Carbonaceous Chondrite Meteorites: the Chronicle of a Potential Evolutionary Path between Stars and Life

High-spatial resolution functional chemistry of nitrogen compounds in the observed UK meteorite fall Winchcombe

Organic matter in extraterrestrial samples is a complex material that might have played an important role in the delivery of prebiotic molecules to the early Earth. We report here on the identification of nitrogen-containing compounds such as amino acids and N-heterocycles within the recent observed meteorite fall Winchcombe by high-spatial resolution spectroscopy techniques. Although nitrogen contents of Winchcombe organic matter are low (N/C ~ 1-3%), we were able to detect the presence of these compounds using a low-noise direct electron detector. These biologically relevant molecules have therefore been tentatively found within a fresh, minimally processed meteorite sample by high spatial resolution techniques conserving the overall petrographic context. Carbon functional chemistry investigations show that sizes of aromatic domains are small and that abundances of carboxylic functional groups are low. Our observations demonstrate that Winchcombe represents an important addition to the collection of carbonaceous chondrites and still preserves pristine extraterrestrial organic matter.

Quantifying Global Origin-Diagnostic Features and Patterns in Biotic and Abiotic Acyclic Lipids for Life Detection

Lipids are a geologically robust class of organics ubiquitous to life as we know it. Lipid-like soluble organics are synthesized abiotically and have been identified in carbonaceous meteorites and on Mars. Ascertaining the origin of lipids on Mars would be a profound astrobiological achievement. We enumerate origin-diagnostic features and patterns in two acyclic lipid classes, fatty acids (i.e., carboxylic acids) and acyclic hydrocarbons, by collecting and analyzing molecular data reported in over 1500 samples from previously published studies of terrestrial and meteoritic organics. We identify 27 combined (15 for fatty acids, 12 for acyclic hydrocarbons) molecular patterns and structural features that can aid in distinguishing biotic from abiotic synthesis. Principal component analysis (PCA) demonstrates that multivariate analyses of molecular features (16 for fatty acids, 14 for acyclic hydrocarbons) can potentially indicate sample origin. Terrestrial lipids are dominated by longer straight-chain molecules (C4-C34 fatty acids, C14-C46 acyclic hydrocarbons), with predominance for specific branched and unsaturated isomers. Lipid-like meteoritic soluble organics are shorter, with random configurations. Organic solvent-extraction techniques are most commonly reported, motivating the design of our novel instrument, the Extractor for Chemical Analysis of Lipid Biomarkers in Regolith (ExCALiBR), which extracts lipids while preserving origin-diagnostic features that can indicate biogenicity.

Setting the geological scene for the origin of life and continuing open questions about its emergence

The origin of life is one of the most fundamental questions of humanity. It has been and is still being addressed by a wide range of researchers from different fields, with different approaches and ideas as to how it came about. What is still incomplete is constrained information about the environment and the conditions reigning on the Hadean Earth, particularly on the inorganic ingredients available, and the stability and longevity of the various environments suggested as locations for the emergence of life, as well as on the kinetics and rates of the prebiotic steps leading to life. This contribution reviews our current understanding of the geological scene in which life originated on Earth, zooming in specifically on details regarding the environments and timescales available for prebiotic reactions, with the aim of providing experimenters with more specific constraints. Having set the scene, we evoke the still open questions about the origin of life: did life start organically or in mineralogical form? If organically, what was the origin of the organic constituents of life? What came first, metabolism or replication? What was the time-scale for the emergence of life? We conclude that the way forward for prebiotic chemistry is an approach merging geology and chemistry, i.e., far-from-equilibrium, wet-dry cycling (either subaerial exposure or dehydration through chelation to mineral surfaces) of organic reactions occurring repeatedly and iteratively at mineral surfaces under hydrothermal-like conditions.

