The exclusive presence of β-D-ribofuranose in nucleic acids is still a conundrum in prebiotic chemistry, given that pyranose species are substantially more stable at equilibrium. However, a precise ...characterisation of the relative furanose/pyranose fraction at temperatures higher than about 50 °C is still lacking. Here, we employ a combination of NMR measurements and statistical mechanics modelling to predict a population inversion between furanose and pyranose at equilibrium at high temperatures. More importantly, we show that a steady temperature gradient may steer an open isomerisation network into a non-equilibrium steady state where furanose is boosted beyond the limits set by equilibrium thermodynamics. Moreover, we demonstrate that nonequilibrium selection of furanose is maximum at optimal dissipation, as gauged by the temperature gradient and energy barriers for isomerisation. The predicted optimum is compatible with temperature drops found in hydrothermal vents associated with extremely fresh lava flows on the seafloor.
Biosignatures in early terrestrial rocks are highly relevant in the search for traces of life on Mars because the early geological environments of the two planets were, in many respects, similar and, ...thus, the potential habitats for early life forms were similar. However, the identification and interpretation of biosignatures in ancient terrestrial rocks has proven contentious over the last few years. Recently, new investigations using very detailed field studies combined with highly sophisticated analytical techniques have begun to document a large range of biosignatures in Early Archaean rocks. Early life on Earth was diversified, widespread and relatively evolved, but its traces are generally, but not always, small and subtle. In this contribution I use a few examples of morphological biosignatures from the Early-Mid Archaean to demonstrate their variety in terms of size and type: macroscopic stromatolites from the 3.443 Ga Strelley Pool Chert, Pilbara; a meso-microscopic microbial mat from the 3.333 Ga Josefsdal Chert, Barberton; microscopic microbial colonies and a biofilm from the 3.446 Ga Kitty’s Gap Chert, Pilbara; and microscopic microbial corrosion pits in the glassy rinds of 3.22–3.48 Ga pillow lavas from Barberton. Some macroscopic and microscopic structures may be identifiable in an
in situ
robotic mission to Mars and
in situ
methods of organic molecule detection may be able to reveal organic traces of life. However, it is concluded that it will probably be necessary to return suitably chosen Martian rocks to Earth for the reliable identification of signs of life, since multiple observational and analytical methods will be necessary, especially if Martian life is significantly different from terrestrial life.
The Middle Marker – horizon H1 of the Hooggenoeg Formation – is the oldest sedimentary horizon in the Barberton greenstone belt and one of the oldest sedimentary horizons on Earth. Herein, we ...describe a range of carbonaceous microstructures in this unit which bear resemblance to phototrophic microbial biofilms, biosedimentary structures, and interpreted microfossils in contemporaneous greenstone belts from the Early Archaean. Post-depositional iron-rich fluid cycling through these sediments has resulted in the precipitation of pseudo-laminated structures, which also bear resemblance, at the micron-scale, to certain microbial mat-like structures, although are certainly abiogenic. Poor preservation of multiple putative microbial horizons due to coarse volcaniclastic sedimentation and synsedimentary fragmentation by hydrothermal fluid also makes a conclusive assessment of biogenicity challenging. Nonetheless, several laminated morphologies within volcaniclastic sandstones and siltstones and coarse-grained volcaniclastic sandstones are recognisable as syngenetic photosynthetic microbial biofilms and microbially induced sedimentary structures; therefore, the Middle Marker preserves the oldest evidence for life in the Barberton greenstone belt. Among these biosignatures are fine, crinkly, micro-tufted, laminated microbial mats, pseudo-tufted laminations and wisp-like carbonaceous fragments interpreted as either partially formed biofilms or their erosional products. In the same sediments, lenticular objects, which have previously been interpreted as bona fide microfossils, are rare but recurrent finds whose biogenicity we question. The Middle Marker preserves an ancient record of epibenthic microbial communities flourishing, struggling and perishing in parallel with a waning volcanic cycle, an environment upon which they depended and through which they endured. Direct comparisons can be made between environment-level characters of the Middle Marker and other Early Archaean cherts, suggesting that shallow-water, platformal, volcanogenic-hydrothermal biocoenoses were major microbial habitats throughout the Archaean.
