Interstellar dust,chirality, comets and the origins of life: Life from dead stars?

The physical, chemical and astrophysical processes by which chiral prebiotic molecules can be produced in interstellar dust and later delivered safely to the earth are considered. A laboratory analog experiment on the irradiation by circularly polarized UV light of mirror image molecules at the low temperatures of interstellar dust demonstrates that a substantial degree of chirality can be produced by irradiation of the dust by circularly polarized light from pulsars whose mean brightness and distribution in the Milky Way provide the energetic photons. The chirality is then preserved by cold aggregation of the dust into low density fragile nuclei. The thermal evolution of comets following them from birth through billions of years in the Oort cloud and back to the inner solar system results in preservation of dust organics in largely pristine form — even including effects of radiogenic heating. Physical justification for the cushioned transfer of fragments of the fluffy comets impacting the earth's atmosphere provides a conceptual basis for depositing significant concentrations of interstellar prebiotic molecules. Chiral amplification in water on the earth is presumed to be enhanced by this local concentration. If chiral molecules are discovered in comet nucleus material which will some day be returned to the laboratory, we may have in our hands the same building blocks from which we evolved.     

THE SEARCH FOR INTERSTELLAR GLYCINE

Millimeter arrays can be used to identify hot young molecular cores which contain large, highly saturated interstellar molecules, including biomolecules. These cores are prime locations for searching for interstellar glycine. The current status of the interstellar glycine search is discussed.     

Interstellar molecules

Summary The study of interstellar molecules broadly includes two areas of interest. One area uses the unique ability of molecules to act as probes of the physical conditions in the cold, dense, visually opaque component of the interstellar medium. The physical properties of this and other components of the interstellar medium are summarized. The other area deals with the chemistry of interstellar molecules, recent aspects of which are emphasized in this review. Gas-phase chemistry, shock chemistry, and grain surface chemistry are discussed in the context of recent observations. No present observations suggest that surface reactions are relevant, but neither can they be ruled out. Ion-molecule reactions are clearly operative, at least for the simpler species. Chemical isotope fractionation is reviewed, andd it is concluded that the complexities of the chemistry allow no cosmological conclusions to be drawn from observations of deuterium in interstellar molecules, while the presence of¹³C in interstellar molecules permits an estimate of the¹²C/¹³C ratio which is consistent with the current concepts of the nucleosynthesis history of the Galaxy. Possible connections between interstellar molecules and the early molecular history of the solar system are discussed.This is the first of a series of papers which will appear in the Synthesis section of the Journal of Molecular Evolution pertaining to the topic Organic Molecules in the Solar System and Beyond.Operated by Associated Universitites, Inc., under contract with the National Science Foundation     

Extraterrestrial Organic Matter: A review

We review the nature of the widespread organic material present in the Milky Way Galaxy and in the Solar System. Attention is given to the links between these environments and between primitive Solar System objects and the early Earth, indicating the preservation of organic material as an interstellar cloud collapsed to form the Solar System and as the Earth accreted such material from asteroids, comets and interplanetary dust particles. In the interstellar medium of the Milky Way Galaxy more than 100 molecular species, the bulk of them organic, have been securely identified, primarily through spectroscopy at the highest radio frequencies. There is considerable evidence for significantly heavier organic molecules, particularly polycyclic aromatics, although precise identification of individual species has not yet been obtained. The so-called diffuse interstellar bands are probably important in this context. The low temperature kinetics in interstellar clouds leads to very large isotopic fractionation, particularly for hydrogen, and this signature is present in organic components preserved in carbonaceous chondritic meteorites. Outer belt asteroids are the probable parent bodies of the carbonaceous chondrites, which may contain as much as 5% organic material, including a rich variety of amino acids, purines, pyrimidines, and other species of potential prebiotic interest. Richer in volatiles and hence less thermally processed are the comets, whose organic matter is abundant and poorly characterized. Cometary volatiles, observed after sublimation into the coma, include many species also present in the interstellar medium. There is evidence that most of the Earth's volatiles may have been supplied by a late bombardment of comets and carbonaceous meteorites, scattered into the inner Solar System following the formation of the giant planets. How much in the way of intact organic molecules of potential prebiotic interest survived delivery to the Earth has become an increasingly debated topic over the last several years. The principal source for such intact organics was probably accretion of interplanetary dust particles of cometary origin.     

