Cosmic Dust Synthesized in Reducing Environments

Extract

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 CH₄ and NH₃, with 2.6% H₂0 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 × 10⁸ ohms-cm and does not change up to a pressure of 10⁵ bars.