The origin of organic matter in the Martian meteorite ALH84001.

Stable carbon isotope measurements of the organic matter associated with the carbonate globules and the bulk matrix material in the ALH84001 Martian meteorite indicate that two distinct sources are present in the sample. The delta 13C values for the organic matter associated with the carbonate globules averaged -26% and is attributed to terrestrial contamination. In contrast, the delta 13C values for the organic matter associated with the bulk matrix material yielded a value of -15%. The only common carbon sources on the Earth that yield similar delta 13C values, other then some diagenetically altered marine carbonates, are C4 plants. A delta 13C value of -15%, on the other hand, is consistent with a kerogen-like component, the most ubiquitous form of organic matter found in carbonaceous chondrites such as the Murchison meteorite. Examination of the carbonate globules and bulk matrix material using laser desorption mass spectrometry (LDMS) indicates the presence of a high molecular weight organic component which appears to be extraterrestrial in origin, possibly derived from the exogenous delivery of meteoritic or cometary debris to the surface of Mars.     

Amino acids in meteorites.

Carbonaceous chondrites carry a record of chemical evolution that is unparalleled among presently accessible natural materials. Within the complex suite of organic compounds that characterize these meteorites, amino acids occur at a total concentration that may reach 0.6 micromole g-1 meteorite (approximately 60 ppm). Both free amino acids and acid-labile amino acid derivatives have been found in hot-water extracts of a CI1 and seven CM2 chondrites. Although the amino acid composition of all CM2 chondrites is not the same, differences may be largely explicable on the basis of spontaneous and biologically-caused decomposition occurring during their terrestrial residence. The amino acids of the Murchison meteorite (CM2) have been extensively analyzed and 52 amino acids have been positively identified. Thirty three of these amino acids are unknown in natural materials other than carbonaceous chondrites. Thus the Murchison meteorite has recently been the major source of new naturally-occurring amino acids. The Murchison amino acids comprise a mixture of C2 through C8 cyclic and acyclic monoamino alkanoic and alkandioic acids of nearly complete structural diversity. Within the acyclic monoamino alkanoic acid series, primary alpha-amino alpha-branched amino acids are predominant. The concentrations of individual amino acids decline exponentially with increasing carbon number within homologous series. Amino acid enantiomers are found in approximately equal amounts. Eight of the terrestrial protein amino acids have been found.     

The chemical conditions on the parent body of the murchison meteorite: Some conclusions based on amino, hydroxy and dicarboxylic acids

Amino and hydroxy acids have been identified in the Murchison meteorite. Their presence is consistent with a synthetic pathway involving aldehydes, hydrogen cyanide and ammonia in an aqueous environment (Strecker-cyanohydrin synthesis). From the various equilibrium and rate constants involved in this synthesis, four independent estimates of the ammonium ion concentrations on the parent body at the time of compound synthesis are obtained; all values are about 2 × 10^?3 M. Succinic acid and β-alanine have also been detected in the Murchison meteorite. Their presence is consistent with a synthesis from acrylonitrile, hydrogen cyanide and ammonia. Using the equilibrium and rate constants for this synthetic pathway, and the succinic acid/β-alanine ratio measured in the Murchison meteorite, an estimate of the hydrogen cyanide concentration of 10^?3 to 10^?2 M is obtained. Since hydrogen cyanide hydrolyzes relatively rapidly in an aqueous environment ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{< 10}{}^4\text{yrs}$) this high concentration implies a period of synthesis of organic compounds as short as 10⁴ years on the Murchison meteorite parent body.     

Characteristics and formation of amino acids and hydroxy acids of the Murchison meteorite.

Eight characteristics of the unique suite of amino acids and hydroxy acids found in the Murchison meteorite can be recognized on the basis of detailed molecular and isotopic analyses. The marked structural correspondence between the alpha-amino acids and alpha-hydroxy acids and the high deuterium/hydrogen ratio argue persuasively for their formation by aqueous phase Strecker reactions in the meteorite parent body from presolar, i.e., interstellar, aldehydes, ketones, ammonia, and hydrogen cyanide. The characteristics of the meteoritic suite of amino acids and hydroxy acids are briefly enumerated and discussed with regard to their consonance with this interstellar-parent body formation hypothesis. The hypothesis has interesting implications for the organic composition of both the primitive parent body and the presolar nebula.     

