An update on the Jupiter water mystery

Water makes up about 0.25% of the molecules in the atmosphere at Jupiter’s equator, almost three times that of the Sun.

That estimate, published in the journal Nature Astronomy, comes courtesy of NASA’s Juno mission, and provides a critical missing piece to the puzzle of our Solar System’s formation.

It is the first information on the planet’s abundance of water since NASA’s 1995 Galileo mission suggested it might be extremely dry compared to the Sun – a comparison based not on water, but on the presence of its components, oxygen and hydrogen, in the Sun.

Jupiter was likely the first planet to form, and 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 implications for the gas giant’s meteorology 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 has remained elusive.

Before the Galileo probe stopped transmitting in December 1995, it sent out spectrometer measurements to a depth of about 120 kilometres, and scientists working on the data were dismayed to find 10 times less water than expected.

Surprisingly, the amount of water 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.

“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,” says the mission’s principal investigator, Scott Bolton.  “No one would have guessed that water might be so variable across the planet.”

In the latest work, scientists used data collected during Juno’s first eight Jupiter flybys 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 radiometer was able to collect data from 150 kilometres into Jupiter’s atmosphere 

“We found the water in the equator to be greater than what the Galileo probe measured,” says Cheng Li, from 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.”