Lifetime, size and emission of laser-induced plasmas for in-situ laser-induced breakdown spectroscopy on Earth, Mars and Moon

Abstract

The spectroscopic technique of laser-induced breakdown spectroscopy (LIBS) is a powerful method to perform rapid chemical analysis of geologic samples with short measurement times and no need for sample preparation. After the ChemCam instrument aboard NASA’s MSL rover proved its suitability for space missions that explore planetary surfaces in 2012, the interest in LIBS instruments as payloads has grown and several subsequent missions have successfully used this technique since. The characteristics of a LIBS plasma depend on experimental and environmental parameters as well as on sample properties, including atmospheric conditions, laser irradiance and sample lithology. Consequently, LIBS instruments need to be designed and optimized specifically for each use case to maximize their science output. To aid in the development of new LIBS instruments for space exploration, we investigate the influence of atmospheric conditions, laser irradiance and sample lithology on the lifetime, size and emission of laser-induced plasmas. In our measurements, we use a plasma imaging setup with high temporal resolution of down to 2 ns to investigate the evolution of the plasma from its ignition to its decay. We present a comparable data set recorded at terrestrial, Martian and airless atmospheric conditions, covering irradiances between 0.79 GW/mmˆ2 and 1.43 GW/mmˆ2 and samples with diverse properties, namely basalt and soapstone, as well as the lunar regolith simulants LHS-1 and LMS-1. Our measurements show the strong influence of atmospheric conditions on the plasma size and emission, while the lithologies and laser irradiances covered in this work play a minor role. This shows that instruments designed to work at certain atmospheric conditions can be used for a range of laser parameters and sample properties. Furthermore, we demonstrate that the decay of the plasma emission and the expansion of the plasma plume parallel to the sample surface can be described well by a power law and a drag model, respectively.