2020同步年報

Chemical Science 021 A stronomical observations performed with airborne telescopes, equipped with an infrared spectrometer, show absorption bands of volatile species in the solid phase and a broad absorption due to silicates. These spectra correspond to (sub)micrometer silicate dust in interstellar and protoplanetary environments. Here the dust tempera- tures near 10 K allow the growth of an ice mantle on the dust grains, which is composed of simple species found to absorb in the infrared region, i.e ., water (H 2 O) and smaller amounts of carbon monoxide (CO), carbon dioxide (CO 2 ), methanol (CH 3 OH), methane (CH 4 ), ammonia (NH 3 ) and other molecules. Laboratory simulations of multicomponent ice mantles performed at 10 K under ultra-high- vacuum conditions include irradiation with ultraviolet or X-ray photons. The irradiated ice samples are warmed to allow diffusion of the generated radicals and other reactive species. After subli- mation of volatile components, an organic residue remains at room temperature. Such a residue contains a plethora of organic molecules of astrobiological interest; among them are amino acids, nucleobases, sugars, carboxylic acids etc . It is generally thought that such agglomeration of icy dust grains leads to formation of cometesimals and plane- tesimals. Indeed, some meteorites contain similar organic species. Cometary missions such as Stardust and Rosetta detected glycine, acetamide and several other prebiotic species in comets that are readily formed in experiments on ice irradiation. Cometary and asteroid impacts on the early Earth delivered water and such organic species that likely contributed to the origin of life. Protoplanetary discs The formation of such complicated organic substances would have added value if it occurred in the gas surround- ing a young solar-type star, implying that their synthesis would be coeval with the formation of planets in a proto- planetary system. Star formation is a violent and chaotic event in which a gas flows in and is ejected outwards at speeds up to hundreds of km per second. This effect oc- curs because the gravitational infall is locally opposed by thermal, turbulent and magnetic pressures, by dynamical outflows, and, as the parent cloud is rotating, by effects of angular momentum. As a consequence of all such compet- Organic Chemistry in Space – X-ray Processing of a Realistic Ice Mantle The absence of complicated species from the cold gas in protoplanetary disks, and the presence of abundant CO, HCO and H 2 CO and negligible CH 3 OH, is compatible with the simulated realistic ice with X-ray. The particularly small abundances of other COMs in the cold parts of the disk are formed in the ice bulk but not ejected into the gaseous phase. ing processes, the contracting cloud forms a swirling disc. Circumstellar discs are an inevitable consequence of the conservation of angular momentum during the formation of a star through gravitational collapse. Initially, discs rap- idly funnel material onto the star but, as the surrounding molecular core is consumed or otherwise disperses, the rate of accretion decreases; only a small proportion of the original material persists in the disc. That these discs can be considered protoplanetary is apparent not only in the geometry of the Solar System but also in the large rate of detection of exoplanets. Observing discs around solar-type stars, we might catch a glimpse of the chemical evolution preceding the onset of life on our planet. Young solar-type stars emit X-rays at a level 3–4 orders of magnitude greater than the present-day Sun. For a 100-Myr-old star the X-ray flux is larger than the vacuum- and extreme-ultraviolet emission; their ratio remains within a factor two for stars as old as 1 Gyr. X-rays, being much more penetrating than ultraviolet radiation, might illuminate cold regions of a disc and, therefore, are a major agent in processing circumstel- lar material. The molecular content of protoplanetary discs The physical conditions in a protoplanetary disc vary great- ly, with hot and dense regions of gas and dust near the star and much colder material at greater distances from it. In protoplanetary discs, the inner edge of the region at which the temperature falls below the condensation tem- perature of a volatile substance is referred to as the snow line (for that species). Each volatile has a distinct location of snow line, water ice being nearest the host star, farther on CO 2 and then CO. As a consequence, the ices within a disc are organized in a bi-layered structure of segregated polar (water-rich) and apolar (water-poor) components. The initial water-rich layer is thought to form early in the disc through hydrogenation of atomic oxygen. The bulk of solid CH 4 and NH 3 is likely also formed at this stage, through hydrogenation of carbon and nitrogen. As the disc gradu- ally cools, free-flying molecules are removed from the gas, of which the main component (after volatile H 2 not sticking to dust) is carbon monoxide. The formation of a layer of CO ice provides a feedstock for the formation of icy methanol through hydrogenation of CO. The birth of methanol marks the first generation of complicated species.