Juno plumbs the depths of Jupiter's atmospheric jet streams
NASA's Juno orbiter continues to return data from Jupiter that is changing our understanding of how the giant planet works. Having completed 10 close passes as near as 2,200 miles (3,500 kilometers) above the Jovian cloud tops, the unmanned probe has found evidence that the famous cloud bands extend deep into the atmosphere, as well as clues to the nature of the planet's core.
Ever since the first telescopes were trained on Jupiter, the multicolored bands that run horizontally east and west across the face of the gas giant have puzzled astronomers. They're now known to be globe-circling jet streams that change width, color, and intensity, but remain stable for long periods of time – much longer than similar phenomenon on Earth or other planets.
Why this is so remained unknown until Juno entered Jupiter's orbit on July 4, 2016. As the probe follows its highly elliptical orbit that takes it 8.1 million km (5.0 million mi) away from Jupiter and then close enough to pass under the planet's intense radiation belts, its Gravity Science experiment transmits and receives radio signals from NASA's Deep Space Network. By using the Doppler effect on the signals to measure changes in the spacecraft's velocity, it's possible to measure variations in Jupiter's gravitational field, which allows scientists to map and analyze the planet and its atmosphere.
Juno found that the bands don't sit on the top of the Jovian atmosphere, but are like icebergs – most of their mass is deep beneath the surface with the weather layers plunging to depths of 1,900 mi (3,000 km) in huge climactic cells that make up one percent of Jupiter's mass or the equivalent of three Earths. To put this into perspective, the Earth's entire atmosphere makes up only one millionth of our planet's mass.
Another finding was that Jupiter's atmosphere is asymmetrical with marked differences between the massive cyclones at the north and south poles, as are the jet stream bands. This is because the deeper the jets, the more massive they are. According to NASA, the magnitude of the gravitational asymmetry determines how deep the jet streams go.
"Galileo viewed the stripes on Jupiter more than 400 years ago," says Yohai Kaspi, Juno co-investigator from the Weizmann Institute of Science in Israel. "Until now, we only had a superficial understanding of them and have been able to relate these stripes to cloud features along Jupiter's jets. Now, following the Juno gravity measurements, we know how deep the jets extend and what their structure is beneath the visible clouds. It's like going from a 2D picture to a 3D version in high definition."
Another instrument, the Jovian Infrared Auroral Mapper (JIRAM), sent back infrared images that were able to study the weather layers down to a depth of 30 to 45 mi (50 to 70 km). The polar regions are very different from the middle latitudes with their bands. The poles are dominated by central cyclones – the north pole's has eight circumpolar cyclones ranging in diameter from 2,500 to 2,900 mi (4,000 to 4,600 km) and the south has five cyclones that are 3,500 to 4,300 mi (5,600 to 7,000 km) wide. Despite being packed close enough to touch one another, these cyclones didn't disrupt one another after seven months of observation.
"Prior to Juno we did not know what the weather was like near Jupiter's poles. Now, we have been able to observe the polar weather up-close every two months," says Alberto Adriani, Juno co-investigator from the Institute for Space Astrophysics and Planetology in Italy. "Each one of the northern cyclones is almost as wide as the distance between Naples, Italy and New York City – and the southern ones are even larger than that. They have very violent winds, reaching, in some cases, speeds as great as 220 mph (350 km/h). Finally, and perhaps most remarkably, they are very close together and enduring. There is nothing else like it that we know of in the solar system.
"The question is, why do they not merge? We know with Cassini data that Saturn has a single cyclonic vortex at each pole. We are beginning to realize that not all gas giants are created equal."
In addition, Juno revealed that the core of Jupiter under the weather layer rotates almost as if it's a rigid body – a fact that may have impact far beyond the one planet.
"This is really an amazing result, and future measurements by Juno will help us understand how the transition works between the weather layer and the rigid body below," says Tristan Guillot, a Juno co-investigator from the Université Côte d'Azur, Nice, France. "Juno's discovery has implications for other worlds in our solar system and beyond. Our results imply that the outer differentially-rotating region should be at least three times deeper in Saturn and shallower in massive giant planets and brown dwarf stars."
The video below shows how the interior of Jupiter's atmosphere is structured.