Organic matter in carbonaceous chondrites

Near-infrared absorption and radiative cooling of naphthalene dimers (C10H8)2

The radiative cooling of naphthalene dimer cations, (C10H8)2+ was studied experimentally through action spectroscopy using two different electrostatic ion-beam storage rings, DESIREE in Stockholm and Mini-Ring in Lyon. The spectral characteristics of the charge resonance (CR) band were observed to vary significantly with a storage time of up to 30 seconds in DESIREE. In particular, the position of the CR band shifts to the blue, with specific times (inverse of rates) of 0.64 s and 8.0 s in the 0-5 s and 5-30 s storage time ranges, respectively. These long-time scales indicate that the internal energy distribution of the stored ions evolves by vibrational radiative cooling, which is consistent with the absence of fast radiative cooling via recurrent fluorescence for (C10H8)2+. Density functional based tight binding calculations with local excitations and configuration interactions (DFTB-EXCI) were used to simulate the absorption spectrum for ion temperatures between 10 and 500 K. The evolution of the bandwidth and position with temperature is in qualitative agreement with the experimental findings. Furthermore, these calculations yielded linear temperature dependencies for both the shift and the broadening. Combining the relationship between the CR band position and the ion temperature with the results of the statistical model, we demonstrate that the observed blue shift can be used to determine the radiative cooling rate of (C10H8)2+.

Quantifying the Potential for Nitrate-Dependent Iron Oxidation on Early Mars: Implications for the Interpretation of Gale Crater Organics

Geological evidence and atmospheric and climate models suggest habitable conditions occurred on early Mars, including in a lake in Gale crater. Instruments aboard the Curiosity rover measured organic compounds of unknown provenance in sedimentary mudstones at Gale crater. Additionally, Curiosity measured nitrates in Gale crater sediments, which suggests that nitrate-dependent Fe2+ oxidation (NDFO) may have been a viable metabolism for putative martian life. Here, we perform the first quantitative assessment of an NDFO community that could have existed in an ancient Gale crater lake and quantify the long-term preservation of biological necromass in lakebed mudstones. We find that an NDFO community would have the capacity to produce cell concentrations of up to 106 cells mL-1, which is comparable to microbes in Earth's oceans. However, only a concentration of <104 cells mL-1, due to organisms that inefficiently consume less than 10% of precipitating nitrate, would be consistent with the abundance of organics found at Gale. We also find that meteoritic sources of organics would likely be insufficient as a sole source for the Gale crater organics, which would require a separate source, such as abiotic hydrothermal or atmospheric production or possibly biological production from a slowly turning over chemotrophic community.

Deuterium Isotope Fractionation of Polycyclic Aromatic Hydrocarbons in Meteorites as an Indicator of Interstellar/Protosolar Processing History

The stable isotope composition of soluble and insoluble organic compounds in carbonaceous chondrites can be used to determine the provenance of organic molecules in space. Deuterium enrichment in meteoritic organics could be a residual signal of synthetic reactions occurring in the cold interstellar medium or an indicator of hydrothermal parent-body reactions. δD values have been measured in grains and bulk samples for a wide range of meteorites; however, these reservoirs are highly variable and may have experienced fractionation during thermal and/or aqueous alteration. Among the plethora of organic compounds in meteorites are polycyclic aromatic hydrocarbons (PAHs), which are stable and abundant in carbonaceous chondrites, and their δD ratio may preserve evidence about their formation environment as well as the influence of parent-body processes. This study tests hypotheses about the potential links between PAHs-deuteration concentrations and their formation conditions by examining the δD ratio of PAHs in three CM carbonaceous chondrites representing an aqueous alteration gradient. We use deuterium enrichments in soluble 2–5-ring PAHs as an indicator of either photon-driven deuteration due to unimolecular photodissociation in warm regions of space, gas-phase ion–molecule reactions in cold interstellar regions of space, or UV photolysis in ices. We also test hypothesized reaction pathways during parent-body processing that differ between partially and fully aromatized PAHs. New methodological approaches were developed to extract small, volatile PAHs without fractionation. Our results suggest that meteoritic PAHs could have formed through reactions in cold regions, with possible overprinting of deuterium enrichment during aqueous parent-body alteration, but the data could not rule out PAH alteration in icy mantles as well.

