  Spectral observations of Neptune made in 2019 with the MUSE instrument at the
Very Large Telescope in Chile have been analysed to determine the spatial
variation of aerosol scattering properties and methane abundance in Neptune's
atmosphere. The darkening of the South Polar Wave (SPW) at $\sim$ 60$^\circ$S,
and dark spots such as the Voyager 2 Great Dark Spot is concluded to be due to
a spectrally-dependent darkening ($\lambda < 650$nm) of particles in a deep
aerosol layer at $\sim$ 5 bar and presumed to be composed of a mixture of
photochemically-generated haze and H$_2$S ice. We also note a regular
latitudinal variation of reflectivity at wavelengths of very low methane
absorption longer than $\sim$ 650 nm, with bright zones latitudinally separated
by $\sim$ 25$^\circ$. This feature, similar to the spectral characteristics of
a discrete deep bright spot DBS-2019 found in our data, is found to be
consistent with a brightening of the particles in the same $\sim$5-bar aerosol
layer at $\lambda > 650 $ nm. We find the properties of an overlying
methane/haze aerosol layer at $\sim$ 2 bar are, to first-order, invariant with
latitude, while variations in the opacity of an upper tropospheric haze layer
reproduce the observed reflectivity at methane-absorbing wavelengths, with
higher abundances found at the equator and also in a narrow `zone' at
$80^\circ$S. Finally, we find the mean abundance of methane below its
condensation level to be 6--7% at the equator reducing to $\sim$3% south of
$\sim$25$^\circ$S, although the absolute abundances are model dependent.
