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P.N. Romani (NASA - GSFC), E. Lellouch, B. B\'{e}zard (Observatoire de Paris, Section de Meudon), H. Feuchtgruber (Max-Planck Inst. fur Extraterrestrische Physik)
ISO SWS observations of H2O and CO2 on Neptune (Feuchtgruber et al. 1997, Nature 389, 159) coupled with ground based millimeter observations of CO (Marten et al., 1993, Ap. J. 406 285) provide strong constraints on the photochemistry of these compounds in Neptune's stratosphere. Additional constraints on the photochemistry and vertical mixing come from the rich suite of hydrocarbons observed by ISO: CH4, C2H2, C2H6 (Bézard, 1998, Ann. Geophysicae 16, Sup. III C1037), CH3 (Bézard, et al. Ap. J. 515, 868), and recently C2H4 (Encrenaz et al. this DPS). We will use a one-dimensional global average photochemical model to analyze these compounds. In particular, we will better constrain the estimated external flux of H2O to Neptune's stratosphere given in Feuchtgruber et al., which is important in understanding the source of the observed stratospheric CO2.