Prof. Patrick Irwin

Abstract:

We present an analysis of IRTF-TEXES spectra of Jupiter's mid-to-high latitudes in order to test the hypothesis that the CH4 homopause altitude is higher in Jupiter's auroral regions compared to elsewhere on the planet. A family of photochemical models, based on Moses & Poppe (2017), were computed with a range of CH4 homopause altitudes. Adopting each model in turn, the observed TEXES spectra of H2 S(1), CH4, and CH3 emission measured on 2019 April 16 and August 20 were inverted, the vertical temperature profile was allowed to vary, and the quality of the fit to the spectra was used to discriminate between models. At latitudes equatorward of Jupiter's main auroral ovals (>62°S, <54°N, planetocentric), the observations were adequately fit assuming a homopause altitude lower than ~360 km (above 1 bar). At 62°N, inside the main auroral oval, we derived a CH4 homopause altitude of ${461}_{-39}^{+147}$ km, whereas outside the main oval at the same latitude, a 1σ upper limit of 370 km was derived. Our interpretation is that a portion of energy from the magnetosphere is deposited as heat within the main oval, which drives vertical winds and/or higher rates of turbulence and transports CH4 and its photochemical by-products to higher altitudes. Inside the northern main auroral oval, a factor of ~3 increase in CH3 abundance was also required to fit the spectra. This could be due to uncertainties in the photochemical modeling or an additional source of CH3 production in Jupiter's auroral regions.

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.

Abstract:

We aim at investigating whether the chemical composition of the outer region of the protosolar nebula can be consistent with current estimates of the elemental abundances in the ice giants. To do so, we use a self-consistent evolutionary disc and transport model to investigate the time and radial distributions of H₂O, CO, CO₂, CH₃OH, CH₄, N₂ and H₂S, i.e. the main O-, C-, N and S-bearing volatiles in the outer disc. We show that it is impossible to accrete a mixture composed of gas and solids from the disc with a C/H ratio presenting enrichments comparable to the measurements (approx. 70 times protosolar). We also find that the C/N and C/S ratios measured in Uranus and Neptune are compatible with those acquired by building blocks agglomerated from solids condensed in the 10-20 arb. units region of the protosolar nebula. By contrast, the presence of protosolar C/N and C/S ratios in Uranus and Neptune would imply that their building blocks agglomerated from particles condensed at larger heliocentric distances. Our study outlines the importance of measuring the elemental abundances in the ice giant atmospheres, as they can be used to trace the planetary formation location, the origin of their building blocks and/or the chemical and physical conditions of the protosolar nebula. This article is part of a discussion meeting issue 'Future exploration of ice giant systems'.