Geobiologists attempt to answer such questions as: when and under what conditions did life begin, how can we verify biogenicity in the geologic record, and what governs the relation between the living world and the mineral world? Raman spectroscopy can characterize and identify both organic and inorganic phases, typically nondestructively, at the (sub-)micrometer scale and, thereby, can provide key information to tackle these questions. This article illustrates contributions that Raman spectroscopy has made to understanding mineralization processes in mollusks, corals, and bones. Raman spectroscopy can also be used in the search for earliest terrestrial life and life on other planets. Some challenges for the three Raman instruments to be deployed on Mars are discussed.

The search for molecular biosignatures at the surface of Mars is complicated by an intense irradiation in the mid- and near-ultraviolet (UV) spectral range for several reasons: (i) many astrobiologically relevant molecules are electronically excited by efficient absorption of UV radiation and rapidly undergo photochemical reactions; (ii) even though the penetration depth of UV radiation is limited, aeolian erosion continually exposes fresh material to radiation; and (iii) UV irradiation generates strong oxidants such as perchlorates that can penetrate deep into soils and cause subsurface oxidative degradation of organics. As a consequence, it is crucial to investigate the effects of UV radiation on organic molecules embedded in mineral matrices mimicking the martian soil, in order to validate hypotheses about the nature of the organic compounds detected so far at the surface of Mars by the NASA Mars Science Laboratory's (MSL) Curiosity rover, as well as organics that will be possibly found by the next rover missions Mars 2020 (NASA) and ExoMars 2022 (ESA-Roscosmos). In addition, studying the alteration of possible molecular biosignatures in the martian environment will help to redefine the molecular targets for life detection missions and devise suitable detection methods. Here we report the results of mid- and near-UV irradiation experiments of Mars soil analog samples obtained adsorbing relevant organic molecules on a clay mineral that is quite common on Mars, i.e. montmorillonite, doped with 1 wt% of magnesium perchlorate. Specifically, we chose to investigate the photostability of a plausible precursor of the chlorohydrocarbons detected on Mars by the Curiosity rover, namely phthalic acid, along with the biomarkers of extant life L-phenylalanine and L-glutamic acid, which are proteomic amino acids, and adenosine 5'-monophosphate, which is a nucleic acid component. We monitored the degradation of these molecules adsorbed on montmorillonite through in situ spectroscopic analysis, investigating the reflectance properties of the samples in the Near InfraRed (NIR) spectral region. Such spectroscopic characterization of molecular alteration products provides support for two upcoming robotic missions to Mars that will employ NIR spectroscopy to look for molecular biosignatures, through the instruments SuperCam on board Mars 2020, ISEM, Ma_Miss and MicrOmega on board ExoMars 2022.

Stromatolites have been a major focus in the search for ancient microbial life, however, the organic carbon biosignatures of dolomitized stromatolites have not yet been fully characterized or correlated with their dolomitizing conditions. Although dolomitization rarely preserves microbial morphology, the presence of organic carbon can provide valuable information for characterization of fossils biogenicity, syngenicity, and indigeneity to their host rock. The Cambrian Allentown Formation in New Jersey, USA, is an excellent example of dolomitized stromatolites and thromboiites containing diagenetically modified microbial biosignatures. Based on XRD and EPMA data, the dolomite composition is typically stoichiometric, with varying degrees of cationic ordering. The outcrop underwent early dolomitization in a marginal-marine setting and later burial diagenesis resulting in multi-generational dolomite formation: (1) microspar dolomite formed by early diagenetic replacement at or near the surface, (2) zoned dolomite formed penecontemporaneously with the microspar phase as rhombohedral crystals by mulling primary pore spaces within the microspar matrix. The rhombic crystals continued to grow outward in alternating stages of Fe-enriched and -depleted fluids, which were preserved in zoned rims and revealed by cathodoluminescence, and (3) saddle dolomite formed during late stage deep burial with Fe- and Mn-rich fluids, and occurs as a void-filling, high-temperature phase. Organic carbon, characterized using confocal Raman microscopy, has an exclusive distribution within the microspar dolomite, and the D and G bands' characteristics reveal similar thermal alteration to the host rock, indicating that the mapped organic carbon is indigenous and syngenetic with the Cambrian carbonates. The findings presented in this study reveal organic matter found within microspar of various dolomitized fades deriving from different source pools of organic carbon. This study sheds light on biosignatures in secondary dolostones and may aid biosignature detection in older carbonate rocks on Earth and Mars. (C) 2020 Elsevier B.V. All rights reserved.

