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Author(s): Xi Zhang (corresponding author) [1]; Darrell F. Strobel [2]; Hiroshi Imanaka [3, 4] Plutos atmosphere is cold and hazy [1, 2, 3]. Recent observations [1] have shown it to [...]

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.08.031 Byline: Yasuhito Sekine (a), Hiroshi Imanaka (b)(c), Takafumi Matsui (d), Bishun N. Khare (e)(f), Emma L.O. Bakes (c)(e), Christopher P. McKay (e), Seiji Sugita (d) Keywords: Titan; Organic chemistry; Satellites; atmospheres; Atmospheres; chemistry Abstract: In Titan's atmosphere consisting of N.sub.2 and CH.sub.4, large amounts of atomic hydrogen are produced by photochemical reactions during the formation of complex organics. This atomic hydrogen may undergo heterogeneous reactions with organic aerosol in the stratosphere and mesosphere of Titan. In order to investigate both the mechanisms and kinetics of the heterogeneous reactions, atomic deuterium is irradiated onto Titan tholin formed from N.sub.2 and CH.sub.4 gas mixtures at various surface-temperatures of the tholin ranging from 160 to 310 K. The combined analyses of the gas species and the exposed tholin indicate that the interaction mechanisms of atomic deuterium with the tholin are composed of three reactions; (a) abstraction of hydrogen from tholin resulting in gaseous HD formation (HD recombination), (b) addition of D atom into tholin (hydrogenation), and (c) removal of carbon and/or nitrogen (chemical erosion). The reaction probabilities of HD recombination and hydrogenation are obtained as I*.sub.abst=1.9([+ or -]0.6)x10.sup.-3xexp(-300/T) and I*.sub.hydro=2.08([+ or -]0.64)xexp(-1000/T), respectively. The chemical erosion process is very inefficient under the conditions of temperature range of Titan's stratosphere and mesosphere. Under Titan conditions, the rates of hydrogenation > HD recombination a' chemical erosion. Our measured HD recombination rate is about 10 times (with an uncertainty of a factor of 3-5) the prediction of previous theoretical model. These results imply that organic aerosol can remove atomic hydrogen efficiently from Titan's atmosphere through the heterogeneous reactions and that the presence of aerosol may affect the subsequent organic chemistry. Author Affiliation: (a) Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan (b) Department of Chemistry, University of Arizona, 1306 E. University Blvd., Tucson, AZ 85721-0041, USA (c) SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA (d) Department of Complexity Science & Engineering, Graduate School of Frontier Science, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 227-8562, Japan (e) Space Science Division, NASA Ames Research Center, Moffett Field, CA 94035-1000, USA (f) The Carl Sagan Center for the Study of Life in the Universe, SETI Institute, 515 N. Whisman Road, Mountain View, CA 94043, USA Article History: Received 9 February 2007; Revised 1 August 2007

Titan, the largest satellite of Saturn, has a thick nitrogen/methane atmosphere with a thick global organic haze. A laboratory analogue of Titan's haze, called tholin, was formed in an inductively coupled plasma from nitrogen/methane = 90/10 gas mixture at various pressures ranging from 13 to 2300 Pa. Chemical and optical properties of the resulting tholin depend on the deposition pressure in cold plasma. Structural analyses by IR and UV/VIS spectroscopy, microprobe laser desorption/ionization mass spectrometry, and Raman spectroscopy suggest that larger amounts of aromatic ring structures with larger cluster size are formed at lower pressures (13 and 26 Pa) than at higher pressures (160 and 2300 Pa). Nitrogen is more likely to incorporate into carbon networks in tholins formed at lower pressures, while nitrogen is bonded as terminal groups at higher pressures. Elemental analysis reveals that the carbon/nitrogen ratio in tholins increases from 1.5-2 at lower pressures to 3 at 2300 Pa. The increase in the aromatic compounds and the decrease in C/N ratio in tholin formed at low pressures indicate the presence of the nitrogen-containing polycyclic aromatic compounds in tholin formed at low pressures. Tholin formed at high pressure (2300 Pa) consists of a polymer-like branched chain structure terminated with -C[H.sub.3], -N[H.sub.2], and -C[equivalent to]N with few aromatic compounds. Reddish-brown tholin films formed at low pressures (13-26 Pa) shows stronger absorptions (almost 10 times larger k-value) in the UV/VIS range than the yellowish tholin films formed at high pressures (160 and 2300 Pa). The tholins formed at low pressures may be better representations of Titan's haze than those formed at high pressures, because the optical properties of tholin formed at low pressures agree well with that of Khare et al. (1984a, Icarus 60, 127-137), which have been shown to account for Titan's observed geometric albedo. Thus, the nitrogen-containing polycyclic aromatic compounds we find in tholin formed at low pressure may be present in Titan's haze. These aromatic compounds may have a significant influence on the thermal structure and complex organic chemistry in Titan's atmosphere, because they are efficient absorbers of UV radiation and efficient charge exchange intermediaries. Our results also indicate that the haze layers at various altitudes might have different chemical and optical properties. Keywords: Titan; Haze; Tholin; Spectroscopy; Organic chemistry; Nitrogen-containing polycyclic aromatic compounds

Haze particles exert a significant influence over the thermodynamics and radiation absorption properties of the Titan haze, as well as its complex organic chemistry. Characterization of both the molecular and the submicrometer components of the haze is therefore vital for understanding the global properties of Titan. We have carried out a Titan tholin synthesis experiment and measured the time variation of the infrared spectrum of the product as a thin film developed. Also, to examine the possibility of oxygen contamination, we compared the infrared spectrum of the tholin film with that of a tholin film exposed to dry air and laboratory air. The objective of this study is to understand the chemical processes related to how simple organic molecules are processed into more complex haze particles. The progressive development of features characteristic of amines, aromatic and aliphatic hydrocarbons, and nitriles in the experimental mixture is clear. Of particular interest is the formation of aromatic rings after only a few seconds of glow discharge, indicating that these compounds appear to be intermediates between simple haze molecules and microphysical aerosols. The early dominance of aromatic ring structures is accompanied during the later stages of the experiment by the appearance of nitrile and amine compounds. This time-dependent succession of chemical structures provides vital clues to the possible chemical formation pathways of Titan haze aerosols. Key Words: Titan; tholins; stratospheric chemistry; aromatics; amines; alkanes; hydrocarbons; aerosols.

The taxonomy of the partner strain ofthe mixed culture in the simulated production of biphenomycin A and the role ofeach microorganism in the mixed culture are discussed. High-performance liquid chromatography was used for the taxonomic study of the strain which was isolated from soil samples. Only a small amount of biphenomycin A was produced by Streptomyces griseorubiginosus.

A successful fictionalization of single-walled carbon nanotubes (SWNTs), through a microwave discharge of ammonia was carried out. The results of ab initio calculations are reported to better identify the experimental products that are formed.