The CO Abundance on Neptune from HST Observations

We have marginally detected two CO absorption bands in the UV reflection 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 Å. The interpretation of these features with a radiative transfer model—including Rayleigh, Raman, and particulate scattering processes—implies a CO mole fraction of (2.7 ± 1.8) × 10−6in the 30–800 mbar pressure range. The total uncertainty takes into account the statistical error bars as well as systematic effects (correction for straylight in the FOS data, temperature dependence of the CO absorption). Our CO mole fractions are in agreement with previous determinations made from millimeter measurements of CO rotational lines both in the stratosphere and in the troposphere. Carbon monoxide thus appears to be well-mixed in Neptune's atmosphere, which would confirm its internal origin. We have also examined the possible influence of NH3and HC3N on Neptune's spectrum in the 1950–2150 Å range, but could only estimate upper limits for these species (respectively 3 × 10−4and 6 × 10−3cm-am) that are much larger than the abundances predicted by current photochemical models.