Origin of organic compounds in carbonaceous chondrites.

Carbonaceous chondrites, a class of primitive meteorite, have long been known to contain their complement of carbon largely in the form of organic, i.e., hydrocarbon-related, matter. Both discrete organic compounds and an insoluble, macromolecular material are present. Several characteristics of these materials provide evidence for their abiotic origin. The principal formation hypotheses have invoked chemistry occurring either in the solar nebula or on the parent body. However, recent stable isotope analyses of the meteorite carboxylic acids and amino acids indicate that they may be related to interstellar cloud compounds. These results suggest a formation scheme in which interstellar compounds were incorporated into the parent body and subsequently converted to the present suite of meteorite organics by the hydrothermal process believed to have formed the clay minerals of the meteorite matrix.     

Biogeochemical evidence of microbial activity on Mars.

We suggest a new interpretation of the data on so-called SNC meteorites and delta 13C values of the calcium carbonate minerals and organic matter discovered in them. The delta 13C value of calcite (up to 15 ppt) is accounted for by the microbial reaction CO2 + H2 ---> CH4 + H2O. Methane-forming bacteria also synthesize organic carbon (in the form of biomass) from CO2, and this process is accompanied by 12C fractionation. Therefore, the organic carbon of SNC meteorites is enriched with 12C (delta 13C as low as -35 ppt). The environmental conditions under which the calcite of SNC meteorites was formed were favorable for the activity of methanogens.     

Stable carbon isotope fractionation in the search for life on early Mars.

Isotopic measurements and, more specifically, ratios of 13C to 12C in organic relative to inorganic deposits, are useful in reconstructing past biological activity on Earth. Organic matter has a lower ratio of 13C to 12C due largely to the preferential fixation of 12C over the heavier isotope by the major carbon-fixation enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase, although other factors (e.g., availability of source carbon, fixation by other carboxylating enzymes and diagenesis of organic material) also contribute to fractionation. Would carbon isotope discrepancies between inorganic and organic carbon indicate past biological activity on Mars? In order to answer this question, we analyse what is known about terrestrial biologic and abiologic carbon fixation and its preservation in the fossil record, and suggest what the isotope discrimination during possible biologic and abiologic carbon fixation on Mars might have been like. Primarily because isotopic signatures of abiotically fixed carbon overlap with those of biotic fixation, but also because heterotrophy does not significantly alter the isotopic signature of ingested carbon, fractionation alone would not be definitive evidence for life. However, a narrow range of fractionation, including no fractionation, would suggest biotic processes. Never-the-less, isotopic ratios in organic deposits on Mars would be extremely useful in analysing prebiotic, if not biotic, carbon transformations on Mars.     

The atmosphere of the primitive earth and the prebiotic synthesis of organic compounds.

The prebiotic synthesis of organic compounds using a spark discharge on various simulated prebiotic atmospheres at 25 degrees has been studied. Methane mixtures contained H2 + CH4 + H2O + N2 + NH3 with H2/CH4 molar ratios from 0 to 4 and pNH3 = 0.1 torr. A similar set of experiments without added NH3 was performed. The yields of amino acids (1.2 to 4.7% based on the carbon) are approximately independent of the H2/CH4 ratio and the presence of added NH3, and a wide variety of amino acids are obtained. Mixtures of H2 + CO + H2O + N2 and H2 + CO2 + H2O + N2, with and without added NH3, all give about 2% yields of amino acids at H2/CO and H2/CO2 ratios of 2 to 4. For the H2/CO and H2/CO2 ratios less than 1, the yields fall off drastically to as low as 10(-3)%. Glycine is almost the only amino acid produced from CO and CO2 atmospheres. These results show that the maximum yield is about the same for the three carbon sources at high H2/carbon ratios, but that CH4 is superior at low H2/carbon ratios. In addition, CH4 gives a much greater variety of amino acids than either CO or CO2. If it is assumed that amino acids more complex than glycine were required for the origin of life, then these results indicate the need for CH4 in the primitive atmosphere. The yields of cyanide and formaldehyde parallel the amino acid results, with yields of HCN and H2CO as high as 13% based on the carbon. Ammonia is also produced from N2 in experiments with no added NH3 in yields as high as 4.9%. These results show that large amounts of NH3 would have been synthesized on the primitive earth by electric discharges. The amount of ammonia formed by hydrolysis of HCN and various nitriles may have exceeded that formed directly in electric discharges.     

