LIBS plasma diagnostics with SuperCam on Mars: Implications for quantification of elemental abundances

The Perseverance rover landed in Jezero Crater, Mars, in 2021 to explore an ancient delta for signs of past life and collect samples for a future Mars Sample Return Mission. SuperCam, onboard Perseverance, uses Laser Induced Breakdown Spectroscopy (LIBS) to quantify the elements in the rocks/soils encountered along the rover's traverse. LIBS is desirable for planetary science missions because of its effectiveness at a distance and with different target types (rock, soil, etc). However, decreased laser irradiance with distance and changing coupling efficiency in different targets could cause the physical conditions of the plasma plume to vary, with important implications for SuperCam's elemental calibration. This study examines the characteristics of laser-induced plasmas on Mars by estimating apparent temperature via the multiline Boltzmann plot method and electron density using Stark broadening of the H-α line. We find that apparent plasma temperatures do not decrease with distance or vary systematically with target type (rock vs soil). The variability in plasma temperatures seen on Mars is fully represented in the laboratory dataset used for SuperCam's elemental calibration, which suggests that our elemental calibration is likely robust against observed changes in apparent plasma temperature. These results imply that SuperCam can make reliable LIBS observations to at least 8 m. Estimated electron density is 1.4× higher in soils than rock targets, which is likely related to the dependence of the H-α line on topographic relief, a poorly understood mechanism which contributes to the difficulty of quantifying hydrogen abundance in Mars spectra.