Perspectives from the Juno Microwave Radiometer: Probing Jupiter?s Deep Rock Clouds, Thermochemistry and High-Energy Electron Precipitation over Northern Aurora

Abstract

Microwave remote sensing from Earth and space-borne instruments have been crucial to understanding the atmospheric dynamics of Jupiter and monitoring its space environment. Probing Jupiter’s atmosphere below the water cloud region offers valuable insights into Jupiter’s deep convective-diffusive transport and overall energy balance. The inventory of heavy elements in the Jovian envelope, including condensable volatile species, provides us with inference about the origin and evolution of Jupiter. Jupiter’s magnetic field interacts with its upper atmosphere through precipitation of energetic electrons and ions. The contribution of high-energy electrons to auroral heating, ionization, and chemical kinetic processes serves to couple the Jovian magnetosphere to its upper atmosphere.

The Juno Microwave Radiometer instrument measures the thermal radiation emitted by Jupiter’s atmosphere at six different frequency bands. It is sensitive to changes in the microwave opacity of the Jovian atmosphere. The 600 MHz channel is sensitive to microwave opacity sources originating between 100–1000 bar deep into the atmosphere. Rock cloud-forming alkali metals such as sodium and potassium, are expected to undergo thermal ionization at pressures exceeding 100 bar, providing free electrons opaque to microwave radiation. Measurement of Jovian thermal emission and its angular dependence constrains the free-electron density of the deep atmosphere. Free electrons produced by the thermal ionization of alkali vapors can be leveraged to constrain the abundance of sodium and potassium, revealed to be 10$^{-2}$ to 10$^{-5}$ times their solar photospheric abundances. This result contrasts with the enrichment of heavy elements at pressures above 20 bar measured by the Galileo Probe Mass Spectrometer, hinting at a potential compositional gradient. It has implications for heat and mass transport within the Jovian envelope.

Deep below the water clouds, the alkali metals react with other constituents in the Jovian atmosphere. The thermochemistry of alkali metals with other condensable volatile affects the concentration of charge carriers and alkali salt-cloud thickness. Numerical modeling of Jovian thermochemical equilibrium reveals the formation of stable anions such as HS$^{-}$ and Cl$^{-}$ driven by partial decomposition of atmospheric H$_{2}$S and HCl, and consequent electron attachment process. These high-temperature chemical reactions remove a proportion of free electrons, elevating the alkali metal abundances $sim$ 0.1 times their solar abundance. Alternatively, enrichment of either Na or K partially matches with MWR observations, contingent on the existence of alkali metals sequestered at levels deeper than pressure regimes sensitive to the MWR instrument. This analysis negates the possibility of a deep radiative layer on the basis of contemporary estimations of Jupiter's atmospheric opacity. Knowledge of atmospheric chemistry improves the understanding of heavy-element reservoirs in Jupiter's envelope.

The MWR measurements within the main auroral oval show low brightness temperatures compared to the deep thermal emission. Electron precipitation-driven ionization is expected to cause an increase in ionospheric electron density that leads to the absorption of microwaves near 1 GHz in a highly collisional medium. Electron energies with 1 MeV or higher penetrate below the methane homopause to increase microwave opacity, exhibiting a strong variation over tens of seconds. The microwave data provides a complementary perspective to understand magnetosphere-ionosphere-atmosphere coupling in addition to the in-situ electron flux and ultraviolet auroral emissions observed by Juno. Atmospheric models of microwave radiative transfer, chemical kinetics, and electron precipitation are applied to present a comprehensive theory of Jupiter’s deep atmosphere and polar aurorae, informed by synergistic observations from Juno.