B. N. Khare

Tholins: organic chemistry of interstellar grains and gas

A complex organic solid produced from cosmically abundant molecules helps to explain many properties of the interstellar grains and gas.

Long-wavelength ultraviolet photoproduction of amino acids on the primitive Earth.

Amino acids are produced under possible primitive Earth conditions by irradiation of gas mixtures with long-wavelength ultraviolet light, representing the most abundant useful energy source for prebiological organic synthesis. Hydrogen sulfide is the initial photon acceptor in this work; superthermal atomic hydrogen photodissociation products appear to initiate reactions leading to amino acid synthesis with an overall quantum yield on the order of 5X10-5

Polycyclic aromatic hydrocarbons in the atmospheres of Titan and Jupiter

Polycyclic aromatic hydrocarbons (PAHs) are important components of the interstellar medium and carbonaceous chondrites, but have never been identified in the reducing atmospheres of the outer solar system. Incompletely characterized complex organic solids (tholins) produced by irradiating simulated Titan atmospheres reproduce well the observed UV/visible/IR optical constants of the Titan stratospheric haze. Titan tholin and a tholin generated in a crude simulation of the atmosphere of Jupiter are examined by two-step laser desorption/multiphoton ionization mass spectrometry. A range of two- to four-ring PAHs, some with one to four alkylation sites are identified, with net abundance approximately 10(-4) g g-1 (grams per gram) of tholins produced. Synchronous fluorescence techniques confirm this detection. Titan tholins have proportionately more one- and two-ring PAHs than do Jupiter tholins, which in turn have more four-ring and larger PAHs. The four-ringed PAH chrysene, prominent in some discussions of interstellar grains, is found in Jupiter tholins. Solid state 13C NMR spectroscopy suggests approximately equal to 25% of the total C in both tholins is tied up in aromatic and/or aliphatic alkenes. IR spectra indicate an upper limit in both tholins of approximately equal to 6% by mass in benzenes, heterocyclics, and PAHs with more than four rings. Condensed PAHs may contribute at most approximately 10% to the observed detached limb haze layers on Titan. As with interstellar PAHs, the synthesis route of planetary PAHs is likely to be via acetylene addition reactions.

Cosmic Dust Synthesized in Reducing Environments

A variety of complex dark brown organic solids called tholins have been produced from a mixture of cosmically abundant gases upon irradiation with ultraviolet light or electrical discharges (Sagan and Khare 1979). Such tholins were probably produced in the primitive solar nebula that contributed to the present composition of carbonaceous chondrites, comets, interstellar grains and gas. Spark tholins produced from approximately equimolar mixtures of CH 4 and NH 3 , with 2.6% H 2 0 exhibit 50% thermal dissociation temperature of about 900°C. Sequential and non-sequential pyrolysis followed by gas chromatography/mass spectrometry are employed to study both UV (Khare et al. 1978) and spark tholins. Typical pyrolyzates of spark tholins include alkanes, alkenes, aromatic hydrocarbon, abundant nitriles, pyrroles, pyrazines and alkylbenzenes. Organic molecules uncovered by microwave line spectroscopy of the interstellar gas are plausibly derived as spallation products of interstellar tholins comparable to the origin we propose for cometary nitriles and aldehydes as the spallation products of the cometary tholins. Spark tholins are highly insulating. Their measured resistivity is 71.4 × 10 8 ohms-cm and does not change up to a pressure of 10 5 bars.