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. The visible–near-infrared spectrum of the dark spot that appeared on Neptune in 2018 indicates the presence of material that makes the aerosol layer at 5 bar darker at visible wavelengths. Such material can come from deeper layers via upwelling or by sublimation of H2S ice that reveals the darker condensation nuclei.