The search for traces of life is one of the principal objectives of Mars exploration. Central to this objective is the concept of habitability, the set of conditions that allows the appearance of ...life and successful establishment of microorganisms in any one location. While environmental conditions may have been conducive to the appearance of life early in martian history, habitable conditions were always heterogeneous on a spatial scale and in a geological time frame. This "punctuated" scenario of habitability would have had important consequences for the evolution of martian life, as well as for the presence and preservation of traces of life at a specific landing site. We hypothesize that, given the lack of long-term, continuous habitability, if martian life developed, it was (and may still be) chemotrophic and anaerobic. Obtaining nutrition from the same kinds of sources as early terrestrial chemotrophic life and living in the same kinds of environments, the fossilized traces of the latter serve as useful proxies for understanding the potential distribution of martian chemotrophs and their fossilized traces. Thus, comparison with analog, anaerobic, volcanic terrestrial environments (Early Archean >3.5-3.33 Ga) shows that the fossil remains of chemotrophs in such environments were common, although sparsely distributed, except in the vicinity of hydrothermal activity where nutrients were readily available. Moreover, the traces of these kinds of microorganisms can be well preserved, provided that they are rapidly mineralized and that the sediments in which they occur are rapidly cemented. We evaluate the biogenicity of these signatures by comparing them to possible abiotic features. Finally, we discuss the implications of different scenarios for life on Mars for detection by in situ exploration, ranging from its non-appearance, through preserved traces of life, to the presence of living microorganisms.
Mars-Early Earth-Anaerobic chemotrophs-Biosignatures-Astrobiology missions to Mars.
Palaeontology is an essential tool for tracing the history of life in the geological record. However, access to the origin oflife is blocked because of the lack of preservation of suitable rocks ...dating from the fi rst billion years of Earth’s history. Nevertheless, studyof Early Archaean rocks (~4-3.3 Ga) indicates that the environmental conditions of the early Earth, upon which life emerged, were verydifferent to those of today and provides essential information for guiding investigations into the origin of life in terms of realistic environmental scenarios and possible timing of the appearance of life. Microbial palaeontology investigations of well-preserved, Early Archaean rocks ~3.5to 3.3 Ga show that the earliest preserved life was diverse and widespread and suggest that it probably appeared in the Hadean, as soon asthe Earth’s surface was habitable. The extreme, anaerobic conditions characterising the early Earth, together with the ingredients of life,i.e. carbon molecules, liquid water and energy, were common on other planets and satellites in the early Solar System. Considering carbonand water-based life forms to be a cosmically frequent phenomenon, it is hypothesised that life could have emerged on some of these bodiesand that traces of its appearance may still be preserved, for instance on Mars, Europa or Enceladus. Microbial palaeontology as well asinformation gleaned from extant extremophiles and experimental data provides us with essential information about what kinds of extant orfossilised life forms to look for on another planet or satellite. Moreover, the methods evolved to study and understand the remains of fossiltraces of primitive microbial life will aid the search for life and its origins on Mars or other satellites. The perspective of returning to Earthrocks from Mars (or other samples from Europa or Enceladus?) containing potential traces of extraterrestrial life, most likely primitiveanaerobic chemotrophs, will be a challenge for microbial palaeontology that we need to start addressing now. Most importantly, it will openup the possibility of establishing the universality of life.
