Raman Measurements under Simulated Martian conditions

Raman spectroscopy is generally regarded as nondestructive. It is easy to apply, as no extensive sample preparation is necessary. As part of the ExoMars mission 2018, a compact Raman laser spectrometer (RLS) will analyze the mineral composition of the Martian soil and in particular search for organic matter [1]. Considering the possibility that life once evolved on Mars, its chemical traces may be detectable in Martian mineral matrices [2]. Our investigations on biomolecules have shown that high laser powers can influence the spectral outcome and even lead to complete sample destruction. To ascertain parameters and sample preparations favorable for an application on Mars, we developed a new measuring set-up simulating Martian environmental factors. Using a cryostat as simulation chamber, the samples were cooled down stepwise to 200 K. To minimize the oxygen level, a special pump created a stable vacuum of ca. 10–6 mbar. Different sample types (powders, pellets) have been measured with increasing laser power. The results are quite revealing as they show a major influence of the physical properties of the samples

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