LIBS Plasmas on Mars: Temperature and Electron Density with SuperCam over a large range of distances, rock compositions, and morphologies

Abstract

The Perseverance rover landed in Jezero Crater, Mars in 2021 with the goal of exploring an ancient delta for signs of past life. SuperCam, on the mast of Perseverance, uses Laser Induced Breakdown Spectroscopy (LIBS) to quantify elements present in the rocks/soils we encounter, shot at distances from 2-15 meters.

LIBS is appealing for planetary science missions because of its effectiveness at a distance and with different target types (rock, soil, etc). However, because of decreased laser irradiance at longer distances and changing coupling efficiency in different targets, the physical conditions (temperature and electron density) of the plasma plume could vary. If this were the case, it would have implications for our elemental calibration.

In this work, we use apparent temperature and electron density to explore the dynamics of plasmas generated by SuperCam on Mars. We estimate apparent temperature using the multiline Boltzmann plot method and calculate electron density using Stark broadening of the H-α line. Apparent plasma temperatures do not decrease with distance or vary significantly with target type (rock vs soil). The variability in plasma temperatures that we do see on Mars is fully captured by the laboratory dataset used for SuperCam’s elemental calibration. This suggests that our elemental calibration is robust against observed changes in apparent plasma temperature.

Estimated electron density is 1.5x higher in soils than rock, corresponding to a FWHM increase in the H-α line of ~0.4 nm. We hypothesize that the difference in electron density is related to the dependence of the H-α signal on topographic relief, a poorly understood mechanism which contributes to the difficulty of quantifying hydrogen abundance on Mars. Future work with spatiotemporally resolved LIBS should be able to determine the physical mechanism(s) behind this phenomenon.