The search for organic biosignatures on Mars will depend on finding material protected from the destructive ambient radiation. Solar ultraviolet can induce photochemical degradation of organic compounds, but certain clays have been shown to preserve organic material. We examine how the SHERLOC instrument on the upcoming Mars 2020 mission will use deep-ultraviolet (UV) (248.6nm) Raman and fluorescence spectroscopy to detect a plausible biosignature of adenosine 5'-monophosphate (AMP) adsorbed onto Ca-montmorillonite clay. We found that the spectral signature of AMP is not altered by adsorption in the clay matrix but does change with prolonged exposure to the UV laser over dosages equivalent to 0.2-6 sols of ambient martian UV. For pure AMP, UV exposure leads to breaking of the aromatic adenine unit, but in the presence of clay the degradation is limited to minor alteration with new Raman peaks and increased fluorescence consistent with formation of 2-hydroxyadenosine, while 1wt % Mg perchlorate increases the rate of degradation. Our results confirm that clays are effective preservers of organic material and should be considered high-value targets, but that pristine biosignatures may be altered within 1 sol of martian UV exposure, with implications for Mars 2020 science operations and sample caching.

The search for organic biosignatures on Mars will depend on finding material protected from the destructive ambient radiation. Solar ultraviolet can induce photochemical degradation of organic compounds, but certain clays have been shown to preserve organic material. We examine how the SHERLOC instrument on the upcoming Mars 2020 mission will use deep-ultraviolet (UV) (248.6 nm) Raman and fluorescence spectroscopy to detect a plausible biosignature of adenosine 5 '-monophosphate (AMP) adsorbed onto Ca-montmorillonite clay. We found that the spectral signature of AMP is not altered by adsorption in the clay matrix but does change with prolonged exposure to the UV laser over dosages equivalent to 0.2-6 sols of ambient martian UV. For pure AMP, UV exposure leads to breaking of the aromatic adenine unit, but in the presence of clay the degradation is limited to minor alteration with new Raman peaks and increased fluorescence consistent with formation of 2-hydroxyadenosine, while 1 wt % Mg perchlorate increases the rate of degradation. Our results confirm that clays are effective preservers of organic material and should be considered high-value targets, but that pristine biosignatures may be altered within 1 sol of martian UV exposure, with implications for Mars 2020 science operations and sample caching.

Although the habitability of early Mars is now well established, its suitability for conditions favorable to an independent origin of life (OoL) has been less certain. With continued exploration, evidence has mounted for a widespread diversity of physical and chemical conditions on Mars that mimic those variously hypothesized as settings in which life first arose on Earth. Mars has also provided water, energy sources, CHNOPS elements, critical catalytic transition metal elements, as well as B, Mg, Ca, Na and K, all of which are elements associated with life as we know it. With its highly favorable sulfur abundance and land/ocean ratio, early wet Mars remains a prime candidate for its own OoL, in many respects superior to Earth. The relatively well-preserved ancient surface of planet Mars helps inform the range of possible analogous conditions during the now-obliterated history of early Earth. Continued exploration of Mars also contributes to the understanding of the opportunities for settings enabling an OoL on exoplanets. Favoring geochemical sediment samples for eventual return to Earth will enhance assessments of the likelihood of a Martian OoL.

