Complex organics in laboratory simulations of interstellar/cometary ices.

We present the photochemical and thermal evolution of both non-polar and polar ices representative of interstellar and pre-cometary grains. Ultraviolet photolysis of the non-polar ices comprised of O2, N2, and CO produces CO2, N2O, O3, CO3, HCO, H2CO, and possibly NO and NO2. When polar ice analogs (comprised of H2O, CH3OH, CO, and NH3) are exposed to UV radiation, simple molecules are formed including: H2, H2CO, CO2, CO, CH4, and HCO (the formyl radical). Warming produces moderately complex species such as CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), R-CN and/or R-NC (nitriles and/or isonitriles). Several of these are already known to be in the interstellar medium, and their presence indicates the importance of grain processing. Infrared spectroscopy, 1H and 13C nuclear magnetic resonance (NMR) spectroscopy, and gas chromatography-mass spectrometry demonstrate that after warming to room temperature what remains is an organic residue composed primarily of hexamethylenetetramine (HMT, C6H12N4) and other complex organics including the amides above and polyoxymethylene (POM) and its derivatives. The formation of these organic species from simple starting mixtures under conditions germane to astrochemistry may have important implications for the organic chemistry of interstellar ice grains, comets and the origins of life.     

From the interstellar clouds, through the inner to the outer solar system: a universally distributed complex organic chemistry. Preface.

High molecular weight organic compounds are involved in the chemistry and physics of many astrophysical and planetary objects. They are or should be present in interstellar dust, in comets and meteorites, in the Giant planets and Titan, in asteroids Triton and icy satellites. They represent a class of very complex organic material, part of which may have played a role in the origin of life on Earth. Thus they directly concern prebiotic chemistry and exobiology.     

Circumstellar chemistry from microwave and mm-wave spectroscopy.

Stars in their late stages of evolution often shed matter in the form of a cool wind which is molecular in composition. These winds are a major source of replenishment of the interstellar gas and dust, so they furnish a large part of the raw materials for new generations of stars and planets. The chemistry of the circumstellar envelope depends strongly on the photospheric abundances of the elements, especially C and O. If C/O > 1, a rich organic chemistry is observable in the microwave and mm-wavelength emission lines of the reactions products. This paper reviews the observational evidence for the presence of organic molecules and their formation pathways in circumstellar envelopes, with special emphasis on rotational spectra at microwave and millimeter wavelengths.     

Chemical and biological evolution in space

Astronomical infrared spectra are used to confirm the existence of complex organic molecules produced by ultraviolet photoprocessing of interstellar grain mantles. This material is shown to be the major component of the interstellar grains between the sun and the galactic center and, by inference, constitutes more than 10 million solar masses — or close to one part in a thousand of the entire mass of the milky way galaxy. It may be demonstrated that the primitive chemistry of the earth's surface was dominated by these extraterrestrial molecules after aggregated into comets if the rate of comet impacts with the earth was comparable with that required to account for the extinction of species over the past 300 million years.

Ultraviolet irradiation of bacterial spores has been studied for the first time under simulated interstellar conditions. The inactivation time predicted for the less dense regions of space is at most several hundred years. Within molecukar clouds it is shown on theoretical and experimental grounds that this t the estimated cloud. However survival of spores during their initial exposure to the solar ultraviolet presents a problem for panspermia because it requires that in the process of ejection from the earth's surface they must be enclosed within a cocoon (or mantle) of ultraviolet absorbing material of 0.6 μm thickness. Thus, although panspermia can not be rejected on the basis of lack of interstellar survival there may remain insurmountable obstacles to its occuring because of the very special protective shield requirements during ejection from its planetary source.     

A laboratory model for interstellar chemical evolution.

The chemistry in a supersonic plasma source flow was studied as a laboratory model for interstellar chemical evolution. It is important to match the similarity parameters for cosmic and laboratory conditions, which connect the temporal and spatial scales of the two cases. The apparatus simulated the conditions in a molecular cloud with respect to molecular-ionic reaction fraction, temperature, and non-equilibrium kinetics. The plasma flow was found to be cold enough, by the radical expansion, to produce polyatomic molecules. From the simple atomic plasma as reactant, cyanopolyyne and unsaturated hydrocarbons were synthesized in the present experiment. These molecules are also inherent in molecular clouds. The reaction mechanism is discussed.     

Extraterrestrial organic chemistry: from the interstellar medium to the origins of life. Part 2: complex organic chemistry in the environment of planets and satellites.

