Source: Journal of Geophysical Research: Planets
Among Jupiter’s most notable attributes is its distinctive banded appearance. Planetary scientists call the light, whitish bands “zones” and the darker, reddish ones “belts.” Jupiter’s planetary-scale winds circulate in opposite directions on the boundaries of these alternating regions. A key question is whether the belts and zones are confined to the planet’s cloud tops, or whether they persist with increasing depth.
An investigation of this phenomenon is one of the primary objectives of NASA’s Juno mission, and the spacecraft carries a specially designed microwave radiometer not only to measure emissions from deep within the planet for the first time, but also to examine the nature of the belts and zones. Juno’s microwave radiometer operates in six wavelength channels ranging from 1.4 to 50 centimeters, and these enable Juno to probe the Jovian atmosphere at pressures starting near the cloud tops near 0.6 bar to pressures up to 1,000 bars (about 600 kilometers deep).
Fletcher et al. used data from the microwave radiometer and found that at the cloud tops, Jupiter’s belts are bright with microwave emission, whereas the zones are dark. This configuration persists down to approximately 5 bars. And at pressures deeper than 10 bars, the pattern reverses, with the zones becoming microwave-bright and the belts becoming dark.
The team terms the transition region between 5 and 10 bars the jovicline, a comparison with the thermocline region in Earth’s oceans, where seawater transitions sharply from relative warmth to relative coldness. The researchers observe that the jovicline is nearly coincident with a stable atmospheric layer created by condensing water.
According to the authors, there are two mechanisms that could be responsible for the change in brightness, each implying different physical conclusions. One mechanism is related to the ammonia distribution within the belts and zones. Ammonia is opaque to microwaves; thus, a region with relatively less ammonia will shine brighter in Juno’s observations. This mechanism could imply a stacked system of opposing circulation cells with sinking in belts at shallow depths and upwelling in belts at deeper levels, or vigorous storms and precipitation, moving ammonia gas from place to place.
Another possibility is that the gradient in emissions corresponds to a gradient in temperature, with higher temperatures resulting in greater microwave radiation. If this scenario is correct, then Jupiter’s winds may increase with depth below the clouds until we reach the jovicline, before tapering off into the deeper atmosphere—something that was also suggested by the Galileo probe in 1995.
The likely scenario is that both mechanisms are at work simultaneously, each contributing to part of the observed brightness variation. According to the authors, new atmospheric models based on the Juno discovery could help discriminate the relative importance of each mechanism, and this will eventually lead to broader understanding of how circulation cells, winds, and precipitation work in giant planet atmospheres. (Journal of Geophysical Research: Planets, https://doi.org/10.1029/2021JE006858, 2021)
—Morgan Rehnberg, Science Writer