Role of interstellar molecules in prebiological evolution

Three generations of organic molecules in space are considered: interstellar molecules, molecules synthesised in protosolar cloud and molecules synthesised on the Earth. It is shown that there is no possibilities for amino acid polymers to be synthesised under interstellar cloud conditions. Molecules of the second generation were disintegrated during the Earth accumulation period. The problem of the origin of life is connected with the evolution of molecules of the third generation.     

Chemical evolution in space

The bulk of complex molecules in the space between the stars is probably contained in small frozen interstellar dust grains. A typical grain is about as old as the earth and has, as a result of photochemical processing, converted a large fraction of its oxygen, carbon and nitrogen bearing mantle into large organic molecules whose maximum molecular weights are limited only by the grain size of about 0.1 m. Laboratory and theoretical methods provide the basis for explaining the evolution of interstellar grains from the time they are formed as seedlings in the atmospheres of cool evolved stars to the time they are destroyed by being incorporated into the material of new stars. The organic dust constitutes about one tenth of a percent of the total mass of the Milky Way and far outweighs any estimates of the total mass of all the planets. A planet like the earth is continually and directly accreting interstellar dust from space. Primitive carbonaceous meteorites show evidence of containing interstellar dust. Since comets are possibly almost pure aggregated interstellar dust they also provide a source of interstellar organic material on the earth.     

Interstellar Ices as a Source of CN-Bearing Molecules in Protoplanetary Disks

A reliable model for the composition and evolution of interstellar ices inregions of active star formation is fundamental to our quest to determinethe organic inventory of planetesimals in the early Solar System. This hasbecome a realistic goal since the launch of the Infrared Space Observatory,which provides a facility for infrared spectroscopy unhindered by telluricabsorption over the entire spectral range of vibrational modes in solids ofexobiological interest. Interstellar molecules detected in the solid phaseto date include H_{_2O}, NH₃, CO, CO₂,CH₃OH, CH₄, H₂CO, OCS andHCOOH, together with a CN-bonded absorber generically termed`XCN'. In this article, we focus on cosmic synthesis of CN-bearing species,as this important class of prebiotic molecules may not have formedendogenously in significant quantities on early Earth if conditions were nothighly reducing. Experiments in which interstellar ice analogs are subjectto UV photolysis or energetic ion bombardment yield CN-rich residues with aspectral signature that matches a corresponding feature observed in youngprotostars enshrouded in dust and gas. CN-bearing species are also presentin cometary ices, with a combined abundance comparable to the lower end of therange observed in protostars. Energetic processing of interstellar ices isthus a viable and potentially significant source of CN compounds inprotoplanetary disks.     

The Role of Supernova Neutrinos on Molecular Homochirality

Electroweak parity violating interaction between supernova (SN) neutrinos and electrons of a simple chiral molecule is studied related to the origin of molecular homochirality. Appearance of supernova remnants inside molecular clouds favours the interaction of SN-neutrinos with interstellar molecules, leading to a energetic difference between the two enantiomers of the order of 10^–5 eV. This energetic difference is closer to the thermic energy of the interstellar medium, so molecular homochirality could be enhanced in molecular clouds containing supernova remnants inside it due to neutrino interaction.     

Theoretical interstellar and prebiotic organic chemistry: A tentative methodology

A theoretical methodology for the systematic study of the interstellar molecules is proposed.Some examples, dealing with formaldehyde excited states, formyl radical and ion, reactivity of the excited states of formic acid, methyl cyanide and methyl acetylene, as well as the reaction path of formaldehyde photodecomposition are presented.Quantum chemical methods appear to be a powerful tool to study the structure and behaviour of molecules related with interstellar space and the Origin of Life.     

On the formation and observation of complex interstellar molecules

As of the present, a significant number of small molecules have been discovered in the interstellar medium. The largest molecule unambiguously detected, HC₁₁N, has only thirteen atoms. In this article, the prospects for observing far more complex species than this in interstellar clouds are discussed as are the mechanisms by which such complex species might be synthesized.     

