▪ 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...

/pdf/organic-molecules-in-the-interstellar-medium-comets-and-14nzw41xt5.pdf

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

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.

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.

/pdf/chemical-evolution-of-star-forming-regions-2j830kxyvn.pdf

Chemical evolution of star-forming regions

This work reviews factors which are important for the evolution of habitable Earth-like planets such as the effects of the host star dependent radiation and particle fluxes on the evolution of atmospheres and initial water inventories. We discuss the geodynamical and geophysical environments which are necessary for planets where plate tectonics remain active over geological time scales and for planets which evolve to one-plate planets. The discoveries of methane–ethane surface lakes on Saturn’s large moon Titan, subsurface water oceans or reservoirs inside the moons of Solar System gas giants such as Europa, Ganymede, Titan and Enceladus and more than 335 exoplanets, indicate that the classical definition of the habitable zone concept neglects more exotic habitats and may fail to be adequate for stars which are different from our Sun. A classification of four habitat types is proposed. Class I habitats represent bodies on which stellar and geophysical conditions allow Earth-analog planets to evolve so that complex multi-cellular life forms may originate. Class II habitats includes bodies on which life may evolve but due to stellar and geophysical conditions that are different from the class I habitats, the planets rather evolve toward Venus- or Mars-type worlds where complex life-forms may not develop. Class III habitats are planetary bodies where subsurface water oceans exist which interact directly with a silicate-rich core, while class IV habitats have liquid water layers between two ice layers, or liquids above ice. Furthermore, we discuss from the present viewpoint how life may have originated on early Earth, the possibilities that life may evolve on such Earth-like bodies and how future space missions may discover manifestations of extraterrestrial life.

/pdf/what-makes-a-planet-habitable-4fal2p0i4z.pdf

What makes a planet habitable

Carbonaceous chondrites comprise a unique subset of meteorites. Two classes of carbonaceous chondrites, the so-called CI1 and CM2 chondrites, are particularly interesting, in part because of their relatively high carbon content and the fact that most of this carbon is present as organic matter. This material is largely macromolecular but also contains a complex mixture of organic compounds that include carboxylic acids, dicarboxylic acids, amino acids, hydroxy acids, sulfonic acids, phosphonic acids, amines, amides, nitrogen heterocycles including purines and a pyrimidine, alcohols, carbonyl compounds, and aliphatic, aromatic, and polar hydrocarbons. The organic-rich CI1 and CM2 chondrites also contain an extensive clay mineralogy and other minerals that are believed to be indicative of an early episode of hydrous activity in the meteorite parent body. Recent stable isotope measurements have shown the organic matter in general, to be substantially enriched in deuterium and the discrete organic compounds to be enriched in 15N and somewhat enriched in 13C relative to terrestrial matter. These findings suggest that the organic matter is comprised of, or is closely related to, interstellar organic compounds. The organic chemistry of these meteorites is consistent with a formation scheme in which (1) a parent body was formed from volatile-rich icy planetesimals containing interstellar organic matter, (2) warming of the parent body led to an extensive aqueous phase in which the interstellar organics underwent various reactions, and (3) residual volatiles were largely lost leaving behind the suite of nonvolatile compounds that now characterize these meteorites.

Organic matter in meteorites: molecular and isotopic analyses of the Murchison meteorite.

Photolysis of mixtures of CO:NH3:H2O at 12 K results in the formation of an organic residue which is not volatile in high vacuum at room temperature. Analysis of this fraction by GC-MS resulted in the detection of C2-C3 hydroxy acids and hydroxy amides, glycerol, urea, glycine, hexamethylene tetramine, formamidine and ethanolamine. Use of isotopically labeled gases made it possible to establish that the observed products were not contaminants. The reaction pathways for the formation of these products were determined from the position of the isotopic labels in the mass spectral fragments. The significance of these findings to the composition of comets and the origins of life is discussed.

Comet Halley as an aggregate of interstellar dust and further evidence for the photochemical formation of organics in the interstellar medium

Interstellar grains and gas comprise about 10% of the mass of the stars in our galaxy. A typical grain consists of a silicate core and a mantle. If the grain is present in a diffuse cloud of interstellar gas, the mantle consists of relatively nonvolatile higher molecular weight organic compounds. Grains in dense clouds of interstellar dust are believed to be coated with volatile inorganic and organic compounds such as H20, CH4, CO2, CO, H2S, $2, NH3, ions such as OCNand higher molecular weight organics.

P. J. McCain

Papers

Comet Halley as an aggregate of interstellar dust and further evidence for the photochemical formation of organics in the interstellar medium

Photochemical reactions on interstellar grains