Gas-phase synthesis of racemic helicenes and their potential role in the enantiomeric enrichment of sugars and amino acids in meteorites

The molecular origins of homochirality on Earth is not understood well, particularly how enantiomerically enriched molecules of astrobiological significance like sugars and amino acids might have been synthesized on icy grains in space preceding their delivery to Earth. Polycyclic aromatic hydrocarbons (PAHs) identified in carbonaceous chondrites could have been processed in molecular clouds by circularly polarized light prior to the depletion of enantiomerically enriched helicenes onto carbonaceous grains resulting in chiral islands. However, the fundamental low temperature reaction mechanisms leading to racemic helicenes are still unknown. Here, by exploiting synchrotron based molecular beam photoionization mass spectrometry combined with electronic structure calculations, we provide compelling testimony on barrierless, low temperature pathways leading to racemates of [5] and [6]helicene. Astrochemical modeling advocates that gas-phase reactions in molecular clouds lead to racemates of helicenes suggesting a pathway for future astronomical observation and providing a fundamental understanding for the origin of homochirality on early Earth.

An open source computational workflow for the discovery of autocatalytic networks in abiotic reactions

A central question in origins of life research is how non-entailed chemical processes, which simply dissipate chemical energy because they can do so due to immediate reaction kinetics and thermodynamics, enabled the origin of highly-entailed ones, in which concatenated kinetically and thermodynamically favorable processes enhanced some processes over others. Some degree of molecular complexity likely had to be supplied by environmental processes to produce entailed self-replicating processes. The origin of entailment, therefore, must connect to fundamental chemistry that builds molecular complexity. We present here an open-source chemoinformatic workflow to model abiological chemistry to discover such entailment. This pipeline automates generation of chemical reaction networks and their analysis to discover novel compounds and autocatalytic processes. We demonstrate this pipeline's capabilities against a well-studied model system by vetting it against experimental data. This workflow can enable rapid identification of products of complex chemistries and their underlying synthetic relationships to help identify autocatalysis, and potentially self-organization, in such systems. The algorithms used in this study are open-source and reconfigurable by other user-developed workflows.

Synthesis of Phospholipids Under Plausible Prebiotic Conditions and Analogies with Phospholipid Biochemistry for Origin of Life Studies

Phospholipids are essential components of biological membranes and are involved in cell signalization, in several enzymatic reactions, and in energy metabolism. In addition, phospholipids represent an evolutionary and non-negligible step in life emergence. Progress in the past decades has led to a deeper understanding of these unique hydrophobic molecules and their most pertinent functions in cell biology. Today, a growing interest in "prebiotic lipidomics" calls for a new assessment of these relevant biomolecules.

Integrated Molar Absorptivity of Mid- and Far-Infrared Spectra of Alanine and a Selection of Other Five Amino Acids of Astrobiological Relevance

Alanine and other five proteinogeninc amino acids produced quite easily in exogenous and/or endogenous prebiotic processes, that is, valine, serine, proline, glutamic acid, and aspartic acid (Ala, Val, Ser, Pro, Glu, and Asp, respectively) were studied in the mid- and far-infrared spectral range. This work is an extension of the previous one where other proteinogenic amino acids glycine, isoleucine, phenylalanine, tyrosine, and tryptophan (Gly, Ile, Phe, Tyr, and Trp, respectively) were studied in the mid-infrared and in the far-infrared with the purpose to facilitate the search and identification of these astrobiological and astrochemical relevant molecules in space environments. The molar extinction coefficients (ɛ) of all mid- and far-infrared bands were determined as well as the integrated molar absorptivities (ψ). The mid-infrared spectra of Ala, Val, Ser, Pro, Glu, and Asp were recorded also at three different temperatures from -180°C to nearly ambient temperature and at 200°C. With the reported values of ɛ and ψ, it will be possible to estimate the relative abundance of these molecules in space environments.

Possible Ribose Synthesis in Carbonaceous Planetesimals

The origin of life might be sparked by the polymerization of the first RNA molecules in Darwinian ponds during wet-dry cycles. The key life-building block ribose was found in carbonaceous chondrites. Its exogenous delivery onto the Hadean Earth could be a crucial step toward the emergence of the RNA world. Here, we investigate the formation of ribose through a simplified version of the formose reaction inside carbonaceous chondrite parent bodies. Following up on our previous studies regarding nucleobases with the same coupled physico-chemical model, we calculate the abundance of ribose within planetesimals of different sizes and heating histories. We perform laboratory experiments using catalysts present in carbonaceous chondrites to infer the yield of ribose among all pentoses (5Cs) forming during the formose reaction. These laboratory yields are used to tune our theoretical model that can only predict the total abundance of 5Cs. We found that the calculated abundances of ribose were similar to the ones measured in carbonaceous chondrites. We discuss the possibilities of chemical decomposition and preservation of ribose and derived constraints on time and location in planetesimals. In conclusion, the aqueous formose reaction might produce most of the ribose in carbonaceous chondrites. Together with our previous studies on nucleobases, we found that life-building blocks of the RNA world could be synthesized inside parent bodies and later delivered onto the early Earth.

