minerals, as pure mineral compositions can be easily determined by the occurrence of only the elements present in the given mineral. Postlanding calibrations will be done with the onboard standards described above, and by comparison with both the preflight calibrations and with Mars analog samples analyzed in terrestrial LIBS laboratories. Comparisons between ChemCam LIBS spectra and LIBS spectra from terrestrial laboratories need to be studied, as does the effect of distance on calibrations. For example, different emission lines respond to distance differently likely based on their activation energies. We are actively studying these effects, and we expect to continue these studies into the mission phase.
Operation of the instrument is expected to be shared 50/50 between the U.S. and France. After transition to remote operations, the team will operate on a 38 day cycle during which the Mars time shifts relative to Earth time. The French team will be responsible for instrument operation during the half of the cycle in which downlinks are too late in the day in the US, while the U.S. team members will operate the instrument during times when downlinks are too late in the day in France. Staffing of science theme groups will be done by both countries regardless of who has instrument operation responsibilities at any given time, so that scientists from both countries are involved in decisions at all times.
ChemCam science team members are expected to help staff the Science Operations Working Group and ChemCam Payload Downlink Lead positions, especially given the expected frequent operation of the instrument. RMI data reduction will use JPL imaging tools. LIBS data reduction will consist of preprocessing such as background subtraction, wavelength calibration, and distance corrections. Data processing will use multivariate analysis techniques, relying heavily on comparison with spectra of high-fidelity Mars analogs analyzed in terrestrial laboratories. We envision using principal components analyses (PCA) and related techniques to classify and compare samples, while we expect to use partial least squares (PLS) to calibrate and quantitatively determine elemental compositions (e.g., Clegg et al., 2009). Payload Downlink Leads will be expected to carry out preprocessing and sample classification in order to present results to the SOWG for tactical use.
Engineering Qualification Model Mast Unit, Engineering Model Body Unit, and Engineering Model rover-mounted calibration targets. Photo credits: LANL and JPL.