Low Metallicity Alkali Metals Inferred from Juno MWR Observations

Limb darkening and brightness temperatures recorded by the Juno microwave radiometer (MWR) indicate an additional source of opacity in the deep atmosphere below 100 bar other than the conventional water and ammonia vapor. Alkali metals (Na and K) in the gas giants are expected to form clouds (Na₂S and KCl) in the deep atmosphere and undergo ionization to provide free electrons. It is shown that the free electrons corresponding to metallicities of -2 to -4 can act as the additional source of opacity and are in agreement with the MWR observations at 600 MHz. The electron density of the deep atmosphere is expected to be greater than the H-He system. The depleted abundance of alkali metals is found to be in contrast to the enrichment of the other heavy elements including C, N, S, and the noble gases Ar, Kr, and Xe.