Latitudinal variation of methane mole fraction above clouds in Neptune's atmosphere from VLT/MUSE-NFM: Limb-darkening reanalysis

Abstract

We present a reanalysis of visible/near-infrared (480-930 nm) observations of Neptune, made in 2018 with the MUSE instrument at the Very Large Telescope (VLT) in Narrow Field Adaptive Optics mode, reported by Irwin et al., Icarus, 311, 2019. We find that the inferred variation of methane abundance with latitude in our previous analysis, which was based on central meridian observations only, underestimated the retrieval errors when compared with a more complete assessment of Neptune's limb darkening. In addition, our previous analysis introduced spurious latitudinal variability of both the abundance and its uncertainty, which we reassess here. Our reanalysis of these data incorporates the effects of limb-darkening based upon the Minnaert approximation, which provides a much stronger constraint on the cloud structure and methane mole fraction, makes better use of the available data and is more computationally efficient. We find that away from discrete cloud features, the observed reflectivity spectrum from 800-900 nm is very well approximated by a background cloud model that is latitudinally varying, but zonally symmetric, consisting of a H$_2$S cloud layer, based at 3.6-4.7 bar with variable opacity and scale height, and a stratospheric haze. The background cloud model matches the observed limb darkening seen at all wavelengths and latitudes and we find that the mole fraction of methane at 2-4 bar, above the H$_2$S cloud, but below the methane condensation level, varies from 4-6\% at the equator to 2-4\% at near the south pole, consistent with previous analyses, with a equator/pole ratio of $1.9 \pm 0.2$ for our assumed cloud/methane model. The spectra of discrete cloudy regions are fitted, to a very good approximation, by the addition of a single vertically thin methane ice cloud with opacity ranging from 0 - 0.75 and pressure less than $\sim 0.4$ bar.