Limb darkening values of Jupiter’s atmosphere at 600 MHz measured by Juno Microwave Radiometer

Sensing potential microwave opacity sources well below the water cloud potentially allows us to probe whether rock-forming species are present in Jupiter—here, specifically, the alkali metals.  The measurement of limb darkening (relative change of the brightness temperature from nadir viewing to limb viewing at the 45-degree emission angle) by the Juno Microwave Radiometer (MWR) is very precise ( 0.1%) due to the stability of the instrument. We analyzed the MWR data from perijove 1 to perijove 12 and found that the 600 MHz channel of the MWR observed a consistent limb darkening value of around 14% from 40^oS to 40^oN for Jupiter’s atmosphere while thermodynamic models predict that the limb darkening should be about 18%. The 4% difference is well above the uncertainty of the measurement. We construct end-member models to investigate the possible cause. We have examined the effect of 1) ammonia depletion, 2) the existence of a deep radiative layer between 1000 ~ 2000 K, 3) concentration of alkali metals, 4) opacity models of water vapor continuum and 5) opacity models of ammonia and concluded that the most likely cause is the presence of alkali metals, which thermally dissociate at temperatures > 1000 K. Other factors may also contribute to the anomalous limb darkening. To fully resolve the degeneracy, laboratory measurements of the opacities of ammonia and water at high temperatures are recommended.