Complex organics in meteorites.

Complex macromolecular organic matter is present in carbonaceous chondrites as the most abundant organic matter and may be present in interstellar dusts and comets. With this view, our studies of the complex organic matter isolated from six CM2 chondrites, namely Yamato-74662, Yamato-791198, Yamato-793321, Yamato-86720, Belgica-7904, and Murchison are introduced and discussed in the text. The complex organic matter is acid-insoluble and organic solvent-insoluble, and therefore, it was examined by heating experiments to obtain information on its chemical constituents and characteristics. Three chondrites, Yamato-74662, Yamato-791198, and Murchison which have solvent-extractable organic compounds, such as amino acids, carboxylic acid, hydrocarbons, etc. possess thermally labile organic fraction in the complex organic matter. Organic compounds detected in the pyrolyzate of the complex organic matter number over 130 of which aromatic hydrocarbons are dominant. They appeared around 300 degrees C, and disappeared at about 600 degrees C with a maximum at 400-500 degrees C during the heating. On the other hand, the other three chondrites do not have the extractable organic compounds nor a thermally labile organic fraction. The presence or absence of the fraction in the complex organic matter likely indicates the presence or absence of the solvent-extractable organic compounds and relates to thermal history of the chondrite.     

Fluorescent organic matter in carbonaceous chondrites.

Fluorescent organic matter in carbonaceous chondrites was investigated using a microscope equipped with a fluorescence spectrophotometer. Fluorescent particles were observed in powdered CM2 carbonaceous chondrites (Y-74662, Y-7791198, and Murchison) without carbon enrichment by acid treatments. Although it was difficult to find fluorescent particles in powdered sample of C3 chondrites (ALH-77307, Y-791717, and Allende) without acid treatments, many fluorescent particles were observed after carbon enrichment by acid treatments. Fluorescence of coronene and shock-altered graphite were observed using the same microscope and the same conditions as those for carbonaceous chondrites.     

Quantum chemical calculations of infrared spectra for the identification of unknown compounds by GC/FTIR/MS in exobiological simulation experiments.

Gas chromatography/Fourier transform IR spectroscopy/mass spectrometry (GC/FTIR/MS) is a powerful tool for the separation and unambiguous identification of complex mixtures of organic compounds, where the use of two kinds of spectra allows to significantly increase identification reliability. The simplest situation is when acquired spectra can be found in IR and MS databases, or appropriate standards are available; but this is not always the case. Some simulation experiments related to the origins of life and exobiology (e.g., simulation of amino acid pyrolysis during atmospheric entry of space bodies) can be a typical example when one encounters with numerous unknown compounds. To assist their identification by GC/FTIR/MS, recently we suggested quantum chemical calculations of infrared spectra in order to compare them to IR spectra acquired experimentally. The present work summarizes the results obtained by semi-empirical and ab initio methods, discusses their advantages and limitations, considering as test compounds some cyclic amides and amidines derived from amino acids, saturated and unsaturated nitriles (including those of interest for the Titan atmospheric chemistry), acetylenes and some other nitrogen compounds.     

Organic analysis of hydrogen cyanide polymers: prebiotic and extraterrestrial chemistry.