During COSPAR'00 in Warsaw, Poland, in the frame of Sub-Commission F.3 events (Planetary Biology and Origins of Life), part of COSPAR Commission F (Life Sciences as Related to Space), and Commission B events (Space Studies of the Earth-Moon System, Planets, and Small Bodies of the Solar System) a large joint symposium (F.3.4/B0.8) was held on extraterrestrial organic chemistry. Part 2 of this symposium was devoted to complex organic chemistry in the environment of planets and satellites. The aim of this event was to cover and review new data which have been recently obtained and to give new insights on data which are expected in the near future to increase our knowledge of the complex organic chemistry occurring in several planets and satellites of the Solar System, outside the earth, and their implications for exobiology and life in the universe. The event was composed of two main parts. The first part was mainly devoted to the inner planets and Europa and the search for signatures of life or organics in those environments. The second part was related to the study of the outer solar system.     

Hot atoms in cosmic chemistry

High energy chemical reactions and atom molecule interactions might be important for cosmic chemistry with respect to the accelerated species in solar wind, cosmic rays, colliding gas and dust clouds and secondary knock-on particles in solids. “Hot” atoms with energies ranging from a few eV to some MeV can be generated via nuclear reactions and consequent recoil processes. The chemical fate of the radioactive atoms can be followed by radiochemical methods (radio GC or HPLC). Hot atom chemistry may serve for laboratory simulation of the reactions of energetic species with gaseous or solid interstellar matter. Due to the effective measurement of 10⁸–10¹⁰ atoms only it covers a low to medium dose regime and may add to the studies of ion implantation which due to the optical methods applied are necessarily in the high dose regime.

Experimental results are given for the systems: C/H₂O (gas), C/H₂O (solid, 77 K), N/CH₄ (solid, 77K) and C/NH₃ (solid, 77 K). Nuclear reactions used for the generation of 2 to 3 MeV atoms are: ¹⁴N(p, ) ¹¹C, ¹⁶O(p, pn) ¹¹C and ¹²C(d,n) ¹³N with 8 to 45 MeV protons or deuterons from a cyclotron. Typical reactions products are: CO, CO₂, CH₄, CH₂O, CH₃OH, HCOOH, NH₃, CH₃NH₂, cyanamide, formamidine, guanidine etc. Products of hot reactions in solids are more complex than in corresponding gaseous systems, which underlines the importance of solid state reactions for the build-up of precursors for biomolecules in space. As one of the major mechanisms for product formation, the simultaneous or fast consecutive reactions of a hot carbon with two target molecules (reaction complex) is discussed.     

Simulated cometary matter as a test for enantiomer separating chromatography for use on comet 46P/Wirtanen.

The Cometary Sampling and Composition Experiment on board of European Space Agency's cornerstone mission ROSETTA is designed to identify organic molecules in cometary matter in situ by a combined pyrolysis gas chromatographic and mass spectrometric technique. Its capillary columns coated with chiral stationary phases received considerable attention, because they are designed for separations of non-complex enantiomers to allow the determination of enantiomeric ratios of cometary chiral organic compounds and consequently to provide information about the origin of molecular parity violation in biomolecules. To get gas chromatographic access to organic compounds on the comet, where macromolecules and complex organic polymers of low volatility are expected to make up the main organic ingredients, the combination of two injection techniques will be applied. The pyrolysis technique performed by heating cometary samples stepwise to defined temperatures in specific ovens resulting in thermochemolysis reactions of polymers and a chemical derivatization technique, in which the reagent dimethylformamide dimethylacetal assists pyrolysis derivatization reactions in producing methyl esters of polar monomers. The combination of the reagent assisted pyrolysis gas chromatographic technique with enantiomer separating chromatography was tested with laboratory-produced simulated cometary matter.     

Very low temperature formaldehyde reactions and the build-up of organic molecules in comets and interstellar ices.

We have investigated thermally promoted reactions of formaldehyde (H2CO) in very low temperature ices. No such reactions occurred in ices of pure formaldehyde. However, addition of trace amounts of ammonia (NH3) were sufficient to catalyze reactions at temperatures as low as 40 K. Similar reactions could take place in interstellar ices and in Comets and produce considerable amounts of organic molecules.     

On the possible detection of solid O2 and O3 interstellar grains with ISO

In various models of interstellar grain chemistry, solid O₂ is formed by accretion as well as by surface reactions on grains. In dense molecular cloud models, at a later stage of the evolution, the O₂ molecule may become a substantial grain mantle constituent. Since IR dipole vibrational transitions for the homonuclear diatomic molecule O₂ are forbidden, the abundance of this potentially important grain mantle component can not be determined. However, embedded in a dirty ice matrix, the fundamental vibration of O₂ at 1550 cm⁻¹ becomes observable at 10 K, due to interactions with surrounding molecules, which break the symmetry of molecular oxygen. This process might be applicable for the dust mantle environment of interstellar grains. We have studied the role of solid O₂ and O₃ in astrophysically relevant ice mixtures and discuss the possible detection of solid O₂ and its major photolysis product O₃ in interstellar grains, in dense molecular clouds. Both molecules represent a specific target to be observed by the ISO satellite in the near future.     

