The infrared spectrum of Neptune at 3.5–4.1 microns: Search for H$\mathsf{_{3}^{+}}$ and evidence for recent meteorological variations

Abstract

The infrared spectrum of Neptune at $3.5{-}3.75~\mu$m and $3.87{-}4.1~\mu$m 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 ${\rm CH_4}$ spectrum can be fitted by a multilayer model assuming that the solar light is partly reflected above several layers, including the ${\rm CH_4}$ haze and stratospheric photochemical hazes. An upper limit to a disk averaged column density of ${\rm H}_{3}^{+}$ of $2.9\left({{+7.1}\atop{-1.8}}\right)\times10^{10}~{\rm cm^{-2}}$ is reported, consistent with present ionospheric models.