  Observations of Neptune, made in 2018 using the new Narrow Field Adaptive
Optics mode of the Multi Unit Spectroscopic Explorer (MUSE) instrument at the
Very Large Telescope (VLT) from 0.48 - 0.93 micron, are analysed here to
determine the latitudinal and vertical distribution of cloud opacity and
methane abundance in Neptune's observable troposphere (0.1 - ~3 bar). Previous
observations at these wavelengths in 2003 by HST/STIS (Karkoschka and Tomasko
2011, Icarus 205, 674-694) found that the mole fraction of methane above the
cloud tops (at ~2 bar) varied from ~4% at equatorial latitudes to ~2% at
southern polar latitudes, by comparing the observed reflectivity at wavelengths
near 825 nm controlled primarily by either methane absorption or H2-H2/H2-He
collision-induced absorption. We find a similar variation in cloud-top methane
abundance in 2018, which suggests that this depletion of methane towards
Neptune's pole is potentially a long-lived feature, indicative of long-term
upwelling at mid-equatorial latitudes and subsidence near the poles. By
analysing these MUSE observations along the central meridian with a retrieval
model, we demonstrate that a broad boundary between the nominal and depleted
methane abundances occurs at between 20 - 40S. We also find a small depletion
of methane near the equator, perhaps indicating subsidence there, and a local
enhancement near 60 - 70S, which we suggest may be associated with South Polar
Features (SPFs) seen in Neptune's atmosphere at these latitudes. Finally, by
the use of both a reflectivity analysis and a principal component analysis, we
demonstrate that this depletion of methane towards the pole is apparent at all
locations on Neptune's disc, and not just along its central meridian.
