Water makes up about 0.25% (almost three times that of the Sun) of the molecules in Jupiter’s atmospheric molecules at the gas giant’s equator, according to a new analysis of data collected by NASA’s Juno spacecraft.
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 December 16, 2017. Imager credit: NASA / JPL-Caltech / SwRI / MSSS / Kevin M. Gill.
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 fueled 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.
Before NASA’s Galileo probe stopped transmitting 57 minutes into its Jovian descent in December 1995, it radioed out spectrometer measurements of the amount of water in the gas giant’s atmosphere down to a depth of about 120 km (75 miles), where the atmospheric pressure reached about 320 pounds per square inch (22 bar).
The scientists working on the data were dismayed to find ten times less water than expected.
Even more surprising: the amount of water the Galileo probe measured appeared to be still increasing at the greatest depth measured, far below where theories suggest the atmosphere should be well mixed.
In a well-mixed atmosphere, the water content is constant across the region and more likely to represent a global average; in other words, it’s more likely to be representative of water planetwide.
When combined with an infrared map obtained at the same time by a ground-based telescope, the results suggested the probe mission may have just been unlucky, sampling an unusually dry and warm meteorological spot on Jupiter.
“Just when we think we have things figured out, Jupiter reminds us how much we still have to learn,” said Juno principal investigator Dr. Scott Bolton, a researcher at the Southwest Research Institute.
“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.”
Dr. Bolton and colleagues used data collected by Juno’s Microwave Radiometer (MWR) instrument during the orbiter’s first eight science flybys of Jupiter to generate the findings.
They initially concentrated on the equatorial region because the atmosphere there appears more well-mixed, even at depth, than in other regions.
From its orbital perch, the MWR was able to collect data from a far greater depth into Jupiter’s atmosphere than the Galileo probe — 150 km (93 miles), where the pressure reaches about 480 pounds per square inch (33 bar).
“We found the water in the equator to be greater than what the Galileo probe measured,” said Juno scientist Dr. Cheng Li, from the University of California, Berkeley.
“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 flyby.
The Juno 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.
Juno’s 24th science flyby of Jupiter occurred on February 17, 2020. The next science flyby takes place on April 10, 2020.
“Every science flyby is an event of discovery. With Jupiter there is always something new. Juno has taught us an important lesson: we need to get up close and personal to a planet to test our theories,” Dr. Bolton said.
The results were published in the journal Nature Astronomy.
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C. Li et al. The water abundance in Jupiter’s equatorial zone. Nat Astron, published online February 10, 2020; doi: 10.1038/s41550-020-1009-3
