Jupiter's Temperate Belt/Zone Contrasts Revealed at Depth by Juno Microwave Observations

TL;DR

Juno's microwave observations reveal that Jupiter's belts and zones exhibit contrasting properties at different depths, with a transition layer indicating changes in ammonia distribution and temperature, affecting the planet's atmospheric dynamics.

Contribution

This study identifies a depth-dependent transition in Jupiter's belt/zone contrasts using microwave data, linking it to ammonia and temperature gradients and their impact on atmospheric shear.

Findings

The transition layer (jovicline) is between 4 and 10 bars deep.

Belts are ammonia-depleted and warmer above the jovicline, and ammonia-enriched and cooler below.

Vertical shear in zonal winds varies depending on whether temperature or ammonia gradients dominate.

Abstract

Juno Microwave Radiometer (MWR) observations of Jupiter's mid-latitudes reveal a strong correlation between brightness temperature contrasts and zonal winds, confirming that the banded structure extends throughout the troposphere. However, the microwave brightness gradient is observed to change sign with depth: the belts are microwave-bright in the $p<5$ bar range and microwave-dark in the $p>10$ bar range. The transition level (which we call the jovicline) is evident in the MWR 11.5 cm channel, which samples the 5-14 bar range when using the limb-darkening at all emission angles. The transition is located between 4 and 10 bars, and implies that belts change with depth from being NH$_3$-depleted to NH$_3$-enriched, or from physically-warm to physically-cool, or more likely a combination of both. The change in character occurs near the statically stable layer associated with water condensation. The implications of the transition are discussed in terms of ammonia redistribution via meridional circulation cells with opposing flows above and below the water condensation layer, and in terms of the `mushball' precipitation model, which predicts steeper vertical ammonia gradients in the belts versus the zones. We show via the moist thermal wind equation that both the temperature and ammonia interpretations can lead to vertical shear on the zonal winds, but the shear is $\sim50\times$ weaker if only NH$_3$ gradients are considered. Conversely, if MWR observations are associated with kinetic temperature gradients then it would produce zonal winds that increase in strength down to the jovicline, consistent with Galileo probe measurements; then decay slowly at higher pressures.