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[|DOI]
 * Sagan, Carl & B.N. Khare (1979). Tholins: organic chemistry of interstellar grains and gas. Nature 277, 102-107.**


 * Sagan and Khare start by explaining the challenges in characterizing the brown, sticky residue created when mixtures of cosmically abundant gases were exposed to UV light or spark discharge. A new term, “tholins”, is introduced to categorize these molecules; this comes from the Greek word for “muddy”, but can also mean “vault” or “dome”.
 * UV tholins originate when near-UV light photodissociates H2C or HCHO, while electrical discharges create spark tholins. The authors found amino acid precursors in these tholins and were able to isolate amino acids such as cystine upon hydrolysis. They state the spallation products of tholins give rise to the organic molecules found in interstellar gas.
 * It is noted that the abundance of interstellar organics do not decline much with heavy atom numbers, suggesting an origin from dissociation products rather than in-situ formation. Interstellar grains may have a composition resembling carbonaceous chondrites and cometary nuclei.
 * Experimental near-UV irradiation at 77K of HCHO, H2O, NH3 and C2H6 on a silicate matrix resulted in the formation of simple organic compounds. This indicated that in-situ organic synthesis on ice-coated grains is possible. The Frank-Rabinowitch principle predicts polycyclic aromatics, complexly branched aliphatics, and matrices of mixed rings and aliphatics would be the most dominant molecules.
 * IR absorption data on organic polymers matches interstellar absorption. Hoyle and Wickramasinghe believed polyoxymethylene and polysaccharides were major components of interstellar grains, but no mechanism of production has been proposed. Such molecules lack N and S, which would be expected based on their cosmic abundance.
 * IR spectra varies greatly between galactic sources. There are problems when attempting to attribute certain spectral features to water/ammonia ice or silicates. The authors say tholins can help to account for these anomalies.
 * Spark and UV tholins produce similar IR spectra. At higher temperatures (450 C), the shorter bands disappear. Long polymer tholins may give different spectra than those of monomers.
 * 6.0 and 6.8 um features in the IR spectrum of NGC7538E corresponds with early tholin experiments in which long-chain alkanes (at 3.42 um and 6.87 um) and alkenes (at 6.0 um) were present. Many IR sources in the galaxy were showed evidence of C-C and C-N triple bonds, and these are attributable to typical cosmic abundances.
 * Good agreement was observed between Mokoia carbonaceous chondrite and Hoyle’s best fit galactic spectrum, but more studies need to be done to determine the chemistry of other carbonaceous chondrite samples.
 * The observed UV interstellar absorption at 2200 angstroms might be attributable to graphite or polycyclic aromatic hydrocarbons, but these are not expected to be identifiable because well-ordered structures are unlikely in such an environment. A 2200 angstrom feature is also present in hexafluoroisopropanol, indicating the presence of molecules with conjugated double bonds.
 * In the opticial region, at 4428 angstroms, a previously unidentified line has been hypothesized to be the Soret band of porphyrins (tetrapyrroles), but this theory has complications. A more plausible explanation may be long-chain C molecules; pyrolysis fragments show dienes and longer polyenes are likely to be present in tholins.
 * Larger tholin particles would not be ejected during the Hayashi or red giant stages of stellar evolution, are therefore stay in circumstellar space.
 * Sagan and Khare conclude that organic molecules in microwave line spectroscopy are generally the result of spallation, spluttering or photodissociation of tholins in interstellar grains. The lifetime of these grains is 10^8 to 10^9 years, after which the UV-induced vaporization products of condensates, or the pyrolyzates of tholins, should be evident.
 * Observations of cometary spectra have revealed nitriles and aldehydes which could have originated from tholins during the formation of the solar system. The authors propose conducting in situ cometary studies through a flyby spacecraft mission. Additionally, tholin fragments should be studied further in the lab using UV, visible, IR and microwave spectroscopy.
 * The widespread production of these complex molecules in a preplanetary cloud suggests that biologically important molecules would have easily formed on the primordial Earth, after the planet had cooled sufficiently. It is unlikely that indigenous organisms from before this point would have survived. While similar organic chemistry may exist on other worlds, any extraterrestrial organisms would likely have evolved with very different biochemistry.
 * The research described in the paper was funded by a grant from NASA’s Planetary Biology and Planetary Protection program.