Degradation of beta-carotene under mineral thin sections during proton irradiation monitored in situ by Raman spectroscopy

Abstract

More than 3.5 billion years ago, the surface of Mars experienced habitable conditions compatible with the emergence and development of primitive microbial life. Consequently, in the absence of plate tectonics, ancient biosignatures dating back several billion years could still be present at the surface of the red planet. Nevertheless, Mars has been continuously exposed to UV radiation and solar and galactic cosmic rays, which may have degraded these putative biosignatures at the surface over time. The European Space Agency's ExoMars Rosalind Franklin mission, expected to land on Mars in 2030, is thus equipped with a drill to collect samples up to 2 m deep to increase the chances of detecting well preserved molecules.

Here, based on previous models, we first estimate the dose delivered in the first meters of Mars regolith over geological time. We then describe experiments in which beta-carotene (a pigment commonly used by microorganisms) was irradiated using a 2.8 MeV proton beam and its degradation with increasing dose studied by Raman spectroscopy, using a unique device allowing in situ measurements to be carried out within the irradiation chamber. Specific sample preparation, consisting of placing a thin mineral layer on top of the pigment, was also developed to take into account the dose distribution profile within the sample. Finally, based on the dose estimated for Mars, we correlate the change in the Raman signal-to-noise ratio of beta-carotene to an equivalent depth and time on Mars.