Dr. Imke dePater - Radio Observations

Microwave spectrum of Uranus, with superimposed model calculations. The dashed line is a model with a composition that is approximately solar. The coloured lines are for models in which the abundances of H2O, H2S and CH4 in the planet's deep atmosphere are enhanced by a factor of 10 over the solar O, S and C values. Ammonia was kept at the solar N value and is therefore removed above the NH4SH cloud layer. The cyan line shows a calculation in which only opacity by NH3 (together with collision-induced opacity from hydrogen gas) is included. The dark blue curve shows the result when opacity from H2S is included and NH3 is ignored; the red curve has opacity from all gases included. (from: de Pater, I., 2018. Selective enrichment of volatiles confirmed. Nature Astronomy, 2, 364-365.

Excerpts from de Pater, 2018: Compared with the Sun, the giant planets are enriched in elements heavier than helium by factors of 5-15 for Jupiter and Saturn, and about 300 for Uranus and Neptune. The differences between Jupiter/Saturn and Uranus/Neptune can then readily be explained by a much slower accretion rate of the latter planets due to their larger distance from the Sun, where the protoplanetary disk was less dense. With the removal of gas from the protoplanetary nebula 1-10 Myr after the proto-Sun formed, Uranus and Neptune never had the time to grow into gas giants like Jupiter and Saturn. But details of the formation and evolutionary processes remain active areas of research today. A key parameter in all of these studies is the precise composition of a planet. At the temperatures and pressures prevalent in the giant planet atmospheres, the elements carbon, oxygen, nitrogen and sulfur are present in the form of water (H2O), methane (CH4), NH3 and H2S gases. Detection and quantification of these gases, which condense out and form clouds in the upper layers of the atmosphere, allows us to get information on the bulk elemental abundance. For example, at about 40 bar for Uranus/Neptune (or 2-5 bar for Jupiter/ Saturn), one molecule of NH3 will combine with one molecule of H2S to form solid NH4SH. This process will deplete all the H2S or NH3, whichever molecule is less abundant, which in turn depends on the N/S bulk ratio. At solar abundances N/S ~ 5, which would lock all H2S up within the NH4SH cloud, leaving nothing above it. At higher altitudes, NH3 and/or H2S will condense into their own ice clouds. Microwave observations probe layers in giant planets' atmospheres at and below their cloud layers, because clouds are relatively transparent at these wavelengths. The opacity at radio wavelengths is dominated by NH3 gas. Radio spectra of Jupiter and Saturn can be matched reasonably well with models of a near-solar coposition atmosphere, or one in which all heavy elements are enriched by a factor of a few. Radio spectra of a solar composition atmosphere for Uranus and Neptune, however, are much too cold compared with observations, as shown by the dashed line in the figure above.This implies that a solar-composition atmosphere has too much NH3 gas. This dilemma can be reconciled by enhancing all of the species in the atmosphere except for NH3, implying a bulk N/S ratio less than 1. This value would lock up all NH3 gas in the NH4SH cloud layer. Detailed modeling showed that in addition to opacity by NH3 gas, H2S gas must be a major absorber as well, shown by te variious colored lines above. The presence of H2S gas above the NH4SH cloud has been confirmed via direct spectroscopic measurements in the infrared by Irwin et al. (2018). See de Pater, 2018 for more details.

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