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

TL;DR

This study uses spectroscopic observations to analyze Neptune's dark spots, revealing they are caused by a darkening of a deep aerosol layer around 5 bars, and identifies a bright spot linked to circulation patterns.

Contribution

It provides the first detailed spectral analysis of Neptune's dark spots, pinpointing their cause to a deep aerosol layer and revealing new insights into their vertical structure.

Findings

Dark spots are caused by darkening of a ~5-bar aerosol layer.

Bright spots are linked to brightening of the same layer at different wavelengths.

Spectroscopy reveals the vertical structure and composition of Neptune's dark features.

Abstract

Previous observations of dark vortices in Neptune's atmosphere, such as Voyager-2's Great Dark Spot, have been made in only a few, broad-wavelength channels, which has hampered efforts to pinpoint their pressure level and what makes them dark. Here, we present Very Large Telescope (Chile) MUSE spectrometer observations of Hubble Space Telescope's NDS-2018 dark spot, made in 2019. These medium-resolution 475 - 933 nm reflection spectra allow us to show that dark spots are caused by a darkening at short wavelengths (< 700 nm) of a deep ~5-bar aerosol layer, which we suggest is the H$_2$S condensation layer. A deep bright spot, named DBS-2019, is also visible on the edge of NDS-2018, whose spectral signature is 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.