Time-resolved spectral imaging of LIBS plasma at low pressures for the exploration of Solar System bodies

Abstract

LIBS is an increasingly important tool in the exploration of Solar System bodies. It has met great success on Mars with the ChemCam instrument on board of the NASA Mars Science Laboratory. SuperCam, the successor of the ChemCam instrument, will use LIBS in combination with Raman and fluorescence spectroscopy on the Mars2020 mission. The Chandrayaan-2 mission of India will feature a lunar rover with an on-board LIBS instrument. Additionally, concepts for LIBS instruments that could be used on other planetary bodies in the Solar System have been proposed. The reduced atmospheric pressure on many Solar System bodies has a strong influence on the LIBS plasma. At lower pressures, the plasma plume becomes more optically thin, resulting in less plasma shielding and more material ablation. At the same time, the lifetime and the emissivity of the plasma plume decrease as it expands more quickly. This tradeoff yields maximum signal intensities at around 10 mbar, close to Martian atmospheric pressure. Below 1 µbar, the pressure of the LIBS plasma is significantly larger than the atmospheric pressure and a stable low-intensity plateau is reached. Laboratory studies at this pressure should therefore be a good approximation of planetary bodies without an atmosphere. In this study, we investigate plasma plumes of geological samples at pressures between 1 µbar and 1 bar with a custom time-resolved plasma imaging setup. We compare spatial distributions of atomic, ionic and molecular species inside the expanding plasma plumes for different pressures and calculate profiles for the temperature and the electron density. The results will indicate which approximations and assumptions still hold in low-pressure conditions, which is important for the analysis of LIBS data from Mars and other planetary exploration missions.