▪ Abstract Our understanding of the evolution of organic molecules, and their voyage from molecular clouds to the early solar system and Earth, has changed dramatically. Incorporating recent observ...

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Organic Molecules in the Interstellar Medium, Comets, and Meteorites: A Voyage from Dark Clouds to the Early Earth

Various present and future specialized applications of magnets require monodisperse, small magnetic particles, and the discovery of molecules that can function as nanoscale magnets was an important development in this regard. These molecules act as single-domain magnetic particles that, below their blocking temperature, exhibit magnetization hysteresis, a classical property of macroscopic magnets. Such 'single-molecule magnets' (SMMs) straddle the interface between classical and quantum mechanical behaviour because they also display quantum tunnelling of magnetization and quantum phase interference. Quantum tunnelling of magnetization can be advantageous for some potential applications of SMMs, for example, in providing the quantum superposition of states required for quantum computing. However, it is a disadvantage in other applications, such as information storage, where it would lead to information loss. Thus it is important to both understand and control the quantum properties of SMMs. Here we report a supramolecular SMM dimer in which antiferromagnetic coupling between the two components results in quantum behaviour different from that of the individual SMMs. Our experimental observations and theoretical analysis suggest a means of tuning the quantum tunnelling of magnetization in SMMs. This system may also prove useful for studying quantum tunnelling of relevance to mesoscopic antiferromagnets.

Exchange-biased quantum tunnelling in a supramolecular dimer of single-molecule magnets

Amino acids are the essential molecular components of living organisms on Earth, but the proposed mechanisms for their spontaneous generation have been unable to account for their presence in Earth's early history1. The delivery of extraterrestrial organic compounds has been proposed as an alternative to generation on Earth2,3,4,5, and some amino acids have been found in several meteorites6,7,8,9. Here we report the detection of amino acids in the room-temperature residue of an interstellar ice analogue that was ultraviolet-irradiated in a high vacuum at 12 K. We identified 16 amino acids; the chiral ones showed enantiomeric separation. Some of the identified amino acids are also found in meteorites. Our results demonstrate that the spontaneous generation of amino acids in the interstellar medium is possible, supporting the suggestion that prebiotic molecules could have been delivered to the early Earth by cometary dust, meteorites or interplanetary dust particles.

Amino acids from ultraviolet irradiation of interstellar ice analogues.

Stellar nucleosynthesis of heavy elements, followed by their subsequent release into the interstellar medium, enables the formation of stable carbon compounds in both gas and solid phases. Spectroscopic astronomical observations provide evidence that the same chemical pathways are widespread both in the Milky Way and in external galaxies. The physical and chemical conditions—including density, temperature, ultraviolet radiation and energetic particle flux—determine reaction pathways and the complexity of organic molecules in different space environments. Most of the organic carbon in space is in the form of poorly-defined macromolecular networks. Furthermore, it is also unknown how interstellar material evolves during the collapse of molecular clouds to form stars and planets. Meteorites provide important constraints for the formation of our Solar System and the origin of life. Organic carbon, though only a trace element in these extraterrestrial rock fragments, can be investigated in great detail with sensitive laboratory methods. Such studies have revealed that many molecules which are essential in terrestrial biochemistry are present in meteorites. To understand if those compounds necessarily had any implications for the origin of life on Earth is the objective of several current and future space missions. However, to address questions such as how simple organic molecules assembled into complex structures like membranes and cells, requires interdisciplinary collaborations involving various scientific disciplines. Introduction Life in the Universe is the consequence of the increasing complexity of chemical pathways which led to stable carbon compounds assembling into cells and higher organisms.

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A voyage from dark clouds to the early Earth

Recent advances in the understanding of the chemical processes that occur during all stages of the formation of stars, from the collapse of molecular clouds to the assemblage of icy planetesimals in protoplanetary accretion disks, are reviewed. Observational studies of the circumstellar material within 100‐10,000 AU of the young star with (sub)millimeter single-dish telescopes, millimeter interferometers, and ground-based as well as space-borne infrared observatories have only become possible within the past few years. Results are compared with detailed chemical models that emphasize the coupling of gas-phase and grain-surface chemistry. Molecules that are particularly sensitive to different routes of formation and that may be useful in distinguishing between a variety of environments and histories are outlined. In the cold, low-density prestellar cores, radicals and long unsaturated carbon chains are enhanced. During the cold collapse phase, most species freeze out onto the grains in the high-density inner region. Once young stars ignite, their surroundings are heated through radiation and/or shocks, whereupon new chemical characteristics appear. Evaporation of ices drives a “hot core” chemistry rich in organic molecules, whereas shocks propagating through the dense envelope release both refractory and volatile grain material, resulting in prominent SiO, OH, and H2O emission. The role of future instrumentation in further developing these chemical and temporal diagnostics is discussed.

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Chemical evolution of star-forming regions

A set of laboratory experiments is presented that study the evolution of solid organic matter at low temperature as a result of exposure to photons and ions, with the aim of simulating the formation and evolution of organic refractory mantles on interstellar grains. Organic residue is a product of UV photolysed ice mantles in dense clouds and consists of 2-3 C atom containing compounds. From laboratory experiments, the efficiency of organic residue formation is given as a function of UV radiation dose. The yield of organic residue is so high that it probably is the major source of solid carbonaceous matter in the interstellar medium

Carbon dust formation on interstellar grains

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.

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

The photochemical effects of the interstellar radiation field on the ice mantles of molecular-cloud dust grains are investigated by means of laboratory simulations. The apparatus and analytical methods described by Hagen et al. (1979) and Hagen (1982) are employed, and the results are presented as spectra and discussed in detail. Consideration is given to the strong IR absorption bands of OCN(-) and NH4(+) which appear after photolysis and heating of H2O/NH3/CO/O2 ices, the detection of similar bands toward W33A, the critical role of NH3 in the photochemical production of ions, and the use of calculated integrated absorption-band strengths to estimate astrophysical column densities.

Infrared spectroscopy of astrophysical ices - New insights in the photochemistry

The stability of the polycyclic aromatic hydrocarbon (PAH) coronene has been studied under conditions representing those in interstellar space. It is concluded that although polycyclic aromatic material can survive in diffuse clouds, it is rapidly destroyed after accretion on interstellar dust in dense clouds. This limits the lifetime of such material in space to less than or equal to 5 x 10(exp 7) years.

J. M. Greenberg

Papers

Carbon dust formation on interstellar grains

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

Infrared spectroscopy of astrophysical ices - New insights in the photochemistry

The fate of polycyclic aromatic material in space