Changes in the Raman and Fluorescence Spectroscopic Signatures of Irradiated Organic-Mineral Mixtures: Implications for Molecular Biosignature Detection on Mars

The search for potential molecular biosignatures on Mars is complicated by its harsh radiation environment that can alter or destroy the primary molecular features diagnostic of an organic compound's origins. In this work, mixtures of Mars-relevant minerals and organic material representing different types and different chemical states of sedimentary organic material common in the terrestrial geologic record were irradiated with 200 MeV protons to simulate the effect of exposure to galactic cosmic rays and solar energetic particles over geological timescales and characterized using a deep UV Raman and fluorescence spectrometer analogous to the Scanning Habitable Environments with Raman & Luminescence for Organics & Chemicals (SHERLOC) instrument on the Mars 2020 Perseverance Rover. We found that exposure to ionizing radiation generally results in the loss of molecular features diagnostic of an organic material's origins in favor of increasingly aromatic compounds or macromolecules. However, these radiolytic effects can be mitigated by the formation of macromolecular structures that are more resistant to radiolysis compared to individual compounds, and potentially through associations with specific minerals that enable increased polymerization. Based on these results, rocks observed by the SHERLOC instrument with fluorescence or Raman features associated with non-aromatic molecular features and/or kerogen-like structures may indicate less radiolytically damaged organic material that should be prioritized for return as it may retain some primary, diagnostic molecular features.