The Altitude of an Infrared-bright Cloud Feature on Neptune from Near-Infrared Spectroscopy

We present 2.03-2.30 μm near-infrared spectroscopy of Neptune taken 1999 June 2 (UT) with the W. M. Keck Observatory's near-infrared spectrometer (NIRSPEC) during the commissioning of the instrument. The spectrum is dominated by a bright cloud feature, possibly a storm or upwelling, in the southern hemisphere at approximately 50° south latitude. The spectrum also includes light from a dimmer northern feature at approximately 30° north latitude. We compare our spectra (λ/Δλ~2000) of these two features with a simple model of Neptune's atmosphere. Given our model assumption that the clouds are flat reflecting layers, we find that the top of the bright southern cloud feature sat at a pressure level of 0.14^+0.05_-0.03 bars, and thus this cloud did not extend into the stratosphere (P<~0.1 bars). A similar analysis of the dimmer northern feature yields a cloud-top pressure of 0.084+/-0.026 bars. This suggests that the features we observed efficiently transport methane to the base of the stratosphere but do not directly transport methane to the upper stratosphere (P<10^-2 to 10^-3 bars), where photolysis occurs. Our observations do not constrain how far these clouds penetrate down into the troposphere. We find that our model fits to the data restrict the fraction of H₂ in ortho-para thermodynamic equilibrium to greater than 0.8. Data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.