Hydrogen cyanide polymerizes readily to a black solid from which a yellow-brown powder can be extracted by water and further hydrolyzed to alpha-amino acids. These macromolecules could be major components of the dark matter observed on many bodies in the outer solar system, including comets and asteroids. Primitive Earth might therefore have been covered with HCN polymers through bolide bombardment or be terrestrial synthesis. Several instrumental methods were used for the separation and identification of these intriguing polymeric materials, including photoacoustic Fourier transform infrared spectroscopy, supercritical fluid extraction chromatography and pyrolysis mass spectrometry. Our integrated analytical approach revealed fragmentation patterns and chemical functionalities consistent with the presence of polymeric peptide precursors both in HCN polymers and in the Murchison meteorite.     

安徽亳县陨石的热释光研究

毫县陨石于1977年10月20日下午2时30分左右降落于安徽亳县张沃公社吝子门大队.陨石表面灰黑色,有0.5mm厚的烧蚀层,具有明显的球粒结构.已查明[1]该陨石的透明矿物有镁橄榄石、贵橄榄石、透铁橄榄石、古铜辉石、顽火辉石、斜顽辉石、顽火透辉石、易变辉石、斜长石、正长石、白磷钙矿、石英、黑云母、白云母、方解石、刚玉等;不透明矿物有陨硫铁、铁纹石、镍纹石、铬铁矿、镁铁尖晶石、钛铁矿、镍黄铁矿、石墨、磁铁矿、方铁矿、自然铜和张衡矿等.经研究[1]该陨石的化学-岩石类型为LL₄,属于平衡球粒陨石.      

Origins of amino acids in the early solar system.

Synthesis of meteoritic amino acids probably took place in the aqueous sub-surface regions of one or more asteroid-sized parent bodies. Starting material for those reactions apparently consisted of a population of more simple compounds including a suite of aliphatic precursors characterised by (1) complete structural diversity, (2) prevalence of branched- over straight-chain species, (3) exponential decrease in abundance with increasing C number, (4) large enrichment in D, and, probably, (5) systematic decrease in 13C/12C with increasing C number. Those properties were apparently acquired during synthesis of the precursors by ion-molecule reactions in a presolar molecular cloud.     

Similarity of the infrared spectrum of an Orgueil organic polymer with interstellar organic compounds in the line of sight towards IRS 7.

We present a comparison between the IR spectrum of the galactic center source IRS 7 and the spectrum of a carbonaceous polymer from the Orgueil meteorite. We have obtained an almost perfect match between the two spectra in the region between 3020-2790 cm-1, which suggests that the chemical composition of the interstellar organic matter and that of the meteorite polymer are similar or that the meteoritic polymer could be a well preserved interstellar organic molecule. Assuming that the meteoritic polymer has the same C/H ratio as these interstellar molecules, we find that 45 % of the total abundance of carbon in the line of sight toward IRS 7 is trapped in such an interstellar organic grain material.     

Formation of bioorganic compounds in simulated planetary atmospheres by high energy particles or photons.

Various types of organic compounds have been detected in Jupiter, Titan, and cometary coma. It is probable that organic compounds were formed in primitive Earth and Mars atmospheres. Cosmic rays and solar UV are believed to be two major energy sources for organic formation in space. We examined energetics of organic formation in simulated planetary atmospheres. Gas mixtures including a C-source (carbon monoxide or methane) and a N-source (nitrogen or ammonia) was irradiated with the followings: High energy protons or electrons from accelerators, gamma-rays from 60Co, UV light from a deuterium lamp, and soft X-rays or UV light from an electron synchrotron. Amino acids were detected in the products of particles, gamma-rays and soft X-rays irradiation from each gas mixture examined. UV light gave, however, no amino acid precursors in the gas mixture of carbon monoxide, nitrogen and nitrogen. It gave only a trace of them in the gas mixture of carbon monoxide, ammonia and water or that of methane, nitrogen and water. Yield of amino acid precursors by photons greatly depended on their wavelength. These results suggest that nitrogen-containing organic compounds like amino acid precursors were formed chiefly with high energy particles, not UV photons, in Titan or primitive Earth/Mars atmospheres where ammonia is not available as a predominant N-source.     

