Detection of Amorphous Carbon Enclosed in Hydrothermal Gypsum Deposits by Combined LIBS and Raman Spectroscopy: Implications for Biosignature Detection in Planetary Exploration

Abstract

The potential of laser-induced breakdown spectroscopy (LIBS) for detecting and characterizing carbonaceous materials in mineral matrices is attracting increasing interest in planetary exploration [1]. This work presents the first evidence of amorphous carbon enclosed within hydrothermal gypsum (CaSO₄·2H₂O) crystals from the Mina Rica mine (Pulpí, Almería, Spain), a recognized Martian analog site. The study integrates LIBS, Raman spectroscopy, and SEM-EDX to investigate the composition and distribution of carbon-bearing inclusions.

LIBS analyses revealed C(I) (247.8 nm) atomic and CN (Δv = 0) molecular emissions in both dark inclusions and siderite-free regions, indicating the presence of carbonaceous material trapped within gypsum cleavage planes. Depth profiling confirmed the shallow distribution of carbon, while spectral ratios were used to discriminate Fe emissions from siderite. Complementary Raman spectroscopy identified the broad D (1332 cm⁻¹) and G (1602 cm⁻¹) bands that are characteristic of amorphous carbon with a low degree of graphitization. On the other hand, SEM-EDX mapping shown micron-sized, carbon-rich particles dispersed within the gypsum matrix.

These findings demonstrate the capability of LIBS, especially when combined with Raman spectroscopy, to differentiate between organic and inorganic carbon sources in sulfate minerals. The results highlight the suitability of gypsum as a potential preservative host for biosignatures, reinforcing the applicability of LIBS for in situ astrobiological exploration on Mars and other planetary bodies.