Clays and the Origin of Life: The Experiments

There are three groups of scientists dominating the search for the origin of life: the organic chemists (the Soup), the molecular biologists (RNA world), and the inorganic chemists (metabolism and transient-state metal ions), all of which have experimental adjuncts. It is time for Clays and the Origin of Life to have its experimental adjunct. The clay data coming from Mars and carbonaceous chondrites have necessitated a review of the role that clays played in the origin of life on Earth. The data from Mars have suggested that Fe-clays such as nontronite, ferrous saponites, and several other clays were formed on early Mars when it had sufficient water. This raised the question of the possible role that these clays may have played in the origin of life on Mars. This has put clays front and center in the studies on the origin of life not only on Mars but also here on Earth. One of the major questions is: What was the catalytic role of Fe-clays in the origin and development of metabolism here on Earth? First, there is the recent finding of a chiral amino acid (isovaline) that formed on the surface of a clay mineral on several carbonaceous chondrites. This points to the formation of amino acids on the surface of clay minerals on carbonaceous chondrites from simpler molecules, e.g., CO2, NH3, and HCN. Additionally, there is the catalytic role of small organic molecules, such as dicarboxylic acids and amino acids found on carbonaceous chondrites, in the formation of Fe-clays themselves. Amino acids and nucleotides adsorb on clay surfaces on Earth and subsequently polymerize. All of these observations and more must be subjected to strict experimental analysis. This review provides an overview of what has happened and is now happening in the experimental clay world related to the origin of life. The emphasis is on smectite-group clay minerals, such as montmorillonite and nontronite.

Chirality and the Origin of Life

The origin of life, based on the homochirality of biomolecules, is a persistent mystery. Did life begin by using both forms of chirality, and then one of the forms disappeared? Or did the choice of homochirality precede the formation of biomolecules that could ensure replication and information transfer? Is the natural choice of L-amino acids and D-sugars on which life is based deterministic or random? Is the handedness present in/of the Universe from its beginning? The whole biosystem on the Earth, all living creatures are chiral. Many theories try to explain the origin of life and chirality on the Earth: e.g., the panspermia hypothesis, the primordial soup hypothesis, theory of parity violation in weak interactions. Additionally, heavy neutrinos and the impact of the fact that only left-handed particles decay, and even dark matter, all have to be considered.

Integrated Molar Absorptivity of Mid- and Far-Infrared Spectra of Glycine and Other Selected Amino Acids

A selection of five proteinogenic amino acids-glycine, isoleucine, phenylalanine, tyrosine, and tryptophan-were studied in the mid-infrared and in the far-infrared with the purpose to facilitate the search and identification of these astrobiologically and astrochemically relevant molecules in space environments. The molar extinction coefficients (ɛ) of all mid- and far-infrared bands were determined as well as the integrated molar absorptivities (ψ). The mid-infrared spectra of the five selected amino acids were recorded also at three different temperatures from -180°C to ambient temperature to +200°C. We measured the wavelength shift of the infrared bands caused by temperature; and for the most relevant or temperature-sensitive infrared bands, a series of linear equations were determined relating wavelength position with temperature. Such equations may provide estimates of the temperature of these molecules once detected in astrophysical objects; and with the reported values of ɛ and ψ, it will be possible to estimate the relative abundance of these molecules in space environments.