The Kumtag meteorite strewn field

The Kumtag meteorite strewn field was found in the Kumtag desert, 132 km south of Hami city in the Xinjiang province, China. It is an ellipse of 2.5 × 7.9 km, with a long axis extending along the northeast-southwest direction. The largest individual meteorite of the strewn field weighs about 10 kg; the smallest individual has a mass of only 27 g. In total, more than 100 individuals with a total mass of more than 180 kg were collected. The location and the distribution of the fragments suggest that the Kumtag meteoroid entered the atmosphere in the direction Northeast-Southwest. All meteorites collected in this strewn field are samples from the same unique meteorite shower. The Kumtag meteorite is an H5 ordinary chondrite with a shock stage S2, and a weathering grade W2. The cosmic ray exposure age of Kumtag is 6.7 ± 0.8 Ma, which is rather typical for H chondrites and which indicates that Kumtag was derived from the massive impact event on its parent body ~7 Ma ago. A significant amount of He has been lost during certain unknown processe(s) before the Kumtag meteorite was ejected from its parent body.     

Formation of amino acid precursors in cometary ice environments by cosmic radiation.

Cometary ices are believed to contain water, carbon monoxide, methane and ammonia, and are possible sites for the formation and preservation of organic compounds relating to the origin of life. Cosmic rays, together with ultraviolet light, are among the most effective energy sources for the formation of organic compounds in space. In order to study the possibility of the formation of amino acids in comets or their precursory bodies (interstellar dust grains), several types of ice mixtures made in a cryostat at 10 K ("simulated cometary ices") were irradiated with high energy protons. After irradiation, the volatile products were analyzed with a quadrupole mass spectrometer, while temperature of the cryostat was raised to room temperature. The non-volatile products remaining in the cryostat at room temperature were collected with water. They were acid-hydrolyzed, and analyzed by ion-exchange chromatography. When an ice mixture of carbon monoxide (or methane), ammonia and water was irradiated, some hydrocarbons were formed, and amino acids such as glycine and alanine were detected in the hydrolyzate. These results suggest the possible formation of "amino acid precursors" (compounds yielding amino acids after hydrolysis) in interstellar dust grains by cosmic radiation. We previously reported that amino acid precursors were formed when simulated primitive planetary atmospheres were irradiated with cosmic ray particles. It will be of great interest to compare the amount of bioorganic compounds that were formed in the primitive earth and that brought by comets to the earth.     

Survivability of biomolecules during extraterrestrial delivery: new results on pyrolysis of amino acids and poly-amino acids.

The hypothesis on exogenous origin of organic matter on the early Earth is strongly supported by the detection of a large variety of organic compounds (including amino acids and nucleobases) in carbonaceous chondrites. Whether such complex species can be successively delivered by other space bodies (comets, asteroids and interplanetary dust particles) is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. Recent simulation experiments on amino acid and nucleic acid base pyrolysis under oxygen-free atmosphere demonstrated that simple representatives of these (considered thermally unstable) compounds can survive at 1-10% level a rapid heating at 500-600 degrees C. In the present work, we report on new data on the pyrolysis of amino acids and their homopolymers and discuss implications of their thermal behavior for extraterrestrial delivery.     

Transport of extraterrestrial biomolecules to the Earth: problem of thermal stability.

The idea of extraterrestrial delivery of organic matter to the early Earth is especially attractive at present and is strongly supported by the detection of a large variety of organic compounds, including amino acids and nucleobases, in carbonaceous chondrites. Whether these compounds can be delivered by other space bodies is unclear and depends primarily on capability of the biomolecules to survive high temperatures during atmospheric deceleration and impacts to the terrestrial surface. In the present study we estimated survivability of simple amino acids (alpha-aminoisobutyric acid, L-alanine, L-valine and L-leucine), purines (adenine and guanine) and pyrimidines (uracil and cytosine) under rapid heating to temperatures of 400 to 1000 degrees C under N2 or CO2 atmosphere. We have found that most of the compounds studied cannot survive the temperatures substantially higher than 700 degrees C; however at 500-600 degrees C, the recovery can be at a per cent level (or even 10%-level for adenine, uracil, alanine, and valine). Implications of the data for extraterrestrial delivery of the biomolecules are discussed.