Molecular dynamics reveals formation path of benzonitrile and other molecules in conditions relevant to the interstellar medium

Polycyclic aromatic hydrocarbons and polycyclic aromatic nitrogen heterocycles are believed to be widespread in different areas of the interstellar medium. However, the astronomical detection of specific aromatic molecules is extremely challenging. As a result, only a few aromatic molecules have been identified, and very little is known about how they are formed in different areas of the interstellar medium. Recently, McGuire et al. [Science 359, 202-205 (2018)] detected the simple aromatic molecule benzonitrile in Taurus Molecular Cloud-1. Although benzonitrile has been observed, the molecular mechanism for its formation is still unknown. In this study, we use quantum chemistry and ab initio molecular dynamics to model ionization processes of van der Waals clusters containing cyanoacetylene and acetylene molecules. We demonstrate computationally that the clusters' ionization leads to molecular formation. For pure cyanoacetylene clusters, we observe bond formation among two and three monomer units, whereas in mixed clusters, bond formation can also occur in up to four units. We show that the large amount of energy available to the system after ionization can lead to barrier crossing and the formation of complex molecules. Our study reveals the rich chemistry that is observed upon ionization of the clusters, with a wide variety of molecules being formed. Benzonitrile is among the observed molecules, and we study the potential energy path for its formation. These results also offer insights that can guide astronomers in their search for aromatic molecules in the interstellar medium.

Mechanisms of the Formation of Adenine, Guanine, and Their Analogues in UV-Irradiated Mixed NH3:H2O Molecular Ices Containing Purine

We investigated the formation mechanisms of the nucleobases adenine and guanine and the nucleobase analogues hypoxanthine, xanthine, isoguanine, and 2,6-diaminopurine in a UV-irradiated mixed 10:1 H₂O:NH₃ ice seeded with precursor purine by using ab initio and density functional theory computations. Our quantum chemical investigations suggest that a multistep reaction mechanism involving purine cation, hydroxyl and amino radicals, together with water and ammonia, explains the experimentally obtained products in an independent study. The relative abundances of these products appear to largely follow from relative thermodynamic stabilities. The key role of the purine cation is likely to be the reason why purine is not functionalized in pure ammonia ice, where cations are promptly neutralized by free electrons from NH₃ ionization. Amine group addition to purine is slightly favored over hydroxyl group attachment based on energetics, but hydroxyl is much more abundant due to higher abundance of H₂O. The amino group is preferentially attached to the 6 position, giving 6-aminopurine, that is, adenine, while the hydroxyl group is preferentially attached to the 2 position, leading to 2-hydroxypurine. A second substitution by hydroxyl or amino group occurs at either the 6 or the 2 position depending on the first substitution. Given that H₂O is far more abundant than NH₃ in the experimentally studied ices (as well as based on interstellar abundances), xanthine and isoguanine are expected to be the most abundant bi-substituted photoproducts. Key Words: Astrophysical ice-Abiotic organic synthesis-Nucleic acids-Origin of life-RNA world. Astrobiology 17, 771-785.

The Activation of Free Dipeptides Promoted by Strong Activating Agents in Water Does not Yield Diketopiperazines

The activation of dipeptides was studied in the perspective of the abiotic formation of oligopeptides of significant length as a requirement for secondary structure formation. The formation of piperazin-2,5-diones (DKP), previously considered as a dead end when activating free dipeptides, was shown in this work to be efficiently suppressed when using strong activating agents (e.g., carbodiimides). This behaviour was explained by the fast formation of a 5(4H)-oxazolone intermediate at a rate that exceeds the time scale of the rotation of the peptide bond from the predominant trans-conformation into the cis-isomer required for DKP formation. No DKP was observed when using strong activating agents whereas phosphate mixed anhydrides or moderately activated esters were observed to predominantly yield DKP. The DKP side-reaction no longer constitutes a drawback for the C-terminus elongation of peptides. These results are considered as additional evidence that pathways involving strong activation are required to drive the emergence of living entities rather than close to equilibrium processes.

