THE CO ABUNDANCE ON NEPTUNE FROM HST OBSERVATIONS

We have marginally detected two CO absorption bands in the UV reflecti on spectrum of Neptune measured by the HST Faint Object Spectrograph ( FOS). They are the (1, 0) and (0, 0) Cameron bands at 1992 and 2063 An gstrom. The interpretation of these features with a radiative transfer model--including Rayleigh, Raman, and particulate scattering processe s--implies a CO mole fraction of(2.7 +/- 1.8) x 10(-6) in the 30-800 m bar pressure range. The total uncertainty takes into account the stati stical error bars as well as systematic effects (correction for strayl ight in the FOS data, temperature dependence of the CO absorption). Ou r CO mole fractions are in agreement with previous determinations made from millimeter measurements of CO rotational lines both in the strat osphere and in the troposphere. Carbon monoxide thus appears to be wel l-mixed in Neptune's atmosphere, which would confirm its internal orig in. We have also examined the possible influence pf NH3 and HC3N on Ne ptune's spectrum in the 1950-2150 Angstrom range, but could only estim ate upper limits for these species (respectively 3 x 10(-4) and 6 x 10 (-3) cm-am) that are much larger than the abundances predicted by curr ent photochemical models. (C) 1996 Academic Press, Inc.