Using portable Raman spectrometers for the identification of organic compounds at low temperatures and high altitudes: exobiological applications

Organic minerals, organic acids and NH-containing organic molecules represent important target molecules for astrobiology. Here, we present the results of the evaluation of a portable hand-held Raman spectrometer to detect these organic compounds outdoors under field conditions. These measurements were carried out during the February-March 2009 winter period in Austrian Alpine sites at temperatures ranging between -5 and -25 degrees C. The compounds investigated were detected under field conditions and their main Raman spectral features were observed unambiguously at their correct reference wavenumber positions. The results obtained demonstrate that a miniaturized Raman spectrometer equipped with 785 nm excitation could be applied with advantage as a key instrument for investigating the presence of organic minerals, organic acids and nitrogen-containing organic compounds outdoors under terrestrial low-temperature conditions. Within the payload designed by ESA and NASA for several missions focusing on Mars, Titan, Europa and other extraterrestrial bodies, Raman spectroscopy can be proposed as an important non-destructive analytical tool for the in situ identification of organic compounds relevant to life detection on planetary and moon surfaces or near subsurfaces.

Microbial colonization of halite from the hyper-arid Atacama Desert studied by Raman spectroscopy

The hyper-arid core of the Atacama Desert (Chile) is the driest place on Earth and is considered a close analogue to the extremely arid conditions on the surface of Mars. Microbial life is very rare in soils of this hyper-arid region, and autotrophic micro-organisms are virtually absent. Instead, photosynthetic micro-organisms have successfully colonized the interior of halite crusts, which are widespread in the Atacama Desert. These endoevaporitic colonies are an example of life that has adapted to the extreme dryness by colonizing the interior of rocks that provide enhanced moisture conditions. As such, these colonies represent a novel example of potential life on Mars. Here, we present non-destructive Raman spectroscopical identification of these colonies and their organic remnants. Spectral signatures revealed the presence of UV-protective biomolecules as well as light-harvesting pigments pointing to photosynthetic activity. Compounds of biogenic origin identified within these rocks differed depending on the origins of specimens from particular areas in the desert, with differing environmental conditions. Our results also demonstrate the capability of Raman spectroscopy to identify biomarkers within rocks that have a strong astrobiological potential.

Application of portable Raman instruments for fast and non-destructive detection of minerals on outcrops

Raman spectral signatures have been obtained in situ for a series of minerals using portable Raman instruments. Cerussite, anglesite, wulfenite, titanite, calcite, tremolite, andradite and quartz were detected using portable Raman spectrometer First Defender XL (Ahura). Baryte, almandine and realgar Raman spectra obtained by this instrument in the field were compared to the data measured by the other mobile Raman instrument Inspector Raman (DeltaNu). Bench Raman dispersive microspectrometer (InVia Reflex, Renishaw) was used for comparative purposes. All spectra were obtained using a 785nm diode excitation. Although displaying lower spectral resolution comparing with the laboratory confocal instrument both portable instruments permit unambiguous detection of minerals in the field. These possibilities designate portable Raman machines as excellent tools for field geological applications. Miniaturised Raman instrument combined with LIBS will be included in the payload of the EXO Mars mission and would open interesting research possibilities in other in situ field planetary studies.

Raman spectra of organic compounds kladnoite (C6H4(CO)2NH) and hoelite (C14H8O2)--rare sublimation products crystallising on self-ignited coal heaps

As minerals, aromatic compounds occur very rarely in nature. Not more than 10 of such minerals are known and most of them were described in the coal deposits where they were formed as a result of coal bed fires or burning of coal waste heaps. Raman spectra of kladnoite C(6)H(4)(CO)(2)NH (natural phthalimide) and hoelite C(14)H(8)O(2), (natural 9,10-anthraquinone) display complex features. Raman microspectroscopy help to detect these phases non-destructively directly in the frame of rare samples. Investigated minerals are transformation products formed from gaseous phase originating in natural pyrolytical process occurring in the frame of coal heaps and coal series outcrops. It is recommended to include Raman spectroscopic characteristics of similar materials in databases for exobiological studies.

Raman spectroscopic study of mellite--a naturally occurring aluminium benzenehexacarboxylate from lignite--Claystone series of the tertiary age

Raman spectra have been obtained for crystals of the organic mineral mellite, from three different sites. Mellite occurs in the frame of the Tertiary series including lignite and coaly slates at Artern (Thuringia), Tula (Russia) and Bílina (Northern Bohemia). Mellite, Al(2)C(6)(COO)(6) x 16H(2)O, can be considered as evidence of previous biological activity in the geological record, similar to other salts of carboxylic acids such as whewellite and weddellite. Assignments of the major Raman features of mellite are proposed on the basis of comparison with the parent, mellitic acid, C(6)(COOH)(6). During diagenesis and epigenesis, mellite is formed from the reaction between organic carbon rich solutions with aluminosilicates, hence, with the current interest in the adoption of Raman spectroscopy for incorporation into robotic instrumentation for space mission landers, it is important that organic minerals be included into a spectroscopic database for the recognition of biomolecular signatures for remote life-detection experiments.

Influence of laminar flow on preorientation of coal tar pitch structural units: Raman microspectroscopic study

In order to estimate the role of laminar flow of viscous, aromatic matter of carbonaceous precursor on microtextural preorientation in pregraphitization stage, we performed experiments with coal tar pitch (CTP). The principal hypothesis of preorientation of basic structural units (BSUs) in the case of laminar flow (pressure impregnation of CTP into porous matrix) and secondary release of volatiles during carbonization were studied. Glass microplates, planar porous medium with average distance between single microplates 5 microm were used as suitable porous matrix. Samples of CTP were carbonized up to 2500 degrees C. Optical microscopy reveals large flow domains in the sample of cokes carbonized between glass microplates. Raman microspectroscopy and high resolution transmission electron microscopy (HRTEM) show that at nanometric scale, the samples do not support the proposed hypotheses. With increasing temperature of pyrolysis, the graphitization of CTP impregnated into porous matrix proceeds to lower degree of structural ordering in comparison with single pyrolyzed CTP. This is explained by the release of volatile matter during carbonization in geometrically restricted spaces. More evident structural changes were discovered with the sample of single coke, where parts of fine grain mosaics, relicts of 'so called QI parts', reveal higher structural organization, in comparison with large and prolonged flow domains, similar to flow domains of cokes from microplates.