The Diffuse Interstellar Bands: A Tracer For Organics In The Diffuse Interstellar Medium?

The diffuse interstellar bands (DIBs) are absorption bands seen in the spectra of stars obscured by interstellar dust. DIBs are recognized as a tracer for free, organic molecules in the diffuse interstellar medium (ISM). The potential molecular carriers for the DIBs are discussed with an emphasis on neutral and ionized polycyclic aromatic hydrocarbons (PAHs) for which the most focused effort has been made to date. From the combined astronomical, laboratory and theoretical study, it is concluded that a distribution of free neutral and ionized complex organics (PAHs, fullerenes, unsaturated hydrocarbons) represents the most promising class of candidates to account for the DIBs. The case for aromatic hydrocarbons appears particularly strong. The implied widespread distribution of complex organics in the diffuse ISM bears profound implications for our understanding of the chemical complexity of the ISM, the evolution of prebiotic molecules and its impact on the origin and the evolution of life on early Earth through the exogenous delivery (cometary encounters and metoritic bombardments) of prebiotic organics.     

Fluorescence Resonance Energy Transfer Near Thin Films on Surfaces

A theory is developed for resonant energy transfer between donor and acceptor molecules outside of a solid coated with a thin film. The energy transfer rate is expressed in terms of a second-rank tensor, allowing one to consider all possible orientations of the transition dipole moments of the molecules. The theory of images is employed to construct expressions valid in the near-field approximation. This theory is extended to the full electrodynamic theory valid over all distances. Connections are made between the expressions for the image charges and the Fresnel coefficients from optics. It is found that the energy transfer rates are strongly influenced by surface resonances, including the interfacial surface plasmons and the two-dimensional plasmon of a metallic film. The possibility of the film supporting Fabry–Perot resonator modes is discussed.     

CIRCUMSTELLAR AND INTERSTELLAR SYNTHESIS OF ORGANIC MOLECULES

We review the formation and evolution of complex circumstellar and interstellar molecules. A number of promising chemical routes are discussed which may lead to the formation of polycyclic aromatic hydrocarbon molecules, fullerenes, and unsaturated hydrocarbon chains in the outflows from stars. Some of the problems with these chemical schemes are pointed out as well. We also review the role of grains in the formation of complex molecules in interstellar molecular clouds. This starts with the formation of simple molecules in an ice grain mantle. UV photolysis and/or thermal polymerization can convert some of these simple molecules into more complex polymeric structures. Some of these species may be released to the gas phase, particularly in the warm regions around newly formed stars. Methanol and formaldehyde seem to play an important role in this drive towards molecular complexity and their chemistry is traced in some detail.     

Reactivity and Survivability of Glycolaldehyde in Simulated Meteorite Impact Experiments

Sugars of extraterrestrial origin have been observed in the interstellar medium (ISM), in at least one comet spectrum, and in several carbonaceous chondritic meteorites that have been recovered from the surface of the Earth. The origins of these sugars within the meteorites have been debated. To explore the possibility that sugars could be generated during shock events, this paper reports on the results of the first laboratory impact experiments wherein glycolaldehyde, found in the ISM, as well as glycolaldehyde mixed with montmorillonite clay, have been subjected to reverberated shocks from ~5 to >25 GPa. New biologically relevant molecules, including threose, erythrose and ethylene glycol, were identified in the resulting samples. These results show that sugar molecules can not only survive but also become more complex during impact delivery to planetary bodies.     

