Rapid Elemental Analysis at Stand-off Distances Using the Libs Concept from the Mars Instrument Development Program

Introduction: The elemental composition of rocks and soils is one of the most fundamental types of information needed to understand geologic contexts and to search for likely locations of biological activity. Nearly all methods for determining elemental composition involve in-situ analysis, requiring time-consuming maneuvering of a rover to acquire the desired sample. By contrast, analyses at stand-off distances allow nearly a tenfold increase in the number of samples obtainable over in-situ techniques [1]. Additionally , methods such as APXS have difficulty distinguishing between the pristine sample and rock coatings of either dust or weathering products [2]. Under the auspices of the Mars Instrument Development Program, we have built and are testing a prototype LIBS (Laser-Induced Breakdown Spectroscopy) instrument which can rapidly determine elemental compositions at a distance. Additionally, by depth profiling µm to mm into the sample, LIBS can distinguish between dust or weathering products and the pristine sample. Here we summarize the LIBS concept, describe the initial performance of our prototype, including work at the combined rover tests in Nevada, and summarize potential LIBS contributions to the Mars exploration program. The LIBS Concept: In the LIBS method [3], powerful laser pulses are focused on the target sample to form a laser spark or plasma. Material within the spark is the result of vaporization/atomization of a small amount of target material. The spark light contains the emission spectra of the elements within the plasma. Collection of the plasma light, followed by spectral dispersion and detection, permit identification of the elements via their unique spectral signatures. When calibrated, concentrations can be determined. Advantages of the method compared to more conventional elemental analysis methods include: (1) rapid analysis (one measurement/pulse); (2) simultaneous multi-element detection; (3) ability to detect all elements (high and low z); (4) ability to clean dust or weathering layers off of sample surfaces; and (5) stand-off analysis capability [4]. Stand-off analysis is possible because the laser pulses can be focused at a distance to generate the laser sparks on a solid. The distance that can be achieved depends on characteristics of the laser and the optics used to focus the pulses on the target. Recent LIBS Results: We have recently shown [5,6] using a laboratory instrument that a) semi-quantitative results (e.g., 10-20% accuracy) can be obtained for nearly all elements at stand-off distances of up to 20 m. using a compact laser and a 4" objective