Microwave Sounding of Saturn and Uranus: Comparing Gas- and Ice-Giant Planets

Ground-based radio observations of the giant planets at millimeter to meter wavelengths probe the atmospheres from pressures near 0.1 bar to kilo-bars. We are sensitive to the abundances and distribution of NH₃, H₂O, and H₂S. These species are important because they are diagnostic of atmospheric dynamics and chemistry, and are potentially diagnostic of planetary formation models.

Saturn observations indicate that the tropospheric NH₃ abundance is 5 to 10x a solar abundance. If NH₃ is near the bottom of that range, H₂O must be ≥10x solar. If NH₃ is near the top of that range, we cannot constrain water. These results are broadly consistent with earlier works (e.g. de Pater and Massie, Icarus 1985), though details have changed due to improved models of atmospheric opacity, additional data, and perhaps because of temporal variations. Favored formation models for Saturn predict abundances comparable to what is observed (Atreya et al. 2018 and references therein).

At Uranus, we find strong vertical abundance variations indicative of cloud formation. Near pressures of 50 bar (just above the liquid water cloud) the NH₃ abundance is 0.2 to 0.6 times solar, and H₂S is 1 to 2 times solar. Water is not well constrained but is probably >3x solar. These results are consistent with earlier work on the upper troposphere (e.g. Gulkis et al, Icarus 1978), but are charting new territory at depth. If the retrieved abundances reflect bulk mixing ratios, they are inconsistent with most (but not all) formation models for Uranus, which favor mixing ratios >50x solar. Our preferred interpretation, however, is that a super-ionic water ocean deep within Uranus traps both NH₃ and H₂S.

Our results highlight three points. 1) The composition, chemistry, and interior structure of the gas giant planets are very different from the ice giants. 2) Microwave observations are useful probes of the dynamics, chemistry, and composition of the deep atmospheres of giant planets. 3) On Uranus, either unidentified atmospheric processes are modifying the abundance of species, or favored planetary formation models are incorrect.

Portions of this work were carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.