The prebiotic molecules observed in the interstellar gas

Over 130 molecules have been identified in the interstellar gas and circumstellar shells, the largest among them is a carbon chain with 13 atoms and molecular weight of 147 (twice that of the simplest amino acid glycine). The high reliability of astronomical identifications, as well as the fairly accurate quantitative analysis which can often be achieved, is emphasized. Glycine itself has been claimed, but a recent analysis indicates that few, if any, of the astronomical radio lines attributed to glycine are actually from that molecule. Polycyclic aromatic hydrocarbons (PAHs) have long been proposed as the source of the unidentified infrared bands between 3 and 16 microm, but no single PAH has been identified in space, partly because PAHs generally have weak or non-existent radio spectra. A remarkable exception is the non-planar corannulene molecule (C20H10) that has a strong radio spectrum; in the rich molecular cloud TMC-1, it is found that less than 10-5 of the carbon is contained in this molecule, suggesting that PAHs are not the dominant large molecules in the interstellar gas, as has been claimed. Owing to inherent spectroscopic limitations, determining the structures of the large molecules in space may require capture of the dust grains, which are continually entering the outer Solar System.     

A Relativistic Neutron Fireball from a Supernova Explosion as a Possible Source of Chiral Influence

We elaborate on a previously proposed idea that polarized electrons produced from neutrons, released in a supernova (SN) explosion, can cause chiral dissymmetry of molecules in interstellar gas-dust clouds. A specific physical mechanism of a relativistic neutron fireball with Lorentz factor of the order of 100 is assumed for propelling a great number of free neutrons outside the dense SN shell. A relativistic chiral electron–proton plasma, produced from neutron decays, is slowed down owing to collective effects in the interstellar plasma. As collective effects do not involve the particle spin, the electrons can carry their helicities to the cloud. The estimates show high chiral efficiency of such electrons. In addition to this mechanism, production of circularly polarized ultraviolet photons through polarized-electron bremsstrahlung at an early stage of the fireball evolution is considered. It is shown that these photons can escape from the fireball plasma. However, for an average density of neutrals in the interstellar medium of the order of 0.2 cm⁻³ and at distances of the order of 10 pc from the SN, these photons will be absorbed with a factor of about 10⁻⁷ due to the photoeffect. In this case, their chiral efficiency will be about five orders of magnitude less than that for polarized electrons.     

Cell junction and cyclic AMP: II. Modulations of junctional membrane permeability,dependent on serum and cell density

Summary Junctional molecular transfer (as indexed by the number of cell interfaces transferring fluorescent-labelled molecules) and concentration of endogenous cAMP were determined in mammalian cells in culture at varying serum concentration and cell density. In several cell types, on stepping the serum concentration from 10% (the concentration to which the cells had been adapted) to zero, the junctional transfer rose (reversibly) within 48 hr, as the endogenous cAMP concentration rose. The junctional transfer was inversely related to serum concentration over a range, most steeply so the transfer of large and charged molecules. one cell type showed no junctional change in response to serum; it showed also no endogenous cAMP change. Junctional transfer varied inversely with cell density over the range of 0.7–7 (10⁴ cells/cm²) in 3T3 cells. In cultures seeded to various densities, or growing to various densities on their own, junctional transfer fell with rising density, and so did the endogenous cAMP concentration. Upon downstep from high density, junctional transfer rose over 24–48 hr. In B cells, junctional transfer was independent of cell density over the aforementioned range, and so was the endogenous cAMP concentration. These results, in conjunction with the effects of exogenous cAMP described in the preceding paper of this series, point to a cAMP-mediated junctional effect; a possible teleonomy for control of membrane junction is discussed.     

Cosmic Carbon Chemistry: From the Interstellar Medium to the Early Earth

Astronomical observations have shown that carbonaceous compounds in the gas and solid state, refractory and icy are ubiquitous in our and distant galaxies. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surprisingly large number of molecules that are used in contemporary biochemistry on Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites, and interplanetary dust particles. In this article we review the current knowledge of abundant organic material in different space environments and investigate the connection between presolar and solar system material, based on observations of interstellar dust and gas, cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter. Current challenges in astrochemistry are discussed and future research directions are proposed.Carbon is a key element in the evolution of prebiotic material (Henning and Salama 1998), and becomes biologically interesting in compounds with nitrogen, oxygen and hydrogen. Our understanding of the evolution of organic molecules—including such compounds—and their voyage from molecular clouds to the early solar system and Earth provides important constraints on the emergence of life on Earth and possibly elsewhere (Ehrenfreund and Charnley 2000). Figure 1 shows the cycle of organic molecules in the universe. Gas and solid-state chemical reactions form a variety of organic molecules in circumstellar and interstellar environments. During the formation of the solar system, this interstellar organic material was chemically processed and later integrated in the presolar nebula from which planets and small solar system bodies formed. The remnant planetesimals in the form of comets and asteroids impacted the young planets in the early history of the solar system (Gomes et al. 2005). The large quantities of extraterrestrial material delivered to young planetary surfaces during the heavy bombardment phase may have played a key role in life''s origin (Chyba and Sagan 1992, Ehrenfreund et al. 2002). How elements are formed, how complex carbonaceous molecules in space are, what their abundance is and on what timescales they form are crucial questions within cosmochemistry.Open in a separate windowFigure 1.Carbon pathways between interstellar and circumstellar regions and the forming solar system.     

