Evolution of Interstellar Ices

Abstract. Infrared observations, combined with realistic laboratory simulations, have revolutionized our understanding of interstellar ice and dust, the building blocks of comets. Ices in molecular clouds are dominated by the very simple molecules H2O, CH3OH, NH3, CO, CO2, and proba-bly H2CO and H2. More complex species including nitriles, ketones, and esters are also present, but at lower concentrations. The evidence for these, as well as the abundant, carbon-rich, inter-stellar, polycyclic aromatic hydrocarbons (PAHs) is reviewed. Other possible contributors to the interstellar/pre-cometary ice composition include accretion of gas-phase molecules and in situ pho-tochemical processing. By virtue of their low abundance, accretion of simple gas-phase species is shown to be the least important of the processes considered in determining ice composition. On the other hand, photochemical processing does play an important role in driving dust evolution and the composition of minor species. Ultraviolet photolysis of realistic laboratory analogs read-ily produces H2, H2CO, CO2, CO, CH4, HCO, and the moderately complex organic molecules: CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), R-CN (nitriles), and hexamethylenetetramine (HMT, C6H12N4), as well as more complex species including amides, ke-tones, and polyoxymethylenes (POMs). Inclusion of PAHs in the ices produces many species simila

Analyses of Aliphatic Aldehydes and Ketones in Carbonaceous Chondrites

Aliphatic aldehydes and ketones are essential building blocks for the synthesis of more complex organic compounds. Despite their potentially key role as precursors of astrobiologically important molecules, such as amino acids and carboxylic acids, this family of compounds has scarcely been evaluated in carbonaceous chondrites. The paucity of such analyses likely derives from the low concentration of aldehydes and ketones in the meteorites and from the currently used chromatographic methodologies that have not been optimized for meteorite analysis. In this work, we report the development of a novel analytical method to quantify the molecular distribution and compound-specific isotopic analysis of 29 aliphatic aldehydes and ketones. Using this method, we have investigated the molecular distribution and ¹³C-isotopic composition of aldehydes and ketones in 10 carbonaceous chondrites from the CI, CM, CR, and CV groups. The total concentration of carbonyl compounds ranged from 130 to 1000 nmol g⁻¹ of meteorite with formaldehyde, acetaldehyde, and acetone being the most abundant species in all investigated samples. The ¹³C-isotopic values ranged from −67 to +64‰ and we did not observe clear relationships between ¹³C-content and molecular weight. Accurately measuring the relative abundances, determining the molecular distribution, and isotopic composition of chondritic organic compounds is central in assessing both their formation chemistry and synthetic relationships