Modern biological dependency on trace elements is proposed to be a consequence of their enrichment in the habitats of early life together with Earth's evolving physicochemical conditions; the ...resulting metallic biological complement is termed the metallome. Herein, we detail a protocol for describing metallomes in deep time, with applications to the earliest fossil record. Our approach extends the metallome record by more than 3 Ga and provides a novel, non-destructive method of estimating biogenicity in the absence of cellular preservation. Using microbeam particle-induced X-ray emission (µPIXE), we spatially quantify transition metals and metalloids within organic material from 3.33 billion-year-old cherts of the Barberton greenstone belt, and demonstrate that elements key to anaerobic prokaryotic molecular nanomachines, including Fe, V, Ni, As and Co, are enriched within carbonaceous material. Moreover, Mo and Zn, likely incorporated into enzymes only after the Great Oxygenation Event, are either absent or present at concentrations below the limit of detection of µPIXE, suggesting minor biological utilisation in this environmental setting. Scanning and transmission electron microscopy demonstrates that metal enrichments do not arise from accumulation in nanomineral phases and thus unambiguously reflect the primary composition of the carbonaceous material. This carbonaceous material also has δ
C between -41.3‰ and 0.03‰, dominantly -21.0‰ to -11.5‰, consistent with biological fractionation and mostly within a restricted range inconsistent with abiotic processes. Considering spatially quantified trace metal enrichments and negative δ
C fractionations together, we propose that, although lacking cellular preservation, this organic material has biological origins and, moreover, that its precursor metabolism may be estimated from the fossilised "palaeo-metallome". Enriched Fe, V, Ni and Co, together with petrographic context, suggests that this kerogen reflects the remnants of a lithotrophic or organotrophic consortium cycling methane or nitrogen. Palaeo-metallome compositions could be used to deduce the metabolic networks of Earth's earliest ecosystems and, potentially, as a biosignature for evaluating the origin of preserved organic materials found on Mars.
Scientists use the Earth as a tool for astrobiology by analyzing planetary field analogues (i.e. terrestrial samples and field sites that resemble planetary bodies in our Solar System). In addition, ...they expose the selected planetary field analogues in simulation chambers to conditions that mimic the ones of planets, moons and Low Earth Orbit (LEO) space conditions, as well as the chemistry occurring in interstellar and cometary ices. This paper reviews the ways the Earth is used by astrobiologists: (i) by conducting planetary field analogue studies to investigate extant life from extreme environments, its metabolisms, adaptation strategies and modern biosignatures; (ii) by conducting planetary field analogue studies to investigate extinct life from the oldest rocks on our planet and its biosignatures; (iii) by exposing terrestrial samples to simulated space or planetary environments and producing a sample analogue to investigate changes in minerals, biosignatures and microorganisms. The European Space Agency (ESA) created a topical team in 2011 to investigate recent activities using the Earth as a tool for astrobiology and to formulate recommendations and scientific needs to improve ground-based astrobiological research. Space is an important tool for astrobiology (see Horneck et al. in Astrobiology, 16:201–243,
2016
; Cottin et al.,
2017
), but access to space is limited. Complementing research on Earth provides fast access, more replications and higher sample throughput. The major conclusions of the topical team and suggestions for the future include more scientifically qualified calls for field campaigns with planetary analogy, and a centralized point of contact at ESA or the EU for the organization of a survey of such expeditions. An improvement of the coordinated logistics, infrastructures and funding system supporting the combination of field work with planetary simulation investigations, as well as an optimization of the scientific return and data processing, data storage and data distribution is also needed. Finally, a coordinated EU or ESA education and outreach program would improve the participation of the public in the astrobiological activities.
•Extraterrestrial organic matter is detected by EPR in 3.33 Ga sediments.•It is associated with Ni-Cr-rich ferrite “cosmic” spinel nanoparticles.•A challenge for the research for organic traces of ...extinct life in Mars.
