Galileo got it wrong – official: Jupiter actually wet, not super-dry: 'No one would have guessed that water might be so variable across the planet'

The 1990s spacecraft, that is

Jupiter contains more water than a previous study suggested, according to recordings from NASA's Juno probe, which were published in Nature Astronomy this month.

Juno, launched in 2011, has performed several flybys of Jupiter. Its Microwave Radiometer (MWR) instrument used six antennas to measure the temperature of the planet’s atmosphere at various depths simultaneously, allowing boffins to figure out how much water was present in the gas giant's atmosphere along its equator.

At 93 miles below the top of the atmosphere, Juno was able to collect more accurate information compared to NASA’s Galileo spacecraft at 75 miles in the 1990s. Galileo's data suggested the alien world was rather dry, much to scientists' disappointment: water readings from the satellite's spectrometer were ten times lower than expected. Now Juno has provided what looks like a correct measurement, we're told.

“We found the water in the equator to be greater than what the Galileo probe measured," said Cheng Li, first author of the study and a research fellow at the University of California, Berkeley, this week.

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According to the US space agency, "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," according to the Juno readings. The agency also clarified the comparison with our star "is based not on liquid water but on the presence of its components, oxygen and hydrogen, present in the Sun."

Measuring the abundance of water on the gas giant allows eggheads to piece together how it formed. Scientists believe it was the first planet to take shape when the Solar System began forming about 4.5 billion years ago, and Jupiter contains most of the dust and gas that wasn’t sucked up by the Sun.

The team believe the latest measurements show the planetesimals that glommed together to form Jupiter were not made out of clathrate hydrates, a type of water-rich solid that resembles ice.

Water also affects the planet's meteorology, including lightning and wind flows. Lightning requires moisture, and although bolts hundreds of times more powerful than those on Earth have been observed multiple times on the gas giant, it has been tricky to estimate how much water its atmosphere contains.

The Galileo spacecraft readings suggested the amount of water seemed to increase at deeper depths where there was more atmospheric pressure. An infrared heat map revealed Galileo probably just analysed a spot that was unusually warm and dry.

Juno’s measurements provide further proof that Jupiter’s atmosphere is patchy: some regions contain more water than others. Scientists are still trying to figure out why.

"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,” said Scott Bolton, co-author of the research and a principal investigator on the Juno mission at the Southwest Research Institute. “No one would have guessed that water might be so variable across the planet." ®

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