Spatial variations in Neptune’s aerosol and methane abundances from VLT/MUSE observations.

We present spectral observations of Neptune (covering 473 to 933 nm) made in 2019 with the MUSE Integral Field Unit Spectrometer at the Very Large Telescope in Chile. The observations were made in Narrow Field Mode with a pixel size of 0.025 arcsec, giving ~100 pixels across Neptune’s disc at a spatial resolution of ~0.1 arcsec. These spectral imaging observations have been analysed to determine the spatial variation of aerosol scattering properties and methane abundance in Neptune’s atmosphere, which also required the development of a novel spatial deconvolution algorithm. We find we are able to fit the observations very well with a slightly modified form of the ‘holistic’ aerosol model of Irwin et al., 2022 ( https://doi.org/10.1029/2022JE007189), which consists of three distinct layers: 1) an aerosol layer at ~5 bar, assumed to be composed of H₂S ice and photochemical haze; 2) a layer of photochemical haze and methane ice at the methane condensation level near ~2 bar; and 3) an extended layer of photochemical haze, based at 1-2 bar and reaching up through to the stratosphere. The darkening of the South Polar Wave (SPW) at ~60°S, and dark spots such as Voyager-2’s Great Dark Spot and the more recent NDS-2018 spot, visible in our data, is concluded to be due to a spectrally-dependent darkening (λ < 650 nm) of particles in the 5-bar aerosol layer. We also note a regular latitudinal variation of reflectivity in the southern hemisphere at wavelengths of very low methane absorption longer than ~650 nm, with brighter zones latitudinally separated by ~25°. This feature has spectral characteristics similar to that of an unusual deep bright spot DBS-2019 found in our data, and is found to be consistent with a brightening of the particles in the same 5-bar aerosol layer at λ > 650 nm. We find the properties of the overlying 2-bar aerosol layer is, to first-order, invariant with latitude, while variations in the opacity of the upper tropospheric haze layer reproduce well the observed reflectivity at methane-absorbing wavelengths, with higher abundances found at the equator and also in a narrow ‘zone’ at 80°S. Finally, we find the mean abundance of methane below its condensation level to be 6 – 7% at the equator reducing to ~3% polewards of ~25°S, although the absolute abundances are model dependent.