Growth on Carbohydrates from Carbonaceous Meteorites Alters the Immunogenicity of Environment-Derived Bacterial Pathogens

The last decade has witnessed a renewed interest in space exploration. Public and private institutions are investing considerable effort toward the direct exploration of the Moon and Mars, as well as more distant bodies in the solar system. Both automated and human-crewed spacecraft are being considered in these efforts. As inevitable fellow travelers on the bodies of astronauts, spaceships, or equipment, terrestrial microorganisms will undoubtedly come into contact with extraterrestrial environments, despite stringent decontamination. These microorganisms could eventually adapt and grow in their new habitats, where they might potentially recolonize and lead to the infection of the human space travelers. In this article, we demonstrate that clinically relevant bacterial species found in the environment are able to grow in minimal media with sugar compounds identified in extraterrestrial carbon sources. As a surrogate model, we used carbohydrates previously isolated from carbonaceous meteorites. The bacteria underwent an adaptation process that caused structural modifications in the cell envelope that sparked changes in pathogenic potential, both in vitro and in vivo. Understanding the adaptation of microorganisms exposed to extraterrestrial environments, with subsequent changes in their immunogenicity and virulence, requires a comprehensive analysis of such scenarios to ensure the safety of major space expeditions in the decades to come.

A Weakened Immune Response to Synthetic Exo-Peptides Predicts a Potential Biosecurity Risk in the Retrieval of Exo-Microorganisms

Simple Summary

We tested the immune response of T cells of the mammalian immune system towards protein antigens that includes the unusual amino acids isovaline and α-aminoisobutyric. Those amino acids have been found in high abundance on carbonaceous meteorites but are extremely rare in proteomes of earth organisms. We hypothesised that proteins of non-terrestrial alien life forms might contain such amino acids and tested whether chemically synthesised “exopeptides” that contain these amino acids could be detected by the immune system. Our assays, based on the responses of CD8⁺ T cells to these exopeptides, indicated that antigen cleavage, processing, and subsequent T cell activation still occurred, but were less efficient than the response to control peptides that lacked these amino acids. We therefore speculate that the encounter of putative exo-microorganisms of an unusual antigenic repertoire might pose an immunological risk for space missions aiming to retrieve potentially biotic samples from exoplanets and moons.

Abstract

The discovery of liquid water at several locations in the solar system raises the possibility that microbial life may have evolved outside Earth and as such could be accidently introduced into the Earth’s ecosystem. Unusual sugars or amino acids, like non-proteinogenic isovaline and α-aminoisobutyric acid that are vanishingly rare or absent from life forms on Earth, have been found in high abundance on non-terrestrial carbonaceous meteorites. It is therefore conceivable that exo-microorganisms might contain proteins that include these rare amino acids. We therefore asked whether the mammalian immune system would be able to recognize and induce appropriate immune responses to putative proteinaceous antigens that include these rare amino acids. To address this, we synthesised peptide antigens based on a backbone of ovalbumin and introduced isovaline and α-aminoisobutyric acid residues and demonstrated that these peptides can promote naïve OT-I cell activation and proliferation, but did so less efficiently than the canonical peptides. This is relevant to the biosecurity of missions that may retrieve samples from exoplanets and moons that have conditions that may be permissive for life, suggesting that accidental contamination and exposure to exo-microorganisms with such distinct proteomes might pose an immunological challenge.

Immune recognition of putative alien microbial structures: Host-pathogen interactions in the age of space travel

Human space travel is on the verge of visiting Mars and, in the future, even more distant places in the solar system. These journeys will be also made by terrestrial microorganisms (hitchhiking on the bodies of astronauts or on scientific instruments) that, upon arrival, will come into contact with new planetary environments, despite the best measures to prevent contamination. These microorganisms could potentially adapt and grow in the new environments and subsequently recolonize and infect astronauts. An even more challenging situation would be if truly alien microorganisms will be present on these solar system bodies: What will be their pathogenic potential, and how would our immune host defenses react? It will be crucial to anticipate these situations and investigate how the immune system of humans might cope with modified terrestrial or alien microbes. We propose several scenarios that may be encountered and how to respond to these challenges.

