Latitudinal Variations in Methane Abundance, Aerosol Opacity and Aerosol Scattering Efficiency in Neptune's Atmosphere Determined From VLT/MUSE

Spectral observations of Neptune made in 2019 with the Multi Unit Spectroscopic Explorer (MUSE) instrument at the Very Large Telescope (VLT) in Chile have been analyzed to determine the spatial variation of aerosol scattering properties and methane abundance in Neptune's atmosphere. The darkening of the South Polar Wave at similar to 60 degrees 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 similar to 5 bar and presumed to be composed of a mixture of photochemically generated haze and H2S ice. We also note a regular latitudinal variation of reflectivity at wavelengths of very low methane absorption longer than similar to 650 nm, with bright zones latitudinally separated by similar to 25 degrees. This feature, which has similar spectral characteristics to 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 similar to 5-bar aerosol layer at lambda > 650 nm. We find the properties of an overlying methane/haze aerosol layer at similar to 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 degrees S. Finally, we find the mean abundance of methane below its condensation level to be 6%-7% at the equator reducing to similar to 3% south of similar to 25 degrees S, although the absolute abundances are model dependent.