Miniaturized Laser - induced Breakdown Spectroscopy for planetary surface analysis

Introduction: Laser-induced Breakdown spectroscopy (LIBS) is currently under development for future lander missions to Mars [1–10] and other planets and moons, like Venus or Europa [11, 12]. LIBS spectroscopically determines the visible atomic emission from a plasma that is generated by pulsed laser light focused on a sample surface. The intensities of the many characteristic spectral lines of the elements are direct measures of their concentration in the sample. For Martian environmental conditions it was convincingly demonstrated that quantitative results can be obtained applying suitable calibration methods [2-4]. LIBS serves a tool for rapid elemental analyses and has some advantages compared to established methods in space exploration: • Rapid analysis («1 minute) • High lateral resolution (≥ 50 μm) • Needs neither sample preparation nor pumps • Detects simultaneously and quantitatively major, minor and trace elements • Removes dust layers and ”drills” through weathered rock surfaces. • Allows depth profiling • Synergetic integration with Raman spectroscopy [5,6] or microscopy easily feasible • Profitable in combination with Mössbauer or APXS instruments Furthermore, we have demonstrated [7,8] that LIBS directly detects ice and liquid pore or adsorption water inside rocks and on rock/soil surfaces. The stringent space mission requirements (weight, size, power consumption) call for the development of a lightweight LIBS instrument which we currently are pursuing in the framework of the GENTNER project for surface missions to Mars or other planets and moons [9,10]. Here we report on our study of different instrument parameters that are of importance for LIBS specifically in the Martian environment. Experimental: For our studies we utilized a chamber with “Martian” atmosphere (95.55% CO2, 2.7% N2, 1.6% Ar, 0.15% O2) at 7 mbar. We used a miniaturized prototype Nd:YAG laser operated at 1064 nm, with a pulse width of 2 ns and normal incidence onto the sample surface. As the laser mass directly correlates with the laser energy, we first determined with our experimental set up the minimum energy required for LIBS quantitative analyses. To this end, the laser energy was varied from 0.1 to 2.8 mJ and the repetition rate from 1 to 50 Hz. To ensure a sufficiently high irradiance, the laser prototype had a spot diameter of ~ 50 μm. The plasma emission was collected parallel to the incident laser beam and detected with an Echelle spectrometer equipped with an ICCD detector. The latter was used without amplification and temporal resolution to best simulate a probably compact and “simple” flight-spectrometer. Results: Optimization of instrumental parameters. In order to analyze the influence of the instrumental parameters of the miniaturized LIBS laser, laser wavelength, spot diameter, pulse duration and ICCD settings were kept constant. We found that the best results for this fixed configuration are achieved with • Laser energy: ≥ 1.2 mJ • Laser frequency: 10 Hz • Pulse number: 20–50 for soils, ≥ 50 for rocks • Normal laser incidence and detection angle • Focal plane slightly below sample surface A short section of a basalt LIBS spectrum (laser energy 1.8 mJ, 50 pulses, other settings from above) is shown in Fig. 1. In addition to major element lines also the ionic emission of the minor element Sr is visible. Note that a full spectrum (250–900 nm) contains hundreds of useful neutral and ionic element lines – a highly over-determined system.