Jupiter is not known as a water world since it has no oceans, lakes or other kinds of liquid water or ice. It doesn’t even have a solid surface beneath its deep, turbulent atmosphere. But there is some water in the giant planet’s atmosphere, and now thanks to NASA’s Juno spacecraft, we have a much better idea just how much.
The first science results regarding the amount of water have just been published in the journal Nature Astronomy. These are the first direct measurements taken since the Galileo mission in 1995.
Juno found that, at the equator, water molecules make up 0.25% of Jupiter’s atmosphere. This isn’t liquid water as we think of on Earth, not even as rain, but rather the molecules, oxygen and hydrogen, that make up water.
Even the Sun has some water molecules, but about three times less than Jupiter.
Juno uses a new kind of instrument – the Microwave Radiometer (MWR) – to obtain data on water abundance in Jupiter’s atmosphere. MWR has six antennas that can measure the temperature of the atmosphere at various depths simultaneously. It can do this across large portions of the planet.
How does that work? The instrument makes use of a fact utilized by microwave ovens, that water absorbs certain wavelengths of microwave radiation. MWR can use the measured temperatures to determine the amount of water, and ammonia, in different regions deep in the atmosphere. Ammonia molecules can also absorb microwave radiation.
The water data was collected by Juno during the first eight of its science flybys of Jupiter, so there is a lot of data to be studied. The first data came from Jupiter’s equatorial region since the atmosphere is more well-mixed there than in other regions of the planet. Juno was able to obtain data from as deep as 93 miles (150 kilometers), where the pressure reaches about 480 pounds per square inch (33 bar), compared to 75 miles (120 kilometers) for Galileo, where the atmospheric pressure is about 320 psi (22 bar).
“We found the water in the equator to be greater than what the Galileo probe measured,” said Cheng Li, a Juno scientist at 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.”
Scientists have been wanting to know more about Jupiter’s water ever since Galileo took its first initial measurements. The water content can provide valuable clues as to how Jupiter and the rest of the Solar System formed. Current theories about how Jupiter formed depend on the amount of water it still contains in its atmosphere. This water content also affects how winds work in the planet’s turbulent atmosphere. Until now, scientists have had limited information as to how much water there is deep in the atmosphere.
One surprise found previously by Galileo was that the amount of water appeared to keep increasing with depth, below the point where scientists thought the atmosphere was well-mixed. Later comparison with an infrared map, obtained at the same time by an Earth-based telescope, suggested that Galileo may have just found an unusually dry and warm spot in the atmosphere.
“Just when we think we have things figured out, Jupiter reminds us how much we still have to learn,” said Scott Bolton, Juno principal investigator at the Southwest Research Institute in San Antonio. “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.”
Each orbit of Juno around Jupiter takes 53 days, and that orbit is now gradually moving farther north on the planet. The science team is interested in how the water amounts vary with latitude, especially near the poles, where the atmosphere is dominated by powerful cyclones.
Juno’s 24th science flyby of Jupiter was on Feb. 17, 2020 and the next science flyby takes place on April 10, 2020. These close-up studies of Jupiter have revolutionized our understanding of the largest planet in the Solar System. As is common in planetary science, there have been surprises, and Juno has indeed revealed “a whole new Jupiter.”
“Every science flyby is an event of discovery,” said Bolton. “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.”
Juno was launched on Aug. 5, 2011 on an Atlas V rocket from the Cape Canaveral Air Force Station in Florida.
More information about Juno is available on the mission website.