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Prediction for the deep compositions of Uranus and Neptune

Presentation #409.01 in the session Uranus and Neptune Systems (Oral Presentation)

Published onOct 23, 2023
Prediction for the deep compositions of Uranus and Neptune

Uranus and Neptune are the outermost giant planets of our solar system. The apparent size of these two planets in the sky is roughly a factor of 10 smaller than Jupiter and Saturn, making their physical properties much more difficult to characterize. The bulk compositions of Uranus and Neptune are poorly known. Interior models suggest that these planets contain approximately 10–25% of H–He by mass, where the exact number depends on the assumed composition of the heavy elements. Therefore, although there are estimates for the total heavy-element mass, the available data are insufficient to discriminate among different elemental compositions, implying that the water-to-rock ratio remains unknown. Often models assume a large region of the planet presenting high volatiles enrichments, as shown by the measurement of the C abundance, found to be enriched approximately 100 times its protosolar value in both Uranus and Neptune. By contrast, recent measurements suggest that N/H and S/H ratios might be instead subsolar in their envelopes. However, the potential presence of hidden reservoirs of nitrogen and sulfur at deeper levels would imply that volatiles are not homogeneously mixed in the envelopes. At present, it remains unclear whether the measured C abundance represents the planetary bulk. Often it is assumed that this is the case, which corresponds to fully mixed (convective) planetary interiors. However, updated formation, evolution and structure models of the ice giants suggest that both Uranus and Neptune are unlikely to be fully mixed. Nevertheless, it remains useful to use the measured abundances and link them to possible formation scenarios.

Here, we investigate the deep composition of Uranus assuming the planet formed at the location of the carbon monoxide iceline in the protosolar nebula, in agreement with the measured high carbon enrichment. To do so, we use a self-consistent evolutionary disk and transport model to investigate the time and radial distributions of H2O, CO, CO2, CH3OH, CH4, N2, NH3, H2S, PH3, Ar, Kr, and Xe, i.e. the main volatiles of interest in the protosolar nebula. To do so, we use a disk and transport model that accounts for the transport of dust particles and vapors within an evolving protoplanetary disk. Trace species are considered in four distinct forms: vapors, pure condensates, entrapped in clathrates, or forming a monohydrate (case for NH3 only). The sublimation and condensation of pure ices, as well as clathrate destabilization and formation within the disk, are also considered in the model.

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