Light hydrocarbons from plasma discharge in H2-He-CH4: first results and Uranian auroral chemistry.

Voyager 2 found that the Uranian magnetosphere has a substantial flux of energetic charged particles, which becomes rich in higher energies at low magnetospheric L near the orbit of Miranda. The electrons precipitate to produce aurorae, which have been observed in the ultraviolet. The more energetic component of the precipitating electrons can initiate radiation chemistry in the methane-poor stratosphere, near 0.1 mbar where the CH4 mole fraction XCH4 approximately equal to 10(-5). We present laboratory results for cold plasma (glow) discharge in continuous flow H2-He-CH4 atmospheres with mol fractions XCH4 = 10(-2) to 10(-3) and total pressure p = 60 to 0.6 mbar. The yields of simple hydrocarbons in these experiments and an estimate of precipitating electron flux consistent with the Voyager ultraviolet spectroscopy results indicate the globally averaged auroral processing rate of CH4 to higher hydrocarbons approximately equal to 3 x 10(6) C cm-2 s-1, comparable to the globally averaged photochemical production rate. The local rate approximately 2 x 10(8) C cm-2 s-1 in the auroral zones (approximately 20 degrees in diameter) at 15 degrees S and 45 degrees N latitude greatly exceeds the photochemical rate. Even at very low XCH4 approximately equal to 10(-3) the yield (summed over all products) G > approximately 10(-2) C/100 eV and the average slope alpha = <log10¿eta sigma [C eta Hx]/(eta - 1) sigma [C eta - 1 Hx]¿> > approximately -0.4, where the summation is over all product molecules of a given carbon number eta and the square brackets denote abundance. The yield therefore decreases slowly with molecular complexity: hydrocarbons through C7Hx should be present in auroral zones at abundances > approximately 10(-2) of the simplest C2 hydrocarbons. Saturated hydrocarbons (C2H6, C3H8, C4H10, etc.) are mostly shielded from photodissociation by C2H2 and will therefore persist at the sunlit, as well as the currently dark, magnetic polar regions.