Plasma-atmosphere interactions and implications for the characterization of carbon-bearing samples with LIBS on Mars

Kurzfassung

Carbon is a key element on Mars, where it is present in the atmosphere, cryosphere and rocks. It has influenced the Martian climate over billions of years, and records the conditions of past habitable environments. Laser-Induced Breakdown Spectroscopy (LIBS) is an efficient technique for fast in situ analysis of the chemical composition of rocks on the Martian surface, used on several Mars exploration missions already. However, to characterize the carbon content of rocks with LIBS on Mars, one must take into account the contribution of the CO2-dominated Martian atmosphere to the signal recorded in LIBS spectra. In support of LIBS analyses on Mars, we investigate this atmospheric contribution using a plasma-imaging setup in the laboratory. We study the temporal and spatial dynamics of the carbon and oxygen signals in different experimental configurations, where these elements originate from either the sample or the atmosphere. The significance of the atmospheric contribution to the carbon and oxygen signal in Martian atmospheric conditions is highlighted. We show that C I emissions come from different areas of the plasma depending on the origin of the carbon; such difference is not observed in the spatial distributions of C II, O I or O II. The C II / C I ratios are different in atmospheric and sample contributions, with a higher relative abundance of C I in the atmospheric contribution. To recognize carbonate in Martian LIBS data, we recommend using C I / O I if possible, or C II / O I ratios, with appropriate precautions.