The search for a deterministic origin for the presence of nonracemic amino-acids in meteorites: a computational approach

Amino-acid enantiomeric excesses (ee's) have been detected in different types of carbonaceous chondrites, all in favor of the L enantiomer. In this article, we discuss possible deterministic causes to the presence of these amino-acid ee's in meteorites and evaluate in particular enantioselective photolysis by circularly polarized light (CPL). The electronic circular dichroism spectra of a set of amino- and hydroxy-acids, all detected in chondritic matter but some with ee's and others without ee's, were calculated and compared. The spectra were calculated for the most stable conformation(s) of the considered molecules using quantum mechanical methods (density functional theory). Our results suggest that CPL photolysis in the gas phase was perhaps not at the origin of the presence of ee's in meteorites and that the search for another, but still unknown, deterministic cause must be seriously undertaken.

A concept for NASA's Mars 2016 astrobiology field laboratory

The Mars Program Plan includes an integrated and coordinated set of future candidate missions and investigations that meet fundamental science objectives of NASA and the Mars Exploration Program (MEP). At the time this paper was written, these possible future missions are planned in a manner consistent with a projected budget profile for the Mars Program in the next decade (2007-2016). As with all future missions, the funding profile depends on a number of factors that include the exact cost of each mission as well as potential changes to the overall NASA budget. In the current version of the Mars Program Plan, the Astrobiology Field Laboratory (AFL) exists as a candidate project to determine whether there were (or are) habitable zones and life, and how the development of these zones may be related to the overall evolution of the planet. The AFL concept is a surface exploration mission equipped with a major in situ laboratory capable of making significant advancements toward the Mars Program's life-related scientific goals and the overarching Vision for Space Exploration. We have developed several concepts for the AFL that fit within known budget and engineering constraints projected for the 2016 and 2018 Mars mission launch opportunities. The AFL mission architecture proposed here assumes maximum heritage from the 2009 Mars Science Laboratory (MSL). Candidate payload elements for this concept were identified from a set of recommendations put forth by the Astrobiology Field Laboratory Science Steering Group (AFL SSG) in 2004, for the express purpose of identifying overall rover mass and power requirements for such a mission. The conceptual payload includes a Precision Sample Handling and Processing System that would replace and augment the functionality and capabilities provided by the Sample Acquisition Sample Processing and Handling system that is currently part of the 2009 MSL platform.

Electrospray ionization ion mobility spectrometry of carboxylate anions: ion mobilities and a mass-mobility correlation

A number of carboxylate anions spanning a mass range of 87-253 amu (pyruvate, oxalate, malonate, maleate, succinate, malate, tartarate, glutarate, adipate, phthalate, citrate, gluconate, 1,2,4-benzenetricarboxylate, and 1,2,4,5-benzenetetracarboxylate) were investigated using electrospray ionization ion mobility spectrometry. Measured ion mobilities demonstrated a high correlation between mass and mobility in both N2 and CO2 drift gases. Such a strong mass-mobility correlation among structurally dissimilar ions suggests that the carboxylate functional group that these ions have in common is the source of the correlation. Computational analysis was performed to determine the most stable conformation of the studied carboxylate anions in the gas phase under the current experimental conditions. This analysis indicated that the most stable conformations for multicarboxylate anions included intramolecular hydrogen-bonded ring structures formed between the carboxylate group and the neutral carboxyl group. The carboxylate anions that form ring confirmations generally show higher ion mobility values than those that form extended conformations. This is the first observation of intramolecular hydrogen-bonded ring conformation of carboxylate anions in the gas phase at atmospheric pressure.

