Artifact formation during Raman measurements and its relevance to the search for chemical biosignatures on Mars

Raman spectroscopy will be a powerful tool in the in situ search for Martian biosignatures within the ESA/Roscosmos ExoMars and NASA Mars 2020 missions. However, a Raman laser can alter the chemical nature of a sample. This prompted us to investigate the stability of potential biosignatures during Raman measurements. For our study, we selected the photosynthetic pigment beta carotene, the biological membrane component 1,2 dioleoyl sn glycero 3 phosphoethanolamine (DOPE), the iron porphyrin hemin, and the electron transfer protein cytochrome c. The excitation wavelength was 532 nm, which is the wavelength at which the lasers of the RLS (ExoMars) and SuperCam (Mars 2020) instruments will operate. We found that beta-carotene and DOPE were stable up to 7.0 mW, which was the maximum laser power in our experiments, corresponding to an irradiance of 378 kW/cm2. Hemin and cytochrome c, by contrast, decomposed when the energy input exceeded a certain threshold. For example, hemin started to decompose in the 0.05–0.8 mW range (2.5–40 kW/cm2) under Mars-like conditions (200 K, vacuum, 50 s total irradiation time). Carbonaceous materials were the final decomposition products of both compounds. Our experiments also showed that low temperatures near the average Martian surface temperature of ~210 K can delay the decomposition of biomolecules. In addition to loose powders, we studied thin layers pressed on NaCl pellets, where NaCl served as a model mineral matrix. In the case of hemin and cytochrome c on NaCl, the measurements could be performed with higher laser powers because of more efficient heat dissipation by the salt. For comparison, spectra were also recorded under standard laboratory conditions, i.e., at room temperature and atmospheric pressure. A major conclusion of this work is that Raman lasers used on Mars may alter biomolecules by heating the sample and, in specific cases, transform them into carbonaceous matter. The resulting spectra may be misinterpreted as evidence of extinct rather than extant life or even as evidence of non-biological material