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Spectral determination of the colour and vertical structure of dark spots in Neptune’s atmosphere


Previous observations of dark vortices in Neptune’s atmosphere, such as Voyager 2’s Great Dark Spot (1989), have been made in only a few broad-wavelength channels, hampering efforts to determine these vortices’ pressure levels and darkening processes. We analyse spectroscopic observations of a dark spot on Neptune identified by the Hubble Space Telescope as NDS-2018; the spectral observations were made in 2019 by the Multi Unit Spectroscopic Explorer (MUSE) of the Very Large Telescope (Chile). The MUSE medium-resolution 475–933 nm reflection spectra allow us to show that dark spots are caused by darkening at short wavelengths (<700 nm) of a deep ~5 bar aerosol layer, which we suggest is the H2S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, with a spectral signature consistent with a brightening of the same 5 bar layer at longer wavelengths (>700 nm). This bright feature is much deeper than previously studied dark-spot companion clouds and may be connected with the circulation that generates and sustains such spots.

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Fig. 1: Processed VLT/MUSE spectral observations of Neptune (Obs-6).
Fig. 2: Analysis of NDS-2018 difference spectrum.
Fig. 3: Analysis of DBS-2019 difference spectrum.

Data availability

The raw VLT’s MUSE datasets studied in this paper (under ESO/VLT programme 0104.C-0187) are available from the ESO Portal at The reduced raw and deconvolved ‘cubes’ for the observation IDs 6–10 discussed in this paper are available at Data files associated with this analysis are available at

Code availability

The NEMESIS radiative transfer and retrieval code17 used in this study is Open Access and is available for download from GitHub18 or Zenodo20. The deconvolution code described in this paper is Python-based and still under development. However, the current version of this software is available from the corresponding author upon reasonable request.


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The VLT’s MUSE observations were performed at the European Southern Observatory (ESO), under proposal 0104.C-0187. We are grateful to the United Kingdom Science and Technology Facilities Council for funding this research (P.G.J.I.: ST/S000461/1) and also the United Kingdom Space Agency (N.A.T.: ST/R001367/1). S.P.-H. and A.S.-L. have been supported by the Spanish project PID2019-109467GB-I00 (MINECO/FEDER, UE), Elkartek21/87 KK- 2021/00061 and Grupos Gobierno Vasco IT-1742-22. Some of this research (G.S.O.) was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). L.N.F. and M.T.R. were supported by a European Research Council Consolidator Grant (under the European Union’s Horizon 2020 research and innovation programme, grant agreement No. 723890) at the University of Leicester. I.d.P. and S.L.C. were supported in part by NSF grant AST-1615004 to UC Berkeley. Tracking of the longitude of NDS-2018 and the time variation of DBS-2019 utilized observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. These observations are associated with programmes GO/DD-15502 and GO/DD-16057, which provided support for A.S., M.H.W., G.S.O. and L.S. For the purpose of Open Access, the corresponding author has applied a CC BY public copyright licence to any Author Accepted Manuscript version arising from this submission.

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Authors and Affiliations



The observations reported in this paper were obtained under ESO/VLT programme 0104.C-0187, led by P.G.J.I., and including L.N.F., G.S.O., M.T.R., A.S., N.A.T., D.T. and M.H.W. as co-investigators. The initial data reduction and analysis was performed by P.G.J.I. and the subsequent deconvolution was conducted by J.D. Interpretation of the spectral information was assisted by A.J., M.H.W., L.N.F., M.T.R., N.A.T., D.T., G.S.O., S.P.-H., L.S., A.S., I.d.P. and S.L.C., while interpretation of the dynamical implications were assisted by M.H.W., L.N.F., A.S.-L. and R.M.J. All authors contributed to the writing and editing of the paper.

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Correspondence to Patrick G. J. Irwin.

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Irwin, P.G.J., Dobinson, J., James, A. et al. Spectral determination of the colour and vertical structure of dark spots in Neptune’s atmosphere. Nat Astron 7, 1198–1207 (2023).

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