Determination of alpha-dialkylamino acids and their enantiomers in geological samples by high-performance liquid chromatography after derivatization with a chiral adduct of o-phthaldialdehyde

Derivatization with o-phthaldialdehyde (OPA) and the chiral thiol N-acetyl-L-cysteine (NAC) is a convenient and sensitive technique for the HPLC detection and resolution of protein amino acid enantiomers. The kinetics of the reaction of OPA-NAC with alpha-dialkylamino acids was investigated. The fluorescence yield of alpha-dialkylamino acids was only about 10% of that of protein amino acids when the derivatization was carried out at room temperature for 1-2 min, which is the procedure generally used for protein amino acid analyses. The fluorescence yield of alpha-dialkylamino acids can be enhanced by up to ten-fold when the derivatization reaction time is increased to 15 min at room temperature. The OPA-NAC technique was optimized for the detection and enantiomeric resolution of alpha-dialkylamino acids in geological samples which contain a large excess of protein amino acids. The estimated detection limit for alpha-dialkylamino acids is 1-2 pmol, comparable to that for protein amino acids.

A note on the prebiotic synthesis of organic acids in carbonaceous meteorites

Strong similarities between monocarboxylic and hydroxycarboxylic acids in the Murchison meteorite suggest corresponding similarities in their origins. However, various lines of evidence apparently implicate quite different precursor compounds in the synthesis of the different acids. These seeming inconsistencies can be resolved by postulating that the apparent precursors also share a related origin. Pervasive D enrichment indicates that this origin was in a presolar molecular cloud. The organic acids themselves were probably synthesized [correction of synthesised] in an aqueous environment on an asteroidal parent body, the hydroxy (and amino) acids by means of the Strecker cyanohydrin reaction.

Syntheses and properties of zinc and calcium complexes of valinate and isovalinate: metal alpha-amino acidates as possible constituents of the early Earth's chemical inventory

We have studied the ligand behavior of racemic isovalinate (iva) and valinate (val) towards zinc(II) and calcium(II). The following solid metal amino acidates were obtained from aqueous solutions: Zn3Cl2(iva)4 (1), Zn3Cl2(val)4 (2). Zn(val)2 (3), Zn(iva)2 x 2H2O (4), Zn(iva)2 x 3.25H2O (5), Zn(iva)2 (6), Ca(iva)2x xH2O (7), and Ca(val)2 x H2O (8). Except for complex 3, these were hitherto unknown compounds. The conditions under which they formed, together with current ideas of the conditions on early Earth, support the assumption that alpha-amino acidate complexes of zinc and calcium might have belonged to early Earth's prebiotic chemical inventory. The zinc isovalinates 1, 4, and 5 were characterized by X-ray crystal structure analyses. Complex 1 forms a layer structure containing four- and five-coordinate metal atoms, whereas the zinc atoms in 4 and 5 are five-coordinate. Compound 5 possesses an unprecedented nonpolymeric structure built from cyclic [Zn6(iva)12] complexes, which are separated by water molecules. The thermolyses of solids 1. 3, and 8 at 320 degrees C in an N2 atmosphere yielded numerous organic products, including the cyclic dipeptide of valine from 3 and 8. Condensation, C-C bond breaking and bond formation, aromatization, decarboxylation, and deamination reactions occurred during the thermolyses. Such reactions of metal-bound a-amino acidates that are abiotically formed could already have contributed to an organic-geochemical diversity before life appeared on Earth.

Infrared spectra of substituted polycylic aromatic hydrocarbons

Calculations are carried out using density functional theory (DFT) to determine the harmonic frequencies and intensities of 1-methylanthracene, 9-methylanthracene, 9-cyanoanthracene, 2-aminoanthracene, acridine, and their positive ions. The theoretical data are compared with matrix-isolation spectra for these species also reported in this work. The theoretical and experimental frequencies and relative intensities for the neutral species are in generally good agreement, whereas the positive ion spectra are only in qualitative agreement. Relative to anthracene, we find that substitution of a methyl or CN for a hydrogen does not significantly affect the spectrum other than to add the characteristic methyl C-H and C triple bond N stretches near 2900 and 2200 cm-1, respectively. However, addition of NH2 dramatically affects the spectrum of the neutral. Not only are the NH2 modes themselves strong, but this electron-withdrawing group induces sufficient partial charge on the ring to give the neutral molecule spectra characteristics of the anthracene cation. The sum of the absolute intensities is about four times larger for 2-aminoanthracene than those for 9-cyanoanthracene. Substituting nitrogen in the ring at the nine position (acridine) does not greatly alter the spectrum compared with anthracene.