Electron paramagnetic resonance (EPR) analysis of carbonaceous, volcanic, tidal sediments from the 3.33 Ga-old Josefsdal Chert (Kromberg Formation, Barberton Greenstone Belt), documents the presence of two types of insoluble organic matter (IOM): (1) IOM similar to that previously found in Archean cherts from numerous other sedimentary rocks in the world and of purported biogenic origin; (2) anomalous IOM localized in a 2 mm-thick sedimentary horizon. Detailed analysis by continuous-wave-EPR and pulse-EPR reveals that IOM in this layer is similar to the insoluble component of the hydrogenated organic matter in carbonaceous chondrites, suggesting that this narrow sedimentary horizon has preserved organic matter of extraterrestrial origin. This conclusion is supported by the presence in this thin layer of another anomalous EPR signal at g = 3 attributed to Ni-Cr-Al ferrite spinel nanoparticles, which are known to form during atmospheric entry of cosmic objects. From this EPR analysis, it was deduced that the anomalous sedimentary layer originates from deposition, in a nearshore environment, of a cloud of tiny dust particles originating from a flux of micrometeorites falling through the oxygen-poor Archean atmosphere.
The ExoMars 2020 mission will characterise a Martian locality with potential former habitability – Oxia Planum – and attempt to identify preserved morphological and chemical biosignatures. The ...payload will include a drill retrieving cores from the subsurface (up to 2 m depth), which will be imaged at high resolution by two instruments: the Panoramic Camera High Resolution Camera (PanCam HRC) and CLose UP Imager (CLUPI). These instruments will provide guiding interpretation and govern the approach used by the analytical instruments, which will conduct their analyses after crushing of the core sample. Blind tests using Mars-analogue lithological samples provide valuable mission training in terms of maximising scientific return. Previous blind tests evaluating the abilities of the ExoMars 2020 payload to conduct geological approaches used solid rock hand samples as test specimens. Here, we prepared test samples of ExoMars mission-equivalent shapes and dimensions (3 × 1 cm cores) to evaluate the extent of geological interpretation possible using only the images of the core that will obtained during the mission. In the worst-case scenario, core samples will be different from outcrop rock and these images and associated organogeochemical analyses on crushed powders by the other rover instruments will be the sole basis for their interpretation. Imaging these samples using mission-equivalent original resolutions to avoid image processing artefacts, we found that the difficulty inherent in making definitive geological conclusions using traditional ‘field petrography’ approaches is increased when only limited amounts of core sample are available for observation. Challenges involved in the interpretation of core samples include representativeness problems inherent to small samples, distinction between layer-specific characteristics at the sub-centimetre to centimetre scale, and mechanical effects, such as drill marks and dust covering. These issues vary depending upon the rock type and change as a function of the mechanical properties (and thus composition) of the sample. Despite these challenges, we find that CLUPI and PanCam HRC images alone allow many accurate and detailed geological observations; however, confidence and detail of interpretation are notably increased when additional geochemical data are provided, in this case, Raman spectra reflecting the contribution of the Raman Laser Spectrometer (RLS) instrument. Our results argue for the importance of detailed core imaging during the experimental phase of the rover mission despite the challenges involved in interpretation, since HRC and CLUPI images offer a degree of synergy. Inter-instrumental collaboration will be essential during the ExoMars 2020 rover mission (and indeed in any rover mission), since no single payload instrument is able to perform a comprehensive assessment of a putative biosignature within its geological context, and since the instrument suite provides highly complementary data at multiple scales that are key to maximising scientific return.
•We outline a scientific workflow for the ExoMars 2020 mission and argue for the importance of core imaging using both the PanCam (HRC) and CLUPI instruments prior to crushing of the core samples.•Challenges in interpreting core samples include correlation with surface outcrops, as well as misinterpretation resulting from image resolution constraints, features that are over-represented in cores relative to the bulk rock, and drilling effects.•Despite these challenges, accurate lithological identifications can usually be made from CLUPI and HRC images of the core; such identifications can guide the interpretation of analytical data.•Accuracy of interpretation is increased when both imaging and compositional data are available; thus, inter-instrumental collaboration will be key to maximise scientific return from the subsurface core samples that are the unique focus of the ExoMars 2020 mission.
PDF
