Colour and Tropospheric Cloud Structure of Jupiter from MUSE/VLT: Retrieving a Universal Chromophore

TL;DR

This study tests the universal chromophore hypothesis for Jupiter's coloration using VLT/MUSE spectroscopic data, finding a common chromophore with a steeper blue absorption gradient and a global ammonia ice cloud layer at 1.4 bars.

Contribution

It provides direct spectroscopic evidence supporting a universal chromophore with specific absorption properties and characterizes Jupiter's cloud structure using ground-based observations.

Findings

A single chromophore can model Jupiter's belts and GRS with a steeper blue absorption gradient.

The chromophore is located no deeper than 0.2 bars in the GRS and 0.7 bars elsewhere.

A global cloud layer at 1.4 bars is identified, consistent with ammonia ice.

Abstract

Recent work by Sromovsky et al. (2017, Icarus 291, 232-244) suggested that all red colour in Jupiter's atmosphere could be explained by a single colour-carrying compound, a so-called 'universal chromophore'. We tested this hypothesis on ground-based spectroscopic observations in the visible and near-infrared (480-930 nm) from the VLT/MUSE instrument between 2014 and 2018, retrieving a chromophore absorption spectrum directly from the North Equatorial Belt, and applying it to model spatial variations in colour, tropospheric cloud and haze structure on Jupiter. We found that we could model both the belts and the Great Red Spot of Jupiter using the same chromophore compound, but that this chromophore must exhibit a steeper blue-absorption gradient than the proposed chromophore of Carlson et al. (2016, Icarus 274, 106-115). We retrieved this chromophore to be located no deeper than 0.2+/-0.1 bars in the Great Red Spot and 0.7+/-0.1 bars elsewhere on Jupiter. However, we also identified some spectral variability between 510 nm and 540 nm that could not be accounted for by a universal chromophore. In addition, we retrieved a thick, global cloud layer at 1.4+/-0.3 bars that was relatively spatially invariant in altitude across Jupiter. We found that this cloud layer was best characterised by a real refractive index close to that of ammonia ice in the belts and the Great Red Spot, and poorly characterised by a real refractive index of 1.6 or greater. This may be the result of ammonia cloud at higher altitude obscuring a deeper cloud layer of unknown composition.