DETECTION OF ORGANIC MATTER IN INTERSTELLAR GRAINS

Star formation and the subsequent evolution of planetary systems occurs in dense molecular clouds, which are comprised, in part, of interstellar dust grains gathered from the diffuse interstellar medium (DISM). Radio observations of the interstellar medium reveal the presence of organic molecules in the gas phase and infrared observational studies provide details concerning the solid-state features in dust grains. In particular, a series of absorption bands have been observed near 3.4~2940 cm^–1) towards brightinfrared objects which are seen through large column densities of interstellar dust. Comparisons of organic residues, produced under a variety of laboratory conditions, to the diffuse interstellar medium observations have shown that aliphatic hydrocarbon grains are responsible for the spectral absorption features observed near 3.4 ~2940cm^–1). These hydrocarbons appear to carry the –CH₂– and –CH₃ functional groups in the abundance ratio CH₂/CH₃ ~ 2.5, and theamount of carbon tied up in this component is greater than 4% of the cosmic carbon available. On a galacticscale, the strength of the 3.4 band does notscale linearly with visual extinction, but instead increases more rapidly for objects near the Galactic Center. A similar trend is noted in the strength of the Si–O absorption band near 9.7. The similarbehavior of the C–H and Si–O stretching bands suggests that these two components may be coupled, perhaps in the form of grains with silicate cores and refractory organic mantles. The ubiquity of the hydrocarbon features seen in the near infrared near 3.4throughout our Galaxy and in other galaxies demonstrates the widespread availability of such material for incorporation into the many newly forming planetary systems. The similarity of the 3.4features in any organic material with aliphatic hydrocarbons underscores the need for complete astronomical observational coverage in the 2–30 region, oflines of sight which sample dust in both dense and diffuse interstellar clouds, in order to uniquely specify the composition of interstellar organics. This paper reviews the information available from ground-based observations, although currently the Infrared Satellite Observatory is adding to our body of knowledge on this subject by providing more extensive wavelength coverage. The Murchison carbonaceous meteorite has also been used as an analog to the interstellar observations and has revealed a striking similarity between the light hydrocarbons in the meteorite and the ISM; therefore this review includes comparisons with the meteoritic analog as well as with relevant laboratory residues. Fundamental to the evolution of the biogenic molecules, to the process of planetary system formation, and perhaps to the origin of life, is the connection between the organic material found in the interstellar medium and that incorporated in the most primitive solar system bodies.     

Chirality and life

The crucial role of homochirality and chiral homogeneity in the self-replication of contemporary biopolymers is emphasized, and the experimentally demonstrated advantages of these chirality attributes in simpler polymeric systems are summarized. The implausibility of life without chirality and hence of a biogenic scenario for the origin of chiral molecules is stressed, and chance and determinate abiotic mechanisms for the origin of chirality are reviewed briefly in the context of their potential viability on the primitive Earth. It is concluded that all such mechanisms would be non-viable, and that the turbulent prebiotic environment would require an ongoing extraterrestrial source for the accumulation of chiral molecules on the primitive Earth. A scenario is described wherein the circularly polarized ultraviolet synchrotron radiation from the neutron star remnants of supernovae engenders asymmetric photolysis of the racemic constituents in the organic mantles on interstellar dust grains, whereupon these chiral constituents are transported repetitively to the primative Earth by direct accretion of the interstellar dust or through impacts of comets and asteroids.