Abstract
Recent work by Sromovsky et al. (2017) 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). 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.
Highlights
Spatial and temporal variation in red colour on Jupiter is thought to be due to the presence of red colour-carrying compounds (‘chromophores’) whose origin, composition and altitude remain unknown
We analysed three spectral image cubes of Jupiter, obtained between 2014 and 2018 in the visible and near-infrared (480-930 nm) from the Very Large Telescope (VLT)/Multi-Unit Spectroscopic Explorer (MUSE) instrument, in order to a) characterise the absorption spectrum of Jupiter’s colourcarrying compounds (‘chromophores’) and whether it could be applied uniformly to model all blue-absorption on Jupiter (a ‘universal chromophore’), and b) to analyse spatial variability in colour and cloud/haze structure over Jupiter’s surface
As we were able to fit the shape of the shortwave spectral feature of both the belts and the Great Red Spot (GRS) well using the same chromophore compound, we cannot rule out the possibility of a universal chromophore as proposed by Sromovsky et al (2017)
Summary
Spatial and temporal variation in red colour on Jupiter is thought to be due to the presence of red colour-carrying compounds (‘chromophores’) whose origin, composition and altitude remain unknown. These data include spectra both of the GRS in its current deep red state and of the southern NTB when it was at its reddest in early 2017. We find that we require a systematic decrease in I/F of 20% over all wavelengths in order to have I/F values consistently less than 1 over the whole of Jupiter in this particular dataset This generally results in the MUSE spectra being correct to within 5% of both the Karkoschka and HST/WFC3 data, with some discrepancy in the F502N filter possibly due to changes in the GRS. The relatively poor calibration of the 2014 data needs to be taken into account when interpreting our atmospheric retrievals, of tropospheric cloud opacity which is most affected by systematic offsets in I/F
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