Latitudinal variation in the abundance of methane (CH4) above the clouds in Neptune's atmosphere from VLT/MUSE Narrow Field Mode Observations.

Observations of Neptune, made in 2018 using the new Narrow Field Adaptive Optics mode of the Multi Unit Spectroscopic Explorer (MUSE) instrument at theVery Large Telescope (VLT) from 0.48 - 0.93 microns, are analysed here to determinethe latitudinal and vertical distribution of cloud opacity and methane abundancein Neptune's observable troposphere (0.1 { ~3 bar). Previous observations atthese wavelengths in 2003 by HST/STIS (Karkoschka and Tomasko 2011, Icarus205, 674{694) found that the mole fraction of methane above the cloud tops (at~2 bar) varied from ~% at equatorial latitudes to ~2% at southern polar latitudes, by comparing the observed reectivity at wavelengths near 825 nm controlled primarily by either methane absorption or H2{H2/H2{He collision-inducedabsorption. We nd a similar variation in cloud-top methane abundance in 2018,which suggests that this depletion of methane towards Neptune's pole is potentiallya long-lived feature, indicative of long-term upwelling at mid-equatoriallatitudes and subsidence near the poles. By analysing these MUSE observationsalong the central meridian with a retrieval model, we demonstrate that a broadboundary between the nominal and depleted methane abundances occurs at between20-40 deg. S. We also find a small depletion of methane near the equator,perhaps indicating subsidence there, and a local enhancement near 60 - 70 deg. S,which we suggest may be associated with South Polar Features (SPFs) seen inNeptune's atmosphere at these latitudes. Finally, by the use of both a reflectivity analysis and a principal component analysis, we demonstrate that this depletionof methane towards the pole is apparent at all locations on Neptune's disc, andnot just along its central meridian.