Corona discharge of Titan's troposphere.

The atmosphere of Titan is constantly bombarded by galactic cosmic rays and Saturnian magnetospheric electrons causing the formation of free electrons and primary ions, which are then stabilized by ion cluster formation and charging of aerosols. These charged particles accumulate in drops in cloud regions of the troposphere. Their abundance can substantially increase by friction, fragmentation or collisions during convective activity. Charge separation occurs with help of convection and gravitational settling leading to development of electric fields within the cloud and between the cloud and the ground. Neutralization of these charge particles leads to corona discharges which are characterized by low current densities. These electric discharges could induce a number of chemical reactions in the troposphere and hence it is of interest to explore such effects. We have therefore, experimentally studied the corona discharge of a simulated Titan's atmosphere (10% methane and 2% argon in nitrogen) at 500 Torr and 298 K by GC-FTIR-MS techniques. The main products have been identified as hydrocarbons (ethane, ethyne, ethene, propane, propene + propyne, cyclopropane, butane, 2-methylpropane, 2-methylpropene, n-butene, 2-butene, 2,2-dimethylpropane, 2-methylbutane, 2-methylbutene, n-pentane, 2,2-dimethylbutane, 2-methylpentane, 3-methylpentane, n-hexane, 2,2-dimethylhexane, 2,2-dimethylpentane, 2,2,3-trimethylbutane, 2,3-dimethylpentane and n-heptane), nitriles (hydrogen cyanide, cyanogen, ethanenitrile, propanenitrile, 2-methylpropanenitrile and butanenitrile) and an uncharacterized film deposit. We present their trends of formation as a function of discharge time in an ample interval and have derived their initial yields of formation. These results clearly demonstrate that a complex organic chemistry can be initiated by corona processes in the lower atmosphere. Although photochemistry and charged particle chemistry occurring in the stratosphere can account for many of the observed hydrocarbon species in Titan, the predicted abundance of ethene is to low by a factor of 10 to 40. While some ethene will be produced by charged-particle chemistry, its production by corona processes and subsequent diffusion into the stratosphere appears to be an adequate source. Because little UV penetrates to the lower atmosphere to destroy the molecules formed there, the corona-produced species may be long-lived and contribute significantly to the composition of the lower atmosphere and surface.     

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.     

The seeding of life by comets.

The evidence that living organisms were already extant on the earth almost 4 Gyr ago and that early bombardment by comets and asteroids created a hostile environment up to about this time has revived the question of how it was possible for prebiotic chemical evolution to have provided the necessary ingredients for life to have developed in the short intervening time. The actual bracketed available temporal space is no more than 0.5 Gyr and probably much less. Was this sufficient time for an earth-based source of the first simple organic precursor molecules to have led to the level of the prokaryotic cell? If not, then the difficulty would be resolved if the ancient earth was impregnated by organic molecular seed from outer space. Curiously, it seems that the most likely source of such seeds was the same a one of the sources of the hostile enviroment, namely the comets which bombarded the earth. With the knowledge of comets gained by the space missions it has become clear that a very large fraction of the chemical composition of comet nuclei consists of quite complex organic molecules. Furthermore it has been demonstrated that comets consist of very fluffy aggregates of interstellar dust whose chemistry derives from photoprocessing of simple ice mixtures in space. Thus, the ultimate source of organics in comets comes from the chemical evolution of interstellar dust. An important and critical justification for assuming that interstellar dust is the ultimate source of prebiotic molecular insertion on the earth is the proof that comets are extremely fluffy aggregates, which have the possibility of breaking up into finely divided fragments when the comet impacts the earth's atmosphere. In the following we will summarize the properties of interstellar dust and the chemical and morphological structure of comets indicated by the most recent interpretations of comet observations. It will be shown that the suitable condition for comets having provided abundant prebiotic molecules as well as the water in which they could have further evolved are consistent with theories of the early earth environment.     

Ion irradiation: its relevance to the evolution of complex organics in the outer solar system.

Ion irradiation of carbon containing ices produces several effects among which the formation of complex molecules and even refractory organic materials whose spectral color and molecular complexity both depend on the amount of deposited energy. Here results from laboratory experiments are summarized. Their relevance for the formation and evolution of simple molecules and complex organic materials on planetary bodies in the external Solar System is outlined.     

