Latitudinal variations in methane abundance, aerosol opacity and aerosol scattering efficiency in Neptune's atmosphere determined from VLT/MUSE

Journal article

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 ∼ 60◦S, and dark spots such as the Voyager 2 Great Dark Spot is concluded to be due to a spectrally-dependent darkening (λ < 650nm) of particles in a deep aerosol layer at ∼ 5 bar and presumed to be composed of a mixture of ~ 650 nm, with bright zones latitudinally separated by ∼ 25◦ . 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 ∼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.

Authors: Irwin, P; Dobinson, J; James, A; Michael, W; Leigh, F

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Geophysical Union
• Journal: Journal of Geophysical Research: Planets
• Volume: 128
• Issue: 11
• Article number: e2023JE007980
• Publication date: 2023-11-16
• Acceptance date: 2023-10-20
• DOI: 10.1029/2023JE007980
• EISSN: 2169-9100
• ISSN: 2169-9097
• Language: English
• Keywords: FFR