Far-infrared emission spectra of selected gas-phase PAHs: spectroscopic fingerprints

The emission spectra of the gaseous polycyclic aromatic hydrocarbons (PAHs) naphthalene, chrysene, and pyrene were recorded in the far-infrared (far-IR) region. The vibrational bands that lie in the far IR are unique for each PAH molecule and allow discrimination among the three PAH molecules. The far-IR PAH spectra, therefore, may prove useful in the assignment of unidentified spectral features from astronomical objects.

Stable isotope analysis at the molecular level: a new approach for determining the origins of amino acids in the Murchison meteorite

A combined gas chromatography/isotope ratio mass spectrometry (GC/IRMS) method has been developed that permits the direct stable carbon isotope analysis of N(O)-trifluoroacetyl-isopropyl esters of individual amino acids and their respective enantiomers at nanomole abundances. Calculation of the original delta 13C values of the amino acids is accomplished via a correction for the carbon introduced during the derivatization process. Previous GC/IRMS analyses of individual amino acids in the non-hydrolyzed water extract of an interior sample of a Murchison meteorite stone revealed an enrichment in 13C relative to terrestrial organic matter, in agreement with previous findings for bulk extracts. The range of amino acid delta 13C values (+5 to +30%, PDB) suggests possible kinetic effects during synthesis. In this study, an apparent kinetic isotope effect was also observed for the amino acid products of a spark discharge experiment. These preliminary results are supportive of a similar mechanism for the abiotic synthesis of amino acids in the Murchison meteorite.

Exobiology, the study of the origin, evolution and distribution of life within the context of cosmic evolution: a review

The primary goal of exobiological research is to reach a better understanding of the processes leading to the origin, evolution and distribution of life on Earth or elsewhere in the universe. In this endeavour, scientists from a wide variety of disciplines are involved, such as astronomy, planetary research, organic chemistry, palaeontology and the various subdisciplines of biology including microbial ecology and molecular biology. Space technology plays an important part by offering the opportunity for exploring our solar system, for collecting extraterrestrial samples, and for utilizing the peculiar environment of space as a tool. Exobiological activities include comparison of the overall pattern of chemical evolution of potential precursors of life, in the interstellar medium, and on the planets and small bodies of our solar system; tracing the history of life on Earth back to its roots; deciphering the environments of the planets in our solar system and of their satellites, throughout their history, with regard to their habitability; searching for other planetary systems in our Galaxy and for signals of extraterrestrial civilizations; testing the impact of space environment on survivability of resistant life forms. This evolutionary approach towards understanding the phenomenon of life in the context of cosmic evolution may eventually contribute to a better understanding of the processes regulating the interactions of life with its environment on Earth.

A New Mechanism for the Formation of Meteoritic Kerogen-Like Material

The carbon in ancient carbonaceous chondritic meteorites is mainly in a hydrocarbon composite similar to terrestrial kerogen, a cross-linked structure of aliphatic and aromatic hydrocarbons. Until recently, the composite has been commonly thought to have been produced in the early solar nebula by a Fischer-Tropsch-type process, involving the catalytic synthesis of hydrocarbons from carbon monoxide and hydrogen on grain surfaces. Instead, the aromatic hydrocarbons may form in gas-phase pyrolysis of simple aliphatics like acetylene and methane by a mechanism developed recently to explain formation of soot in combustion and of aromatic molecules in circumstellar envelopes. Nonequilibrium chemical kinetic calculations indicate that this mechanism can produce meteoritic aromatics if the initial concentration of simple hydrocarbons in the solar nebula was sufficiently but not unreasonably high.

