Highly depleted alkali metals in Jupiter's deep atmosphere

TL;DR

This study suggests that alkali metals like sodium and potassium, with very low metallicity, are the likely source of unexplained deep atmospheric opacity in Jupiter, contrasting with other enriched heavy elements.

Contribution

It introduces the hypothesis that ionized alkali metals with sub-solar metallicity account for the deep atmosphere opacity observed by Juno's microwave radiometer.

Findings

Alkali metals can provide the missing opacity in Jupiter's deep atmosphere.

Alkali metals have a metallicity range of -2 to -5, much lower than other heavy elements.

If not from alkali metals, the metallicity at 1000 bars would be even lower.

Abstract

Water and ammonia vapors are known to be the major sources of spectral absorption at pressure levels observed by the microwave radiometer (MWR) on Juno. However, the brightness temperatures and limb darkening observed by the MWR at its longest wavelength channel of 50 cm (600 MHz) in the first 9 perijove passes indicate the existence of an additional source of opacity in the deep atmosphere of Jupiter (pressures beyond 100 bar). The absorption properties of ammonia and water vapor, and their relative abundances in Jupiter's atmosphere do not provide sufficient opacity in deep atmosphere to explain the 600 MHz channel observation. Here we show that free electrons due to the ionization of alkali metals, i.e. sodium, and potassium, with sub-solar metallicity [M/H] (log based 10 relative concentration to solar) in the range of [M/H] = -2 to [M/H] = -5 can provide the missing source of opacity in the deep atmosphere. If the alkali metals are not the source of additional opacity in the MWR data, then their metallicity at 1000 bars can only be even lower. The upper bound of -2 on the metallicity of the alkali metals contrasts with the other heavy elements -- C, N, S, Ar, Kr, and Xe -- which are all enriched relative to their solar abundances having a metallicity of approximately +0.5.