2003A&A...403L...7F

The infrared spectrum of Neptune at 3.5-3.75µm and 3.87-4.1µm has been measured at a spectral resolution of 1200. The observed flux is stronger by a factor ∼3 compared to previous measurements, suggesting important meteorological variations between 1997 and 2002. The flux is detected mostly from a bright belt at mid-southern latitudes. Strong absorptions, identified as methane, are observed over the L band. The observed CH₄ spectrum can be fitted by a multilayer model assuming that the solar light is partly reflected above several layers, including the CH₄ haze and stratospheric photochemical hazes. An upper limit to a disk averaged column density of H₃⁺ of 2.9(+7.1-1.8)x10¹⁰.cm^–2 is reported, consistent with present ionospheric models.