THE TITAN HAZE REVISITED - MAGNETOSPHERIC ENERGY-SOURCES AND QUANTITATIVE THOLIN YIELDS

We present laboratory measurements of the radiation yields of complex organic solids produced from N-2/CH4 gas mixtures containing 10 or 0.1 % CH4. These tholins are thought to resemble organic aerosols produced in the atmospheres of Titan, Pluto, and Triton. The tholin yields are large compared to the total yield of gaseous products: nominally, 13 (C + N)/100 eV for Titan tholin and 2.1 (C + N)/100 eV for Triton thol in. High-energy magnetospheric electrons responsible for tholin produc tion represent a class distinct from the plasma electrons considered i n models of Titan's airglow. Electrons with E > 20 keV provide an ener gy flux similar to 1 x 10(-2) erg cm(-2) sec(-1), implying from our me asured tholin yields a mass flux of 0.5 to 4.0 x 10(-14) g cm(-2) sec( -1) of tholin. (The corresponding thickness of the tholin sedimentary column accumulated over 4 Gyr on Titan's surface is 4 to 30 m.) This f igure is in agreement with required mass fluxes computed from recent r adiative transfer and sedimentation models. If, however, these results , derived from experiments at similar to 2 mb, are applied to lower pr essure levels toward peak auroral electron energy deposition and scale d with pressure as the gas-phase organic yields, the derived tholin ma ss flux is at least an order of magnitude less. We attribute this diff erence to the fact that tholin synthesis occurs well below the level o f maximum electron energy deposition and to possible contributions to tholins from UV-derived C-2-hydrocarbons. We conclude that Titan tholi n, produced by magnetospheric electrons, is alone sufficient to supply at least a significant fraction of Titan's haze-a result consistent w ith the fact that the optical properties of Titan tholin, among all pr oposed materials, are best at reproducing Titan's geometric albedo spe ctrum from near UV to mid-IR in light-scattering models. (C) 1994 Acad emic Press, Inc.