Nanoscale graphene morphologies are reported in the Allende and QUE 94366 CV3-type carbonaceous chondrites via Confocal Raman Imaging Spectroscopy. These morphologies are found embedded in the refractory calcium-aluminum-rich inclusion (CAI) rims in Allende and within a chondrule inclusion in QUE 94366. Earlier investigation already revealed graphite whiskers (GWs) presence in both these meteorites. Further inspection of the meteoritic sections, coupled with advancements in the knowledge of carbon materials, reveal a re-interpretation of Raman features of a subset of the reported GWs and newer analyzed features as graphene. This meteoritic graphene perhaps originated from the same protosolar carbon reservoir that synthesized the GWs. The graphene was most likely synthesized concurrent to the inclusions and CAIs, in a high-temperature zone near the proto-Sun and during the solar system's earliest eon. However, in the case of Allende we cannot totally rule out synthesis during later aqueous alteration of the original mineral CAI assemblage.

Combined UV Raman and laser-induced fluorescence (LIF) spectroscopy instruments will soon be launched onboard missions to planetary surfaces, including Mars, to search for biosignatures. However, the rare earth element Ce3+, found in many common and Mars-relevant minerals, can produce fluorescence features within the spectral window usually attributed to organic compounds in a LIF spectrum. This study explored the detection of Ce3+ as a biosignature mimicker using UV Raman-LIF mission instruments. We assessed how LIF spectra of a suite of synthetic CePO4 samples compare with those of organics, how varying concentrations of both Ce3+ and organics in Martian regolith simulant affect this comparison, and whether two additional data sets obtainable by combined UV Raman-LIF instruments, including time-resolved fluorescence decay lifetimes and Raman spectra, can distinguish Ce3+-containing samples from organics. Results showed that the dominant LIF features of Ce3+ (320 and 338 nm) are similar to those of the aromatic amino acid tryptophan (325 and 340 nm), even when Ce3+ samples were mixed in a Martian regolith simulant at a range of concentrations. Lifetimes were revealed to be 2-9 ns in Ce3+-containing samples, typical for organic fluomphores. These results support the erroneous interpretation that LIF spectra and lifetime values obtained on these samples constitute potential organic signatures. Raman spectroscopy results suggested that with UV laser excitation, Raman is unlikely to identify Ce-bearing samples due to strong absorption of Raman scattered energy by Ce3+. We conclude that biosignature searches using UV LIF and Raman spectroscopy instrumentation may encounter challenges with unambiguously distinguishing spectra of organic compounds from Ce-bearing compounds.

Carbonate rocks record the oldest forms of life on Earth, and their geologic reconstruction requires multiple methods to determine physical and chemical processes before conclusions of ancient biosignatures are made. Since crystal orientation within rock fabric may be used to infer geologic settings, we present here a complementary Raman method to study the orientation of calcite (CaCO3) and dolomite [CaMg (CO3)(2)] minerals. The relative peak intensity ratio of the carbonate lattice E-g modes T and L reveals the crystallographic orientation of calcite and dolomite with respect to the incident light polarization. Our results for calcite show that when the incident laser light propagates down the crystallographic a/b axis: (1) the L mode is always greater in intensity than the T mode (I-T < I-L), and (2) the spectra are most intense at 45 degrees and least intense at 90 degrees polarization angles measured from around the c axis. Our results for dolomite show that (1) I-T > I-L when the incident light propagation is down the crystallographic c axis and (2) I-T < I-L when the incident light propagation is down the crystallographic a/b axis. This study reveals mineral orientation variation related to deposition and paragenesis within limestone and dolostone samples. The method presented yields information related to growth and deformation during diagenetic and metamorphic alteration and may be used in research seeking to identify the fabric parameters of any calcite or dolomite containing rock. The compositional and structural data obtained from Raman mapping is useful in structural geology, materials science, and biosignature research.