Interstellar hydrogen bonding

This paper reports the first extensive study of the existence and effects of interstellar hydrogen bonding. The reactions that occur on the surface of the interstellar dust grains are the dominant processes by which interstellar molecules are formed. Water molecules constitute about 70% of the interstellar ice. These water molecules serve as the platform for hydrogen bonding. High level quantum chemical simulations for the hydrogen bond interaction between 20 interstellar molecules (known and possible) and water are carried out using different ab-intio methods. It is evident that if the formation of these species is mainly governed by the ice phase reactions, there is a direct correlation between the binding energies of these complexes and the gas phase abundances of these interstellar molecules. Interstellar hydrogen bonding may cause lower gas abundance of the complex organic molecules (COMs) at the low temperature. From these results, ketenes whose less stable isomers that are more strongly bonded to the surface of the interstellar dust grains have been observed are proposed as suitable candidates for astronomical observations.     

Photoinduced catalytic synthesis of biologically important metabolites from formaldehyde and ammonia under plausible “prebiotic” conditions

The article analyzes new and previously reported data on several catalytic and photochemical processes yielding biologically important molecules. UV-irradiation of formaldehyde aqueous solution yields acetaldehyde, glyoxal, glycolaldehyde and glyceraldehyde, which can serve as precursors of more complex biochemically relevant compounds. Photolysis of aqueous solution of acetaldehyde and ammonium nitrate results in formation of alanine and pyruvic acid. Dehydration of glyceraldehyde catalyzed by zeolite HZSM-5-17 yields pyruvaldehyde. Monosaccharides are formed in the course of the phosphate-catalyzed aldol condensation reactions of glycolaldehyde, glyceraldehyde and formaldehyde. The possibility of the direct synthesis of tetroses, keto- and aldo-pentoses from pure formaldehyde due to the combination of the photochemical production of glycolahyde and phosphate-catalyzed carbohydrate chain growth is demonstrated. Erythrulose and 3-pentulose are the main products of such combined synthesis with selectivity up to 10%. Biologically relevant aldotetroses, aldo- and ketopentoses are more resistant to the photochemical destruction owing to the stabilization in hemiacetal cyclic forms. They are formed as products of isomerization of erythrulose and 3-pentulose. The conjugation of the concerned reactions results in a plausible route to the formation of sugars, amino and organic acids from formaldehyde and ammonia under presumed ‘prebiotic’ conditions.     

Photochemical growing of complex organics in planetary atmospheres

UV induced syntheses of organic compounds from the main atmospheric constituents can be a very important source of organics in a given planetary environment provided the atmosphere is in a reduced state. The evolution of a CO₂ rich medium only produces very low yields of formaldehyde and related oxygenated compounds. Considering a CO rich atmosphere, the photochemical yield of O-organics formation is much higher, when the synthesis of N-organics remains difficult. The most favourable atmosphere as far as photochemical organic synthesis is concerned is a CH₄ rich milieu.. The photochemical evolution of such a CH₄ atmosphere under UV irradiation leads to a chain of various organics, the complexity of which increases together with the number of pathways involved in their formation. Their complexity also closely correlates with their UV photoabsorption spectrum: the more complex they are, the more shifted is their UV spectrum toward the visible range. Direct photodissociation of methane requires UV photon of wavelengths shorter than about 145 nm. It mainly produces ethane which absorbs UV at wavelengths shorter than about 160 nm, and acetylene, that presents an absorption spectrum extending up to 200 nm. This shift still continuously increases with further increase in number of C atoms. Unsaturated hydrocarbons with 4 and more C atoms have UV absorption characteristics including noticeable band structures in the 250–300 nm range. This trend has very important implication in the photochemical behaviour of a CH₄-rich planetary atmosphere, as it induces many catalytic processes. The occurrence of such processes is closely related to vertical atmospheric and energy deposition profiles. Titan provides a very good example of such a UV-directed organic atmospheric chemistry.     

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.     

New insights into Titan's organic chemistry in the gas and aerosol phases.

Titan, the largest satellite of Saturn, with a dense atmosphere very rich in organics, and many couplings in the various parts of its "geofluid", is a reference for studying prebiotic chemistry on a planetary scale. New data have been obtained from experiments simulating this organic chemistry (gas and aerosol phases), within the right ranges of temperature and a careful avoiding of any chemical contamination. They show a very good agreement with the observational data, demonstrating for the first time the formation of all the organic species already detected in Titan atmosphere including, at last, C4N2, together with many other species not yet detected in Titan. This strongly suggests the presence of more complex organics in Titan's atmosphere and surface, including high molecular weight polyynes and cyanopolyynes. The NASA-ESA Cassini-Huygens mission has been successfully launched in October 1997. The Cassini spacecraft will reach the Saturn system in 2004 and become an orbiter around Saturn, while the Huygens probe will penetrate into Titan's atmosphere. In situ measurements, in particular from Huygens GC-MS and ACP instruments, will provide a detailed analysis of the organics present in the air, aerosols, and surface. This very ambitious mission should yield much information of crucial importance for our knowledge of the complexity of Titan's chemistry, and, more generally for the field of exobiology.