Published recently in the journal Nature Astronomy, the Juno results estimate that at the equator, water makes up about 0.25 per cent of the molecules in Jupiter's atmosphere – almost three times that of the sun.
These are also the first findings on the gas giant's abundance of water since the agency's 1995 Galileo mission suggested Jupiter might be extremely dry compared with the sun (the comparison is based not on liquid water but on the presence of its components, oxygen and hydrogen, present in the sun).
An accurate estimate of the total amount of water in Jupiter's atmosphere has been on the wish lists of planetary scientists for decades: the figure in the gas giant represents a critical missing piece to the puzzle of our solar system's formation.
Jupiter was likely the first planet to form, and it contains most of the gas and dust that wasn't incorporated into the sun.
The leading theories about its formation rest on the amount of water the planet soaked up. Water abundance also has important implications for the gas giant's meteorology (how wind currents flow on Jupiter) and internal structure.
While lightning – a phenomenon typically fuelled by moisture – detected on Jupiter by Voyager and other spacecraft implied the presence of water, an accurate estimate of the amount of water deep within Jupiter's atmosphere remained elusive.
Scott Bolton, Juno principal investigator at the Southwest Research Institute in San Antonio, said, "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."
A rotating, solar-powered spacecraft, Juno launched in 2011. Because of the Galileo probe experience, the mission seeks to obtain water abundance readings across large regions of the immense planet.
A new kind of instrument for deep space planetary exploration, Juno's microwave radiometer (MWR) observes Jupiter from above using six antennas that measure atmospheric temperature at multiple depths simultaneously.
The MWR takes advantage of the fact that water absorbs certain wavelengths of microwave radiation, the same trick used by microwave ovens to quickly heat food.
The measured temperatures are used to constrain the amount of water and ammonia in the deep atmosphere, as both molecules absorb microwave radiation.
Cheng Li, a Juno scientist at the University of California, Berkeley, said, "We found the water in the equator to be greater than what the Galileo probe measured. Because the equatorial region is very unique at Jupiter, we need to compare these results with how much water is in other regions."
Juno's 53-day orbit is slowly moving northward, as intended, bringing more of Jupiter's northern hemisphere into sharper focus with each fly-by.
The science team is eager to see how atmospheric water content varies by latitude and region, as well as what the cyclone-rich poles can tell them about the gas giant's global water abundance.
NASA's Jet Propulsion Laboratory in Pasadena, California, manages the Juno mission for Bolton,.
Juno is part of NASA's New Frontiers Program, which is managed at NASA's Marshall Space Flight Center in Huntsville, Alabama, for NASA's Science Mission Directorate. The Italian Space Agency contributed the Jovian Infrared Auroral Mapper and the Ka-Band translator system.
Lockheed Martin Space in Denver built and operates the spacecraft.
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