Pre-biotic organic matter from comets and asteroids

Several authors have suggested that comets or carbonaceous asteroids contributed large amounts of organic matter to the primitive Earth, and thus possibly played a vital role in the origin of life. But organic matter cannot survive the extremely high temperatures (>10(4) K) reached on impact, which atomize the projectile and break all chemical bonds. Only fragments small enough to be gently decelerated by the atmosphere--principally meteors of 10(-12)-10(-6) g--can deliver their organic matter intact. The amount of such 'soft-landed' organic carbon can be estimated from data for the infall rate of meteoritic matter. At present rates, only approximately 0.006 g cm-2 intact organic carbon would accumulate in 10(8) yr, but at the higher rates of approximately 4 x 10(9) yr ago, about 20 g cm-2 may have accumulated in the few hundred million years between the last cataclysmic impact and the beginning of life. It may have included some biologically important compounds that did not form by abiotic synthesis on Earth.

The carbon budget in the outer solar nebula

Detailed models of the internal structures of Pluto and Charon, assuming rock and water ice as the only constituents, indicate that the mean silicate mass fraction of this two-body system is on the order of 0.7; thus the Pluto/Charon system is significantly "rockier" than the satellites of the giant planets (silicate mass fraction approximately 0.55). This compositional contrast reflects different formation mechanisms: it is likely that Pluto and Charon formed directly from the solar nebula, while the circumplanetary nebulae that produced the giant planet satellites were derived from envelopes that surrounded the forming giant planets (envelopes in which icy planetesimals dissolved more readily than rocky planetesimals). Simple cosmic abundance calculations, and the assumption that the Pluto/Charon system formed directly from solar nebula condensates, strongly suggest that the majority of the carbon in the outer solar nebula was in the form of carbon monoxide; these results are consistent with (1) inheritance from the dense molecular clouds in the interstellar medium (where CH4/CO < 10(-2) in the gas phase) and/or (2) of the Lewis and Prinn kinetic inhibition model of solar nebula chemistry. Theoretical predictions of the C/H enhancements in the atmospheres of the giant planets, when compared to the actual observed enhancements, suggest that 10%, or slightly more, of the carbon in the outer solar nebula was in the form of condensed materials (although the amount of condensed C may have dropped slightly with increasing heliocentric distance). Strict compositional limits computed for the Pluto/Charon system using the densities of CH4 and CO ices indicate that these pure ices are at best minor components in the interiors of these bodies, and imply that CH4 and CO ices were not the dominant C-bearing solids in the outer nebula. Clathrate-hydrates could not have appropriated enough CH4 or CO to be the major form of condensed carbon, although such clathrates may be necessary to explain the presence of methane on Pluto after its formation from a CO-rich nebula. Laboratory studies of carbonaceous chondrites, and spacecraft observations of Comet Halley, strongly suggest that of the remaining possibilities, organic material, rather than elemental carbon, is the most likely candidate for the dominant C-bearing solid in the outer solar nebula. We conclude that the majority of the carbon in the outer solar nebula was in gaseous CO; 10% to a few tens of percent of the C was in condensed organic materials; and at least a trace amount of carbon was in methane gas.

Application of Two-Step Laser Mass Spectrometry to Cosmogeochemistry: Direct Analysis of Meteorites

Polycyclic aromatic hydrocarbons in C1, C2, and C3 carbonaceous chondrites and in some ordinary chondrites have been directly analyzed by two-step laser desorption/ laser multiphoton ionization mass spectrometry, a selective and sensitive method requiring only milligram samples. At the ionization wavelength of 266 nanometers, parent ion peaks of polycyclic aromatic hydrocarbons dominate the mass spectra. Quantitative analysis is possible; as an example, the concentration of phenanthrene in the Murchison meteorite was determined to be 5.0 parts per million.