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

Abstract

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 ∼60°S, and dark spots such as the Voyager 2 Great Dark Spot is concluded to be due to a spectrally dependent darkening ( λ < 650 nm) of particles in a deep aerosol layer at ∼5 bar and presumed to be composed of a mixture of photochemically generated haze and H ₂S ice. We also note a regular latitudinal variation of reflectivity at wavelengths of very low methane absorption longer than ∼650 nm, with bright zones latitudinally separated by ∼25°. 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 ∼5‐bar aerosol layer at λ > 650 nm. We find the properties of an overlying methane/haze aerosol layer at ∼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°S. Finally, we find the mean abundance of methane below its condensation level to be 6%–7% at the equator reducing to ∼3% south of ∼25°S, although the absolute abundances are model dependent.

Plain Language Summary

Observations of Neptune in visible light, made with the Multi Unit Spectroscopic Explorer (MUSE) instrument at ESO's Very Large Telescope, reveal the different layers of clouds and gases within this Ice Giant atmosphere, and how they change with height and latitude. The properties of the 1–2‐bar layer of methane ice and haze are found to be roughly constant with latitude. However, a diffuse upper layer is thickest at the equator and near the south pole, indicating air rising at mid‐latitudes and descending near the equator and poles. Conversely, the distribution of methane between the deep 5‐bar clouds (hydrogen sulphide ice and haze) and the middle layers decreases from 6% to 7% at the equator to ∼3% near the south pole, suggesting instead that air is rising at the equator and descending elsewhere. Finally, a blue‐green darkening of the particles in the deep layer can explain Neptune's dark spots and the dark “South Polar Wave” at 60°S, whereas a brightening of the same particles at red and infrared wavelengths matches occasional discrete deep bright spots and a set of previously unnoticed bright “zones”, separated by 25° latitude. All this is evidence that the atmospheric circulation changes as a function of height and latitude in complex and surprising ways.

Key Points

• Neptune Multi Unit Spectroscopic Explorer (MUSE) visible/near‐infrared spectra are well fitted by a simple aerosol model comprised of three distinct layers

• A darkening of particles at blue‐green wavelengths in a deep aerosol layer near 5 bar can explain dark spots and the dark “South Polar Wave”

• A brightening of the same particles at red‐infrared wavelengths can explain bright zones and spots seen in longwave narrow reflectance peaks