POLYCYCLIC AROMATIC-HYDROCARBONS IN THE ATMOSPHERES OF TITAN AND JUPITER

Polycyclic aromatic hydrocarbons (PAHs) are important components of th e interstellar medium and carbonaceous chondrites, but have never been identified in the reducing atmospheres of the outer solar system. Inc ompletely characterized complex organic solids (tholins) produced by i rradiating simulated Titan atmospheres reproduce well the observed UV/ visible/IR optical constants of the Titan stratospheric haze. Titan th olin and a tholin generated in a crude simulation of the atmosphere of Jupiter are examined by two-step laser desorption/multiphoton ionizat ion mass spectrometry. A range of two- to four-ring PAHs, some with on e to four alkylation sites are identified, with net abundance approxim ately 10(-4) g g-1 (grams per gram) of tholins produced. Synchronous f luorescence techniques confirm this detection. Titan tholins have prop ortionately more one- and two-ring PAHs than do Jupiter tholins, which in turn have more four-ring and larger PAHs. The four-ringed PAH chry sene, prominent in some discussions of interstellar grains, is found i n Jupiter tholins. Solid state C-13 NMR spectroscopy suggests congruen t-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 congruent-to 6% by mass in benzenes, heterocyclics, and PAHs with m ore than four rings. Condensed PAHs may contribute at most approximate ly 10% to the observed detached limb haze layers on Titan. As with int erstellar PAHs, the synthesis route of planetary PAHs is likely to be via acetylene addition reactions.