Enceladus: First Observed Primordial Soup Could Arbitrate Origin-of-Life Debate

A recent breakthrough publication has reported complex organic molecules in the plumes emanating from the subglacial water ocean of Saturn's moon Enceladus (Postberg et al., 2018, Nature 558:564-568). Based on detailed chemical scrutiny, the authors invoke primordial or endogenously synthesized carbon-rich monomers (<200 u) and polymers (up to 8000 u). This appears to represent the first reported extraterrestrial organics-rich water body, a conceivable milieu for early steps in life's origin ("prebiotic soup"). One may ask which origin-of-life scenario appears more consistent with the reported molecular configurations on Enceladus. The observed monomeric organics are carbon-rich unsaturated molecules, vastly different from present-day metabolites, amino acids, and nucleotide bases, but quite chemically akin to simple lipids. The organic polymers are proposed to resemble terrestrial insoluble kerogens and humic substances, as well as refractory organic macromolecules found in carbonaceous chondritic meteorites. The authors posit that such polymers, upon long-term hydrous interactions, might break down to micelle-forming amphiphiles. In support of this, published detailed analyses of the Murchison chondrite are dominated by an immense diversity of likely amphiphilic monomers. Our specific quantitative model for compositionally reproducing lipid micelles is amphiphile-based and benefits from a pronounced organic diversity. It thus contrasts with other origin models, which require the presence of very specific building blocks and are expected to be hindered by excess of irrelevant compounds. Thus, the Enceladus finds support the possibility of a pre-RNA Lipid World scenario for life's origin.

Multiwall and bamboo-like carbon nanotubes from the Allende chondrite: A probable source of asymmetry

This study presents multiwall and bamboo-like carbon nanotubes found in samples from the Allende carbonaceous chondrite using high-resolution transmission electron microscopy (HRTEM). A highly disordered lattice observed in this material suggests the presence of chiral domains in it. Our results also show amorphous and poorly-graphitized carbon, nanodiamonds, and onion-like fullerenes. The presence of multiwall and bamboo-like carbon nanotubes have important implications for hypotheses that explain how a probable source of asymmetry in carbonaceous chondrites might have contributed to the enantiomeric excess in soluble organics under extraterrestrial scenarios. This is the first study proving the existence of carbon nanotubes in carbonaceous chondrites.

Formation of Cyanamide-Glyoxal Oligomers in Aqueous Environments Relevant to Primeval and Astrochemical Scenarios: A Spectroscopic and Theoretical Study

The condensation of cyanamide and glyoxal, two well-known prebiotic monomers, in an aqueous phase has been investigated in great detail, demonstrating the formation of oligomeric species of varied structure, though consistent with generalizable patterns. This chemistry involving structurally simple substances also illustrates the possibility of building molecular complexity under prebiotically plausible conditions, not only on Earth, but also in extraterrestrial scenarios. We show that cyanamide-glyoxal reactions in water lead to mixtures comprising both acyclic and cyclic fragments, largely based on fused five- and six-membered rings, which can be predicted by computation. Remarkably, such a mixture could be identified using high-resolution electrospray ionization (ESI) mass spectrometry and spectroscopic methods. A few mechanistic pathways can be postulated, most involving the intermediacy of glyoxal cyanoimine and further chain growth, thus increasing the diversity of the observed products. This rationale is supported by theoretical analyses with clear-cut identification of all of the stationary points and transition-state structures. The properties and structural differences of oligomers obtained under thermodynamic conditions in water as opposed to those isolated by precipitation from organic media are also discussed.

A Statistical Approach to Illustrate the Challenge of Astrobiology for Public Outreach

In this study, we attempt to illustrate the competition that constitutes the main challenge of astrobiology, namely the competition between the probability of extraterrestrial life and its detectability. To illustrate this fact, we propose a simple statistical approach based on our knowledge of the Universe and the Milky Way, the Solar System, and the evolution of life on Earth permitting us to obtain the order of magnitude of the distance between Earth and bodies inhabited by more or less evolved past or present life forms, and the consequences of this probability for the detection of associated biosignatures. We thus show that the probability of the existence of evolved extraterrestrial forms of life increases with distance from the Earth while, at the same time, the number of detectable biosignatures decreases due to technical and physical limitations. This approach allows us to easily explain to the general public why it is very improbable to detect a signal of extraterrestrial intelligence while it is justified to launch space probes dedicated to the search for microbial life in the Solar System.