Investigation on the origin of molecular emissions in laser-induced breakdown spectroscopy under Mars-like atmospheric conditions of isotope-labeled compounds of interest in astrobiology

Abstract

The search for life or evidence of ancient biological activity on the surface of Mars has been the main focus of recent missions to the Red Planet. The rover Perseverance was launched in July 2020 with the aim of collecting and analyzing core samples from the Mars surface using a suite of inspection technologies including LIBS, Raman, UV-VIS, infrared, molecular fluorescence and X-ray fluorescence spectroscopies. LIBS is primarily devoted to the analysis of inorganic materials and to establish the atomic composition of rocks, minerals and soils. Recent research has demonstrated, though, that significant information may appear in the LIBS spectrum to assist in the identification of organic molecules. Recognition of organic residues in geological materials by LIBS in presence of carbon dioxide from the atmosphere is not without challenges due to the interaction of the surrounding gases with the plasma and to the complexity of the reactions of formation, fragmentation and recombination of the involved emitting species during plasma formation and subsequent evolving stages. The present work aims at contributing to the assessment of LIBS to identify emissions features that may arise in the sampling and analysis of materials containing organic residues in a Martian environment, and to uncover the origin of such emissions registered in the spectroscopic data. For this purpose, a systematic study on the effect of a variety of background gases (Earth atmosphere, pure CO2 gas and simulated Mars atmosphere) on the emission features of a selection of organic compounds (urea, pyrene and benzamide) was carried out. Investigations were centered on elucidating the possible reaction mechanisms -and ultimately, the dominant ones- giving rise to three molecular species, namely CN, C2 and NH. In order to gain deeper insight into the main pathways, measurements with isotopically labeled molecules (15N-benzamide, D4-urea and 13C-urea) were also conducted. This study demonstrates that the likely detrimental effect of the atmosphere is only marginal and that fingerprinting of organic compounds is readily accessible on the LIBS spectral information.