We’re learning more about the composition of Jupiter’s atmosphere, and in particular, the amount of water therein, as a result of data from the Juno mission. The data come in the 1.25 to 22 GHz range from Juno’s microwave radiometer (MWR), depicting the deep atmosphere in the equatorial region. Here, water (considered in terms of its component oxygen and hydrogen) makes up about 0.25 percent of the molecules in Jupiter’s atmosphere, almost three times the percentage found in the Sun. All of this gets intriguing when compared to the results from Galileo.

You’ll recall that the Galileo probe descended into the Jovian atmosphere back in 1995, sending back spectrometer measurements of the amount of water it found down to almost 120 kilometers, where atmospheric pressure reached 320 pounds per square inch (22 bar). Unlike Juno, Galileo showed that Jupiter might be dry compared to the Sun — there was in fact ten times less water than expected — but it also found water content increasing even as it reached its greatest depth, an oddity given the assumption that mixing in the atmosphere would create a constant water content. Did Galileo run into some kind of meteorological anomaly?

A new paper in Nature Astronomy looks at the matter as part of its analysis of the Juno results, which also depict an atmosphere not well mixed:

The findings of the Galileo probe were puzzling because they showed that where ammonia and hydrogen sulfide become uniformly mixed occurs at a level much deeper (~10 bar) than what was predicted by an equilibrium thermochemical model. The concentration of water was subsolar and still increasing at 22 bar, where radio contact with the probe was lost, although the concentrations of nitrogen and sulfur stabilized at ~3 times solar at ~10 bar. The depletion of water was proposed to be caused by meteorological effects at the probe location. The observed water abundance was assumed not to represent the global mean water abundance on Jupiter, which is an important quantity that distinguishes planetary formation models and affects atmospheric thermal structure.

Now Juno has found water content greater than what Galileo measured. But the fact that Galileo showed a water concentration that was still increasing when the probe no longer could send data makes its results inconclusive. The matter is important for those interested in planet formation because as the likely first planet to form, Jupiter would have contained the great bulk of gas and dust that did not go into the composition of the Sun. Thus planet formation models are keyed to factors like the amount of water the young planet would have assimilated. Scott Bolton, Juno principal investigator at the Southwest Research Institute in San Antonio, comments:

“Just when we think we have things figured out, Jupiter reminds us how much we still have to learn. Juno’s surprise discovery that the atmosphere was not well mixed even well below the cloud tops is a puzzle that we are still trying to figure out. No one would have guessed that water might be so variable across the planet.”

Image: The JunoCam imager aboard NASA’s Juno spacecraft captured this image of Jupiter’s southern equatorial region on Sept. 1, 2017. The image is oriented so Jupiter’s poles (not visible) run left-to-right of frame. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.

The research team, led by Cheng Li (JPL/Caltech) used data from Juno’s first eight science flybys, focusing on the equatorial region first because the atmosphere appears to be better mixed there than in other regions. Juno’s microwave radiometer can measure the absorption of microwave radiation by water at multiple depths at the same time. Using these methods, Juno could collect data from deeper in the atmosphere than Galileo, where pressures reach about 480 psi (33 bar). The next move will be to compare this with other regions, giving us a picture of water abundance as Juno coverage extends deeper into Jupiter’s northern hemisphere. Of particular interest will be what Juno will find at the planet’s poles.

From the paper:

We have shown that the structure of Jupiter’s EZ [equatorial zone] is steady, relatively uniform vertically and close to a moist adiabat [a region where heat does not enter or leave the system]; from this we have derived its water abundance. The thermal structure outside of the equator is still ambiguous owing to the non-uniform distribution of ammonia gas, for which we do not know the physical origin. Deriving the thermal structure outside of the equator in the future not only hints about the water abundance on Jupiter at other latitudes but also places constraints on the atmospheric circulation model for giant planets in the Solar System and beyond.

Image: Thick white clouds are present in this JunoCam image of Jupiter’s equatorial zone. These clouds complicate the interpretation of infrared measurements of water. At microwave frequencies, the same clouds are transparent, allowing Juno’s Microwave Radiometer to measure water deep into Jupiter’s atmosphere. The image was acquired during Juno’s flyby of the gas giant on Dec. 16, 2017. Credit: NASA/JPL-Caltech/SwRI/MSSS/Kevin M. Gill.

The authors add that Juno has already revealed a deep atmosphere that is surprisingly variable as a function of latitude, highlighting the need to tread cautiously before making any assumptions about the planet’s overall water abundance. Extending these observations into other regions of the planet will be useful because oxygen is the most common element after hydrogen and helium in Jupiter’s atmosphere, and as water ice may thus have been the primary condensable in the protoplanetary disk. Consider this a deep probe into planet formation.

The paper is Li et al., “The water abundance in Jupiter’s equatorial zone,” Nature Astronomy 10 February 2020 (abstract).